Content:

SM – Seismology

SM1.1 – General Contributions on Earthquakes, Earth Structure, Seismology

EGU2020-12702 | Displays | SM1.1 | Highlight | Arne Richter Award for Outstanding ECS Lecture

Environmental seismology: Listening to landslides whispering

Wei-An Chao

The rapidly emerging field of Environmental Seismology (EnviroSeis) uses seismological techniques to monitor geomorphic processes at Earth’s surface, providing non-invasive, relatively inexpensive, continuous constraints on physical properties and dynamics of surface processes including landslides, debris flows, snow avalanches, river sediment transport, and variations in groundwater table. EnviroSeis has direct ties to real-time geohazards monitoring and provides timely warnings for the hazard mitigation and assessment. Nowadays, the places in world with real-time seismic networks are ready to implement EnviroSeis. The major topic focused on here is how to provide relevant information on the deep-seated landslides associated to the three-time stages:

  • Pre-slide: (1) seismic precursor and (2) seismic velocity changes corresponding to basal sliding behavior.
  • Sliding: A real-time landquake monitoring (RLMS; http://collab.cv.nctu.edu.tw/main.html)
  • After-slide: (1) near-real-time monitoring of river sediment transport and (2) early warning of the landslide-generated tsunami.

How to cite: Chao, W.-A.: Environmental seismology: Listening to landslides whispering, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12702, https://doi.org/10.5194/egusphere-egu2020-12702, 2020.

EGU2020-10161 | Displays | SM1.1

Seismological Observations of the Seasonal Rain and Aquifer Induced Seismicity in Southeastern Brazil

Jaime A. Convers, Marcelo Assumpção, and Jose R. Barbosa

We update our analysis on the ongoing seasonal induced microseismic activity in southeastern Brazil, in the interior of the state of Sao Paulo.  This is an area that not evidenced any active seismicity before 2016. We monitor this phenomenon as it is similar to other episodes of seasonal seismicity in other regions of Brazil, under similar aquifer and host rock conditions, commonly associated with those of the Parana Basin. 

This induced seismicity is seemingly triggered yearly during the high-rain season in Southeast Brazil, between December and May, and ceases as soon as the heavy rain season ends each year.  In these periods of increased precipitation during the annual onset of seismicity, we have found more than 1500 seismic events of magnitudes up to M2.0 in since 2017, after we deployed seismic stations in this area. Using phase weighing earthquake locations algorithms, we examine the clustering of the seismicity around recently drilled water wells, and seismicity rate changes, as it is modified by variations in the precipitation.

We perform full moment tensor analysis when possible to find the seismic activity is not only clustering horizontally, but at depth as well.  We identify two main regions where events are more frequently occurring and have mostly prevalent sub-horizontal dipping planes: The shallow events between 100 and 200 m and from 600 to 700 m depth. 

This phenomenon is facilitated mainly by the inadequate water well perforation practices in the region. Uncased water wells promote the transport of both rainwater and groundwater from upper to lower aquifers during higher precipitation months. The stress conditions of the fractured basaltic rock inside the confined aquifers are affected by the intrusion and percolation of significant amounts of water, which produce pore-pressure changes inside the host rock, and facilitates stress release though the microseisms.  This implies that the confined aquifer characteristics of intermittent sandstone layers and fractured basalt rocks from the Parana Basin condition the characteristics of the seismicity occurring in this region of Brazil. 

How to cite: Convers, J. A., Assumpção, M., and Barbosa, J. R.: Seismological Observations of the Seasonal Rain and Aquifer Induced Seismicity in Southeastern Brazil, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10161, https://doi.org/10.5194/egusphere-egu2020-10161, 2020.

The average stress drop during an earthquake is a parameter fundamental to ground motion prediction and earthquake source physics, but it has proved hard to measure accurately. This has limited our understanding of earthquake rupture, as well as the spatiotemporal variations of fault strength. In this study, we investigate the resolution limits of spectral analysis based on synthetic spectra with similar magnitude range, average stress drop and frequency bands to a fluid-injection induced earthquake sequence in Oklahoma near Guthrie.

Synthetic tests using joint spectral fitting method define the resolution limit of corner frequency as a function of maximum frequency for both individual spectra and averaged spectra from multiple stations. Synthetic tests based on stacking analysis find that the improved stacking approach can recover the true input stress drop if the corner frequencies are within the resolution limit defined by joint spectral fitting.

The improved approach is applied to the Guthrie sequence, different wave types and different signal-to-noise criteria are examined to understand the stability of the stress drop distributions. The results suggest no systematic scaling relationship for stress drop for M≤ 3.1 earthquakes, but larger events M≥3.5 tend to have higher average stress drops. Results with lower signal-to-noise ratio requirement and direct P-wave tend to have higher scaling factor compared to results with high signal-to-noise ratio and S-waves.

Comparison of results from several different methods suggest that the average stress drop is well resolved and not subject to tradeoff with attenuation. Some robust spatiotemporal variations can be linked to triggering processes and indicate possible stress heterogeneity within the fault zone. Tight clustering of low stress drop events at the beginning stage of the sequence suggests that pore pressure influences earthquake source processes. Events at shallow depth have much lower stress drop compared to deeper events. The largest earthquake occurred within a cluster of high stress drop events, and involved cascading failure of several sub-events.   

How to cite: Chen, X., Abercrombie, R., and Wu, Q.: Earthquake stress drop: what can we resolve from observations, and what can we infer about earthquake triggering processes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10935, https://doi.org/10.5194/egusphere-egu2020-10935, 2020.

EGU2020-5052 | Displays | SM1.1

Responding to Media Inquiries About Remote Triggering Interactions

Lingling Ye, Hiroo Kanamori, and Thorne Lay

In the aftermath of a significant earthquake, seismologists are frequently asked questions by the media and public regarding possible interactions with recent prior events, including events at great distances away, along with prospects of larger events yet to come, both locally or remotely.  For regions with substantial earthquake catalogs that provide information on the regional Gutenberg-Richter magnitude-frequency relationship, Omori temporal aftershock statistical behavior, and aftershock productivity parameters, probabilistic responses can be provided for likelihood of nearby future events of larger magnitude (as well as expected behavior of the overall aftershock sequence). However, such procedures do not provide answers to inquiries about long-range interactions, either retrospectively for interaction with prior remote large events or prospectively for interaction with future remote large events. Dynamic triggering that may be involved in such long-range interactions occurs, often with significant temporal delay, but is not well-understood, making it difficult to respond to related inquiries. One approach to addressing such inquiries is to provide retrospective or prospective occurrence histories for large earthquakes based on global catalogs; while not providing quantitative understanding of any physical interaction, experience-based guidance on the (typically very low) chances of causal interactions can inform public understanding of likelihood of specific scenarios they are commonly very interested in.

How to cite: Ye, L., Kanamori, H., and Lay, T.: Responding to Media Inquiries About Remote Triggering Interactions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5052, https://doi.org/10.5194/egusphere-egu2020-5052, 2020.

EGU2020-15417 | Displays | SM1.1

A probabilistic, multi-parametric real-time earthquake location method

Alessandro Caruso, Aldo Zollo, Simona Colombelli, Luca Elia, and Grazia De Landro

For network-based Earthquake Early Warning Systems (EEWS), the real-time earthquake location is crucial for a correct estimation of event location/magnitude and therefore, for a reliable prediction of the potential expected shaking at the target sites in terms of predicted maximum ground shaking. Different approaches have been recently proposed for the real-time location which mainly use absolute (or differential) P-wave travel times at a set of minimum available stations or measurement of the initial P-wave arrival time (Elarms, Presto, Horiuchi), polarization (Eiserman and Bock) or amplitude and time (Yamada). In this work, we propose a new method which is able to exploit the continuous, real-time information available from both time, amplitude and polarization of initial P-wave signals acquired by dense three component arrays deployed in the source zones. The methodology we propose is an evolutionary and Bayesian probabilistic technique that combines three different observed parameters: 1) the differential arrival times of P-waves (which are computed using a 1D velocity model for the estimation of the theoretical arrival times); 2) the differential P-wave amplitudes in terms of P-wave peak velocity) [reference]  (which are computed using an existing P-peak motion prediction equation) and 3) the real-time estimation of back-azimuthal direction, measured shortly after the P-wave arrival. These three parameters are measured in real-time and are used as prior and conditional information to estimate the posterior probability of the event location parameters, e.g. the hypocenter coordinates and the origin time. The method is evolutive, since it updates the location parameters as new data are acquired by more and more distant stations as the P-wavefront propagates across the network. The output is a multi-dimensional Probability Density Function (PDF), which contains the complete information about the maximum likelihood parameter estimation with their uncertainty. The method is computationally efficient and optimized for running in real-time applications, where the earthquake location has to be retrieved in a very short time window (around 1 sec) after data acquisition. We tested the proposed strategy on a sequence of 29 earthquakes of the 2016-2017 central Italy seismic sequence acquired by the RAN (Rete Accelerometrica Nazionale) network with a magnitude range of 4.2-6.5. For the testing phase, we also simulated non-optimal conditions in terms of source-to-receiver geometry. Specifically, we tested the method  by ssimulating the case of “offshore” earthquakes recorded by a coastal network and in the case of a linear “barrier-type” geometry of the network. Our approach turned out to be suitable to work in condition of a sparse network, with a limited number of nodes and poor azimuthal coverage. In most of the cases, reliable location errors, less than 10 km, are achieved within few seconds from the first recorded P wave. As compared to other classical location techniques (i.e RTLOC in PRESTo) our approach shows an improvement of the solutions, especially for the first instants (2 seconds after the first P-wave arrival at network) when a poor number of stations (less than 4) is available.

How to cite: Caruso, A., Zollo, A., Colombelli, S., Elia, L., and De Landro, G.: A probabilistic, multi-parametric real-time earthquake location method, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15417, https://doi.org/10.5194/egusphere-egu2020-15417, 2020.

EGU2020-20059 | Displays | SM1.1

EMS and MCS macroseismic intensities assessed in Italy are equivalent?

Gianfranco Vannucci, Paolo Gasperini, Gulia Laura, and Lolli Barbara

The most of intensity assessments provided by the large (more than 100000 intensity observations) Italian macroseismic database (DBMI15) were made using the traditional Mercalli-Cancani-Sieberg (MCS) scale but in most recent macroseismic surveys in Italy even the European Macroseismic Scale (EMS) scale was used by some research groups. In principle, MCS and EMS scales should give almost the same intensities if only damage to traditional masonry buildings is considered for MCS estimates. Some doubts remain on this equivalence even if MCS and EMS intensities were actually used as they were coincident, as in the case of or the compilation of the CPTI15 catalog used for seismic hazard assessment in Italy. In this work we compared intensity estimates made using both scales for the traditional (expert) estimates made for the same localities of some recent earthquakes as well as community intensities provided by on line questionnaires “Hai Sentito Il Terremoto” (HSIT) collected by INGV. We computed linear regressions between the two sets of intensity estimates and also compared the earthquake parameters (locations magnitude and fault orientations) computed by the Boxer code, using independently the two sets of intensities.

How to cite: Vannucci, G., Gasperini, P., Laura, G., and Barbara, L.: EMS and MCS macroseismic intensities assessed in Italy are equivalent?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20059, https://doi.org/10.5194/egusphere-egu2020-20059, 2020.

EGU2020-19437 | Displays | SM1.1

The future strong motion national seismic networks in Central America designed for earthquake early warning.

Frederick Massin, John Clinton, Roman Racine, Maren Bose, Yara Rossi, Griselda Marroquin, Wilfried Strauch, Mario Arroyo, Lepolt Linkimer, Esteban Chavez, Marino Protti, and Robin Yani

The national seismic networks in Central America have been developing network-based early warning since 2016 for Nicaragua, 2018 for El Salvador and 2019 for Costa Rica. This effort is part of a project with the Swiss Seismological Service (ETH Zurich) including funds for accelerograph deployment. At each network, delay for first earthquake parameter estimations have been significantly reduced by optimizing data acquisition, metadata quality, and configuration of the EEW algorithms implemented in SeisComP3, i.e. Virtual Seismologist and the Finite fault rupture Detector. Issues remain with significant numbers of deployed instrumentation that for a variety of reasons, do not optimally contribute to the EEW systems. Building on our experience so far, we design national network upgrades that will optimize the earthquake early warning performance in the Central America region, mitigating the current issues with velocimeter clipping during large events, datalogger delays, and incomplete network coverage. The new instruments have been selected after testing all available EEW-capable accelerographs natively compatible with SeisComP3 including class A force balance accelerometers as well as MEMs. To justify our instrument selection, we summarize the performance of these different instruments. We model and discuss reference maps for performance expectations, and present planned instrument vaults. Our primary focus is on minimizing first alert times but we also wish to accentuate the broad value of the network upgrade for seismological monitoring showing changes in the magnitude of completeness in the region. We demonstrate the value of the network upgrade for earthquake early warning with real-time processing simulation using synthetic data for the maximum magnitude earthquake expected for the Central America subduction zone.

How to cite: Massin, F., Clinton, J., Racine, R., Bose, M., Rossi, Y., Marroquin, G., Strauch, W., Arroyo, M., Linkimer, L., Chavez, E., Protti, M., and Yani, R.: The future strong motion national seismic networks in Central America designed for earthquake early warning., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19437, https://doi.org/10.5194/egusphere-egu2020-19437, 2020.

EGU2020-20668 | Displays | SM1.1

Impact of earthquakes and its dependence on magnitude: testing the Greek seismicity

Ioanna Triantafyllou, Gerassimos Papadopoulos, and Efthimios Lekkas

Strong earthquakes cause significant impact on both the built and natural environment. Impact databases are of fundamental importance for seismic risk assessment in a region. Such data include human and property losses as well as secondary effects including ground failures and tsunamis. The earthquake impact, EI, depends on many factors, one of the most important being the earthquake magnitude, M. To test the dependence of EI on M we selected the Greek seismicity which is the highest in the Mediterranean region with record of earthquakes since the antiquity. Although various descriptive and parametric earthquake catalogues as well as inventories of intensity observation points are available for Greece no database for EI has been organized so far. For a first time we organized a Greek Earthquake Impact Database (GEID) which covers the time interval from 1800 to 2019 and includes earthquake parameters and three main quantitative impact elements: building damage, fatalities and injuries. Data on tsunami impact are also included in the GEID. A long number of sources have been utilized, some of them remaining unknown so far in the seismological community. To select the most appropriate magnitude for each earthquake event occurring in the instrumental period of seismology, i.e. from 1900 onwards, we compared the catalogues produced by the ISC-GEM and by three academic institutions. After completeness testing and examination for magnitude homogeneity we performed magnitude closeness analysis and produced formulas for magnitude conversion from one catalogue to another. For the 19th century earthquakes we again compared various catalogues, collected new data from documentary sources and compiled a new catalogue by re-calculating macroseismic magnitudes equivalent to Mw from intensity/M relations developed for Greek earthquakes of the instrumental period. We found that for single earthquake events the level of impact generally depends on magnitude but this is not valid for offshore events. However, the time distribution of the three impact elements over the period examined showed a relative decrease of the totally collapsed buildings which implied drastic decrease of the fatality rate but not of the injuries rate. This is attributed to the gradual improvement of the building construction particularly after the enforcement of antiseismic building codes in the country. Τhe first author was supported by the Hellenic Foundation for Research and Innovation (HFRI) and the General Secretariat for Research and Technology (GSRT), under the HFRI PhD Fellowship grant (GA. no. 490).

How to cite: Triantafyllou, I., Papadopoulos, G., and Lekkas, E.: Impact of earthquakes and its dependence on magnitude: testing the Greek seismicity , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20668, https://doi.org/10.5194/egusphere-egu2020-20668, 2020.

Yellow Sea and East Sea regions near Korea are two of the most seismically active marginal seas in the Far East.  While offshore earthquakes in the Yellow Sea may be attributed to potential micro-plate boundaries, East Sea earthquakes may be associated to the seaward extension of many active faults on land or the deformation boundary between oceanic and continental crust.  However, offshore earthquake locations using local seismic network are always subjecting to large uncertainties due to poor spatial coverage of seismic stations, discrepancies on velocity models, and limitations on traditional location technologies.  For instance, it is not uncommon that the same earthquake within Yellow Sea may be reported independently more than tens to hundreds of km apart in Chinese and Korean catalogs while there is no mechanism for earthquake data exchange between the two countries.   Multiple seismic array method can be applied to improve epicenter location of offshore earthquakes.  Seismic stations in Korea can be integrated into three arrays based on their latitude. Apparent azimuths and apparent velocities of the incoming seismic waves (mainly Pn) from a regional earthquake to each array can be reliably determined.  Epicenter of a regional earthquake can thus be located by tracing seismic rays following the back azimuths derived from multiple arrays.  Offshore earthquakes in the East Sea and Yellow Sea regions are located at shallow depth within crust that Pn waves are expected to be the first arrival phase at many Korean stations.  Thus, offshore earthquakes can be reasonably located using Pn arrivals.  In the Yellow Sea case, the apparent velocity ~8.0 km/sec is observed for all arrays suggesting a typical continental Pn waves propagating across the continent-continent transition region into Korea.  In the East Sea case, the apparent velocity of ~6.8 km/sec or lower is observed for all arrays suggesting a typical oceanic Pn wave propagating across the oceanic-continental margin into Korea.  A better relocated earthquake location in the offshore region is essential for our understanding of regional tectonics and earthquake hazard assessment.

How to cite: Chiu, S.-C., Chiu, J.-M., Kim, K., and Kang, S.: Relocation of Offshore Earthquakes around the Korean Peninsula using Multiple Seismic Arrays: Case Examples for East Sea and Yellow Sea Regions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13610, https://doi.org/10.5194/egusphere-egu2020-13610, 2020.

Owing to developments of geodetic observation using satellite systems such as GNSS, we can now estimate slip-deficit rate distribution at plate interfaces. There are roughly two types of attempts to predict possible scenarios for future megathrust earthquakes based on the estimated slip deficit rates. One is kinematic modeling, in which coseismic slip distribution is modeled by multiplying the estimated slip deficit rates by the recurrence time (e.g., Baranes et al. 2018 GRL; Watanabe et al, 2018 JGR). The rupture area and seismic moment can be easily modeled, but the model is not always consistent with the mechanics of fault rupture. The other is dynamic modeling, in which source models are obtained via dynamic rupture simulations using shear stress calculated from the slip deficit rates and assuming frictional parameters (e.g., Hok et al., 2011 JGR; Lozos et al., 2015 GRL; Yang et al., 2019 JGR). The method reasonably predicts the rupture processes based on the mechanics of fault rupture, but generally needs a lot of computing resources for parametric studies of the frictional parameters because of the difficulty to estimate them. In this study, we propose a mechanics-based method to bridge the gap between the kinematic and dynamic modeling. The method predicts possible static slip models with a small computational load, and then examines whether each model actually happens from the viewpoint of the mechanics of fault rupture.

First, we calculated shear stress change rates at the plate interface from the slip-deficit rate distribution estimated from GNSS data (Noda et al., 2018 JGR). In each scenario, we assumed a rupture region and obtained stress drop distribution by multiplying the shear stress change rates in the region by accumulation period. The coseismic slip distribution of each scenario was estimated from the assumed stress drop distribution by using an inversion method. We created scenarios for various rupture regions and various accumulation periods. Next, we investigated the possibility that the scenario happens based on the conservation law of energy. Fault rupture releases shear strain energy accumulated in the lithosphere and the released strain energy is consumed as the radiated energy and the dissipated energy. We assumed some plausible frictional constitutive relations for the plate interface to evaluate the dissipated energy for each case. We calculated the strain energy released by shear faulting in each scenario and compared it with the dissipated energy considering that the released strain energy is necessarily larger than the dissipated energy in earthquake occurrence. If the released strain energy is smaller than the dissipated energy, we find that the scenario will not happen in terms of earthquake mechanics.

We applied this method to the subduction zone along the Nankai trough, southwest Japan, where great thrust earthquakes have repeatedly occurred with a recurrence time of about 100 years. Based on possible scenarios predicted in this region, we discussed the necessary condition of fault strength and accumulation period for earthquake generation.

How to cite: Noda, A., Saito, T., Fukuyama, E., and Urata, Y.: Mechanics-based scenarios for great thrust earthquakes in subduction zones using GNSS data analysis: Released strain energy and dissipated energy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12581, https://doi.org/10.5194/egusphere-egu2020-12581, 2020.

EGU2020-20659 | Displays | SM1.1

Crust-mantle velocity structure in Shanxi rift, Central North China Craton

Yan Cai and Jianping Wu

North China Craton is the oldest craton in the world. It contains the eastern, central and western part. Shanxi rift and Taihang mountain contribute the central part. With strong tectonic deformation and intense seismic activity, its crust-mantle deformation and deep structure have always been highly concerned. In recent years, China Earthquake Administration has deployed a dense temporary seismic array in North China. With the permanent and temporary stations, we obtained the crust-mantle S-wave velocity structure in the central North China Craton by using the joint inversion of receiver function and surface wave dispersion. The results show that the crustal thickness is thick in the north of the Shanxi rift (42km) and thin in the south (35km). Datong basin, located in the north of the rift, exhibits large-scale low-velocity anomalies in the middle-lower crust and upper mantle; the Taiyuan basin and Linfen basin, located in the central part, have high velocities in the lower crust and upper mantle; the Yuncheng basin, in the southern part, has low velocities in the lower crust and upper mantle velocities, but has a high-velocity layer below 80 km. We speculate that an upwelling channel beneath the west of the Datong basin caused the low velocity anomalies there. In the central part of the Shanxi rift, magmatic bottom intrusion occurred before the tension rifting, so that the heated lithosphere has enough time to cool down to form high velocity. Its current lithosphere with high temperature may indicate the future deformation and damage. There may be a hot lithospheric uplift in the south of the Shanxi rift, heating the crust and the lithospheric mantle. The high-velocity layer in its upper mantle suggests that the bottom of the lithosphere after the intrusion of the magma began to cool down.

How to cite: Cai, Y. and Wu, J.: Crust-mantle velocity structure in Shanxi rift, Central North China Craton, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20659, https://doi.org/10.5194/egusphere-egu2020-20659, 2020.

EGU2020-20958 | Displays | SM1.1

Improving Stoneley-mode constraints on the structures near the core-mantle boundary

Harry Matchette-Downes, Robert D. van der Hilst, Jingchen Ye, Jia Shi, Jiayuan Han, and Maarten V. de Hoop

Although observations of seismic normal modes provide constraints on the structure of the entire Earth, the core-mantle boundary region remains poorly understood. Stoneley modes should offer better constraints, because they are confined near to the fluid-solid interface, but this property also makes them difficult to detect. In this study, we use recently-developed finite-element approach to show that Stoneley modes can be excited and detected, but only in certain special cases. We first investigate the physical explanation for these cases. Next, we describe how they could be detected in seismic data, and the sensitivity of these data to the material properties. We illustrate this sensitivity by calculating the modes of a three-dimensional Earth model containing a large low-shear-velocity province (LLSVP). Finally, we present some preliminary observations. We hope that this new understanding will lead to new constraints on the material properties and morphology of the core-mantle boundary region. In turn, this information, especially the constraints on density, should help to answer questions about the Earth, for example in mantle convection (are LLSVPs thermally or chemically buoyant? Primordial or slab graveyards? Passive or active?) and core convection (does the outermost core have a stable stratification?).

How to cite: Matchette-Downes, H., van der Hilst, R. D., Ye, J., Shi, J., Han, J., and de Hoop, M. V.: Improving Stoneley-mode constraints on the structures near the core-mantle boundary, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20958, https://doi.org/10.5194/egusphere-egu2020-20958, 2020.

EGU2020-18164 | Displays | SM1.1

Efficient simulation of prompt elasto-gravity signals (PEGS) based on a spherical self-gravitating earth model

Rongjiang Wang, Shenjian Zhang, Torsten Dahm, and Sebastian Heimann

An earthquake causes a sudden rock-mass redistribution through fault rupture and generates seismic waves that cause bulk density variations propagating with them. For a large earthquake, both processes can induce global gravity perturbations, whose signals propagate with the speed of light and therefore can arrive at remote stations earlier than the fastest elastic P wave. In turn, the gravity perturbations generate secondary seismic sources overall within the earth, a part of which can cause ground motion prior to the direct P wave arrival, too. Recently, these prompt elasto-gravity signals (Vallée et al. 2017) for large earthquakes like Tohoku 2011 Mw 9.1 have been detected in records of  broadband seismometers and superconducting gravimeters. Though the physics of the PEGS has been well understood, the tools used so far for a realistic modelling of them are complicated and computationally intensive. In this study, we present a new and rather simple approach that solves the full-coupled elasto-gravitational boundary-value problem more accurately, but no more complicated than to compute synthetic seismograms in a conventional way. Using the new tool, we simulate the PEGS of the 2011 Tohoku earthquake in both temporal and spatial scales, based on a realistic kinematic finite-fault source model. The temporal results show clearly how the ground motion is inspired by the gravity change in short- and long-term as well as how the combined PEGS behave at different epicentral distances from 400 to 3000 km. The spatial patterns of PEGS, especially that of gravity change, reveal the relationship between the PEGS and the focal mechanism. We also compare our simulation results with the predictions made before and with the observed waveforms and find a good agreement. Furthermore, we show particularly that the moment magnitude, rupture duration and focal mechanism of the 2011 Tohoku earthquake can be estimated robustly using the PEGS measured at a dozen selected stations, which could be helpful for the earthquake and tsunami early warning in the future.

How to cite: Wang, R., Zhang, S., Dahm, T., and Heimann, S.: Efficient simulation of prompt elasto-gravity signals (PEGS) based on a spherical self-gravitating earth model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18164, https://doi.org/10.5194/egusphere-egu2020-18164, 2020.

EGU2020-4453 | Displays | SM1.1

Seismogenic structures of the collision-subduction zone in the eastern Taiwan

Wen-Shan Chen, Yih-Min Wu, Hsiao‑Chin Yang, Po-Yi Yeh, Yi-Xiu Lai, Ming-Chun Ke, Siao-Syun Ke, and Yi-Kai Lin

The Taiwan orogenic belt is relatively young and active with an ongoing arc-continent collision since the middle Miocene. In this study, we systematically investigate the use of seismic tomography, focal-mechanism and distribution of earthquakes to analysis the seismogenic patterns in the collision-subduction zone in the eastern Taiwan, which can be delineated five seismogenic zones of the Longitudinal Valley Fault Seismic Zone (LVFZ), the Central Range Fault Seismic Zone (CRFZ), the Backbone Range Seismic Zone (BRSZ), the Ludao-Lanyu Fault Seismic Zone (LLFZ), and the Wadati-Benioff Seismic Zone (WBSZ).

The LVFZ and CRFZ, formed along the collision zone between the Philippine Sea and the Eurasian Plates, earthquake focal mechanisms show P axes distributed in direction of 285-335°, reflecting the compressive stress field due to the collision. The LVSZ is the collisional boundary between the Philippine Sea and Eurasian plates. The LLFZ is a high-angle, east-dipping reverse fault separating the Luzon Volcanic Arc and the North Luzon Trough. The Eurasian plate (the South China Sea oceanic crust) subducts beneath the Philippine Sea plat in the southeastern Taiwan forming the WBSZ to a depth of 160 km.

The CRFZ, located along the eastern limb of Backbone Range, is formed by a zone of west-dipping reverse fault. In addition, the earthquakes on the BRSZ generated by normal and strike-slip faults at about 5-15 km depth which occur in response to left-lateral transtensional deformation by the collision. Earthquake focal mechanisms show P and T axes distributed in direction of 280-330° and 20-70°, respectively.

How to cite: Chen, W.-S., Wu, Y.-M., Yang, H., Yeh, P.-Y., Lai, Y.-X., Ke, M.-C., Ke, S.-S., and Lin, Y.-K.: Seismogenic structures of the collision-subduction zone in the eastern Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4453, https://doi.org/10.5194/egusphere-egu2020-4453, 2020.

EGU2020-6854 | Displays | SM1.1

The Waveform Characteristics and Classification of Intermediate-depth Earthquakes in Ryukyu Subduction Zone

Yu-Jhen Lin, Tai-Lin Tseng, and Wen-Tzong Liang

Using intraslab earthquakes shallower than 150 km in the southernmost Ryukyu subduction zone, previous studies in Taiwan found the wave guide effect that typically shows a low-frequency (<2Hz) first P arrival followed by sustained high-frequency (3–10 Hz) wave trains. Recently occurred deeper events at depth 150-300 km allow us to better quantify the properties of those seismic waves traveling in the subduction zone. In this study, we aim to systematically scan through the local broadband waveforms of the intermediate depth earthquakes with M>5 between 1997 and 2016. Event are classified based on the waveform characteristics and their frequency contents.

To detect events with similar properties, we applied sliding-window cross-correlation (SCC)​ using three components of P waveform data simultaneously for a set of stations​. The time window used here was 10 s and traces were bandpass filtered in the frequency range 0.5–10 Hz. After the degree of similarity are calculated, e​vents containing comparable waveforms can be sorted into families. The events within a family would have been triggered because they came from the same source region and their paths to a particular receiver should produce similar waveforms. Our results show that most earthquakes are low in waveform similarity, implying no “repeating” behavior for those intermediate intraslab events. However, some events (cc>0.6 threshold) present enough charterers that can be grouped as a family.

One important property is the frequency content of the arrivals that may be related to the speed of structure traveled. We have developed a work scheme to determine the delayed time of higher-frequency energy. On family of events show beautiful dispersion with arrival time smoothly increasing with frequency between 0.5 and 6 Hz.​ Another type of dispersive waveforms appear as two distinct arrivals: low frequency and then high-frequency energy, separated by around 1 s. The time delay seems to be independent of focal depth. The latter case has been reported in the previous study for shallower event and it was interpreted as effect from low-velocity layer or heterogeneity of the subducted slab. On the other hand, the continuous dispersion is a new feature observed by our study, which may infer a thinner layer and/or longer propagation for some kind of reflecting waves to develop such interference.

In addition, we will classify the waveforms according to the frequency content and decay of coda. The variations in P coda properties can be associated with the way in which the seismic energy gets ducted into the stochastic waveguide associated with the lithosphere. With sufficient amount of data, it is possible to further identify the earthquakes with unusual source properties or structure anomaly along specific propagation paths. We expect the classification results can provide a reference for future numerical simulation analysis. 

Keywords: Ryukyu subduction zone, SCC, guide wave, waveform classification, intermediate-depth earthquakes

How to cite: Lin, Y.-J., Tseng, T.-L., and Liang, W.-T.: The Waveform Characteristics and Classification of Intermediate-depth Earthquakes in Ryukyu Subduction Zone , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6854, https://doi.org/10.5194/egusphere-egu2020-6854, 2020.

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Influence of Mantle Structures on Measurements of Anisotropy in the Inner Core

Sandra Beiers and Christine Thomas

The seismological exploration of the Earth’s inner core has revealed some structural complexities such as seismic anisotropy and hemispherical separation. Investigating the travel times of PKP waves from at least two different ray paths, a polar and an equatorial one, is one of the commonly used methods to probe the inner core’s anisotropy. Since the waves are traversing anomalous structures in the lowermost mantle before entering the core, these heterogeneities have to be taken into account when investigating anisotropy in the inner core.

In this study we use data from an equatorial path with events from Indonesia recorded in Morocco and a nearly polar one with earthquakes in New Zealand recorded in England. The two waves used in our study, PKPdf and PKPab, both propagate through mantle and outer core and PKPab additionally traverses the inner core. Within this work, we do not only analyse the travel times of the waves but rather investigate their deviations from the originally assumed path along with their incidence angle. This is done with the methods of array seismology, mainly its two parameters slowness and backazimuth.

The results of this study reveal opposite deviations of slowness and backazimuth of the polar in contrast to the equatorial path. While the polar waves travel shallower and closer to North, the equatorial waves propagate deeper and farther from North than predicted by ak135. Additionally we observe hemispherical differences between waves that sample the eastern and the ones that sample the western hemisphere for both ray paths, PKPdf and PKPab, which leads us to the assumption that the deviations are not caused by the inner core but are rather due to mantle structures.

How to cite: Beiers, S. and Thomas, C.: Influence of Mantle Structures on Measurements of Anisotropy in the Inner Core, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8388, https://doi.org/10.5194/egusphere-egu2020-8388, 2020.

We present the first high resolution seismic images illuminating the hitherto-elusive crustal architecture beneath the Eastern Ghats Mobile Belt (EGMB) using teleseismic receiver functions. Data were collected using 27 broadband seismic stations operated in a continuous mode covering two distinct seismic profiles (~550 km long) during 2015–2018. Several interesting observations and inferences are made through analysis of the receiver functions such as (a) a very thick crust (>40 km) with oppositely dipping Moho beneath the EGMB and Archean Bastar Craton, (b) EGMB formed from amalgamation of different crustal domains thrust over one another possibly during the Pan-African orogeny, (c) the Archean Bastar Craton crust extends (~75 km) eastward beneath the EGMB, from its surficial geological boundary, (d) there is a sharp contrast in the crustal structure (with ~20 km Moho offset) at the contact between the Rengali Province and Singhbhum Craton which does not support southward growth of the Singhbhum Craton through accretion, (e) anorthosite complexes may possibly be created by rising diapirs channeled through the weak zones in the crust, from the magma chambers developed by melting of frontal portion of the underthrusting lower crust. We report a significant change in the crustal architecture just east of the most elevated topography observed along the profile covering the Bastar Craton and the EGMB. It requires further careful petrological investigations to ascertain the relationship of high elevation and its linkages with the deep crust, forming a separate domain. Our results do not support or discard a Grenvillian age (~1 Ga) docking of the EGMB with Proto-India, though it is preferred to explain the present day crustal features with intense Pan-African (0.5–0.6 Ga) reorganization.

How to cite: Singh, A. and Singh, C.: Seismic imaging of the deep crustal structure beneath Eastern Ghats Mobile Belt (India): Crustal growth in the context of assembly of Rodinia and Gondwana supercontinents, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-33, https://doi.org/10.5194/egusphere-egu2020-33, 2020.

As one of the most active intracontinental orogenic belts in the world, the Tien Shan orogenic belt originated in the Paleozoic and then experienced tectonic activities such as plate subduction and closure of the Paleo-Asian Ocean. Previous seismological and geodynamic studies have shown the observed the low-velocity anomaly (LVA) beneath the central Tien Shan at the uppermost mantle, which has a significant influence on the formation and modification of the crust and mantle lithosphere ( Lei et al, 2007). However, the distribution, morphology and physical property of the LVA are highly debatable.

We conduct 2-D forward waveform modeling based on spectral-element method (SEM) to investigate waveform distortions that were generated by the velocity contrast boundary of the LAV. The broadband P- and S- waves from three intermediate-depth earthquakes at Hindu Kush-Pamir were recorded by the Chinese Digital Seismograph Network (Zheng et al., 2010). We use these records to confirm the location, shape and velocity decrement of the LVA by fitting the observed records with the synthetics through SEM based on the 1D velocity structures (TSTB-B) of the central Tien Shan and northern Tarim basin (Gao et al., 2017). We find the LVA at 10~100 km beneath the eastern part of the central Tien Shan. And the northward under-thrusting of the Tarim Basin may trigger some mantle upwelling, contributing to the observed LVA.

Lei, J., Zhao, D. (2007). Teleseismic P-wave tomography and the upper mantle structure of the central Tien Shan orogenic belt. Physics of the Earth and Planetary Interiors, 162, 165-185, doi: 10.1016/j.pepi.200704010.

Zheng, X., Jiao, W., Zhang, C., et al. (2010). Short-Period Rayleigh-Wave Group Velocity Tomography through Ambient Noise Cross-Correlation in Xinjiang, Northwest China. Bulletin of the Seismological Society of America, 100(3): 1350-1355, doi: 10.1785/0120090225.

Gao, Y., Cui, Q., Zhou, Y. (2017). Seismic detection of P-wave velocity structure atop MTZ beneath the Central Tian Shan and Tarim Basin. Chinese Journal of Geophysics ( in Chinese with English Abstract ), 60 (1) : 98-111, doi: 10.6038 /cjg20170109.

How to cite: Cui, R. and Zhou, Y.: Seismic detection of the low-velocity anomaly at the crust and uppermost mantle beneath the central Tien Shan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3300, https://doi.org/10.5194/egusphere-egu2020-3300, 2020.

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An updated crustal thickness map of central South America based on receiver function measurements

Julia Carolina Rivadeneyra-Vera, Marcelo Bianchi, and Marcelo Assumpção

Determining the seismic properties of continental crust is essential in tectonic studies to understand the geological evolution, as well as elaborating velocity models to better monitoring the regional and global seismicity. Since the early 90s many seismic studies have focused on the details of the crust and upper mantle in the Andean region. However most of the stable part of the continent remains poorly sampled due to its complexity and lack of seismic stations. In the previous compilation of crustal structure in South America, areas as the thin crust in the Sub-Andean lowlands and Amazon region have been largely estimated by gravity data. A deployment of 35 temporary seismic stations in southwest Brazil and parts of Bolivia, Paraguay, Argentina and Uruguay filled a significant gap in crustal information of the central part of South America. Additionally, restricted seismic stations of Bolivia and the eastern of Peru have been analyzed to better constraint our results in the Sub-Andean region. Crustal thicknesses and Vp/Vs ratios were estimated with a modified H-k method by producing three stacked traces to enhance the three Moho conversions (the direct Ps and the two multiples Ppps and Ppss). This modified method yields lower uncertainties than previous studies and shows more regional consistency between close stations. Using the temporary stations, the Brazilian permanent network (RSBR), and the restricted stations of Peru and Bolivia we have better characterized the crustal structure in the central part of South America, our results shows a belt thin crust (35-40 km) along the Sub-Andean region, which is narrower the previous works, and a normal crustal thickness average of 40 km in the central part of the South America. This study, combined with other published data, provides an updated crustal thickness map of South America that is useful for future regional studies of seismic wave propagation, gravity modeling and inferences of crustal evolution.

How to cite: Rivadeneyra-Vera, J. C., Bianchi, M., and Assumpção, M.: An updated crustal thickness map of central South America based on receiver function measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6224, https://doi.org/10.5194/egusphere-egu2020-6224, 2020.

The South China Sea (hereafter as SCS) located in the southeastern Asia has been affected by the subduction of the western Pacific, Indo-Australian and Eurasian plates (Sun et al., 2018). Broadband P waveforms from the China Digital Seismograph Network (Zheng et al., 2010) for three intermediate-depth earthquakes that occurred closely in Mindoro, Philippine are used to detect velocity structures of the lowermost upper mantle and mantle transition zone (MTZ) beneath the northern SCS. The study area is divided into five profiles distributed from southwest to northeast azimuthally to reduce the computational costs and concern possible lateral variations (Li et al., 2018), and the corresponding 1-D best-fit velocity models are obtained from the observed and synthetic triplicated waveform fitting based on the iterative grid-search procedure. The searching grid can be described as below, three parameters for the low-velocity layer (LVL) atop the 410 km discontinuity (hereafter as the 410), five parameters for the high-velocity anomaly (HVA) atop the 660 km discontinuity (hereafter as the 660) and one parameter for the velocity perturbation below the 660. After the sensitivity tests of the synthetic waveforms with different parameters, the grid steps of the depth and velocity perturbation are set as 5 km and 0.5%, respectively.

Relative to the reference model IASP91 (Kennett and Engdahl, 1991), our results reveal that there are ubiquitous HVAs in five profiles at the bottom of the MTZ with a velocity increment of 1.5~3.5% and a thickness of 209~219 km, which show no apparent progressive velocity increment or decrement along the southwest-northeast direction. We prefer that the weak and abnormal thick HVAs are induced by the proto-SCS north slab remnants. We also observe an uplift 410 and depressed 660 with the depth change of 5 km and 5~15 km, respectively, which further support the low-temperature anomaly related to the stagnant slab. In addition, our results show there is an LVL atop the MTZ with a velocity decrement of 2.0~2.5% and a thickness of 60~75 km, and can be interpreted by the partial melting induced by upwelling materials from the MTZ, which are hydrated by water released from the stagnant slab. We infer that the LVL with little lateral variations may result from the percolation of the partial melts atop the MTZ under vertical pressure.

 

Kennett B L N, Engdahl E R. 1991. Traveltimes for global earthquake location and phase identification. Geophys. J. Int. 105(2): 429-465, doi:10.1111/j.1365-246X.1991.tb06724.x.

Li W, Wei R, Cui Q, et al. 2018. Velocity structure around the 410 km discontinuity beneath the East China Sea area based on the waveform fitting method. Chinese J. Geophys. 61(1): 150-160, doi:10.6038/cjg2018L0370.

Sun W, Lin C, Zhang L, et al. 2018. The formation of the South China Sea resulted from the closure of the Neo-Tethys: A perspective from regional geology. Acta Petrol. Sin. 34(12): 3467-3478, doi:1000-0569/2018/034(12)-3467-78.

Zheng X, Jiao W, Zhang C, et al. 2010. Short-Period Rayleigh-Wave Group Velocity Tomography through Ambient Noise Cross-Correlation in Xinjiang, Northwest China. Bull. Seismol. Soc. Am. 100(3): 1350-1355, doi:10.1785/0120090225.

 

How to cite: Li, W., Zhou, Y., Wei, R., Li, G., and Cui, Q.: P-wave velocity structures of the upper mantle and mantle transition zone beneath the northern South China Sea based on triplication fitting, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2008, https://doi.org/10.5194/egusphere-egu2020-2008, 2020.

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The impact of crustal structures on multiple frequency and waveform tomography of Antarctica

Maria Tsekhmistrenko and Sergei Lebedev

We present two preliminary tomography models of Antarctica using seismic data recorded globally since 1994. Through combined efforts, several seismic broadband arrays have been deployed in Antarctica in previous decades, enabling the generation of two types of tomography models in this study: a multiple-frequency body-wave tomography and a waveform tomography model. Altogether, more than 2000 global events are collected resolving this region in great detail.

Crustal correction is crucial in seismic tomography, as it can cause the crustal smearing or leakage of shallow heterogeneities into the deep mantle. In global multiple-frequency tomography, synthetic seismograms are calculated on a spherically symmetric earth model (e.g. PREM, IASP91) in which effects of the crust, ellipticity, and topography are neglected. At a later stage, corrections are applied to the measured traveltimes to account for the known deviations from spherically symmetric earth models.

In waveform tomography, the crust has a significant impact on the Rayleigh and Love wave speeds. We invert for the crustal structure and explicitly account for its highly non-linear effects on seismic waveforms. Here, we implement a flexible workflow where different 3D reference crustal models can be plugged in. We test this using the CRUST2.0 and CRUST1.0 models.

In this study, we quantify the effects of these crustal models on two types of inversion techniques with a focus on the mantle structure beneath Antarctica. We compare the mantle structures beneath Antarctica imaged by a multiple-frequency body-wave tomography technique (e.g., Hosseini et al, 2019) and a waveform tomography method (Lebedev et al. 2005; Lebedev and van der Hilst 2008) using CRUST1.0 and CRUST2.0.

References:
K. Hosseini, K. Sigloch, M. Tsekhmistrenko, A. Zaheri, T. Nissen-Meyer, H. Igel, Global mantle structure from multifrequency tomography using P, PPand P-diffracted waves, Geophysical Journal International, Volume 220, Issue 1, January 2020, Pages 96–141, https://doi.org/10.1093/gji/ggz394

S. Lebedev, R. D. Van Der Hilst, Global upper-mantle tomography with the automated multimode inversion of surface and S-wave forms. Geophysical Journal International, Volume 173, Issue 2, May 2008, Pages 505–518, https://doi.org/10.1111/j.1365-246X.2008.03721.x

A. J. Schaeffer, S. Lebedev, Global shear speed structure of the upper mantle and transition zone, Geophysical Journal International, Volume 194, Issue 1, 1 July 2013, Pages 417–449, https://doi.org/10.1093/gji/ggt095

How to cite: Tsekhmistrenko, M. and Lebedev, S.: The impact of crustal structures on multiple frequency and waveform tomography of Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16489, https://doi.org/10.5194/egusphere-egu2020-16489, 2020.

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Automatic Picking of Teleseismic P- and S-Phases using an Autoregressive Prediction Approach

Johannes Stampa, Máté Timkó, Marcel Tesch, and Thomas Meier

In the recent decade, the amount of available seismological broadband data has increased steeply. Picking later arriving phases such as S-phases is difficult, and there are few manual picks available for these phases. Data sets of manual picks can also be problematic, since phase arrival picks are sensitive to the parameters of the filtering, which are often unknown, and the individual picking behavior of the analysts. This neccesitates the adoption of automatic techniques for determining teleseismic phase arrival times consistently over a large data set.

In this work, a robust automatic picking algorithm based on autoregressive prediction in a moving window is explained. In this algorithm, a characteristic function is calculated as the autoregressive prediction error in a moving window. This characteristic function is then transformed with the Akaike-Information Criterion to obtain the phase arrival time estimate. This estimate is further improved in a second iteration of a similiar scheme in a smaller time window.

The algorithm is applied to a global data set including AlpArray stations, covering a time period from 1995 to present, to obtain arrival times for teleseis- mic P- and S-phases. Residuals to theoretical travel times and to local averages are shown. Different methods for automatically evaluating the quality of indi- vidual picks are used, based on signal to noise ratio of the seismic trace and impulsiveness of the arrival. The picking errors are estimated by comparision with manual picks and neighboring stations as well as statistical methods. The quality evaluations suggest potential of using these automatically determined phase arrival times for a travel time tomography.

How to cite: Stampa, J., Timkó, M., Tesch, M., and Meier, T.: Automatic Picking of Teleseismic P- and S-Phases using an Autoregressive Prediction Approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22023, https://doi.org/10.5194/egusphere-egu2020-22023, 2020.

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Seismic noise characterization of the Sos Enattos Mine (Sardinia), a candidate site for the next generation of terrestrial gravitational waves detectors

Carlo Giunchi, Matteo Di Giovanni, Gilberto Saccorotti, and Luca Naticchioni

We present some preliminary results from ongoing seismic measurements aimed to assess the seismic noise levels in the Sos Enattos Mine (Sardina). Due to his geologic setting, low population density and lack of significant industrial activity, Sardinia is characterized by very low anthropogenic noise and very low seismic activity. These unique combinations of factors make Sardinia, and in particular the Sos Enattos site, suitable to host instruments that must be placed in particularly seismically quiet locations in order to meet their targeted sensitivity. This is certainly the case of gravitational waves detectors, whose next generation, called Einstein Telescope (ET), is planned to be able to measure a strain, induced by the passing wave on the interferometer’s arm, of the order of 2x10-25Hz-1/2. Three broadband seismometers has been installed since May 2019 both at surface and at different depths along the mine tunnels. We analyse the spectral distribution of the seismic noise with a special focus on the frequency bands that may affect the operation of a gravitational waves interferometer. We also study the correlation of seismic noise with the observed sea waves in the Mediterranean Sea. The results enlighten very low seismic noise levels at the surface and attenuation at the depths foreseen to build ET. Further, seismic noise levels appear to be strongly correlated with sea waves in NW Mediterranean Sea. We conclude that the selected site may meet the stringent seismic requirements needed to realize the ET infrastructure.

How to cite: Giunchi, C., Di Giovanni, M., Saccorotti, G., and Naticchioni, L.: Seismic noise characterization of the Sos Enattos Mine (Sardinia), a candidate site for the next generation of terrestrial gravitational waves detectors, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9692, https://doi.org/10.5194/egusphere-egu2020-9692, 2020.

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Coda-derived moment magnitudes in central Anatolia

Tuna Eken

A reliable representation of the energy at the earthquake source is vitally important to make reliable seismic hazard assessments in tectonically active areas. The use of coda waves, for this aim, can provide source spectra for robust moment magnitude estimates mainly due to its volume-averaging property sampling the entire focal sphere as this makes these waves insensitive to any source radiation pattern effect. In the present study, we examined local earthquakes beneath central Anatolia earthquakes with magnitudes 2.0≤ML≤5.2 recorded at 69 seismic stations that were operated between 2013 and 2015 within the framework of the Continental Dynamics–Central Anatolian Tectonics (CD–CAT) passive seismic experiment. The inversion scheme used here involved simultaneous modeling of source properties as well as seismic attenuation parameters in five different frequency bands between 0.75 and 12 Hz. Forward modeling of coda waves was achieved through an isotropic acoustic Radiative Transfer Theory approach. A comparison between coda derived (Mw coda) and routinely reported local (ML) magnitudes shows an overall consistency. However, apparent move-out observed around small earthquakes (ML < 3.5) can be attributed to wrong assumptions for anelastic attenuation as well as to the use of seismic recordings with a finite sampling interval.

How to cite: Eken, T.: Coda-derived moment magnitudes in central Anatolia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-877, https://doi.org/10.5194/egusphere-egu2020-877, 2020.

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Acquisition Protocol - Its Impact on Real-time Data Acquisition System Performance

Michael Laporte, Michael Perlin, Ben Tatham, Mojtaba Hosseini, Dario Baturan, Andrew Moores, and Bruce Townsend

A fundamental element of real-time mission critical seismic monitoring networks is the data acquisition system, comprising the underlying protocol and the telemetry solution. Selection of the acquisition protocol can have significant impact on the outcomes sought by the seismic network such as data availability and usability as well as operational cost and even station and data center design.

We examine the performance of various acquisition protocols using a set of standard measures of system performance. Primary measures include bandwidth utilization, data latency and robustness (data completeness). In addition, protocol functionality and features, including support for multiple data types and state-of-health, are assessed for system impact on options for station, telemetry, and data center design as well as the overall functionality of the system solution.

Real-world and system generated data are employed and key quantitative measures of system effectiveness are identified and used as the basis of the analysis. Results of the analysis show the real-world impact of low level aspects like protocol selection on system performance.

How to cite: Laporte, M., Perlin, M., Tatham, B., Hosseini, M., Baturan, D., Moores, A., and Townsend, B.: Acquisition Protocol - Its Impact on Real-time Data Acquisition System Performance, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20351, https://doi.org/10.5194/egusphere-egu2020-20351, 2020.

Seismic attenuation accompanying the velocity structures demonstrates the variations of the physical and chemical properties of the earth. The t* measurement using the seismic body wave spectrum, however, typically encounters the trade-off of corner frequency, t*, and site effect. Ko et al, [2012] proposed the cluster event method (CEM) that reduced the model parameter numbers by grouping the spatial-closed enough events for those traveling to each station along the adjacent paths and sharing one t*. Yet, the site effects among different stations collected in the same cluster bring the challenges on fitting all spectrum. We adapt the cluster strategy to group multiple nearby events recorded by one station only. Moreover, the new iterative CEM algorithm includes both the spectrum and spectral ratio data which provide constraints on seismic moments and corner frequencies of each earthquake inside the cluster, respectively. The final t* and corner frequencies are determined again by including the side effects which are averaging from spectrum residuals in the initial CEM stage. We applied the iterative CEM for earthquakes recorded at dense deployed F-net and Hi-net by NIED in the Tohoku area, Japan. The multitaper spectrums are retrieved from direct P waves with coda wavetrains tapered. Combining the spectral ratio and spectrum data with proper weightings, our new approach increases the stability of t* measurements contributed from better constrains on the corner frequency estimations.

How to cite: Jian, P.-R. and Kuo, B.-Y.: Robust measurements of corner frequency, t* and site effect: the iterative cluster event method, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2694, https://doi.org/10.5194/egusphere-egu2020-2694, 2020.

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The relationship between ML and Mw for small earthquakes (ML < 2-4) in Italy

Barbara Lolli, Gasperini Paolo, Emanuele Biondini, and Gianfranco Vannucci

Several authors empirically observed that the scaling between local magnitude ML and moment magnitude Mw computed by spectral methods is not 1:1 for ML<2-4. In particular ML is found to be about proportional to 1.5 Mw but the exact threshold below which this occurs is argued. Such behavior was explained as due to attenuation and scattering along the path or to a minimum limit in the pulse duration or equivalently a maximum limit to the corner frequency of the observed spectra imposed by surface attenuation. The frequency-magnitude distribution of ML estimates provided by the Italian Seismic Instrumental Database (ISIDe) of INGV show a strictly linear behavior with b-value»1.0 down to about ML 1.4 at least. This implies that for Mw the b-value would be about 1.5 below magnitude 2-4 and 1 above. As the frequency magnitude relationship with b-value»1 in terms of Mw is recognized as a general characteristic of seismicity all over the world, based on both empirical and theoretical considerations, the question arises on the reasons of the observed discrepancy for small shocks. One explanation might be the assumption of incorrect seismic wave attenuation properties for the computation of ML, of spectral Mw or both.

How to cite: Lolli, B., Paolo, G., Biondini, E., and Vannucci, G.: The relationship between ML and Mw for small earthquakes (ML < 2-4) in Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18821, https://doi.org/10.5194/egusphere-egu2020-18821, 2020.

Since the inception of 62 borehole seismic arrays deployed by Central Weather Bureau (CWB) in Taiwan until the end of 2018, a large quantity of strong-motion records have been accumulated from frequently occurring earthquakes around Taiwan, which provide an opportunity to detect micro-seismicity. Each borehole array includes two force balance accelerometers, one at the surface and other at a depth of a few ten-to-hundred (30-492) meters, as well as one broadband seismometer is below the borehole accelerometer. In general, the background seismic noise level are lower at the downhole stations than surface stations. However, the seismograms recorded by the downhole stations are smaller than surface stations due to the near-surface site effect. In Taiwan, the local magnitude (ML) determinations use the attenuation function derived from surface stations. Therefore, the ML will be underestimated by using current attenuation function for downhole stations. In this study, we used 19079 earthquakes to investigate the site amplification at subsurface materials between downhole and surface stations. Results demonstrate the amplification factors ranging from 1.11 to 5.74, provide the site effect parameter at shallow layers and have a strong relationship with Vs30. Further, we apply the amplification factors to revise the station local magnitude for downhole station. The revised ML at downhole stations correlate well with the ML at surface stations. Implement of the downhole station in the ML determination, it enhances the ability to detect the micro-earthquake and makes the earthquake catalog more comprehensive in Taiwan.

How to cite: Lai, T.-S., Wu, Y.-M., and Chao, W.-A.: Application of Site Amplification Factors to Determine the Local Magnitude from Borehole Seismic Stations in Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6310, https://doi.org/10.5194/egusphere-egu2020-6310, 2020.

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EMS98 intensity estimation of the shallow Le Teil earthquake, ML 5.2, by Macroseismic Response Group GIM

Antoine Schlupp, Christophe Sira, Emeline Maufroy, Ludmila Provost, Remi Dretzen, Etienne Bertrand, Elise Beck, and Marc Schaming

BCSF-RéNaSS (Bureau central sismologique français – Réseau national de surveillance sismique) manages the collection of data from the field for any earthquake in mainland France of magnitude greater than 3.7 and ensures their interpretation in terms of macro-seismic intensities (severity of ground shaking) on EMS98, European Macroseismic Scale (Grünthal, 1998). Unlike the magnitude, which is calculated from seismological records, the intensity of the tremor is only known in each location by analysing the observable effects on people, objects and structures. In case of damage, the GIM (Groupe d'intervention macrosismique = Macroseismic Response Group), coordinated by the BCSF-RéNaSS, establishes EMS98 intensities within a short time after the occurrence of the earthquake. It gathers together scientists (researchers and engineers in tectonics, geology, civil engineering, etc.) from various French scientific institutions.

The 2019-11-11 Le Teil earthquake of magnitude ML 5.2 occurred at 10h52 UTC, 11h52 local time. It is a very shallow event, with hypocentre at about 2km depth and a fault rupture that reached the surface. More than 2000 people who felt the tremor responded to the online survey via the www.franceseisme.fr website, allowing a preliminary and rapid estimation of the intensity of the tremor. The day after the event, the BCSF-RéNaSS launched a survey toward the municipal authorities using a collective form designed for the town halls of the municipalities potentially affected. Given the damage described in the answers, the GIM was mobilized to accurately assess the EMS98 intensities of municipalities near the epicentre, based on the effects observed on buildings, people and objects, and taking into account their vulnerability.

Among the almost sixty experts that compose the GIM, seven from IRSN, ISTerre/RESIF-RAP, Cerema, Pacte/UGA, IPGS and EOST/BCSF-RéNaSS answered the call. Divided into teams of 2 or 3, they inspected 24 municipalities between November 18thand 22nd, assisted by mayors or municipal services and sometimes accompanied by the rescue brigade. Several hundred buildings of different vulnerabilities were inspected.

In most cases, many cracks, sometimes significant and open, were observed. Few of the oldest structures built mostly in the 19thcentury, associated to vulnerability A, partially or totally collapsed in the most affected areas such as Le Teil and Viviers. For comparable buildings, more severe damages were observed on top of hills (Saint-Thomé) or on sedimentary filling (Savasse) which attests for local site effects. 

The highest intensities reach locally VIII in La Rouvière and Mélas, two neighbourhood of Le Teil, that are located the closest to the Rouvière fault. These are the highest intensities observed in mainland France since the Arette earthquake in 1967 (Rothé, 1972).

The macroseismic intensities EMS98, estimated during the GIM's field missions, are one of the major input on which is based the decision of the French commission to classify municipalities in a state of natural disasters.  That decision triggers insurance coverage of damages.  Over the 24 analysed by the GIM, the commission classified 19 municipalities during their meetings of November 20thand December 11th. Following commission meetings will examine the other impacted municipalities.

How to cite: Schlupp, A., Sira, C., Maufroy, E., Provost, L., Dretzen, R., Bertrand, E., Beck, E., and Schaming, M.: EMS98 intensity estimation of the shallow Le Teil earthquake, ML 5.2, by Macroseismic Response Group GIM, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3767, https://doi.org/10.5194/egusphere-egu2020-3767, 2020.

The western region of the Pyeongnam Basin has relatively higher e​arthquake activity than the rest of the Korean Peninsula. We analyzed 48 earthquakes in the area, with a magnitude (ML) of 2.0 or more, from January 2009 to June 2019. The hypocentral parameters were re-determined using an iterative algorithm that repeats the calculation until the residual error between the observed and calculated arrival time of a seismic phase at each station is minimized. Using the hypocenters and the optimal 1-D velocity model derived from this process, the focal mechanisms were determined using the first-motion polarities of body waves. Many earthquakes are associated with left-lateral strike-slip faults, with a strike in the NW-SE direction and a normal faulting component. A stress inversion was performed using data of the pressure and tensional axes from the focal mechanisms. The maximum principal stress in the study area acts in the NW-SE direction with high angles of plunge and differs from the maximum horizontal principal stress in the rest of the Korean Peninsula. This stress perturbation is caused by the detachment of a small local stress from the regional stress field due to the presence of weak faults with low shear strength that develop in the sedimentation environment of the Pyeongnam Basin.

How to cite: Kang, T.-S. and Lee, H.: Complex seismic activity and local stress perturbation of the Pyeongnam basin, northern Korean Peninsula, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18365, https://doi.org/10.5194/egusphere-egu2020-18365, 2020.

EGU2020-7557 | Displays | SM1.1

Recent seismic events preserved in lacustrine sediments from the SE Tibetan Plateau

Yongbo Wang, Xuezhi Ma, and Zhenyu Ni

Large earthquakes are regarded as important contributors to long-term erosion rates and considerable hazard to infrastructure and society, which were difficult to track because of the long recurrence time exceeding the time span of historical records. Geological records, especially the continuously accumulated lacustrine sediments, hold the potential to capture signals of prehistoric seismic events, which has been barely reported from the Tibetan Plateau. Here we present lacustrine sediment records recovered from Basom Tso in Southeastern Tibetan Plateau, in which two seismic events were preserved. Sediment lithology, grain size composition, magnetic susceptibility and XRF scanning induced element compositions showed dramatic variations in two turbidite-like sediment segments. Particularly, the grain size showed an abrupt increase at the bottom of the Turbidites which was followed by a fining-up pattern and covered by a fine clay cap, expressing similar sedimentary processes caused by the seiche effect triggered by seismic events. Consistent patterns were recorded in the element contents as well, i.e. obvious bias in the counts of Fe, Zr, Ti, Ca. In addition, scuh pattern were preserved in sediment cores from different part of the lake basin, indicating a basin wide event layer. Finally, according to the dating results from 137Cs and 14C, the two Turbidites were formed around 1950 A.D. and during the late18th/early 19th century respectively. Such information was further confirmed by historical earthquake records that Chayu Earthquake (M=8.6, 1950 A.D.) and Nyingchi Earthquake (M=6.75, 1845 A.D.) have possibly responsible for the slump of underwater sediments and the formation of these two turbidites.

How to cite: Wang, Y., Ma, X., and Ni, Z.: Recent seismic events preserved in lacustrine sediments from the SE Tibetan Plateau, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7557, https://doi.org/10.5194/egusphere-egu2020-7557, 2020.

EGU2020-22107 | Displays | SM1.1

SEISMOLOGICAL AND ENGINEERING PARAMETERS OF 24 and 26 SEPTEMBER, 2019 MARMARA SEA EARTHQUAKES

Eser Çakti, Fatma Sevil Malcioğlu, and Hakan Süleyman

On 24th and 26th  September 2019, two earthquakes of Mw=4.5 and Mw=5.6 respectively took place in the Marmara Sea. They were associated with the Central Marmara segment of the North Anatolian Fault Zone, which is pinpointed by several investigators as the most likely segment to rupture in the near future giving way to an earthquake larger than M7.0. Both events were felt widely in the region. The Mw=5.6 event, in particular, led to a number of building damages in Istanbul, which were larger than expected in number and severity. There are several strong motion networks in operation in and around Istanbul. We have compiled a data set of recordings obtained at the stations of the Istanbul Earthquake Rapid Response and Early Warning operated by the Department of Earthquake Engineering of Bogazici University and of the National Strong Motion Network operated by AFAD. It consists of 148 three component recordings, in total.  444 records in the data set, after correction, were analyzed to estimate the source parameters of these events, such as corner frequency, source duration, radius and rupture area, average source dislocation and stress drop. Duration characteristics of two earthquakes were analyzed first by considering P-wave and S-wave onsets and then, focusing on S-wave and significant durations. PGAs, PGVs and SAs were calculated and compared with three commonly used ground motion prediction models (i.e  Boore et al., 2014; Akkar et al., 2014 and Kale et al., 2015). Finally frequency-dependent Q models were estimated using the data set and their validity was dicussed by comparing with previously developed models.

How to cite: Çakti, E., Malcioğlu, F. S., and Süleyman, H.: SEISMOLOGICAL AND ENGINEERING PARAMETERS OF 24 and 26 SEPTEMBER, 2019 MARMARA SEA EARTHQUAKES, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22107, https://doi.org/10.5194/egusphere-egu2020-22107, 2020.

EGU2020-3960 | Displays | SM1.1

The ML = 6.8 25 October 2018 Earthquake Zakynthos Island (Ionian Sea) and the evolution of the aftershock sequence one year later

Alexandra Moshou, Antonios Konstantaras, Panagiotis Argyrakis, and Nikolaos Sagias

The area of Zakynthos (Ionian Island) is located at a complex plate boundary region where two tectonic plates (Africa-Nubia and Eurasia) converge, thus forming the western Hellenic Arc. On the midnight of 26th October (ML = 6.6, 22:54:49 UTC) a very strong earthquake has struck at the eastern part of Zakynthos Island (Ionian Sea, Western Greece). Epicentral coordinates of the earthquake was determined as 37.3410° N, 20.5123° E and a focal depth at 10 km, according to the manual solution of National Observatory of Athens

(http://bbnet.gein.noa.gr/alerts_manual/2018/10/evman181025225449_info.html).

This earthquake was strongly felt at the biggest shock was felt as far afield as Naples in western Italy, and in Albania, Libya, and the capital Athens. Nobody was injured by these events but there was significant damage to the local port and a 13th Century island monastery south of Zakynthos.

A few minutes later (23:09:20, UTC) a second intermediate earthquake with magnitude ML=5.1 was followed the first event. The M5+ events of 25 October 2019, as well as the rich aftershock sequence of 10.000+ events with magnitudes 1.0<ML<4.9 of the 12 following months have been relocated using the double – difference algorithm HYPODD.

For the aftershocks with 3.7<ML<6.6 we applied the moment tensor inversion to determine the activation of the faulting type, the Seismic Moment (M0) and the Moment Magnitude (Mw). For this purpose, 3–component broadband seismological data from the Hellenic Unified Seismological Network (HUSN) at epicentral distances less than 3˚ were selected and analyzed. The preparation of the data, includes the deconvolution of instrument response, following the velocity was integrated to displacement and finally the horizontal components rotated to radial and transverse. All the focal mechanisms were compared with those from other institutes and they are in agreement. The second part of this study refers to the calculation of the stress tensor using the STRESSINVERSE package by Václav Vavryčuk. The final part of this study includes an extensive kinematic analysis of geodetic data from local GNSS permanent station to further examine the dynamic displacement.

References:

How to cite: Moshou, A., Konstantaras, A., Argyrakis, P., and Sagias, N.: The ML = 6.8 25 October 2018 Earthquake Zakynthos Island (Ionian Sea) and the evolution of the aftershock sequence one year later, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3960, https://doi.org/10.5194/egusphere-egu2020-3960, 2020.

The Matese Massif is the major mountain range of the Sannio-Matese, which is the transition area between central and southern Apennines. The Massif is located among the seismogenic sources of large destructive historical Earthquakes (e.g. 1349, MW =7.0; 1688, MW = 6.6; 1805, MW = 6.8). Previous studies on the instrumental seismicity of the Sannio-Matese have shown that the seismic activity along and close to the Matese Massif is prevalently characterized by the occurrence of sparse low magnitude events (ML<2.5) and by seismic sequences with low to moderate magnitude (MWmax=5.0) with hypocenters within the uppermost crust. Last relevant seismic sequence occurred between the late 2013-early 2014 following an MW=5.0 earthquake. This sequence struck the internal southern side of the Massif in an area where no evidence of active faulting has been recorded so far. Multidisciplinary investigation on this sequence suggest that the sequence has developed along a SW dipping NNW-SSE striking normal fault, ~10 km long, confined in the 10-20 km depth range. The 1805 Earthquake affected the northern slope of the Massif whereas the 1349 and 1688 Earthquakes affected the southern side. The 1349 Earthquake, that includes at least three main shocks, given its age, stands out due to the lack of reliable and sufficiently vast historical documentation. Geological, geomorphological and historical analysis on this Earthquake evidenced a SW dipping 125 striking 22 km length normal fault, named Aquae Iuliae Fault (AIF), as responsible for one of the main shocks of this Earthquake. In order to provide further information on the seismotectonics setting of the southwest sector of the Matese Massif, here is analyzed the instrumental seismicity occurred in 2009-2019 time interval in the area of the 1349 Earthquake. The spatial distribution of the relocated seismicity mainly consists of single events with magnitude ML≤3.5. The single events are localized prevalently nearby AIF and have foci falling generally in the first 15 km of the crust. The focal mechanisms of the most energetic events show normal dip-slip solutions, with NW-SE striking planes and NE-SW striking T-axes. The epicentral distribution of a low magnitude seismic swarm, triggered by an earthquake of ML 3.3 and constituted by about 120 events,  shows a roughly WNW-ESE alignment. The hypocenters, confined in the range 5-15 km depth, roughly depict a SW dipping plane. The fault plane solutions of the very few events of this swarm with ML > 2.0 show both normal dip slip solutions, with a minor strike component, and strike-slip solutions, with a minor dip component. The common element of these focal mechanisms is the presence of a SW dipping fault plane, striking from NW-SE to NNW-SSE. The preliminary results of this study, taking into account the dipping plane of the 2013-2014 sequence and that of the AIF, suggest that the release of seismic energy in the southwest side of the Matese Massif occur on very small fault segments, with SW dipping.

How to cite: Milano, G.: Recent instrumental seismicity of the southwest Matese Massif (Sannio-Matese area - Italy): a contribution on the seismotectonics setting., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15851, https://doi.org/10.5194/egusphere-egu2020-15851, 2020.

EGU2020-12975 | Displays | SM1.1

New seismological insights from the analyses of historical and recent earthquakes at Ischia Island (Southern Italy)

Stefano Carlino, Vincenzo Convertito, Anna Tramelli, Vincenzo De Novellis, and Nicola Alessandro Pino

We report here a first comparative analysis between recent and historical earthquakes, occurred in the island of Ischia (Southern Italy), which produced heavy damages and thousands of fatalities. The island of Ischia is located in the Gulf of Naples, and represents a peculiar case of resurgent caldera in which volcano-tectonic earthquakes, with low magnitude, have generated large damages and catastrophic effects, as is the case for the 4 March 1881 (Imax8-9 MCS) and the 28 July 1883 (Imax10-11 MCS) events. Both the earthquakes struck the northern area of the island, similarly to the recent 21 August 2017 earthquake. The results allowed us to assess the location, as well as the possible dimension and the related maximum magnitude of the seismogenic structure, located in the northern sector of the island, and responsible of damaging earthquakes. Our results also provide an additional framework to interpret mechanisms leading to earthquakes associated with dynamics of calderas.

 

How to cite: Carlino, S., Convertito, V., Tramelli, A., De Novellis, V., and Pino, N. A.: New seismological insights from the analyses of historical and recent earthquakes at Ischia Island (Southern Italy) , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12975, https://doi.org/10.5194/egusphere-egu2020-12975, 2020.

EGU2020-11385 | Displays | SM1.1

Efficiency assessment of seismological information, monitoring system and seismicity study for the Republic of Armenia

Jon Karapetyan, Karlen Ghazaryan, Rudolf Sargsyan, and Roza Karapetyan

To study the deep structure of the earth it is important to have an optimal monitoring network and reliable seismic baseline database for the investigated area. In addition, the territory of Armenia according to its geology and seismological conditions is a full-scale experimental polygon for seismic problems, in particular the study of Earth's deepest structure by seismic methods. For this purpose, the work aims to assess the effectiveness of the existing seismic monitoring system in Armenia, to offer optimal solutions for the station layout, to evaluate the accuracy of seismic information registered in the RA by performing hypocenter recalculation. Then, within of the work organized modern seismic stations in Armenia and Russia towns with seismic equipment made and produced in Armenia (IGES NAS RA). The work was supported by MESCS Science Committee of the Republic of Armenia (grant № 18SH-1E012).

How to cite: Karapetyan, J., Ghazaryan, K., Sargsyan, R., and Karapetyan, R.: Efficiency assessment of seismological information, monitoring system and seismicity study for the Republic of Armenia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11385, https://doi.org/10.5194/egusphere-egu2020-11385, 2020.

EGU2020-11211 | Displays | SM1.1

Present-day seismic activity in the Mugello Basin and adjoining areas (Northern Apennines, Italy)

Rebecca Bruni, Giacomo Corti, Michele D'Ambrosio, Andrea Fiaschi, Carlo Giunchi, Derek Keir, Davide Piccinini, Federico Sani, and Gilberto Saccorotti

The Northern Apennines is a NW-SE striking fold-and-thrust belt composed of a pile of NE-verging tectonic units that developed during Cenozoic collision between the European plate (Corso–Sardinian block) and the Adria plate. Seismicity and geodetic data indicate that contemporaneous crustal shortening (in the external, Adriatic part) and extension (in the internal, Tyrrhenian side) characterize the current tectonic activity of the Apennines. The region around the Mugello basin (Northern Tuscany) represents one of the most important seismogenic areas of the Northern Apennines. Large historical earthquakes have occurred, such as the M=6.0, 1542 and the M=6.4, 1919 events. Its proximity to densely-urbanized areas and the potential impact of strong earthquakes on the cultural heritage in the nearby (~30km) city of Florence makes a better knowledge of the seismicity in the Mugello basin a target of paramount importance. Unresolved issues regard (i) the exact location and geometry of the fault(s) which produced the 1542 and 1919 earthquakes, (ii) the mechanism driving the abrupt transition from an extensional to compressional stress regime at the internal and external sides of the belt, respectively, and (iii) geometry of and role played by a close-by transfer zone oriented transversely (NE-SW) to the main strike of the belt. To address these problems, in early 2019 we initiated a project aiming at improving the knowledge about the seismo-tectonic setting of the basin and adjoining areas. At first, we integrated all the available seismic catalogs for the area, obtaining more than 12000 earthquakes spanning the 2005-2019 time interval. These data have been used to derive a minimum-misfit, 1-D velocity model to be subsequently used for a travel times inversion 3D tomography. At the same time, we Installed 9 temporary seismic stations, complementing the permanent networks presently operating in the area. This new deployment recorded a Mw=4.5 earthquake that struck the NW margin of the basin on Dec. 9, 2019. The mainshock and the ~200 aftershocks precisely delineate a 5-km-long, NW-striking and SW-dipping fault which extends over the 6-9 km depth interval. The focal mechanism of the mainshock yields consistent results, indicating a normal fault striking N105°E and dipping about 45°. This fault appears to be distinct from that (those) activated during the two last important sequences in the area, which occurred in 2008 and 2009. The earthquake caused unexpected, large accelerations (PGA~0.24g at ~7km epicentral range), provoking damages that resulted in the evacuation of more than 150 residents and economic losses of several millions of euro. Sample horizontal-to-vertical spectral ratios at the most damaged sites report significant amplification within the 1-5 Hz frequency range, likely responsible for the anomalous ground shaking. Given the proximity of the aforementioned fault to that inferred for the 1542 (and, possibly, 1919) earthquake(s), a detailed study of the 2019 seismic sequence is expected to shed new light into the overall dynamics of the basin.

How to cite: Bruni, R., Corti, G., D'Ambrosio, M., Fiaschi, A., Giunchi, C., Keir, D., Piccinini, D., Sani, F., and Saccorotti, G.: Present-day seismic activity in the Mugello Basin and adjoining areas (Northern Apennines, Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11211, https://doi.org/10.5194/egusphere-egu2020-11211, 2020.

EGU2020-11945 | Displays | SM1.1

The May 7 - 11, 2016 Earthquake Sequence at Rivera Fault Zone

Francisco J Nunez-Cornu, Diego Cordoba, William L Bandy, Juan José Dañobeitia, Carlos Mortera-Gutierrez, Edgar Alarcon, Diana Nuñez, Claudia B Quinteros-Cartaya, and Carlos Suarez-Plascencia

The geodynamic complexity in the interaction between Rivera, Cocos and NOAM plates is mainly reflected in the high and not well located seismicity of the region. In the framework of TsuJal Project, a study of the passive seismic activity was carried out. A temporal seismic network with 25 Obsidian stations with sensor Le-3D MkIII were deploying from the northern part of Nayarit state to the south of Colima state, including the Marias Islands, in addition to the Jalisco telemetric Seismic Network, being a total of 50 seismic stations on land. Offshore, ten Ocean Bottom Seismographs type LCHEAPO 2000 with 4 channels (3 seismic short period and 1 pressure sensors) were deployed and recover by the BO El Puma from UNAM in an array from the Marias Islands to off coast of the border of Colima and Michoacan state, in the period from 19th April to 7th November 2016.

A seismic sequence started on May 7, 2016 with an earthquake Mw = 5.6 reported by CMT-Harvard, USGS and SSN at the area north of Paleo Rivera Transform fault and west of the Middle America Trench, an area with a very complex tectonics due to the interaction of Rivera, Cocos and NOAM plates.

An analysis of this earthquake sequence from May 7 to May 11 using data from OBS and adequate P-Wave velocity model for Rivera plate is presented, 87 earthquakes were located. Data from onland stations were integrated after a travel-time residual analysis.

We observed that the new location is about 50 km southwest direction, from previous one, between the Paleo Rivera Transform fault and the northern tip of the East Pacific Rise – Pacific Cocos Segment.  This area has a different tectonic stress regime.

How to cite: Nunez-Cornu, F. J., Cordoba, D., Bandy, W. L., Dañobeitia, J. J., Mortera-Gutierrez, C., Alarcon, E., Nuñez, D., Quinteros-Cartaya, C. B., and Suarez-Plascencia, C.: The May 7 - 11, 2016 Earthquake Sequence at Rivera Fault Zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11945, https://doi.org/10.5194/egusphere-egu2020-11945, 2020.

SM1.2 – New seismic data analysis methods for automatic characterization of seismicity

Traditionally, the ability to study seismic phenomena is dependant on both the available hardware and time for processing data needed to produce a research grade catalogue. Consequently, shortages in either of these resources constrain the scope of studies available to the research scientist. This is becoming especially challenging as networks become larger and more dense, and as the community moves towards Large-N networks and arrays. We will look at alternative solutions to address these resource constraints and open the scientist up to a broader field of study.

Ownership of equipment or waiting in a queue for loan pool assets are the two most common methods for acquiring the hardware necessary to conduct a scientific study. Further, once the data has been collected, a good deal of time is spent processing that data to produce a catalogue before the scientific inquiry can begin.   

There is now an alternative model for acquiring and processing data in seismology that shortens the time and effort necessary to produce a research grade catalogue. We will demonstrate how we can customize acquisition arrays to meet experimental goals and apply proven processing models and AI techniques to deliver a bespoke research grade catalogue at a fraction of the time and cost of traditional acquisition and processing methods. This removes several of the challenging aspects of running an experiment in order to enable researchers to get straight to their science and shortening the time to publication.

How to cite: Baturan, D., Townsend, B., and Moores, A.: A new operational model that increases experiment diversity and shortens time to publication for research Seismology, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12535, https://doi.org/10.5194/egusphere-egu2020-12535, 2020.

EGU2020-5874 | Displays | SM1.2

The Coda Calibration and Processing Tool: Java-Based Freeware for the Geophysical Community

Kevin Mayeda, Rengin Gok, Justin Barno, William Walter, and Jorge Roman-Nieves

The coda magnitude method of Mayeda and Walter (1996) provides stable source spectra and moment magnitudes (Mw) for local to regional events from as few as one station that are virtually insensitive to source and path heterogeneity. The method allows for a consistent measure of Mw over a broad range of event sizes rather than relying on empirical magnitude relationships that attempt to tie various narrowband relative magnitudes (e.g., ML, MD, mb, etc.) to absolute Mw derived from long-period waveform modeling. The use of S-coda and P-coda envelopes has been well documented over the past several decades for stable source spectra, apparent stress scaling, and hazard studies. However, up until recently, the method requires extensive calibration effort and routine operational use was limited only to proprietary US NDC software. The Coda Calibration Tool (CCT) stems from a multi-year collaboration between the US NDC and LLNL scientists with the goal of developing a fast and easy Java-based, platform independent coda envelope calibration and processing tool. We present an overview of the tool and advantages of the method along with several calibration examples, all of which are freely available to the public via GitHub (https://github.com/LLNL/coda-calibration-tool). Once a region is calibrated, the tool can then be used in routine processing to obtain stable source spectra and associated source information (e.g., Mw, radiated seismic energy, apparent stress, corner frequency, source discrimination on event type and/or depth). As more events are recorded or new stations added, simple updates to the calibration can be performed. All calibration and measurement information (e.g., site and path correction terms, raw & measured amplitudes, errors, etc.) is stored within an internal database that can be queried for future use. We welcome future collaboration, testing and suggestions by the geophysical community.  

How to cite: Mayeda, K., Gok, R., Barno, J., Walter, W., and Roman-Nieves, J.: The Coda Calibration and Processing Tool: Java-Based Freeware for the Geophysical Community, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5874, https://doi.org/10.5194/egusphere-egu2020-5874, 2020.

A generic approach for real-time magnitude and stress drop is introduced that is based on the omega-squared model (Brune, 1970) and results from Lior and Ziv (2018). This approach leads to approximate expressions for earthquake magnitude and stress drop as functions of epicentral distance and ground motion root-mean-squares (rms). Because the rms of the ground motion (acceleration, velocity and displacement) may be calculated directly from the seismogram in the time domain, the use of this approach for automated real-time processing is rather straightforward. Once the seismic moment and stress drop are known, they may be plugged in the ground motion prediction equations (GMPE) of Lior and Ziv (2018) to map the predicted peak shaking.

This method is generic in the sense that it is readily implementable in any tectonic environment, without having to go through a calibration phase. The potential of these results for automated early warning applications is demonstrated using a large dataset of about 6000 seismograms recorded by strong-motion and broadband velocity sensors from different tectonic environments. Optimal real-time performance is achieved by integrating magnitude and stress drop estimates into an evolutionary algorithm. The result of such an evolutionary calculation for the Mw 7.1 Ridgecrest earthquake indicates close agreement with the true magnitude.

How to cite: ziv, A. and Lior, I.: Generic Source Parameter Determination for Earthquake Early Warning: Theory, Observations and Implications for the Mw 7.1 Ridgecrest Earthquake, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15078, https://doi.org/10.5194/egusphere-egu2020-15078, 2020.

EGU2020-3224 | Displays | SM1.2

Fast acquisition of focal mechanism based on statistical analysis

Marisol Monterrubio-Velasco, José Carlos Carrasco-Jimenez, Otilio Rojas, Juan Esteban Rodríguez, and Josep de la Puente

Earthquake and tsunami early warning systems and post-event urgent computing simulations require of fast and accurate quantification of earthquake parameters such as magnitude, location and Focal Mechanism (FM). Methodologies to estimate earthquake location and magnitude are well-established and in place. However, automatic solutions of FMs are not always provided by operational institutions and are, in some cases, available only after a time-consuming inversion of the wave-forms needed to determine the moment tensor components. This precludes urgent seismic simulations, which aim at providing ground shaking maps with severe time constraints. We propose a new strategy for fast (<60 s) determination of FM based on historical data sets, tested it at five different active seismic regions, Japan, New Zealand, California, Iceland, and Italy. The methodology includes the k-nearest neighbor's algorithm in a spatial dimension domain to search the most similar FMs between the data set. In our research, we focus on moderate to large earthquakes. The comparison algorithm includes the four closest events, and also a hypothetical event building by the median values of strike, dip, and rake of the k-neighbors. The validation stage includes the minimum rotated angle measure to compute the similitude between a pair of FMs. We find three model parameters, such as the minimum number of neighbors, the threshold radius that defines the neighboring sphere, and the magnitude threshold, that could improve the statistical similitude results. Our fast methodology has a 75%-90% agreement with traditional inversion mechanisms, depending on the particular tectonic region and dataset size. Our work is a key component of an urgent computing workflow, where the FM information will be used as input for ground motion simulations. Future work will assess the sensitivity of FM uncertainty in the resulting ground-shaking maps.

How to cite: Monterrubio-Velasco, M., Carrasco-Jimenez, J. C., Rojas, O., Rodríguez, J. E., and de la Puente, J.: Fast acquisition of focal mechanism based on statistical analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3224, https://doi.org/10.5194/egusphere-egu2020-3224, 2020.

EGU2020-18973 | Displays | SM1.2

A user-friendly probabilistic earthquake source inversion framework for joint inversion of seismic, geodetic, and gravitational signals - The Grond toolkit

Sebastian Heimann, Marius Isken, Daniela Kühn, Hannes Vasyura-Bathke, Henriette Sudhaus, Andreas Steinberg, Gesa Petersen, Marius Kriegerowski, Simon Daout, Simone Cesca, and Torsten Dahm

Seismic source and moment tensor waveform inversion is often ill-posed or non-unique if station coverage is poor or signals are weak. Three key ingredients can help in these situations: (1) probabilistic inference and global search of the full model space, (2) joint optimisation with datasets yielding complementary information, and (3) robust source parameterisation or additional source constraints. These demands lead to vast technical challenges, on the performance of forward modelling, on the optimisation algorithms, as well as on visualisation, optimisation configuration, and management of the datasets. Implementing a high amount of automation is inevitable.

To tackle all these challenges, we are developing a sophisticated new seismic source optimisation framework, Grond. With its innovative Bayesian bootstrap optimiser, it is able to efficiently explore large model spaces, the trade-offs and the uncertainties of source parameters. The program is highly flexible with respect to the adaption to specific source problems, the design of objective functions, and the diversity of empirical datasets.

It uses an integrated, robust waveform data processing, and allows for interactive visual inspection of many aspects of the optimisation problem, including visualisation of the result uncertainties. Grond has been applied to CMT moment tensor and finite-fault optimisations at all scales, to nuclear explosions, to a meteorite atmospheric explosion, and to volcano-tectonic processes during caldera collapse and magma ascent. Hundreds of seismic events can be handled in parallel given a single optimisation setup.

Grond can be used to optimise simultaneously seismic waveforms, amplitude spectra, waveform features, phase picks, static displacements from InSAR and GNSS, and gravitational signals.

Grond is developed as an open-source package and community effort. It builds on and integrates with other established open-source packages, like Kite (for InSAR) and Pyrocko (for seismology).

How to cite: Heimann, S., Isken, M., Kühn, D., Vasyura-Bathke, H., Sudhaus, H., Steinberg, A., Petersen, G., Kriegerowski, M., Daout, S., Cesca, S., and Dahm, T.: A user-friendly probabilistic earthquake source inversion framework for joint inversion of seismic, geodetic, and gravitational signals - The Grond toolkit, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18973, https://doi.org/10.5194/egusphere-egu2020-18973, 2020.

EGU2020-20783 | Displays | SM1.2

Seismicity of the Mt. Kinabalu fault system in Sabah, Borneo, revealed using waveform backprojection

Conor Bacon, Amy Gilligan, Nicholas Rawlinson, Felix Tongkul, David Cornwell, Simone Pilia, Omry Volk, and Tim Greenfield

The aim of the Northern Borneo Orogeny Seismic Survey (nBOSS) is to better understand the mechanisms driving the processes that occur in a post-subduction setting. A network of 46 seismometers was deployed across Sabah, Borneo, between March 2018 and January 2020 (22 months) in order to investigate these mechanisms using a suite of seismic imaging techniques.

 

Mt. Kinabalu (~4100 m) is a large granitic pluton that was emplaced between ~7.9 and 7.2 Ma. The region around the mountain experiences infrequent earthquakes, with the M6.0 Sabah earthquake in 2015 being the second largest earthquake to strike the region in the past century. This earthquake caused the loss of 18 lives and an estimated 100 million Ringgit (~€22 million) of damage to buildings, roads and infrastructure. The 2015 earthquake has highlighted the importance of improving our understanding of seismic hazards in northern Borneo. Although both a network of faults striking along the spine of the Crocker range, and a complex network of faults around the Kinabalu massif have been mapped, which of these are currently active remains poorly understood. Using data from the nBOSS seismic network, together with additional data from the Malaysian Meteorological Service, we aim to quantify and categorise the seismicity associated with this fault system.

 

We have used QuakeMigrate, a new, modular, open-source Python package for waveform backprojection to efficiently, automatically and robustly detect and locate microseismicity in the region around Mt. Kinabalu. We provided QuakeMigrate with continuous raw seismic data, a velocity model derived using nBOSS seismic data, and a list of station locations. A realistic estimate of the event location uncertainty, phase picks with uncertainties, and a suite of visual outputs allows for rigorous selection of real events at a sub-SNR detection threshold.

 

Using data from March 7 2018 to 28 August 2018, we have detected and located over 1500 events with hypocentres highly concentrated beneath the Kinabalu massif. Given existing catalogues for the area around Mt. Kinabalu only record on the order of tens of events between 1990 and the present day, our results demonstrate that these catalogues are highly incomplete at low magnitudes and thus existing tectonic and hazard models for the area need to be revised.

How to cite: Bacon, C., Gilligan, A., Rawlinson, N., Tongkul, F., Cornwell, D., Pilia, S., Volk, O., and Greenfield, T.: Seismicity of the Mt. Kinabalu fault system in Sabah, Borneo, revealed using waveform backprojection, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20783, https://doi.org/10.5194/egusphere-egu2020-20783, 2020.

The Alpine chain marks the border between different nations, so it’s important in this area the cooperation, the data sharing and the coordination among institutions operating in contiguous regions and nations that are involved in the observation and the management of natural hazards such as earthquakes affecting large portions of the territory.

As part of the Interreg Alcotra cross-border program, one of the objectives of the RISVAL project concerns the improvement of the seismic hazard assessment and in general of the knowledge of seismicity in the Western Alps. In this area, Italian, French and Swiss stations operate in various national and regional networks, connected to each other, sharing data also with European services (e.g. EIDA). Streaming raw data are the basic type of data shared, since each institution produces its own analyses and computed data, resulting for instance in different seismic catalogs, with of course different characteristics, also in spatio-temporal boundaries.

Furthermore the monitoring and analysis systems have been interested over the years by technological developments, so that the available data grow exponentially and the catalogs derived from the surveillance activities in near-real time show several internal inhomogeneities in the various time intervals, also considering the different sensitivity and subjectivity of the operators who alternate in carrying out the manual review.

Therefore emerges the need to process increasingly large amounts of data available, that could be re-analyzed and updated in a homogeneous way according to new developments. To face this effort we wanted to test the performance of a complete automatic procedure (Scafidi et. al, 2019) to re-compile a portion (2012-2019) of the seismic catalog derived by RSNI network (Regional Seismic network of Northwestern Italy) operating routines, including travel-time and strong-motion parameters dataset.

The procedure, driven by customizable set of parameters suitable for network geometry and seismicity features, relies on a multistep algorithm, that in this work we tested skipping the initial steps concerning the event detection tool on continuous raw data. So we perform it on 21391 already available detected waveform traces for 1549 events: 1) automatic P- and S-phase picker, 2) hypocenter locator (using NonLinLoc package and 3D velocities model), 3) magnitude and strong-motion parameter calculator.

We firstly evaluate the results for the re-compiled catalog both in terms of distributions of errors and other quality parameters and in terms of time-residuals distributions on the basis of azimuth variation for each station, distinguishing shorter and longer epicentral distances, in order to evaluate anomalies in propagation velocities pattern.

Then we compare the new catalog results with manual catalogs available in the area, to point out differences in sources and stations calculated parameters: primarily the original RSNI, confirming the reliability of the method, then the Italian national CPTI by INGV, and, with a closer view in the cross-border Alps area, the French ones (RéNaSS, Sismoazur, SISmalp).

Scafidi D. et al. 2019. A Complete Automatic Procedure to Compile Reliable Seismic Catalogs and Travel-Time and Strong-Motion Parameters Datasets, in Seismological Research Letters, Volume XX, Number XX – 2019, DOI: 10.1785/02201802

How to cite: Bosco, F., Spallarossa, D., and Deschamps, A.: Assessment of performance of an automatic procedure for a review of recent seismicity in Western Alps compiling an homogeneous and reliable catalog, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16477, https://doi.org/10.5194/egusphere-egu2020-16477, 2020.

EGU2020-9095 | Displays | SM1.2

Remarks on the micro-earthquake detection problem: Refining the outcome using stochastic modeling

Athanasios Lois, Fotis Kopsaftopoulos, Dimitrios Giannopoulos, Katerina Polychronopoulou, and Nikos Martakis

Methodologies dealing with the detection of micro-earthquakes and the accurate estimation of body waves’ arrival time constitute, during the last decades, a topic of ongoing research. The extraction and efficient analysis of the useful information from the continuous recordings is of great importance, since it is a prerequisite for reliable interpretations.  Small magnitude seismic events, either naturally-occuring or induced, have been increasingly used in a wide range of industrial fields, with applications ranging from hydrocarbon and geothermal reservoir exploration, to passive seismic tomography surveys.

A great number of algorithms have been proposed and applied up to now for seismic event detection, exploiting specific properties of the seismic signals both in time and in frequency domain, with the energy-based detectors (STA/LTA) to be the most commonly used, due to their simplicity and the low computational cost they require. A significant obstacle emerging at seismological identification problems lies on the fact that such processes usually suffer from a number of false alarms, which is significantly increased in extremely noisy environments.

For that scope, we propose a “Decision-Making” mechanism, independent of the applied detection algorithm, which controls the results obtained during the detection process by minimizing false detections and providing the best possible outcome for further analysis. The specific scenario is based on the comparison among autoregressive models estimated on isolated seismic noise recordings, as well as on the detected intervals that resulted during the event identification procedure. A number of examples, associated with the implementation of the proposed scenario on real data, is presented with the scope of evaluating its performance. Several issues concerning the isolation of the seismic noise from the raw data, the estimation of the autoregressive models, the choice of the orders of the stochastic models etc., are discussed.

How to cite: Lois, A., Kopsaftopoulos, F., Giannopoulos, D., Polychronopoulou, K., and Martakis, N.: Remarks on the micro-earthquake detection problem: Refining the outcome using stochastic modeling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9095, https://doi.org/10.5194/egusphere-egu2020-9095, 2020.

EGU2020-4853 | Displays | SM1.2

Multistage adaptive spectral subtraction of seismic signals

Yousef Rajaeitabrizi, Robabeh Salehiozoumchelouei, Luca D'Auria, and José Luis Sánchez de la Rosa

The detection of microearthquakes is an important task in various seismological applications as volcano seismology, induced seismicity, and mining safety. In this work we have developed a novel technique in order to improve the quality and efficiency of STA/LTA based detection of microearthquakes. This technique consists of different stages of filtering employing an adaptive spectral subtraction method, which allows greatly improving the signal/noise ratio.

The implemented technique consists in a preliminary band-pass filtering of the signal followed by different stages of an adaptive spectral subtraction. The spectral subtraction technique is a non-linear filtering which allows taking into account the actual noise spectrum shape. It allows achieving a good filtering even in cases where the signal and noise spectrum overlaps. In order to take into account of the temporal variation in the background noise spectrum, we designed an adaptive technique. We first divide the incoming signals into short temporal windows. Each window is classified as “noise only” or “meaningful signal” (which can be either a microearthquake or any other relevant transient signal) using different features as the signal energy and the zero-crossing rate. Windows classified as “noise only” are continuously accumulated in a dynamic buffer which allows the average noise spectrum to be estimated and updated in an adaptive manner. This technique can be applied on subsequent stages to further improve the signal/noise ratio. This technique has been implemented in Python for the automatic detection of the microearthquakes on both off-line and near-real time data.

In order to check the efficiency of the results, we compared the results of an STA/LTA based automatic detection on the initial band-pass filtered signal and on the spectral subtracted signals after different stages of filtering. A notable improvement of the quality of the detection process is observed when repeated spectral subtraction stages are applied. 

We applied this procedure to seismic data recorded by Red Sísmica Canaria, managed by Instituto Volcanológico de Canarias (INVOLCAN), on Tenerife (Canary Islands), comparing results from the proposed detection algorithm with standard approaches as well as with manual detections. We present an extensive statistical analysis of the results, determining the percentage of correct detections, novel detections, false positives and false negatives after each stage of filtering. First results have shown that this technique is also able to detect automatically microearthquakes which went undetected after a manual analysis.

How to cite: Rajaeitabrizi, Y., Salehiozoumchelouei, R., D'Auria, L., and Sánchez de la Rosa, J. L.: Multistage adaptive spectral subtraction of seismic signals, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4853, https://doi.org/10.5194/egusphere-egu2020-4853, 2020.

EGU2020-19417 | Displays | SM1.2

Kouvola earthquake swarm - using a cross-correlator to find very small events and cluster them

Tuija Luhta, Kari Komminaho, Kati Oinonen, Timo Tiira, Marja Uski, Toni Veikkolainen, and Tommi Vuorinen

Kouvola area, a part of the Vyborg rapakivi batholith in southeastern Finland, has been experiencing an intraplate earthquake swarm since December 2011. The events have magnitudes ranging from ML -1.2 to 2.8 and they happen in the uppermost two kilometers of the crust. The Vyborg batholith has a long history of earthquake swarms with macroseismic data from 1751 onwards and the first instrumentally recorded swarm in 2003-2004.

Inspired by the ongoing activity, Institute of Seismology of University of Helsinki (ISUH) has installed temporary seismic stations in the area to complement seismic stations of the Finnish National Seismic network (FNSN). The detection threshold of FNSN is ML1.0, not sufficiently low to catch the smallest earthquakes of the swarm.

Several tailored cross-correlators have been developed at the ISUH to lower the event detection threshold. These can be used to detect even very small seismic events well below the current FNSN detection threshold. The method is especially well suited to swarm events, which generate nearly identical signals due to their common origin.

Only the largest events of the swarm can be used to calculate focal mechanisms or other event parameters reliably. One approach to use all data is waveform clustering. Event groups with identical signal can be formed, allowing e.g. calculation of composite focal mechanisms for each event cluster.

How to cite: Luhta, T., Komminaho, K., Oinonen, K., Tiira, T., Uski, M., Veikkolainen, T., and Vuorinen, T.: Kouvola earthquake swarm - using a cross-correlator to find very small events and cluster them, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19417, https://doi.org/10.5194/egusphere-egu2020-19417, 2020.

EGU2020-5099 | Displays | SM1.2

Waveform cross-correlation-based earthquake detection applied to microseismicity near the central Alpine Fault, New Zealand

Konstantinos Michailos, Calum J. Chamberlain, and John Townend

The Alpine Fault is a major plate boundary oblique strike-slip fault, known to fail in large M 7-8 earthquakes, posing a significant seismic hazard to southern and central New Zealand. The central part of the Alpine Fault exhibits low seismic activity when compared to adjacent areas. We have examined the smaller-magnitude earthquake activity occurring along the central portion of the Alpine Fault using data from five temporary seismic networks from late 2008 to early 2017.

We have created the most complete and accurate earthquake catalog at the central Alpine Fault to date (9,111 earthquake locations with magnitudes ranging from ML -1.2 to 4.6). We used this catalog as templates with a matched-filtering earthquake detection method and further extend the earthquake catalog. This even more comprehensive earthquake catalog will provide more definitive evidence for the seismicity characteristics observed and better insights into the fault zone’s geometry. 

Taking advantage of this extensive earthquake catalog, we also aim to examine whether there are any repeating highly similar seismic signals (repeating earthquakes). These repeating earthquakes can potentially help better determine the locked and creeping sections of the Alpine Fault and possibly quantify the total amount of creep taking place with respect to seismic deformation.

How to cite: Michailos, K., Chamberlain, C. J., and Townend, J.: Waveform cross-correlation-based earthquake detection applied to microseismicity near the central Alpine Fault, New Zealand, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5099, https://doi.org/10.5194/egusphere-egu2020-5099, 2020.

In September–November 2013 a seismic swarm occurred in Galati region of southeastern Romania. The area was previously known as characterized by low seismic activity along the major crustal faults. During the period of swarm, between September 23rd and November 5th, over 1000 events with the magnitudes (Ml) of 0.2–4.0, located at the depth of 5–10 km, have been detected. Despite the relatively small magnitude, events generated ground motions that were well felt by local people, leading to panic in the area. The proximity of active oil fields caused additional annoyance.

Advanced seismic monitoring in the region started in 2013 with deployment of mobile seismic stations immediately after the beginning of the swarm. Additionally, active seismic measurements were performed in order to characterize the shallow velocity structure at specific sites. Starting from July 2015 new permanents stations were installed in the area marking the beginning of Galati local network development. The routine seismic catalog derived using the acquired data and applying the standard detection and location techniques pointed that area continues to be seismically active, however with low rate of activity and magnitude of events. These made it a perfect study case for development of new advanced schemes for seismic monitoring of the regions with low and complex seismicity aiming on an understanding of the phenomenon underlying the 2013 seismic swarm as well as the current seismic activity in the area.

We developed and automatic monitoring scheme based on the network-based full waveform detection and location method BackTrackBB (Poiata et al. 2016) that exploits the coherency of signals’ statistical features recorded across the seismic network. Once extracted from the flux of continuous data, seismic events are compared against the database of previously detected events using coherency and allowing to identify potential repeaters or multiplets. The earthquake catalog provided by the system starting from 2017 was compared to the routine ROMPLUS catalog of NIEP showing an increase in the number of detected events by the order of 3. We present the details of the implementation and discuss its advantages and drawbacks.

How to cite: Tataru, D., Poiata, N., and Grecu, B.: Automatic monitoring of crustal seismic activity in Galati region of southeastern Romania using full waveform-based approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21361, https://doi.org/10.5194/egusphere-egu2020-21361, 2020.

EGU2020-13058 | Displays | SM1.2

Towards Real-Time Double-Difference Hypocenter Relocation of Natural and Induced Seismicity

Luca Scarabello, Tobias Diehl, Philipp Kästli, John Clinton, and Stefan Wiemer

In order to assess the fault-geometry and the spatio-temporal evolution of natural and induced seismicity, high-precision (relative) micro-seismic hypocenter locations are key information. From such precise relative hypocenter locations, we can infer e.g. the spatial extent during a seismic sequence, the seismogenic volume affected by stimulation procedures as well as geometries (orientation, segmentation) of potentially activated faults. Additionally, in the case of induced seismicity, the spatio-temporal evolution of seismicity (e.g. migration velocities of seismicity, r-t-diagrams) can be indicative for fluid-flow processes and provides first-order estimates of hydraulic properties of the reservoir as well as on the existence of possible hydraulic connections. Information on spatial extent, geometries and the spatio-temporal evolution of seismogenic structures can help to improve the seismic hazard assessment of natural and induced seismicity in real-time or near-real-time.

However, to make prompt use of information provided by such high-precision hypocenter locations requires relative relocations computed in near-real-time. This can be rather challenging, especially at the beginning of a seismic sequence, when only little or no background seismicity is available for relative relocation. In addition, an automated relative relocation process requires differential times derived from precise and reliable (absolute) automatic picks as well as from waveform cross-correlation.

In this work, we present our strategy towards a near-real-time relative relocation procedure. The procedure follows the methodology described by Waldhauser 2009 (BSSA; doi:10.1785/0120080294) and combines differential times derived from automatic as well as manual picks with waveform cross-correlations measurements. Differential times of new events are inverted for relative locations with respect to a background reference catalog using the double-difference algorithm. We present results derived by a python-based prototype applied to natural and induced earthquake sequences. In addition, the prototype is fully implemented in a new SeisComP3 (SC3) module “scrtdd”, which allows the application in a full real-time environment, using detections and locations from various existing SC3 modules (“scautoloc”, “scanloc”, “screloc”) as input for relative relocation. We outline our implementation strategy, and compare SC3 results with results derived by our software for natural and induced earthquakes monitored be dense near-fault monitoring networks in the Valais (SW Switzerland), St. Gallen (Switzerland) and the Hengill Geothermal Field (SW Iceland).

How to cite: Scarabello, L., Diehl, T., Kästli, P., Clinton, J., and Wiemer, S.: Towards Real-Time Double-Difference Hypocenter Relocation of Natural and Induced Seismicity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13058, https://doi.org/10.5194/egusphere-egu2020-13058, 2020.

EGU2020-7052 | Displays | SM1.2

Discrimination between earthquakes and quarry blasts in the Vertes Hills, Hungary using a correlation detector

Márta Kiszely, Süle Bálint, and István Bondár

Contamination of earthquake catalogues with anthropogenic events largely complicates seismotectonic interpretation. It is especially true for relatively low seismicity areas, such as Hungary. In the present study, we analyze the characteristics of the waveforms of earthquakes and quarry blasts that occurred in the close vicinity of Csokako (CSKK) station between 2017 and 2019 in the Vértes Hills, Hungary.

The objective of this study was to determine the linear discrimination line between the of earthquake and explosion populations. We investigated the effectiveness of P/S amplitude ratios using filtered waveforms at different frequency bands. We applied waveform cross-correlation to build correlation matrices at CSKK and performed hierarchical cluster analysis to identify event clusters. Because most of the quarry blasts were carried out by ripple-fire technology, we computed spectrograms and examined the spectral ratio between low and high frequencies and the steepness of spectra.

Overall, classes of earthquakes and quarry blasts have separated well from each other by combining the amplitude ratio, waveform similarity and the different spectral methods. We created a set of master events for individual quarries to run correlation detectors on past waveforms and identify the explosions of analyzed quarries that were misclassified as earthquakes in the annual Hungarian National Seismological Bulletins.

How to cite: Kiszely, M., Bálint, S., and Bondár, I.: Discrimination between earthquakes and quarry blasts in the Vertes Hills, Hungary using a correlation detector , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7052, https://doi.org/10.5194/egusphere-egu2020-7052, 2020.

EGU2020-7103 | Displays | SM1.2

Localization of Rockfalls at Dolomieu Crater, La Réunion, through Simulation of Seismic Waves on Real Topography

Julian Kuehnert, Anne Mangeney, Yann Capdeville, Emmanuel Chaljub, Eleonore Stutzmann, and Jean-Pierre Vilotte

Rockfall generated seismic signals have been shown to be of great utility in order to detect and monitor rockfall activity. Furthermore, event locations were successfully estimated using methods which rely on either arrival times, amplitudes or polarization of the seismic signal. However, strong surface topography can significantly influence seismic wave propagation and thus flaw the estimates if not taken into account correctly.

On the upside, the imprint of topography on the seismic signal can be characteristic of the source position. We show that this additional information can be used to get a more detailed rockfall location estimation. In order to do so, the seismic impulse response is modeled on a domain with 3D topography using the Spectral Element Method. Subsequently, in order to locate events, station energy ratios of the synthetic seismograms are compared with energy ratios of rockfall signals in a sliding time window.

We test the method on rockfalls which occurred at Dolomieu crater of Piton de la Fournaise, La Réunion. The sensitivity of the method on the resolution of the modeled topography and the underlying velocity model is tested. We propose that the method can be applied for monitoring rockfall activity in a specific area with multiple seismic stations after calculating once the impulse response for the corresponding topography.

How to cite: Kuehnert, J., Mangeney, A., Capdeville, Y., Chaljub, E., Stutzmann, E., and Vilotte, J.-P.: Localization of Rockfalls at Dolomieu Crater, La Réunion, through Simulation of Seismic Waves on Real Topography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7103, https://doi.org/10.5194/egusphere-egu2020-7103, 2020.

EGU2020-19253 | Displays | SM1.2

Real-time monitoring of seismic moment and radiated energy

Davide Scafidi, Daniele Spallarossa, Matteo Picozzi, and Dino Bindi

Understanding the dynamics of faulting is a crucial target in earthquake source physics (Yoo et al., 2010). To study earthquake dynamics it is indeed necessary to look at the source complexity from different perspectives; in this regard, useful information is provided by the seismic moment (M0), which is a static measure of the earthquake size, and the seismic radiated energy (ER), which is connected to the rupture kinematics and dynamics (e.g. Bormann & Di Giacomo 2011a). Studying spatial and temporal evolution of scaling relations between scaled energy (i.e., e = ER/M0) versus the static measure of source dimension (M0) can provide valuable indications for understanding the earthquake generation processes, single out precursors of stress concentrations, foreshocks and the nucleation of large earthquakes (Picozzi et al., 2019). In the last ten years, seismology has undergone a terrific development. Evolution in data telemetry opened the new research field of real-time seismology (Kanamori 2005), which targets are the rapid determination of earthquake location and size, the timely implementation of emergency plans and, under favourable conditions, earthquake early warning. On the other hand, the availability of denser and high quality seismic networks deployed near faults made possible to observe very large numbers of micro-to-small earthquakes, which is pushing the seismological community to look for novel big data analysis strategies. Large earthquakes in Italy have the peculiar characteristic of being followed within seconds to months by large aftershocks of magnitude similar to the initial quake or even larger, demonstrating the complexity of the Apennines’ faults system (Gentili and Giovanbattista, 2017). Picozzi et al. (2017) estimated the radiated seismic energy and seismic moment from P-wave signals for almost forty earthquakes with the largest magnitude of the 2016-2017 Central Italy seismic sequence. Focusing on S-wave signals recorded by local networks, Bindi et al. (2018) analysed more than 1400 earthquakes in the magnitude ranges 2.5 ≤ Mw ≤ 6.5 of the same region occurred from 2008 to 2017 and estimated both ER and M0, from which were derived the energy magnitude (Me) and Mw for investigating the impact of different magnitude scales on the aleatory variability associated with ground motion prediction equations. In this work, exploiting first steps made in this direction by Picozzi et al. (2017) and Bindi et al. (2018), we derived a novel approach for the real-time, robust estimation of seismic moment and radiated energy of small to large magnitude earthquakes recorded at local scales. In the first part of the work, we describe the procedure for extracting from the S-wave signals robust estimates of the peak displacement (PDS) and the cumulative squared velocity (IV2S). Then, exploiting a calibration data set of about 6000 earthquakes for which well-constrained M0 and theoretical ER values were available, we describe the calibration of empirical attenuation models. The coefficients and parameters obtained by calibration were then used for determining ER and M0 of a testing dataset

How to cite: Scafidi, D., Spallarossa, D., Picozzi, M., and Bindi, D.: Real-time monitoring of seismic moment and radiated energy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19253, https://doi.org/10.5194/egusphere-egu2020-19253, 2020.

EGU2020-20268 | Displays | SM1.2

Seismic noise analysis of broadband stations of the Italian Seismic Network by Power Spectral Density

Maria Catania, Antonino D'Alessandro, Luca Greco, Raffaele Martorana, and Salvatore Scudero

The Italian Seismic Network (IV) consists of more than 500 stations located throughout the Italian territory.

The detection capability of  network is constrained by its location performance that is affected by the seismic noise levels variations depending on the characteristics of the dominant source. Discriminating the noise level in each stations may allow to improve in its performance, in order to reduce noisy stations to detect even the smaller energetic seismic events sometimes hidden by high noise values. The main goal of this research has been to establish the characteristics (frequency content) and origin of seismic noise background at these sites and secondly to assess the effects of performance of the network.

For this purpose we have estimated the Power Spectral Density (PSD) of seismic noise selecting only a subset of 233 stations equiped with broadband velocimeters (with minimum period of 40 seconds and with a high sensitivity until to 120s) and operating at least three consecutive years of available data (2015-2017).

The variations of seismic background noise have been investigated using also the relative Probability Density Funcionts (PDF). The data processing of signals carried out with the robust method proposed by McNamara and Buland, (2004). In this study, the analysis was limited in the frequency band from 0.025 to 30 Hz, in accordance with the seismic sensors bandwidth. Four different frequency bands have been identified: 0.025-0.12, 0.12-1.2, 1.2-10 and 10-30 Hz. Each of these has been associated to a main type of source, in agreement with the literature.

A preliminary data analysis has been carried out to understand the statistical properties of the noise power, in the four class identified, both in space and frequency domains. Extracting  the PDFs  all stations, it was produced a representative seismic noise model that it could be considered as a new reference noise for Italian territory. Histograms have been computed for each band, both for vertical and horizontal components and its ratio. In addition, a spatial-statistical analysis was performed showing a good correlation of noise level with some weather conditions and anthropogenic source. Several clustering techniques were applied to the data to identified group of stations with similar PSD level, attributable to the same noise source. Furthermore, a correlation between the noise found at the different stations and spatial data (maps of rainfall, winds, coastlines, ect…) was carried out for a better characterization of the type of source.

How to cite: Catania, M., D'Alessandro, A., Greco, L., Martorana, R., and Scudero, S.: Seismic noise analysis of broadband stations of the Italian Seismic Network by Power Spectral Density, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20268, https://doi.org/10.5194/egusphere-egu2020-20268, 2020.

EGU2020-1846 | Displays | SM1.2

Relationship between water level temporal changes and seismicity in the Mingechevir reservoir (Azerbaijan)

Fakhraddin Gadirov (Kadirov), Luciano Telesca, Gulam Babayev, Gurban Yetirmishli, and Rafig Safarov

Reservoir-induced seismicity has been studied worldwide due to its potential to provoke damage to buildings and constructions, and, more important, human loss. Reservoir-induced seismicity (RIS) is normally related with additional static loading (the weight of the water reservoir and its seasonal variations), tectonic faults, liquefaction and pore pressure variations.The Mingechevir reservoir is located in the north-west of Azerbaijan on the Kurriver. This water reservoir is extended from north-west towards south-east through Kur river valley by 75 km. The area of the dam is 625 km2 with the average width accounting for 6-8 km. The volume of the dam is 16 km3. The dam filling started in 1953. This reservoir is the largest one in the Caucasus and carries a number of geo-hazards interrelated with geodynamics and technogenic factors. The aim of the present study in the Mingechevir reservoir is to investigate relationship between the fluctuations of the water level and the onset of seismicity in the area around the dam more in detail, by using several and independent statistical methods.The temporal variations of the instrumental seismicity (0.5≤ML≤3.5) recorded in the Mingechevir area (Azerbaijan) between January 2010 to April 2018 and its relationship with the level variation of the water reservoir was analysed in this study. Due to the relative high completeness magnitude (MC = 1.6) of the seismic catalogue of the area, only 136 events were selected over a period of more than 8 years. Thus, the monthly number of events was analysed by using the correlogram-based periodogram, the singular spectrum analysis (SSA) and the empirical mode decomposition (EMD), which are robust against the short size of the time series. Our results point out to the following findings: 1) annual periodicity was found in one SSA reconstructed component of the monthly number of events; 2)quasi-annual periodicity was found in one EMD intrinsic mode function of the monthly number of earthquakes. These obtained results could support in a rigorously statistical manner that the seismicity occurring in Minghechevir area could be triggered by the yearly cycle of the water level of the reservoir.

 

Keywords:water reservoir, induced seismicity, water level change, Mingechevir reservoir, Azerbaijan

How to cite: Gadirov (Kadirov), F., Telesca, L., Babayev, G., Yetirmishli, G., and Safarov, R.: Relationship between water level temporal changes and seismicity in the Mingechevir reservoir (Azerbaijan), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1846, https://doi.org/10.5194/egusphere-egu2020-1846, 2020.

SM1.3 – Ambient noise seismology: Topics, targets, tools & techniques

EGU2020-6022 | Displays | SM1.3

What changes when we use ambient noise recorded by fiber optics?

Eileen Martin, Nate Lindsey, Biondo Biondi, Jonathan Ajo-Franklin, and Tieyuan Zhu

Ambient noise seismology has greatly reduced the cost of acquiring data for seismic monitoring and imaging by reducing the need for active sources. For applications requiring time-lapse imaging or continuous monitoring, we desire sensor arrays that require little effort, money, and power to maintain over long periods of time. Distributed Acoustic Sensing repurposes a standard fiber optic cable as a series of single-component strain rate sensors with spacing at the scale of meters over distances of kilometers. With a single location providing the power source and recording all data, along with the ability to use existing underground fiber optic networks, a small team is now able to easily establish a monitoring network and acquire massive amounts of strain rate data continuously.

This talk will explore two conceptual changes when using DAS data for ambient noise interferometry: greatly increased data volumes, and the difference between velocity and distributed strain-rate data. These two challenges will be illustrated in the context of experiments with applications in near-surface Vs imaging with applications in earthquake hazard analysis, permafrost thaw monitoring, and urban geohazard and hydrology monitoring.

On the issue of data volumes: Orders of magnitude more sensors and high sample rates (often in the kilohertz range) quickly result in data quantities that exceed the limits of computational infrastructure and algorithms available to many seismologists, potentially at the petabyte/year scale for modern acquisition instruments. New algorithms focused on reduced data movement are improving our ability to analyze more data with existing resources. This talk will include a brief overview of some recent algorithmic improvements for both ambient noise interferometry for imaging, and interferometry-based event detection.

On the issue of changing from velocity to distributed strain rate data: Because strain rate is a tensor quantity and velocities are a vector quantity, the sensitivity of DAS to seismic sources at different orientations is quite different from typical seismometers. This difference can be clear both in polarity and amplitude of the signal, and is particularly significant in shear and Love wave recordings. We will describe simple models to describe expected changes in how seismometers and DAS record the same noises, and the corresponding changes expected in noise correlation functions. These sensitivity differences are more pronounced in ambient noise correlation functions than they are in raw signal recordings, effectively emphasizing a different distribution of ambient noise sources. Modeling these sensitivities helps determine which sensor orientations are reliable for use in ambient noise interferometry imaging.

How to cite: Martin, E., Lindsey, N., Biondi, B., Ajo-Franklin, J., and Zhu, T.: What changes when we use ambient noise recorded by fiber optics? , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6022, https://doi.org/10.5194/egusphere-egu2020-6022, 2020.

EGU2020-11124 | Displays | SM1.3

In-situ microseism noise generation measured from distributed acoustic sensing on seafloor optical cable

Diane Rivet, Gauthier Guérin, Daniel Mata, Itzhak Lior, Anthony Sladen, and Jean-Paul Ampuero

Measuring seismic and acoustic signals on seafloor telecom cables has proven recently its very high potential for earthquake monitoring but also for beter understanding the interaction between the oceans and the solid earth. A consequence of these interactions is the generation of the primary and secondary microseismic noise on coastal regions and in the deep ocean respectively. These seismic noises that propagate across continents are central to a large fraction of todays' seismic imagery and monitoring campaigns. Compared to previous studies and instrumentation setups, acoustic sensing over oceanic telecom cables offer the unique ability to measure in a very dense manner waves that are generated on the seafloor. We analyse a week long record of ambient noise measurements on the 41.5 km-long seafloor telecom cable offshore Toulon, south of France. At shallow depth, close to the coast, we measure the pressure changes caused by the oceanic gravity waves. The bottom pressure is then compared to an oceanographic buoy located a few kilometers away from the cable. The amplitude and frequency of the pressure are modulated by the gravity waves height and dominant periods. This observation opens the way for a distributed measurement of the oceanic waves characteristics over several kilometers. At depth larger than a 1km, we observe Scholte waves at the ocean-solid earth interface produced by the non-linear interaction of gravity waves. These waves have the double frequency of the gravity waves seen at the coast. We find that the amplitude and frequency change over time, as do the gravity waves observed near the coast. The frequency-wave number decomposition of the signal reveals that the apparent velocity of the Scholte waves does not depend of the azimuth of the fiber. These observations confirm that these deep Scholte waves are secondary microseismic noise, generated locally from the interaction of landward gravity waves with oceanward gravity wave reflected on the coast. Spatially distributed monitoring of the ambient noise wave field at the ocean-solid earth interface provides a better understanding of the noise generation and therefore will allow a better modeling of the ambient noise in the future.

How to cite: Rivet, D., Guérin, G., Mata, D., Lior, I., Sladen, A., and Ampuero, J.-P.: In-situ microseism noise generation measured from distributed acoustic sensing on seafloor optical cable, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11124, https://doi.org/10.5194/egusphere-egu2020-11124, 2020.

EGU2020-6159 | Displays | SM1.3

Ambient noise field and temporal changes on ambient noise auto/cross-correlogram at the sea bottom inferred from ocean-bottom seismic and pressure arrays

Yoshihiro Ito, Miyuu Uemura, Spahr C. Webb, Kimihiro Mochizuki, and Stuart Henrys

The interactions of wind with the ocean surface, ocean wave with acoustic wave, acoustic wave with seismic wave below the sea bottom, and the interplay among them drive important energy flows from the atmosphere to the lithosphere. Uncertainty remains regarding the origin of wind-related noise in the ocean and its coupling to seismic noise below the sea floor. Seismic interferometry is a powerful tool that uses microseisms, or ambient noise within solid earth, to monitor temporal seismic velocity change by referring to the auto/cross-correlation as a Green’s function at the sites, and its temporal change. The most important assumption when detecting seismic velocity changes with seismic interferometry is that mutually uncorrelated noise sources are distributed randomly in space and time without any temporal changes in their density and intensity in a fully diffuse wave field. An effect of temporal variation on the ambit noise field to the retrieval of Green’s function is, however, not fully understood, nor is how reliable temporal changes in interferogram noise are, especially when accompanied by large earthquakes and slow slip events. Here, we show relationships among the temporal changes of sea surface wave, acoustic wave, and seismic wave fields, which are observed in ocean bottom pressure gauges and seismometer arrays installed in New Zealand. The temporal variation in the power spectrum obtained from continuous ocean bottom seismometer and pressure records near 200 mHz correlates with the temporal variation in wind speed above the sites, particularly during wind turbulence of more than 5 m/s. The temporal fluctuation in the ocean bottom pressure caused by the ocean surface wave field correlates to that of a microseism near 200 mHz. The temporal variations in the power spectrum from both continuous ocean bottom pressures and microseisms in the 200–800 mHz range show a positive correlation. After calculating the auto/cross-correlation functions (ACF/CCF) from ambient noise in a 200–800 mHz pass band every 6 h, the temporal variation in the correlation between the ACF/CCFs is investigated every 6 h. The temporal variation in the ACF/CCFs correlates with the time derivative of the temporal changes in the power spectrum amplitude of both the bottom pressure and the microseism rather than the temporal changes in the amplitude of the power spectrum. This suggests that the temporal change that occurs in the seismic interferogram owing to ambient noise, is mostly controlled by the temporal change in the ocean wave field undergoing fluctuations by the atmospheric turbulence over the sea surface. The temporal variations in the noise field in space and time may break the assumption on seismic interferometry, and eventually make the apparent temporal change in interferogram noise.

How to cite: Ito, Y., Uemura, M., Webb, S. C., Mochizuki, K., and Henrys, S.: Ambient noise field and temporal changes on ambient noise auto/cross-correlogram at the sea bottom inferred from ocean-bottom seismic and pressure arrays, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6159, https://doi.org/10.5194/egusphere-egu2020-6159, 2020.

EGU2020-15228 | Displays | SM1.3

On the link between Beamforming and Kernel-based Source Inversion

Daniel Bowden, Korbinian Sager, Andreas Fichtner, and Małgorzata Chmiel

Beamforming and backprojection methods offer a data-driven approach to image noise sources, but provide no opportunity to account for prior information or iterate through an inversion framework. In contrast, recent methods have been developed to locate ambient noise sources based on cross-correlations between stations and the construction of finite-frequency kernels, allowing for inversions over multiple iterations (i.e., Tromp et al., 2010, Ermert et al. 2017, Sager et al. 2018). These kernel-based approaches show great promise, both in mathematical rigour and in results, but may remain difficult to understand or implement for the wider community. Here we show that these two different classes of methods, beamforming and kernel-based inversion, are achieving exactly the same result in certain circumstances. This means existing beamforming and backprojection methods can also incorporate prior information in a mathematically correct manner.

We start with a description of a relatively simple beamforming or backprojection algorithm, based on time-domain shifting and measurement of waveform coherence. Only by changing the order of steps, we begin to resemble the kernel-based approaches. By adding a physical model for the distribution of noise sources, and therefore synthetic correlation functions, we can extend backprojection to an iterative, gradient-based inversion scheme. Adjoint methods and a direct simulation of correlation wavefields can later be used to increase computational efficiency, but we stress that these are not needed to understand the approach.

Given the equivalence of these approaches between these two communities, both sides can benefit from bridging the gap. For example, for kernel-based inversion schemes, a current challenge lies in defining the misfit and time window over which a correlation will be scored; a windowing function based on beamform images offers a more intuitive way to identify significant contributions in the noise wavefield, exploiting more than just the direct surface-wave arrivals.

How to cite: Bowden, D., Sager, K., Fichtner, A., and Chmiel, M.: On the link between Beamforming and Kernel-based Source Inversion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15228, https://doi.org/10.5194/egusphere-egu2020-15228, 2020.

EGU2020-11208 | Displays | SM1.3

Exploiting wind-turbine noise for seismic imaging and monitoring

Elmer Ruigrok, Lisanne Jagt, and Britt van der Vleut

Wind turbines (WTs) have proven to be an increasingly cost-efficient source of sustainable energy. With further cost reductions and growth of environmental awareness, the amount and size of WTs will further expand. In the seismic literature, WTs have mainly been considered a threat rather than an opportunity. WTs act as infrasound and seismic sources, whose wavefield might overwhelm signal from earthquakes. Rather than focusing on the detrimental effects, we embrace the WT revolution and focus on the novel possibilities of the WT seismic source. We show detailed characteristics of this source using recordings over the Groningen seismic network. We further show examples of using the WT seismic noise for extracting medium parameters. Moreover, we exploit the repeatable nature of the source for subsurface monitoring.

How to cite: Ruigrok, E., Jagt, L., and van der Vleut, B.: Exploiting wind-turbine noise for seismic imaging and monitoring, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11208, https://doi.org/10.5194/egusphere-egu2020-11208, 2020.

EGU2020-1680 | Displays | SM1.3

Retrieval of reflectivity images from ambient seismic noise correlations using machine learning as a noise-panel classification tool

Boris Boullenger, Merijn de Bakker, Arie Verdel, and Stefan Carpentier

The theory of ambient seismic noise interferometry offers techniques to retrieve estimates of inter-receiver responses from continuously recorded ambient seismic noise. This is usually achieved by correlating and stacking successive noise panels over sufficiently long periods of time. If the noise panels contain significant body-wave energy, the stacked correlations expected to result in retrieved estimates of the body-wave responses, including reflections. Such application combined with a dense surface seismic array is promising for imaging the subsurface structures at lower cost and lower environmental impact as compared to with controlled seismic sources. Subsequently, this technique can be an alternative to active-source surveys in a range of challenging scenarios and locations, and can also be used to perform time-lapse subsurface characterization.

In this study, we apply seismic body-wave noise interferometry to 30-days of continuous records from a surface line of 31 receivers spaced by 25 meters in the South of the Netherlands with the aim to image subsurface reflectors, at depths from a few hundreds of meters to a few kilometers. As a first step, we compute stacked auto-correlations and compare the retrieved zero-offset section with a co-located stacked section from a past active reflection survey on the site.

Yet, the retrieval of reflectivity estimates relies on the identification and collection of a sufficient number of noise panels with recorded body waves that have travelled from the subsurface towards the array. Even in the case of favorable body-wave noise conditions, the panels are most often contaminated with stronger anthropogenic coherent seismic noise, mainly in the form of surface waves, which in turn prevents the stacked correlations to reveal reflectivity. Because of the limited effect of frequency filtering, the application of seismic body-wave noise interferometry requires in fact extensive effort to identify noise panels without prominent coherent noise from the surface activity. Typically, this leads to disregard a significant amount of actually useful data.

For this reason, we designed, trained and tested a deep convolutional neural network to perform this classification task more efficiently and facilitate the repetition of the retrieval method over long periods of time. We tested several supervised learning schemes to classify the panels, where two classes are defined, according to the presence or absence of prominent coherent noise. The retained classification models achieved close to 90% of prediction accuracy on the test set.

We used the trained classification models to correlate and stack panels which were predicted in the class with coherent noise absent. The resulting stacked correlations exhibit potential reflectors in a larger depth range than previously achieved. The results show the benefits of using machine learning to collect efficiently a maximum amount of favorable noise panels and a way forward to the upscaling of seismic body-wave noise interferometry for reflectivity imaging.

How to cite: Boullenger, B., de Bakker, M., Verdel, A., and Carpentier, S.: Retrieval of reflectivity images from ambient seismic noise correlations using machine learning as a noise-panel classification tool, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1680, https://doi.org/10.5194/egusphere-egu2020-1680, 2020.

EGU2020-7324 | Displays | SM1.3

Structural delineation at the Los Humeros geothermal field, Mexico, by P-wave reflection retrieval from noise

Arie Verdel, Boris Boullenger, Joana E. Martins, Anne Obermann, Tania Toledo, and Philippe Jousset

The overall purpose of the recently finalized GEMex project*, a European-Mexican collaboration, has been to gain an improved understanding of the subsurface at two unconventional geothermal sites: for EGS development at Acoculco and for a superhot resource near Los Humeros. Providing a more precise description of both the geological structure and the geothermal reservoir behavior for these two sites form important requirements for achieving that goal.

For delineating the main structural features at geothermal reservoir level, reflection retrieval from ambient seismic noise can be considered interesting because of its relatively low-cost and low environmental impact as compared to more conventional, controlled-source, seismic surveying practice, where (expensive) active sources are required.

In this study, we present results from the application of ambient noise seismic interferometry (ANSI) to retrieve zero-offset reflected P-waves from continuous seismic data recorded during the second half of 2017 at the Los Humeros geothermal field, Mexico. It is known from noise interferometry theory that reflected P-waves can provide local structural detail at locations directly underneath the employed seismic stations.

We address various data selection and processing aspects related to the retrieval of these reflected P-waves. The reflections are thereafter compared with modelled reflectivities at station locations with sufficient data availability, data quality and proximity to a location at which seismic interval velocity information is available from the literature.

From our study it can be concluded that the ANSI auto-correlation technique that was applied for zero-offset reflectivity retrieval at the Los Humeros site indeed can provide relatively high structural detail: for near-horizontal reflectors in the close vicinity of the selected stations, local depth-estimates of seismic velocity-contrasts were determined. This information can be used to constrain both the geological structure and geothermal reservoir property description.

As such, results from this passive-seismic method may partially complement and partially confirm subsurface information derived from active-seismic, that can only be acquired at a higher cost, which is more labor-intensive and which has more impact on the environment.

We thank the Mexican GEMex team around Angel Figueroa Soto from UMSNH and Marco Calo from UNAM for setting up the seismic network and station maintenance as well as data retrieval. The Comisión Federal de Electricidad (CFE) kindly provided us with access to their geothermal field and permission to install the seismic stations. OGS is thanked for providing us the location details of the four active seismic lines. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 727550 and the Mexican Energy Sustainability Fund CONACYT-SENER, project 2015-04-68074.

* http://www.gemex-h2020.eu/index.php?option=com_content&view=featured&Itemid=101&lang=en

How to cite: Verdel, A., Boullenger, B., E. Martins, J., Obermann, A., Toledo, T., and Jousset, P.: Structural delineation at the Los Humeros geothermal field, Mexico, by P-wave reflection retrieval from noise, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7324, https://doi.org/10.5194/egusphere-egu2020-7324, 2020.

EGU2020-22018 | Displays | SM1.3

Wavefield reconstruction inversion for ambient seismic noise

Sjoerd A.L. de Ridder, James R. Maddison, Ali Shaiban, and Andrew Curtis

With the advent of large and dense seismic arrays, there is an opportunity for novel inversion methods that exploit the information captured by stations in close proximity to each other. Estimating surface waves dispersion is an interest for many geophysical applications using both active and passive seismic data. We present an inversion scheme that exploits the spatial and temporal relationships of the Helmholtz equation to estimate dispersion relations directly from surface wave ambient noise data, while reconstructing the full wavefield in space and frequency. The scheme is a PDE constrained inverse problem in which we jointly estimate the state and parameter spaces of the seismic wavefield. Key to the application on ambient seismic noise recordings is to remove the boundary conditions from the PDE constraint, which renders a conventional waveform inversion formulation singular. With synthetic acoustic and elastic data examples we show that using a variable projection scheme, we can iteratively update an initial estimate of the medium parameters and recover an estimate for the true underlying velocity field. Our examples show that the we can reconstruct the full wavefield even in the case of strong aliasing and irregular sampling. This works forms the basis for a new approach to inverting ambient seismic noise using large and dense seismic arrays.

How to cite: de Ridder, S. A. L., Maddison, J. R., Shaiban, A., and Curtis, A.: Wavefield reconstruction inversion for ambient seismic noise, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22018, https://doi.org/10.5194/egusphere-egu2020-22018, 2020.

EGU2020-481 | Displays | SM1.3

Simultaneous body and surface wave retrieval from the seismic ambient field and discrimination from unavoidably arising spurious artifacts

Ali Riahi, Zaher-Hossein Shomali, Anne Obermann, and Ahmad Kamayestani

We simultaneously extract both, direct P-waves and Rayleigh waves, from the seismic ambient noise field recorded by a dense seismic network in Iran. With synthetics, we show that the simultaneous retrieval of body and surface waves from seismic ambient noise leads to the unavoidable appearance of spurious arrivals that could lead to misinterpretations.

We work with 2 months of seismic ambient noise records from a dense deployment of 119 sensors with interstation distances of 2 km in Iran. To retrieve body and surface waves, we calculate the cross-coherency in low-frequency ranges, i.e. frequencies up to 1.2 Hz, to provide the empirical Green’s functions between each pair of stations. To separate the P and Rayleigh waves, we use the polarization method that also enhances the small amplitude body waves.

We observe both P and Rayleigh waves with an apparent velocity of 4.9±0.3 and 1.8±0.1 km/s in the studied area, respectively, as well as S or higher mode of Rayleigh waves, with an apparent velocity of 4.1±0.1 km/s. Besides these physical arrivals, we also observe two spurious arrivals with similar amplitudes before/after the P and/or Rayleigh waves that render the discrimination challenging.

To better understanding these arrivals, we perform synthetic tests. We show that simultaneously retrieving the body and surface waves from seismic ambient noise sources will unavoidably lead to the appearance of superior arrivals in the calculation of empirical Green’s functions.

How to cite: Riahi, A., Shomali, Z.-H., Obermann, A., and Kamayestani, A.: Simultaneous body and surface wave retrieval from the seismic ambient field and discrimination from unavoidably arising spurious artifacts, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-481, https://doi.org/10.5194/egusphere-egu2020-481, 2020.

EGU2020-10085 | Displays | SM1.3

Imaging azimuthal anisotropy in the alpine crust using noise cross-correlations

Dorian Soergel, Helle Pedersen, Anne Paul, and Laurent Stehly

Imaging azimuthal anisotropy from seismic noise cross-correlations is challenging, especially in very complex tectonic settings such as the Alps. In this region, the focus has been mainly on retrieving anisotropy using SKS-splitting data, but this data does not provide strong depth constraints. In this work, we map the azimuthal anisotropy of Rayleigh-wave velocity in the Alps using seismic noise cross-correlations. This initial study focusses on waves at ~15 s period. The study area is divided into small zones for which all the stations outside are used as virtual sources and all the stations inside are used as receivers. For each virtual source and each zone, we perform time domain beam forming to retrieve the local phase velocity and propagation direction. As the distances between sources and receivers are relatively small, we use an algorithm that takes into account circular wavefronts. The beam forming shows that the waveforms are very coherent for different stations within each small array, and that deviations from great-circle propagation can be significant. The resulting phase velocities in each zone show a variation with azimuth which is in some locations very small (indicating that anisotropy is insignificant) and which in all other locations has a 2θ dependency on azimuth, indicative of well resolved azimuthal anisotropy. Bootstrapping uncertainty estimates show that the results are very stable if a sufficient number of source stations is used. The combination of permanent stations with the temporary AlpArray stations provides us with a very high station density that allows us to carry out this measurement across a large area. The resulting anisotropy maps show a good resolution, with higher uncertainties in the Po plain and the areas of low station density. The clear 2θ azimuth dependency is a sign that our method overcomes both effects related to source directivity (which has an approximate 1θ dependency) and measurement instability which can be significant for Eikonal tomography in the case of irregular networks.

How to cite: Soergel, D., Pedersen, H., Paul, A., and Stehly, L.: Imaging azimuthal anisotropy in the alpine crust using noise cross-correlations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10085, https://doi.org/10.5194/egusphere-egu2020-10085, 2020.

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The Effects of Cracks and Fluids on Post-Seismic Healing

Alison Malcolm, Somayeh Khajehpour Tadavani, and Kristin Poduska

It is now well established that large seismic events change the surrounding velocities, and that these velocities slowly recover over time.  Precisely which mechanisms control the recovery process are less well understood.  We present the results of laboratory experiments to better characterise what properties of the underlying material control the recovery process.  We do this by mixing two waves, one which perturbs the velocity of the sample (as an earthquake does in field data) and one which senses the change in velocity (as in changing noise correlations).  This is an inherently nonlinear experiment as we mix two waves and measure the effects of this wave mixing.  Within our experiments, we vary the properties of the samples to understand which are most important in controlling the nonlinear response.  We focus on two mechanisms.  The first is fractures and how changes in fracture properties change the nonlinear response.  The second is fluids, in particular the effect of low saturations on the nonlinear response.  By changing the fluids and fractures we can turn on and off the nonlinear mechanism, helping us to move toward a better understanding of the underlying mechanisms of these wave-wave interactions.

How to cite: Malcolm, A., Khajehpour Tadavani, S., and Poduska, K.: The Effects of Cracks and Fluids on Post-Seismic Healing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9991, https://doi.org/10.5194/egusphere-egu2020-9991, 2020.

EGU2020-6620 | Displays | SM1.3

Interpreting Coda Wave Decorrelation from ambient seismic noise interferometry, inputs from laboratory experiments

Eric Larose, Romain Thery, Odile Abraham, and Antoine Guillemot

Seismic and ultrasonic waves are sometimes used to track fluid injections, propagation, infiltrations in complex material, including geological and civil engineered ones. In most cases, one use the acoustic velocity changes as a proxy for water content evolution. Here we propose to test an alternative seismic or acoustic observable: the waveform decorrelation. We use a sample of compacted millimetric sand as a model medium of highly porous multiple scattering materials. We fill iteratively the sample with water, and track changes in ultrasonic waveforms acquired for each water level. We take advantage of the high sensitivity of diffuse coda waves (late arrivals) to track small water elevation in the material. We demonstrate that in the mesoscopic regime where the wavelength, the grain size and the porosity are in the same order of magnitude, Coda Wave Decorrelation (waveform change) is more sensitive to fluid injection than Coda Wave Interferometry (apparent velocity change). This observation is crucial to interpret fluid infiltration in concrete with ultrasonic record changes, as well as fluid injection in volcanoes or snow melt infiltration in rocky glaciers. In these applications, Coda Wave Decorrelation might be an extremely interesting tool for damage assessment and alert systems [1].

 

[1] R. Thery, A. Guillemot, O. Abraham, E. Larose, Tracking fluids in multiple scattering and highly porous materials: toward applications in non-destructive testing and seismic monitoring, Ultrasonics, 102, 106019 (2019).

How to cite: Larose, E., Thery, R., Abraham, O., and Guillemot, A.: Interpreting Coda Wave Decorrelation from ambient seismic noise interferometry, inputs from laboratory experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6620, https://doi.org/10.5194/egusphere-egu2020-6620, 2020.

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Sensitivity kernels for coda-wave interferometry in a three-dimensional scalar scattering media

Andres Barajas, Ludovic Margerin, and Michel Campillo

The ambient seismic noise has proven to be a powerful tool to assess velocity changes within the ground using coda-wave interferometry (CWI). CWI is based on the analysis of small waveform changes in the coda of the signals. Localizing and imaging the source that generates changes can be done with the help of sensitivity kernels which contain information on how each part of the surrounding medium contributes to the overall waveform perturbation that is recorded at a receiver. Although progress has been made in the theory of sensitivity kernels in the case of a full elastic space,  the inclusion of a free surface has proven to be difficult. Indeed, the free surface couples body waves and surface waves, which affects the sensitivity of coda waves with respect to the full-space case. Furthermore, one expects the depth sensitivity of coda waves to be strongly dependent on the relative contribution of surface and body waves, which depends on the lapse-time, source-receiver distance and scattering properties of the medium. Using the Monte-Carlo method, we compute traveltime-sensitivity kernels in a 3D scalar problem that includes body and surface waves, based on a recent theoretical model that integrates both through a mixed boundary condition. From these results, we assess the impact of the depth of a velocity perturbation on the recorded signals at the surface. Our results will be compared with previous numerical approaches from the literature. 

How to cite: Barajas, A., Margerin, L., and Campillo, M.: Sensitivity kernels for coda-wave interferometry in a three-dimensional scalar scattering media, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20890, https://doi.org/10.5194/egusphere-egu2020-20890, 2020.

Between 2013 and 2017, the Groningen gas field was monitored by several deployments of an array of geophones in a deep borehole at reservoir level (3 km). Zhou & Paulssen (2017) showed that the P- and S-velocity structure of the reservoir could be retrieved from noise interferometry by cross-correlation. Here we show that deconvolution interferometry of high-frequency train signals from a nearby railroad not only allows determination of the velocity structure with higher accuracy, but also enables time-lapse measurements. We found that the travel times within the reservoir decrease by a few tens of microseconds for two 5-month periods. The observed travel time decreases are associated to velocity increases caused by compaction of the reservoir. However, the uncertainties are relatively large. 
Striking is the large P-wave travel time anomaly (-0.8 ms) during a distinct period of time (17 Jul - 2 Sep 2015). It is only observed for inter-geophone paths that cross the gas-water contact (GWC) of the reservoir. The anomaly started 4 days after drilling into the reservoir of a new well at 4.5 km distance and ended 4 days after the drilling operations stopped. We did not find an associated S-wave travel time anomaly. This suggests that the anomaly is caused by a temporary elevation of the GWC (water replacing gas) of approximately 20 m. We suggest that the GWC is elevated due to pore-pressure variations during drilling. The 4-day delay corresponds to a pore-pressure diffusivity of ~5m2/s, which is in good agreement with the value found from material parameters and the diffusivity of (induced) seismicity for various regions in the world. 

How to cite: Paulssen, H. and Zhou, W.: Time-lapse changes within the Groningen gas field caused reservoir by compaction and distant borehole drilling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3567, https://doi.org/10.5194/egusphere-egu2020-3567, 2020.

EGU2020-43 | Displays | SM1.3

Retrieving the Reflection events from passive signals

Diako Hariri Naghadeh and Chris Bean

To create virtual shot gather from passive signals it is essential to cross-correlate all the signals with the reference trace. Since surface sources dominate the origin of seismic noise, the correlated sections are highly dominated by surface waves. If the target is surface wave inversion general cross-correlation will suit the target. But if the extraction of body waves from those signals is the main objective, coherent ground roll events mask the body waves making it difficult to extract them. To tackle this issue a frequency-spatial nonCoherent filter (FX-NCF) plus a post-correlation processing module are introduced. FX-NCF is a prediction filter and the filter operator is a function of frequency, station interval and the slope of the interested event. In the frequency domain, the filter is looking for the prediction of n-th trace coherence spectrum from the (n-1)-th signal’s coherence spectrum by minimizing the objective function. Hybrid norms used to minimize the error. The coherence spectrum of each trace is the coherency between the reference signal and the desired trace. Applying the FX-NCF on 2D real recorded passive signals shows its superiority over general cross-correlation, deconvolution interferometry, cross-coherence and multi-taper-method-coherence-estimation methods in highlighting surface and body waves also improving the signal-to-noise (S/N) ratio. To show the necessity of post correlation processing (before applying on real recorded signals) to highlight reflection events, hyperbolic Radon transform (HRT) as a suitable post-correlation module applied on correlated section due to applied FX-NCF on simulated passive signals from a simple 2D synthetic model. The result encouraged us to apply the same hybrid modules (FX-NCF plus HRT) on real recorded passive signals to reconstruct wanted reflection events.

How to cite: Hariri Naghadeh, D. and Bean, C.: Retrieving the Reflection events from passive signals , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-43, https://doi.org/10.5194/egusphere-egu2020-43, 2020.

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Investigating the crustal reflections of Taiwan from autocorrelation of seismic noise

Zhuo-Kang Guan and Hao Kuo-Chen

Seismic interferometry is widely applied in various scales to reconstruct seismic signals for investigating Earth interior. The method of Phase Cross Correlation (PCC) takes less pre-processing and is more stable for retrieving of crustal signals than that of the conventional cross correlations by using amplitude information. In order to obtain the crustal reflectors in Taiwan, we applied auto-correlation with PCC to two independent datasets, (1) temporary seismic array in eastern Taiwan with 110 short period seismometers and (2) broadband seismic arrays (BATS and TAIGER) in Taiwan. As a result, the retrieved crustal reflectors, such as Moho reflectors, are stable with different recording time periods and instruments: temporal and spatial signal consistencies in the same site and neighborhood stations, respectively, and also high waveform similarities between short period and broadband seismometers.

Comparing the results with previous studies of velocity model and receiver function, the reflections at 10-12 seconds (roughly 30-40 km) are often observed in most of the results which are correlated to the Moho depths inferred from the receiver function and tomography studies. It is interesting to note that, besides the Moho reflections, some inter-crustal reflectors beneath the Central Range are revealed. The results show that the autocorrelation method has the potential to investigate some signals that are difficult to observe in the past by using other methods.

Another interesting observation from a dense seismic array in eastern Taiwan shows that the chimei fault serves as a sharp boundary to separate the reflectional signals into the northern and southern parts. In the southern part few reflections can be observed and also lack high frequency energies from autocorrelation comparing with those in the northern part. It implies that the distribution of ambient sources or near surface materials could influence the results. After examining the PCC’s feasibility and stability in this study, it is necessary to verify the reliability of results by understanding the source’s properties and local geological situations before interpretation.

How to cite: Guan, Z.-K. and Kuo-Chen, H.: Investigating the crustal reflections of Taiwan from autocorrelation of seismic noise, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21542, https://doi.org/10.5194/egusphere-egu2020-21542, 2020.

To notice key obstacles and suggest effective processing methods for virtual reflection images, numerical modeling was performed by the 2-D finite difference method with time and space intervals of 0.2 ms and 1.25 m, respectively. Vertical sources of the Ricker wavelet with a main frequency of 20 Hz were assumed to be detonated independently at five buried locations with intervals of 500 m. Vertical components of the particle velocity were computed at 99 receivers at 10 m depth with intervals of 20 m. Synthetic data show that maximum amplitudes of reflection signals are less than 2% of those of direct Rayleigh waves on an average. This indicates that the non-reflection events should be attenuated as much as possible before correlating traces to compute virtual seismic data. For attenuating both direct and diffracted Rayleigh waves in the synthetic data, a median filter with a time window of a 0.1-s length was effective. Because stationery-phase source locations for virtual reflections concentrate near receiver locations, only common midpoint gathers close to the sources should be used for good virtual stack images.

How to cite: Kim, K., Song, Y.-S., and Byun, J.: Effects of non-reflection events and stationery source locations on virtual seismic reflection images , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4382, https://doi.org/10.5194/egusphere-egu2020-4382, 2020.

EGU2020-3413 | Displays | SM1.3

Passive Seismic Marchenko Imaging

Zhongyuan Jin

In recent years, seismic interferometry (SI) has been widely used in passive seismic data, it allows to retrieve new seismic responses among physical receivers by cross-correlation or multidimensional deconvolution (MDD). Retrieval of reflected body waves from passive seismic data has been proved to be feasible. Marchenko method, as a new technique, retrieves Green’s functions directly inside the medium without any physical receiver there. Marchenko method retrieves precise Green’s functions and the up-going and down-going Green’s functions can be used in target-oriented Marchenko imaging, and internal multiples related artifacts in Marchenko image can be suppressed. 

Conventional Marchenko imaging uses active seismic data, in this abstract, we propose the method of passive seismic Marchenko imaging (PSMI) which retrieves Green’s functions from ambient noise signal. PSMI employs MDD method to obtain the reflection response without free-surface interaction as an input for Marchenko algorithm, such that free-surface multiples in the retrieved shot gathers can be eliminated, besides, internal multiples don’t contribute to final Marchenko image, which means both free-surface multiples and internal multiples have been taken into account. Although the retrieved shot gathers are contaminated by noises, the up-going and down-going Green’s functions can be still retrieved. Results of numerical tests validate PSMI’s feasibility and robustness. PSMI provides a new way to image the subsurface structure, it combines the low-cost property of passive seismic acquisition and target-oriented imaging property of Marchenko imaging, as well as the advantage that there are no artifacts caused by internal multiples and free-surface multiples.

Overall, the significant difference between PSMI and conventional Marchenko imaging is that passive seismic data is used into Marchenko scheme, which extends the Marchenko imaging to passive seismic field. Passive seismic Marchenko imaging avoids the effects of free-surface multiples and internal multiples in the retrieved shot gathers. PSMI combines the low-cost property of passive seismic acquisition and target-oriented imaging property of Marchenko imaging which is promising in future field seismic survey.

This work is supported by the Fundamental Research Funds for the Central Universities (JKY201901-03). 

How to cite: Jin, Z.: Passive Seismic Marchenko Imaging, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3413, https://doi.org/10.5194/egusphere-egu2020-3413, 2020.

EGU2020-9140 | Displays | SM1.3

Active Noise Tomography in Medical Ultrasound

Ines Ulrich

We propose a translation of widely-used seismic ambient noise tomography to active noise tomography in medical ultrasound. This is intended to eliminate time-consuming transducer calibration and to improve illumination of the target.

Ultrasound computed tomography (USCT) is an emerging visualization modality in medical imaging and is especially apt to screen soft human tissue such as the breast. Currently, USCT applications are developed for breast cancer detection using a collection of ultrasound scans that measure the pressure wavefield emitted by individual transducers. To obtain good coverage, a large number of emitter-receiver pairs is required, as well as careful calibration of transducers using reference measurements in water at constant temperature. Standard acquisition and calibration are time consuming processes, placing major constraints on the integration of USCT for breast cancer detection in medical practice.

We present a novel approach to obtain traveltime measurements between transducer pairs in USCT by applying random field interferometry, as developed in seismic imaging. Since ambient noise sources are absent in the medical application, we generate random wavefields actively by firing sources in a random sequence. Cross-correlation of the recordings provides an approximation of Green’s functions between receivers, from which traveltime measurements can be extracted.

The proposed method has two major benefits: (1) Since cross-correlation eliminates time shifts caused by the a priori unknown source wavelet, the tedious calibration step can be avoided. (2) Coverage improves because the implicit use of reflections off the device boundary overcomes limited illumination caused by the small opening angle of typical ultrasound transducers.

The traveltimes extracted from the Green’s function approximations can be used as new data in a ray-based traveltime tomography. As a proof of concept, we test the algorithm on numerical breast phantoms, and we show that the latter can be reconstructed successfully from the cross-correlation traveltimes. In summary, random field interferometry opens new perspectives to shorten and facilitate the acquisition and tomographic inversion of USCT datasets.

How to cite: Ulrich, I.: Active Noise Tomography in Medical Ultrasound, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9140, https://doi.org/10.5194/egusphere-egu2020-9140, 2020.

EGU2020-21956 | Displays | SM1.3

3D-S wave velocity model of the Los Humeros geothermal field, Mexico, by ambient-noise tomography

Joana Martins, Anne Obermann, Arie Verdel, and Philippe Jousset

Since the successful retrieval of surface-wave responses from the ambient seismic field via cross-correlation, noise-based interferometry has been widely used for high-resolution imaging of the Earth’s lithosphere from all around the globe. Further applications on geothermal fields reveal the potential of ambient noise techniques to either characterize the subsurface velocity field or to understand the temporal evolution of the velocity models due to field operations.

Following the completion of the GeMEX* project, a European-Mexican collaboration to improve our understanding of two geothermal sites in Mexico, we present the results of ambient noise tomography (ANT) techniques over the Los Humeros geothermal field. We used the vertical component of the data recorded by the seismic network active from September 2017 to September 2018. The total network is composed of 45 seismometers from which 25 are Broadband (BB) and the remaining ones short-period stations. From the ambient noise recorded at the deployed seismic network, we extract surface-waves after the computation of the empirical Green’s functions (EGF) by cross-correlation and consecutive stacking. After the cross-correlations, we pick both phase and group velocity arrival times of the ballistic surface-waves for which we derive independent tomographic maps. Finally, using both the retrieved phase and group velocities, we jointly invert the tomographic results from frequency to depth.

We identify positive and negative velocity variations from an average velocity between -15% and 15% for group and between -10% and 10% for phase velocities in the frequency domain. While the velocity variations are consistent for both the phase and group velocities (with expected group velocities lower than the phase velocities), the group velocity anomalies are more pronounced than the phase velocity anomalies. Low-velocity anomalies fall mostly within the inner volcano caldera, the area of highest interest for geothermal energy. This is consistent with the surface temperatures measured at the Los Humeros caldera, indicating the presence of a heat source. Finally, we compare our results with other geophysical studies (e.g geodesy, gravity, earthquake tomography and magnetotelluric) performed during the GeMEX project within the same area.

 

 

 

We thank the European and Mexican GEMex team for setting up the seismic network and station maintenance as well as data retrieval (amongst which Tania Toledo, Emmanuel Gaucher, Angel Figueroa and Marco Calo). We thank the Comisión Federal de Electricidad (CFE) who kindly provided us with access to their geothermal field and permission to install the seismic stations. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 727550 and the Mexican Energy Sustainability Fund CONACYT-SENER, project 2015-04-68074.

 

* http://www.gemex-h2020.eu/index.php?option=com_content&view=featured&Itemid=101&lang=en

How to cite: Martins, J., Obermann, A., Verdel, A., and Jousset, P.: 3D-S wave velocity model of the Los Humeros geothermal field, Mexico, by ambient-noise tomography , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21956, https://doi.org/10.5194/egusphere-egu2020-21956, 2020.

EGU2020-18561 | Displays | SM1.3

Effect of the water layer on seismic noise cross-correlation across the Northeast Atlantic, from Madeira and Canaries to the Atlas-Gibraltar zone

Graça Silveira, Joana Carvalho, Juan Pinzon, Susana Custódio, Carlos Corela, and Luís Matias

One of the aims of project SIGHT (SeIsmic and Geochemical constraints on the Madeira HoTspot system) is to obtain a 3D model of SV-wave velocities of the crust and upper mantle of the Northeast Atlantic area encompassing Madeira and Canary Islands to the Atlas-Gibraltar zone, using seismic noise cross-correlations in the period range 2-100 s. Ambient noise cross-correlation has been successfully applied in a variety of tectonic environments to image the structure of the Earth subsurface. This technique overcomes some limitations ascribed to source–receiver geometry and sparse and irregular earthquake distribution, allowing to image Earth structure with a resolution that mainly depends on the network design. However, the effect of the water layer in the short period Empirical Green Functions, which are obtained by seismic noise cross-correlation, for interstation paths crossing the ocean is still poorly understood.

In several studies, it has been observed that the presence of water and sediments is responsible for later wave-train arrivals. Those later arrivals are frequently disregarded when measuring group velocity, either by considering only longer periods or by specifying a given velocity range.

In this work, we present a systematic study of the influence of the water layer on both vertical and radial synthetic Rayleigh waves, as well as on higher-mode conversion and on the group velocities dispersion measurements.

We show that although the fundamental mode dominates, the presence of the first overtones at short periods (typically below 8 seconds) cannot be neglected. We also show that specifying a given velocity range when retrieving group velocity can result in a mixture of modes. Our tests reveal that, at short periods, the water has a dominant effect on ocean-continent laterally varying media.

This is a contribution to projects SIGHT (Ref. PTDC/CTA-GEF/30264/2017) and STORM (Ref. UTAP-EXPL/EAC/0056/2017). The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020 – IDL.

How to cite: Silveira, G., Carvalho, J., Pinzon, J., Custódio, S., Corela, C., and Matias, L.: Effect of the water layer on seismic noise cross-correlation across the Northeast Atlantic, from Madeira and Canaries to the Atlas-Gibraltar zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18561, https://doi.org/10.5194/egusphere-egu2020-18561, 2020.

EGU2020-7179 | Displays | SM1.3

Reconciling phase velocities from ambient noise and earthquake-generated surface waves by accounting for arrival-angle effects

Giovanni Diaferia, Fabrizio Magrini, Lapo Boschi, and Fabio Cammarano

The shear-wave velocities structure at depth can be unraveled from ambient noise (AN) as well as from earthquake-generated (EQ) surface waves. While the first approach mostly provides information at crustal scale, earthquake-based surface waves sense deeper structures due to their lower frequency content. However, for periods between 20 and 40 s, where the two methods often overlap, a number of studies have shown that phase velocities from EQ surface waves are systematically higher (~1%) than those retrieved from AN. The reason for such systematic bias is still debated; finite-frequency effects, overtone contamination, and off-path propagation of surface waves due to structural inhomogeneities have all been invoked as possible explanations of the discrepancy in question.

We explore the validity of the latter hypothesis, by correcting Rayleigh-wave phase velocities for the effect of off-path arrivals at two stations. The deviation from the theoretical path is estimated by evaluating the resemblance of the vertical with the π/2-shifted radial component of the recorded seismograms. We developed a two-station algorithm implementing such a correction and tested it on a dataset of seismograms collected from more than 350 stations recording 443 earthquake events from 2005 to 2019. We demonstrate that by compensating for the arrival-angle effects, the discrepancy between the two methods is significantly reduced. This result suggests that the off-path propagation between epicenters and receivers due to lateral inhomogeneity in the Earth's structure explains most of the discrepancy between AN and EQ phase velocities previously reported in the literature. Such improvement in determining Rayleigh phase velocities will lead to more reliable seismic tomographies and enhanced interpretations of seismic anomalies in terms of thermo-chemical characteristics.

How to cite: Diaferia, G., Magrini, F., Boschi, L., and Cammarano, F.: Reconciling phase velocities from ambient noise and earthquake-generated surface waves by accounting for arrival-angle effects, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7179, https://doi.org/10.5194/egusphere-egu2020-7179, 2020.

EGU2020-21852 | Displays | SM1.3

Wave propagation and subsurface velocity structure at the Virgo gravitational wave detector (Italy)

Gilberto Saccorotti, Sonja Gaviano, Carlo Giunchi, Irene Fiori, Soumen Koley, and Jo Van den Brand

The performances and sensitivity of gravitational wave (GW) detectors are significantly affected by the seismic environment. In particular, the seismic displacements and density fluctuations of the ground due to seismic-wave propagation introduce noise in the detector output signal; this noise is referred to as gravity-gradient noise, or Newtonian Noise (NN). The development of effective strategies for mitigating the effects of NN requires, therefore, a thorough assessment of seismic wavefields and medium properties at and around the GW detector. In this work, we investigate wave propagation and the subsurface velocity structure at the Virgo GW detector (Italy), using data from a temporary, 50-element array of vertical seismometers. In particular, we analyze the recordings from the catastrophic Mw=6.2 earthquake which struck Central Italy on August 24, 2016, and six of the following aftershocks.  The general kinematic properties of the earthquake wavefields are retrieved from the application of a broad-band, frequency-domain beam-forming technique. This method allows measuring the propagation direction and horizontal slowness of the incoming signal; it is applied to short time windows sliding along the array seismograms, using different subarrays whose aperture was selected in order to match different frequency bands. For the Rayleigh-wave arrivals, velocities range between 0.5 km/s and 5 km/s, suggesting the interference of different wave types and/or multiple propagation modes. For those same time intervals, the propagation directions are scattered throughout a wide angular range, indicating marked propagation effects associated with geological and topographical complexities. These results suggest that deterministic methods are not appropriate for estimating Rayleigh waves phase velocities. By assuming that the gradient of the displacement is constant throughout the array, we then attempt the estimation of ground rotations around an axis parallel to the surface (tilt), which is in turn linearly related to the phase velocity of Rayleigh waves. We calculate the ground tilt over subsequent, narrow frequency bands. Individual frequency intervals are investigated using sub-arrays with aperture specifically tailored to the frequency (wavelength) under examination. From the scaled average of the velocity-to-rotation ratios, we obtain estimates of the Rayleigh-wave phase velocities, which finally allow computing a dispersion relationship. Due to their diffusive nature, earthquake coda waves are ideally suited for the application of Aki’s autocorrelation method (SPAC). We use SPAC and a non-linear fitting of correlation functions to derive the dispersion properties of Rayleigh wave for all the 1225 independent inter-station paths. The array-averaged SPAC dispersion is consistent with that inferred from ground rotations, and with previous estimates from seismic noise analysis.  Using both a semi-analytical and perturbational approaches, this averaged dispersion is inverted to obtain a shear wave velocity profile down to ~1000m depth. Finally, we also perform an inversion of the frequency-dependent travel times associated with individual station pairs to obtain 2-D, Rayleigh wave phase velocity maps spanning the 0.5-3Hz frequency interval. 

How to cite: Saccorotti, G., Gaviano, S., Giunchi, C., Fiori, I., Koley, S., and Van den Brand, J.: Wave propagation and subsurface velocity structure at the Virgo gravitational wave detector (Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21852, https://doi.org/10.5194/egusphere-egu2020-21852, 2020.

EGU2020-16678 | Displays | SM1.3

Imaging shallow structures in Dublin city using seismic interferometry of seismic waves generated by train traffic

Meysam Rezaeifar, Giuseppe Maggio, Yihe Xu, Chris Bean, François Lavoué, Pierre Boué, Laura Pinzon-Rincon, and Florent Brenguier

Although train-induced vibrations are mainly regarded as a source of unwanted noise for classical seismological applications, these vibrations act as powerful sources for seismic imaging using seismic interferometry. Most of the seismic interferometry studies to date have concentrated on using the ambient seismic field generated by natural processes but the appropriate use of train-induced vibrations could result in higher resolution images.

In this study, we present results of seismic interferometry applied on 3 days of railroad traffic data recorded by an array of 3-component seismographs along a railway in Dublin, Ireland. Train-generated waves show a significantly higher frequency range than those recovered from typical ambient noise interferometry. Analysing the recorded signal, we have been able to distinguish between different train types (e.g. cargo vs. passenger trains) and train lengths (3-4, 5-6, 7-9, and/or 10-11 wagons).

For seismic interferometry, a Common Mid-Point – Cross-Correlation (CMP-CC) stack approach has been used to directly image the structures beneath the array. This approach produces a reflection image with interfaces consistent with nearby borehole data at ~450-500 m and ~1350-1400 m depth.

In addition to this reflection image, our results document a strong relation between the ambient source location (trains in this case) and the retrieved seismic reflection image. Since we have train location GPS data, we extracted 2-s time windows for when the train is 1500 m, 1000 m, and 500 m away from the first sensor and we applied the CMP-CC procedure to produce reflection images. As expected, the reflection images are sensitive to the location of the ambient noise source.

Numerical forward modelling of seismic wavefields for various source-receiver configurations also documents a strong correlation between the source location and the retrieved reflection image.

This research emanates from PACIFIC - Passive seismic techniques for environmentally friendly and cost-effective mineral exploration - which has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No~776622. We also acknowledge support from the European Research Council under grant No.~817803, FAULTSCAN.

How to cite: Rezaeifar, M., Maggio, G., Xu, Y., Bean, C., Lavoué, F., Boué, P., Pinzon-Rincon, L., and Brenguier, F.: Imaging shallow structures in Dublin city using seismic interferometry of seismic waves generated by train traffic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16678, https://doi.org/10.5194/egusphere-egu2020-16678, 2020.

EGU2020-16828 | Displays | SM1.3

Understanding seismic waves generated by train traffic via modelling

François Lavoué, Olivier Coutant, Pierre Boué, Laura Pinzon-Rincon, Florent Brenguier, Philippe Dales, Aurélien Mordret, Meysam Rezaeifar, Christopher Bean, and the AlpArray Working Group

Trains have recently been recognised as powerful sources for seismic imaging and monitoring based on the correlation of continuous noise records, but the optimal use of these signals still requires a better understanding of their source mechanisms. In this study, we present a simple approach for modelling train-generated seismic signals inspired from early work in the engineering community, which assumes that seismic waves are emitted by  sleepers regularly spaced along the railway and excited by the passage of the train wheels. 
     As already known in the engineering literature, we exemplify the importance of the spatial distribution of each axle load over the rail track on the high-frequency content of the corresponding source time functions, and therefore of the final seismograms resulting from the contributions of all sleepers. In practice, this high-frequency content mainly depends on ground stiffness beneath the railway.
     Furthermore, we identify two end-member mechanisms to explain the two types of observations documented in the seismological literature. The first is the case of a single stationary source (fixed sleeper) excited by successive wheels of a train. This generates a harmonic spectrum characterised by a narrow spacing between frequency peaks related to a fundamental frequency f1 = Vtrain / Lw controlled by train speed and wagon length. The second is the case of a single moving load (single wheel) exciting all sleepers along the railway. This also yields a harmonic spectrum, but with a larger spacing between frequency peaks, related to a fundamental frequency f2 = Vtrain / Δsleeper  controlled by train speed and sleeper spacing. This moving source also generates a clear Doppler effect. 
     In more realistic cases, considering all wheels and all sleepers, our modelling well reproduces the observations, both in the frequency domain (harmonic spectra) and in the time domain (tremor-like emergent shapes). The dominance of the previously-identified end-member mechanisms depends on sleeper regularity: perfectly-regular sleepers generate signals dominated by the signature of a single moving load with fundamental frequency f2 and a clear Doppler effect, while slightly-irregular sleepers generate signals dominated by the signature of stationary sources with fundamental frequency f1. We speculate that our modelling parameter of sleeper regularity actually depends on the properties of the railway infrastructure in real cases.
     Finally, we discuss the perspectives of this work in view of using train-generated signals for seismic imaging and monitoring. In this regard, an important conclusion is that the frequency content of the signals is dominated by interferences between harmonic waves. Therefore, the exact value of the fundamental frequency at play matters less than the generation and preservation of the high frequencies, which depend on the distribution of the train load over the rail track and on propagation effects (medium heterogeneities, scattering and attenuation). Therefore, most of train traffic worldwide is expected to generate signals with a significant frequency content in the band [1 - 50] Hz of interest for seismic applications, in particular in the case of trains travelling at variable speeds which are expected to produce truly broadband signals. 

How to cite: Lavoué, F., Coutant, O., Boué, P., Pinzon-Rincon, L., Brenguier, F., Dales, P., Mordret, A., Rezaeifar, M., Bean, C., and Working Group, T. A.: Understanding seismic waves generated by train traffic via modelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16828, https://doi.org/10.5194/egusphere-egu2020-16828, 2020.

EGU2020-3418 | Displays | SM1.3

Testing the applicability of ambient noise methods in zones with different degree of anthropogenic sources.

Jordi Diaz, Martin Schimmel, Mario Ruiz, and Ramon Carbonell

The general objectives of the “Seismic Ambient Noise Imaging and Monitoring of Shallow Structures” (SANIMS) project, funded by the Spanish Ministry of Science, Research and Innovation (Ref.: RTI2018-095594-B-I00), are focused into the application and development of methods based on ambient noise seismic data recorded by dense networks to image and monitor natural and human-altered environments. To achieve this objective, temporal seismic networks have been installed since late 2019 in two very different settings; the Cerdanya Basin, a sedimentary basin located in the eastern Pyrenees and the city of Barcelona.

Regarding the Cerdanya Basin, a relatively unaltered setting, a network of up to 25 broad-band stations has been installed for a period of one year. Additionally, a high resolution grid of seismic nodes will be deployed for 2 months in the central part of the basin, with interstation distances of 1.5 km. In order to constraint the uppermost crustal structure using ambient noise, vertical component recordings will be processed using the phase cross-correlation and time-frequency domain phase-weighted stacking to extract fundamental mode Rayleigh waves. The surface waves will then be used to measure inter-station group and phase velocity dispersion curves that will be inverted using the Fast Marching Surface Tomography method. Depending on data quality, we will also process the horizontal components to extract Love waves for joint inversions with Rayleigh waves to constrain radial anisotropy and/or the application of new strategies to perform attenuation tomography.

Regarding areas strongly altered by human activity, we have deployed a network of 15 short-period stations within the city of Barcelona, in most of the cases installed in the basement of secondary schools, for a duration of 9-12 months. The objective of this deployment is twofold; acquire new valuable scientific data and introduce the students in an Earth Science research project. Although the Barcelona area has been investigated using MHVSR methods by different authors, the new data acquired by the SANIMS project will expand the available data and will allow to analyze the time variability of the measurements. This new dataset will also be used to analyze the applicability of the methods based on Rayleigh wave ellipticity inversion of ambient noise and earthquake data to provide S-velocity depth profiles. Under the assumption of an isotropic horizontally layered medium, the ellipticity inversion is not affected by the directivity of the diffusive noise wave field and seems therefore to be a good option to determine local S-velocity depth profiles in areas with little lateral inhomogeneities and uneven distribution of noise sources.

We expect that the use of ambient noise methods will allow to map the basement and to obtain new higher resolution ambient noise tomographic images of the upper crust in the Cerdanya Basin and to better constrain the subsoil properties of Barcelona, hence improving the existing seismic hazard maps. Besides, comparing the results in both areas will allow to compare the performance of the different methods based on ambient noise in quiet and noisy areas.

How to cite: Diaz, J., Schimmel, M., Ruiz, M., and Carbonell, R.: Testing the applicability of ambient noise methods in zones with different degree of anthropogenic sources., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3418, https://doi.org/10.5194/egusphere-egu2020-3418, 2020.

EGU2020-5480 | Displays | SM1.3

Passive seismic velocity monitoring of natural faults: The FaultScan project

Florent Brenguier, Aurelien Mordret, Yehuda Ben-Zion, Frank Vernon, Pierre Boué, Christopher Johnson, and Pieter-Ewald Share

Laboratory experiments report that detectable seismic velocity changes should occur in the vicinity of fault zones prior to earthquakes. However, operating permanent active seismic sources to monitor natural faults at seismogenic depth has been nearly impossible to achieve. The FaultScan project (Univ. Grenoble Alpes, Univ. Cal. San Diego, Univ. South. Cal.) aims at leveraging permanent cultural sources of ambient seismic noise to continuously probe fault zones at a few kilometers depth with seismic interferometry. Results of an exploratory seismic experiment in Southern California demonstrate that correlations of train-generated seismic signals allow daily reconstruction of direct P body-waves probing the San Jacinto Fault down to 4 km depth. In order to study long-term earthquake preparation processes we will monitor the San Jacinto Fault using such approach for at least two years by deploying dense seismic arrays in the San Jacinto Fault region. The outcome of this project may facilitate monitoring the entire San Andreas Fault system using the railway and highway network of California. We acknowledge support from the European Research Council under grant No.~817803, FAULTSCAN.

How to cite: Brenguier, F., Mordret, A., Ben-Zion, Y., Vernon, F., Boué, P., Johnson, C., and Share, P.-E.: Passive seismic velocity monitoring of natural faults: The FaultScan project, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5480, https://doi.org/10.5194/egusphere-egu2020-5480, 2020.

The observations of seismicity, ground deformation, and volcanic gas geochemistry indicate a magmatic unrest of the Changbaishan volcano, northeast China between July 2002 and July 2005. In this study, we collected the continuous waveform data from more than 10 stations of permanent and portable networks around Changbaishan volcano area from 2000 to 2018, and studied the temporal velocity changes beneath the volcano based on both the cross-correlation of station pairs and auto-correlation of singe station method. We adopted the time-frequency domain phase weighted technique to speed up the convergence process of the noise-based Green's function, and improved the time resolution of monitoring from several tens of days to several days. We measured the temporal seismic velocity of the Changbaishan volcano in various frequency bands. The results shown that there were obvious seasonal changes of the seismic velocity for most frequency bands, and for 0.5-1 Hz frequency band a sudden velocity drop was observed starting on June 10, 2002 and the amplitude of velocity changes was up to 0.5%. After that, the number of volcanic events increased significantly. Our results suggest that there may be a precursory velocity drop phenomenon before the magma unrest, which is of great scientific significance for the studies of magma unrest and possible volcanic eruption in the future.

How to cite: Liu, Z.: Temporal changes of seismic velocity associated with a magmatic unrest of Changbaishan volcano, northeast China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6847, https://doi.org/10.5194/egusphere-egu2020-6847, 2020.

EGU2020-12871 | Displays | SM1.3

Noise-Based Monitoring of Spatiotemporal Changes in Crustal Seismic Wavespeed across Southern California

Shujuan Mao, Albanne Lecointre, Qingyu Wang, Robert van der Hilst, and Michel Campillo

Monitoring temporal changes in seismic wavespeed can inform our understanding of the evolution of crustal rocks’ mechanical state caused by perturbations in stress field, damages, and fluids. Furthermore, imaging these time-lapse changes in space can help unravel the response of rocks with different elastic properties. In this study, we analyze the spatiotemporal variations of seismic wavespeed in Southern California from 2007 to 2017. We compute the Green’s functions by daily cross-correlations using ambient noise at over three hundred broadband seismic stations. Instead of calculating simply the linear regressions of travel-time shifts over lag-times, which only resolves homogeneous changes, we scrutinize the variations of travel-time shifts at different lag-times and frequencies using coda-wave sensitivity kernels, in order to probe the spatial distribution of wavespeed changes. The long-term and large-scale analysis allows us to investigate the mechanical response of different crustal materials to various transient processes. As an example we use the 2010 Mw 7.2 El Mayor-Cucapah Earthquake (EMC) and show that large coseismic wavespeed reductions occur in Salton Sea area and the Los Angeles sedimentary basin. In the latter region, the ground motion amplification and high susceptibility of sedimentary materials explain the remote signature of the earthquake. In the Salton Sea region, particularly in the geothermal area with highly pressurized fluids, the non-linear crustal response illustrated by wavespeed changes can be analyzed with regard to the high-level micro-seismicity triggered by EMC.

How to cite: Mao, S., Lecointre, A., Wang, Q., van der Hilst, R., and Campillo, M.: Noise-Based Monitoring of Spatiotemporal Changes in Crustal Seismic Wavespeed across Southern California, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12871, https://doi.org/10.5194/egusphere-egu2020-12871, 2020.

In the last decade, correlation of ambient seismic noise has opened a window to new possibilities for the study of structural properties of the Earth. One such possibility is the monitoring of transient changes in the mechanical properties of the surrounding crustal material following an earthquake. These changes, expressed as variations in seismic velocities, are usually associated to fracture damage and release of fluids due to the earthquakes shaking, but could also be related to deformation associated with afterslip. On April 16, 2016, a Mw 7.8 earthquake struck the coast of Ecuador, rupturing a ~100 km-long segment of the megathrust interface previously identified as highly coupled. Shortly after the mainshock, we deployed a temporary seismic network to monitor the post-seismic phase, in addition to the already in-place permanent Ecuadorian network. Here we present results from cross-correlation of continuous ambient seismic noise during a ~12-months period following the mainshock. Taking advantage of the dense and extensive station network, we investigate the spatio-temporal evolution of the post-seimic seismic velocity changes. Our results show a slow but sustained increase in the average seismic velocities after the earthquake, with a decay in the rate of the increase during the last few months. Spatially, the increase is more notorious nearby the rupture area, whereas the amplitude of the increase diminishes as we move away from the epicenter. We interpret these variations in seismic velocities (steady increase) as the crust’s response to the healing process that takes place during the post-seismic phase, following the sudden coseismic decrease of seismic velocities during the mainshock. This healing process could involve the decrease of fluid-related pore pressures and the healing of fractures and cracks generated during the mainshock, both at the interface and on the overriding plate.

How to cite: Agurto-Detzel, H., Rivet, D., and Charvis, P.: Seismic velocity changes in the epicentral area of the Mw 7.8 Pedernales (Ecuador) earthquake from cross-correlation of ambient seismic noise, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17543, https://doi.org/10.5194/egusphere-egu2020-17543, 2020.

EGU2020-9578 | Displays | SM1.3

Looking for changes in the upper crust associated with large magnitude earthquakes in central Italia using seismic noise autocorrelations

Laurent Stehly, Estelle Delouche, Christophe Voisin, and Piero Poli

In this work, we use seismic noise autocorrelations to monitor the temporal evolution of the upper crust in Central Italia in order to look for changes that could have occured before the 2009 Mw6.3 l'Aquila and the 2016 Mw 6.2 Amatrice earthquake.

To that end, we use the Coherence of Correlated Waveforms [CCW] method, that consists in measuring changes in the waveform of autocorrelations with a temporal resolution of 5 days.

Our measurements of the CCW show that the L'Aquila Earthquake  is preceded by a 150-days oscillation whose amplitude and frequency progressively increases until the rupture. Analysing 17 years of data, we found that this signal occured only before the L'Aquila and the Amatrice earhtquake.  This suggests the existence of a unique nucleation process.

Finally, we compare the results obtained using the CCW method with the temporal evolution of the seismic waves velocity (dv/v) obtained by analysing the coda of seismic noise autocorrelations.

How to cite: Stehly, L., Delouche, E., Voisin, C., and Poli, P.: Looking for changes in the upper crust associated with large magnitude earthquakes in central Italia using seismic noise autocorrelations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9578, https://doi.org/10.5194/egusphere-egu2020-9578, 2020.

EGU2020-15135 | Displays | SM1.3

Temporal Variations of Near-surface seismic structure of Taiwan revealed by coda interferometry

Sheng-Jyun Cai, Li-Wei Chen, Hsin-Yu Lee, Ying-Nien Chen, and Yuan-Cheng Gung

We report the temporal change of the near-surface(<400m) seismic structure of Taiwan revealed by coda interferometry. Following our earlier work (Chen et al., 2017), the Empirical Green’s Functions (EGF) of shear waves extracted from the earthquake coda recorded by the vertical pairs of borehole array, deployed by the Central Weather Bureau, are used to examine the temporal variations of vs and Vs azimuthal anisotropy at the borehole sites. In total, about 700 local events, from 2013 to 2018, are used in this study. The band-passed (3 – 8 hz) EGF extracted from each single event are stacked over variable time period to ensure the reliability of measurements and the desired temporal resolution. The averaged Vs and patterns of Vs azimuthal anisotropy are in good agreement with the site geology, the ambient stress and those reported in our early work. Apparent drop in the Vs isotropic velocities and perturbations in Vs azimuthal anisotropy are observed in few representative borehole sites, and we also noticed that such variations are tightly correlated with the occurrence of major earthquakes in Taiwan. We present the preliminary results and discuss the triggering mechanisms, the healing revolution, and their relationship with the site geology.

How to cite: Cai, S.-J., Chen, L.-W., Lee, H.-Y., Chen, Y.-N., and Gung, Y.-C.: Temporal Variations of Near-surface seismic structure of Taiwan revealed by coda interferometry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15135, https://doi.org/10.5194/egusphere-egu2020-15135, 2020.

EGU2020-18923 | Displays | SM1.3

Passive Reflection Seismic Imaging of the North Anatolian Fault at crustal-scale: A Matrix Framework for Aberrations Correction

Rita Touma, Michel Campillo, Alexandre Aubry, and Thibaud Blondel

To understand fault systems, it is required to identify the structure of the crust and upper mantle. Seismic investigations have long been relying on active sources generating an incident wave-field from the Earth surface. The reflected wave-field is then recorded by sensors deployed at the surface. Nowadays, passive imaging has been adopted as an alternative of this source-receiver configuration by computing the correlations of ambient noise. This process allows to estimate the Green’s function between two receivers. We here present a passive imaging technique applied to data recorded with the Dense Array of North Anatolia [1], which was deployed in western Turkey during 16 months. The array consists of 73 stations covering the two major fault branches of the North Anatolian Fault (NAF). Inspired by previous works in optics and acoustics, we introduce a matrix approach of seismic imaging based on seismic noise cross correlations. Our method applies focusing operations at emission and reception (Blondel et al.,2019) allowing to project the reflection matrix recorded at the surface to depth (redatuming). Although seismic noise is dominated by surface waves, focusing operations allow to extract the body wave components that carry information about the reflectivity of in-depth structures. However, complex velocity distribution of the Earth’s crust results in phase distortions, referred to as aberrations in the imaging process. Phase distortions prevent the imaging of the true reflectivity of the subsurface leading to unphysical features and blurry images. To overcome these issues, we introduce a new operator: the so-called distortion matrix. It connects any virtual source induced by focusing at emission with the distorted part of the reflected wave-front in the spatial Fourier domain. A time-reversal analysis of the distortion matrix allows to correct for high-order aberrations. Crustal-scale 3D images of the fault structure of the North Anatolian Fault are revealed with optimal resolution and contrast.

(1) DANA. Dense array for north anatolia. International Federation of Digital Seismograph Networks doi:10.7914/SN/YH2012, 2012.

How to cite: Touma, R., Campillo, M., Aubry, A., and Blondel, T.: Passive Reflection Seismic Imaging of the North Anatolian Fault at crustal-scale: A Matrix Framework for Aberrations Correction, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18923, https://doi.org/10.5194/egusphere-egu2020-18923, 2020.

EGU2020-15195 | Displays | SM1.3

Monitoring of temporal seismic velocity changes in the North Anatolian Fault zone using data derived scattering properties

Chantal van Dinther, Michel Campillo, Ludovic Margerin, and Albanne Lecointre

Monitoring of temporal seismic velocity changes can provide us with information on the mechanical state of the Earth’s crust due to processes of stress build-up and release. 

In current work, we use the Dense Array of North Anatolia [1], which has been continuously recording from May 2012 until October 2013, to analyse the spatio-temporal variations of seismic velocity changes in the North Anatolian Fault zone (NAF). We compute daily ambient-noise cross-correlation functions for all 63 three-component stations in the frequency band between 0.1 – 1 Hz.

To retrieve spatial distribution of seismic velocity changes in such an inhomogeneous fault zone, we go beyond the simple linear travel-time shifts approximation and homogeneous sensitivity kernel. We therefore invert for the travel-time shifts at different lag-times. Furthermore, we use sensitivity kernels for media with inhomogeneous scattering properties. The scattering properties for the sensitivity kernels are derived from the data: a scattering mean free path inside the fault zone (northern strand of NAF) of ∼ 10 km and ∼ 150 km outside the fault zone, the attenuation coefficient inside and outside the fault zone are 80 and 100 respectively. 

 

[1] DANA. Dense array for north anatolia. International Federation of Digital Seismograph Networks doi:10.7914/SN/YH2012, 2012.

How to cite: van Dinther, C., Campillo, M., Margerin, L., and Lecointre, A.: Monitoring of temporal seismic velocity changes in the North Anatolian Fault zone using data derived scattering properties, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15195, https://doi.org/10.5194/egusphere-egu2020-15195, 2020.

Scattered seismic coda waves are frequently used to characterize small scale medium heterogeneities, intrinsic attenuation or temporal changes of wave velocity. Spatial variability of these properties raises questions about the spatial sensitivity of seismic coda waves. Especially the continuous monitoring of medium perturbations using ambient seismic noise led to a demand for approaches to image perturbations observed with coda waves. An efficient approach to localize the property variations in the medium is to invert the observations from different source-receiver combinations and different lapse times in the coda for the location of the perturbations. The key of such an inversion is calculating the coda-wave sensitivity kernels which describe the connection between observations and the perturbation. Most discussions of sensitivity kernels use the acoustic approximation and assume wave propagation in the diffusion regime.

We model 2-D  elastic multiple nonisotropic scattering in a random medium with spatially variable heterogeneity and attenuation. The Monte Carlo method is used to numerically solve the radiative transfer equation that describes the wave scattering process here. Recording of the specific intensity of the wavefield I(r,n,t) which contains the complete information about the energy at position r at time t with the propagation direction n allows us to calculate sensitivity kernels according to rigorous theoretical derivations. We investigate sensitivity kernels that describe the relationships between changes of the model parameters P- and S-wave velocity, P- and S-wave attenuation, and the strength of fluctuation on the one hand and the observables envelope amplitude, travel time changes and decorrelation on the other hand. These sensitivity kernels reflect the effect of the spatial variations of medium properties on wavefield. Our work offers a direct approach to compute these new expressions and adapt them to spatially variable heterogeneities. The sensitivity kernels we derived are the first step in the development of an inversion approach based on coda waves.

How to cite: Zhang, T. and Sens-Schönfelder, C.: Simulation of seismic wave scattering for the computation of probabilistic coda-wave sensitivity kernels, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5408, https://doi.org/10.5194/egusphere-egu2020-5408, 2020.

EGU2020-7832 | Displays | SM1.3

Optimal processing and unphysical effects in seismic noise correlations

Andreas Fichtner, Daniel Bowden, and Laura Ermert

A wide spectrum of processing schemes is commonly applied during the calculation of seismic noise correlations. This is intended to suppress large-amplitude transient and monochromatic signals, to accelerate convergence of the correlation process, or to modify raw correlations into more plausible approximations of inter-station Green's functions. Many processing schemes, such as one-bit normalisation or various non-linear normalizations, clearly break the linear physics of seismic wave propagation. This naturally raises the question: To what extent are the resulting noise correlations physically meaningful quantities?

In this contribution, we rigorously demonstrate that most commonly applied processing methods introduce an unphysical component into noise correlations. This affects noise correlation amplitudes but also, to a lesser extent, time-dependent phase information. The profound consequences are that most processed correlations cannot be entirely explained by any combination of Earth structure and noise sources, and that inversion results may thus be polluted.

The positive component of our analysis is a new class of processing schemes that are optimal in the sense of (1) completely avoiding the unphysical component, while (2) closely approximating the desirable effects of conventional processing schemes. The optimal schemes can be derived purely on the basis of observed noise, without any knowledge of or assumptions on the nature of noise sources.

In addition to the theoretical analysis, we present illustrative real-data examples from the Irish National Seismic Network and the Lost Hills array in Central California. This includes a quantification of potential artifacts that arise when mapping unphysical traveltime and amplitude variations into images of seismic velocities or attenuation.

How to cite: Fichtner, A., Bowden, D., and Ermert, L.: Optimal processing and unphysical effects in seismic noise correlations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7832, https://doi.org/10.5194/egusphere-egu2020-7832, 2020.

EGU2020-20464 | Displays | SM1.3

The Earth’s Correlation Wavefield: Proof of Concept, Origin and Applications

Hrvoje Tkalčić, Sheng Wang, and Thanh Son Pham

We have recently shown that all features in the earthquake-coda correlogram can be explained by the similarity of seismic phases that have a common slowness for the analysed receiver pair. This includes both the features that have their equivalents in the conventional traveltime stacks, but also those that were previously unexplained. Consequently, the information contained in the correlograms – cross-correlated ground-motion time-series in a two-dimensional representation – can be used to constrain Earth’s internal structure, however, that requires a proof of concept and further investigation into the origin of the correlation wavefield. We thus first decompose relevant correlogram features into discrete constituents with respect to their arrival times and we uniquely identify contributing seismic phases to each constituent. This confirms that the correlation wavefield does not arise due to the reconstruction of body waves between the two receivers (a.k.a. Green’s function) – instead, it is dominated by the interaction of various body waves, and its features are characterised by complex sensitivity kernels.

We demonstrate that the event locations relative to the receivers alter the similarities between the body waves, and may result in significant waveform distortions and inaccuracies in arrival-time predictions. We further show that the nature of source-mechanism and energy-release dynamics are the key influencers responsible for individual correlograms equal in quality to a stack of hundreds of correlograms. In other words, a single seismic event that meets a set of criteria in the presence of multiple receivers can completely `illuminate’ the Earth’s interior. Quantitative kernel-decomposition and identification of body-wave pairs that contribute to a given feature in the correlogram, along with informed choices of seismic events, thus makes the correlation-wavefield tomography and other applications fully feasible. This has the potential to change the course of global seismology in the coming decades.

How to cite: Tkalčić, H., Wang, S., and Pham, T. S.: The Earth’s Correlation Wavefield: Proof of Concept, Origin and Applications, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20464, https://doi.org/10.5194/egusphere-egu2020-20464, 2020.

EGU2020-7662 | Displays | SM1.3

Rapid Global Finite-Frequency Ambient Noise Source Inversion

Jonas Igel, Laura Ermert, and Andreas Fichtner

Common assumptions in ambient noise seismology such as Green’s function retrieval and equipartitioned wavefields are often not met in the Earth. Full waveform ambient noise tomography methods are free of such assumptions, as they implement knowledge of the time- and space-dependent ambient noise source distribution, whilst also taking finite-frequency effects into account. Such methods would greatly simplify near real-time monitoring of the sub-surface. Additionally, the distribution of the secondary microseisms could act as a new observable of the ocean state since its mechanism is well understood (e.g. Ardhuin et al., 2011).

To efficiently forward-model global noise cross-correlations we implement (1) pre-computed high-frequency wavefields obtained using, for example, AxiSEM (Nissen-Meyer et al., 2014), and (2) spatially variable grids, both of which greatly reduce the computational cost. Global cross-correlations for any source distribution can be computed within a few seconds in the microseismic frequency range (up to 0.2 Hz). Similarly, we can compute the finite-frequency sensitivity kernels which are then used to perform a gradient-based iterative inversion of the power-spectral density of the noise source distribution. We take a windowed logarithmic energy ratio of the causal and acausal branches of the cross-correlations as measurement, which is largely insensitive to unknown 3D Earth structures.

Due to its parallelisation on a cluster, our inversion tool is able to rapidly invert for the global microseismic noise source distribution with minimal required user interaction. Synthetic and real data inversions show promising results for noise sources in the North Atlantic with the structure and spatial distribution resolved at scales of a few hundred kilometres. Finally, daily noise sources maps could be computed by combining our inversion tool with a daily data download and processing toolkit.

How to cite: Igel, J., Ermert, L., and Fichtner, A.: Rapid Global Finite-Frequency Ambient Noise Source Inversion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7662, https://doi.org/10.5194/egusphere-egu2020-7662, 2020.

EGU2020-11314 | Displays | SM1.3

Source location and evolution of the 26 s microseism from 3-C beamforming

Charlotte Bruland, Sarah Mader, and Céline Hadziioannou

Source location and evolution of the 26 s microseism from 3-C beamforming

Authors: Charlotte Bruland1, Sarah Mader2, Céline Hadziioannou1
1 Institut für Geophysik, Universität Hamburg, Germany
2 Karlsruher Institut für Technologie, Karlsruhe, Germany

The interest in ambient noise has increased in the recent years due to its applications in imaging and monitoring the subsurface without the use of an active source. One of the major unknowns in this field is the origin of the noise used for these analyses. Better constraints on the location and behavior of noise sources will help us understand the ocean-solid Earth interaction processes driving them and improve our applications of ambient noise. One of the most enigmatic noise sources is the 26 s microseism. This very monochromatic source has been identified in the 1960’s and seems to come from a fixed location in the Gulf of Guinea. The source mechanism of this signal is unknown.

To investigate the origin and physical mechanisms responsible for the 26 s microseism, data from permanent broadband stations in Germany, France and Algeria, and temporary arrays in Morocco and Botswana is used for spectral analysis and 3-component beamforming. The source exhibits a strong temporal variation in spectral amplitude. The signal is not always detectable, but occasionally it becomes so strong it can be detected on stations all around the world. Such burst events can last for a couple of hours up to a couple of days. From January to April 2013, the peak was detected globally 28 percent of the time. The beamforming results confirm that the energy is coming from the Gulf of Guinea, as shown in previous studies, and the direction is temporally stable. Whenever the signal is detectable, both Love and Rayleigh waves are generated. Looking into the 26 s microseism over different time periods and using different arrays, the source is expected to be temporally stable in frequency and location, but varying in energy.

How to cite: Bruland, C., Mader, S., and Hadziioannou, C.: Source location and evolution of the 26 s microseism from 3-C beamforming, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11314, https://doi.org/10.5194/egusphere-egu2020-11314, 2020.

EGU2020-19154 | Displays | SM1.3

Seasonal fluctuations in the secondary microseism wavefield recorded offshore Ireland

Florian Le Pape and Christopher J. Bean

Generated in the ocean, secondary microseisms result from the interaction of opposing ocean wave fronts and represent the strongest ambient seismic noise level measured on land. The recorded noise energy will vary with seasons due to changes in storm activity and associated secondary microseism source locations. Here, ocean bottom seismometer (OBS) data collected offshore Ireland in 2016 have been processed to look into the seasonal variations of the ambient noise wavefield recorded at the seafloor. Daily cross-correlations of OBS pairs located on top of thick sediments in deep water highlight seasonal changes between Rayleigh waves fundamental mode and first overtone for winter and summer months. Comparisons with ocean wave directional spectrum data derived from ocean wave model hindcasts suggest those variations are correlated with changing patterns in ocean waves interactions and therefore microseism source locations. In order to understand those observations in detail, we use 3D numerical simulations to show how the water column but also the subsurface structure below the sea bottom will affect the recorded wavefield at the seafloor for different stations and sources locations. Compared to land stations, the secondary microseism wavefield observed in the ocean and in particular changes in the excitation of Rayleigh modes due to site effects can help characterize the microseism source locations that fluctuate through the seasons.

How to cite: Le Pape, F. and Bean, C. J.: Seasonal fluctuations in the secondary microseism wavefield recorded offshore Ireland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19154, https://doi.org/10.5194/egusphere-egu2020-19154, 2020.

EGU2020-19017 | Displays | SM1.3

Seismic noise characteristics in Chinese mainland

Fang Wang, Weitao Wang, Jianfeng Long, and Leiyu Mu

Using the three-component continuous waveform recordings of 880 broadband seismic stations in China Seismic Network from January 2014 to December 2015, we calculated power spectral densities and probability density functions over the entire period for each station,and  investigated the characteristics of seismic noise in Chinese mainland. The deep analysis on the vertical recordings  indicates that the spatial distribution of noise levels is characterized by obvious zoning for different period bands.  Densely populated areas have higher short-period noise level than sparsely populated ones, suggesting that short-period noise is related to the intensity distribution of human activities such as transportation and industry. Meanwhile,the short-period noise level near the basin is higher than the mountainous areas,which is probably caused by the amplification effect of the sedimentary layer. The microseism energy  gradually decreases from the southeastern coastal lines to the inland regions. Furthermore, horizontal-component noise level  showed a striking constrast with the vertical component at microseismic and long-period bands. In consideration of  the zoning chracteristics and the need of seismic observations, high and low noise models were  acquired for each network , which were proved to be a more effective tool to identify locally abnormal signals including earthquake, instrumental error and various distrubance compared with the global new high and low model. 

How to cite: Wang, F., Wang, W., Long, J., and Mu, L.: Seismic noise characteristics in Chinese mainland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19017, https://doi.org/10.5194/egusphere-egu2020-19017, 2020.

EGU2020-22619 | Displays | SM1.3

Predicting ocean activity from seismic data using machine learning techniques
not presented

Susana Custódio, Francisco Bolrão, Tan Bui, Céline Hadziioannou, Miguel Lima, Diogo Rodrigues, Sheroze Sheriffdeen, Graça Silveira, and Joana Carvalho

The most pervasive seismic signal recorded on our planet – microseismic ambient noise -results from the coupling of energy between atmosphere, oceans and solid Earth. Because it carries information on ocean waves (source), the microseismic wavefield can be advantageously used to image ocean storms. Such imaging is of interest both to climate studies – by extending the record of oceanic activity back into the early instrumental seismic record – and to real-time monitoring – where real-time seismic data can potentially be used to complement the spatially dense but temporally sparse satellite meteorological data.

In our work, we develop empirical transfer functions between seismic observations and ocean activity observations. We start by following the classical approach of Bromirski et al (1999), who computed an empirical transfer function between ground-motion recorded at a coastal seismic station and significant wave height measured at a nearby ocean buoy. We explore further developments by considering other seismic data observations – such as the polarization of seismic ambient noise – and other indicators of ocean activity observations, including the spectra of ocean waves.

In addition to employing the classical approach of empirical transfer functions, we further present preliminary tests using machine learning techniques to: 1) infer which seismic and ocean activity observables are better predictors of each other, and 2) to predict ocean activity given observed ground motion.

The analysis is made using selected datasets around the North Atlantic, namely using seismic data from North America (west Atlantic), the Azores (central Atlantic) and Portugal (east Atlantic).

This work is supported by FCT through projects UIDB/50019/2020 – IDL and UTAP-EXPL/EAC/0056/2017 - STORM.

References:

Bromirski, P. D., Flick, R. E., & Graham, N. (1999). Ocean wave height determined from inland seismometer data: Implications for investigating wave climate changes in the NE Pacific. Journal of Geophysical Research: Oceans, 104(C9), 20753-20766.

 

How to cite: Custódio, S., Bolrão, F., Bui, T., Hadziioannou, C., Lima, M., Rodrigues, D., Sheriffdeen, S., Silveira, G., and Carvalho, J.: Predicting ocean activity from seismic data using machine learning techniques , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22619, https://doi.org/10.5194/egusphere-egu2020-22619, 2020.

EGU2020-13681 | Displays | SM1.3

Statistics on the Performance of Instrument Types and the Significance of HVSR data for Shallow Vs HVSR/DC Joint Inversions - A Result from the Large-N Maupasacq Experiment (Southern France)

Maik Neukirch, Antonio García-Jerez, Antonio Villaseñor, Laurent Stehly, Pierre Boué, Sébastien Chevrot, Matthieu Sylvander, Jordi Díaz, Mario Ruiz, Francisco Luzón, Magali Collin, Sylvain Calassou, Katerina Polychronopoulou, Nikos Martakis, and Adnand Bitri

Horizontal-to-Vertical Spectral Ratios (HVSR) and Rayleigh group velocity dispersion curves (DC) can be used to estimate the shallow S-wave velocity (Vs) structure. Knowing the shallow Vs structure is important for geophysical data interpretation either in order to better constrain data inversions for P-wave velocity (Vp) structures such as travel time tomography or full waveform inversions, or to directly study the Vs structure for geo-engineering purposes (e.g. ground motion prediction). The purpose of this study is to appraise in particular how much information HVSR can add in a large N experiment and how different instrumentation types affect this. 

During the Maupasacq large-scale experiment, 197 three-component short-period stations, 190 geophone nodes and 54 broadband seismometers were continuously operated in Southern France for 6 months (April to October 2017) covering an area of approximately 1500 km2 with a site spacing of approximately 1 to 3 km. On the obtained HVSR and DC data, a statistical Joint inversion is performed for the shallow Vs structure. The results indicate that the addition of HVSR data to the DC inversion reduces the variance of the recovered shallow Vs model and improves the convergence to a smaller data misfit. While broadband and short period instruments delivered similar results, geophone nodes performed significantly worse due to their much higher cut off frequency. 

How to cite: Neukirch, M., García-Jerez, A., Villaseñor, A., Stehly, L., Boué, P., Chevrot, S., Sylvander, M., Díaz, J., Ruiz, M., Luzón, F., Collin, M., Calassou, S., Polychronopoulou, K., Martakis, N., and Bitri, A.: Statistics on the Performance of Instrument Types and the Significance of HVSR data for Shallow Vs HVSR/DC Joint Inversions - A Result from the Large-N Maupasacq Experiment (Southern France), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13681, https://doi.org/10.5194/egusphere-egu2020-13681, 2020.

EGU2020-3999 | Displays | SM1.3

Stability of ambient noise H/V spectra obtained from OBS near the Japan Trench

Atikul Haque Farazi, Emmanuel Soliman M. Garcia, and Yoshihiro Ito

Ocean bottom seismometers (OBS) are widely in use since recent past to monitor seismicity of slow earthquakes as well as that of ordinary earthquakes. Seismic velocity structures, especially of S-wave are essential to estimate hypocenters of them with accuracy. Here we focus on spatial and temporal stability of ambient noise horizontal to vertical spectral ratio (H/V) spectra calculated from ocean bottom seismometers, as the first step toward future application of ambient noise H/V to estimate S-wave velocity structure. We aim to use the Nakamura’s method (1989) for ambient noise H/V spectra using a 3-component OBS array in the Japan Trench, to image deep structure above the plate interface near the trench. To achieve the imaging, it is necessary to examine spatial and temporal stability of the derived H/V spectra from these seismometers. First, we split each 24-hours record into 1-hour windows after removing the instrumental response, Then, Fourier amplitude spectra of each component is taken and smoothed using Konno and Ohmachi (1998) method, with applying downsampling, mean and trend removal, and tapering to each window. Finally, a 1-hour H/V spectral ratio is calculated with taking quadratic mean of two horizontal components. However, a total of 21 OBS, 3 broadband and 18 short-period, stations have been used in this study. A daily variation and stability of the H/V spectra are examined along with comparing them spatially from one station to another. Stability of the H/V spectra from OBS is promising for carrying out our future endevour of deeper observation using the ambient noise H/V method.

How to cite: Farazi, A. H., Garcia, E. S. M., and Ito, Y.: Stability of ambient noise H/V spectra obtained from OBS near the Japan Trench, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3999, https://doi.org/10.5194/egusphere-egu2020-3999, 2020.

SM2.1 – Earthquake Source Processes: Imaging and Numerical Modeling

EGU2020-759 | Displays | SM2.1

The 2017 November 12 Mw 7.3 Sarpol-Zahab (Iran-Iraq border region) earthquake: source model, aftershock sequence and earthquakes triggering

Mohammadreza Jamalreyhani, Mehdi Rezapour, Simone Cesca, Sebastian Heimann, Hannes Vasyura-Bathke, Henriette Sudhaus, Marius Paul Isken, and Torsten Dahm

The Mw 7.3 Sarpol-Zahab earthquake occurred on 12 November 2017 in the Lurestan arc of the Zagros Simply Folded Belt (ZSFB). It is estimated that 600 people were killed and 8000 were injured in this earthquake. This earthquake has been the largest instrumentally recorded earthquake in the ZSFB and its moment, as well as its mechanism, were unexpected. We present an earthquake source study on the Mw 7.3 Sarpol-Zahab earthquake, two large following earthquakes in the region in 2018 and their corresponding aftershock sequences to gain insight of seismotectonic of the Lurestan arc fold-thrust belt.

In this study, we complement previous studies on this earthquake, by non-linear probabilistic optimization of joined geodetic and seismic data using a new, efficient Bayesian bootstrap-based optimization scheme to infer the finite fault geometry and fault slip together with meaningful uncertainty estimates of the model parameters. Our optimization is based on the modeling of ascending and descending Sentinel-1 satellite data, seismological waveform from global seismic networks and the strong motion network of Iran. The posterior mean model of the Sarpol-Zahab earthquake shows that the causative fault plane is centered at is 14±2 km depth and has a low dip angle of 17°±2° and a strike of 350°±10°. The rake angle of 144°±4° points to an oblique thrust mechanism. The rupture area of the uniform-slip, rectangular model is 40±2 km long and 16±2 km width and shows 4.0±0.5 m fault slip, which results in a magnitude estimate of Mw 7.3±0.1.

Later, in August and November 2018, two large earthquakes with Mw 6.0 and Mw 6.4 occurred about 40 km east and 60 km south of the Sarpol-Zahab epicenter, respectively. These earthquakes could have been triggered by the 2017 Sarpol-Zahab earthquake. We apply the same joint inversion modeling to derive the corresponding fault plane solutions. We found strike-slip mechanisms for both events but centroid depths at 10±2 km and 16±2 km for Mw 6.0 and Mw 6.4, respectively.

The 2017 Sarpol-Zahab earthquake and the following studied 2018 earthquakes were followed by a sustained aftershock sequence, with more than 133 aftershocks exceeding Ml 4.0 until December 30, 2019. We rely on the local and regional seismic broad-band stations of Iran and Iraq permanent networks to estimate full-waveform moment tensor solutions of 70 aftershocks down to Ml 4. Most of these aftershocks have shallow centroid depths between 5 and 12 km, so that they occurred in the uppermost part of the basement and/or in the lower sedimentary cover, which is ~8 km thick in this area.

Our results suggest that the Sarpol-Zahab earthquakes activated low-angle thrust faults and shallower strike-slip structures, highlighting that both thin- and thick-skin deformation take place in the fold-thrust belts in the Lurestan arc of the Zagros. Such information on the deformation characteristics is important for the hazard and risk assessment of future large earthquakes in this region.
Additionally, we demonstrate how the joint inversion of different geophysical data can help to better resolve the fault geometry and the earthquake source parameters.

How to cite: Jamalreyhani, M., Rezapour, M., Cesca, S., Heimann, S., Vasyura-Bathke, H., Sudhaus, H., Paul Isken, M., and Dahm, T.: The 2017 November 12 Mw 7.3 Sarpol-Zahab (Iran-Iraq border region) earthquake: source model, aftershock sequence and earthquakes triggering, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-759, https://doi.org/10.5194/egusphere-egu2020-759, 2020.

EGU2020-5312 | Displays | SM2.1

Back-propagating super-shear rupture in the 2016 M7.1 Romanche transform fault earthquake

Stephen Hicks, Ryo Okuwaki, Andreas Steinberg, Catherine Rychert, Nicholas Harmon, Rachel Abercrombie, Petros Bogiaztis, David Schlaphorst, Jiri Zahradnik, J-Michael Kendall, Yugi Yagi, Kousuke Shimizu, and Henriette Sudhaus

Rupture propagation of an earthquake strongly influences potentially destructive ground shaking. Variable rupture behaviour is often caused by complex fault geometries, masking information on fundamental frictional properties. Geometrically smoother ocean transform fault (OTF) plate boundaries offer a favourable environment to study fault zone dynamics because strain is accommodated along a single, wide zone (up to 20 km width) offsetting homogeneous geology comprising altered mafic or ultramafic rocks. However, fault friction during OTF ruptures is unknown: no large (Mw>7.0) ruptures had been captured and imaged in detail. In 2016, we recorded an Mw 7.1 earthquake on the Romanche OTF in the equatorial Atlantic on nearby seafloor seismometers. We show that this rupture had two phases: (1) up and eastwards propagation towards the weaker ridge-transform intersection (RTI), then (2) unusually, back-propagation westwards at super-shear speed toward the fault’s centre. Deep slip into weak fault segments facilitated larger moment release on shallow locked zones, highlighting that even ruptures along a single distinct fault zone can be highly dynamic. The possibility of reversing ruptures is absent in rupture simulations and unaccounted for in hazard assessments.

How to cite: Hicks, S., Okuwaki, R., Steinberg, A., Rychert, C., Harmon, N., Abercrombie, R., Bogiaztis, P., Schlaphorst, D., Zahradnik, J., Kendall, J.-M., Yagi, Y., Shimizu, K., and Sudhaus, H.: Back-propagating super-shear rupture in the 2016 M7.1 Romanche transform fault earthquake, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5312, https://doi.org/10.5194/egusphere-egu2020-5312, 2020.

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Seismic source inversion using Hamiltonian Monte Carlo and a 3-D Earth model in the Japanese Islands

Saulė Simutė, Lion Krischer, Christian Boehm, Martin Vallée, and Andreas Fichtner

We present a proof-of-concept catalogue of full-waveform seismic source solutions for the Japanese Islands area. Our method is based on the Bayesian inference of source parameters and a tomographically derived heterogeneous Earth model, used to compute Green’s strain tensors. We infer the full moment tensor, location and centroid time of the seismic events in the study area.

To compute spatial derivatives of Green’s functions, we use a previously derived regional Earth model (Simutė et al., 2016). The model is radially anisotropic, visco-elastic, and fully heterogeneous. It was constructed using full waveforms in the period band of 15–80 s.

Green’s strains are computed numerically with the spectral-element solver SES3D (Gokhberg & Fichtner, 2016). We exploit reciprocity, and by treating seismic stations as virtual sources we compute and store the wavefield across the domain. This gives us a strain database for all potential source-receiver pairs. We store the wavefield for more than 50 F-net broadband stations (www.fnet.bosai.go.jp). By assuming an impulse response as the source time function, the displacements are then promptly obtained by linear combination of the pre-computed strains scaled by the moment tensor elements.

With a feasible number of model parameters and the fast forward problem we infer the unknowns in a Bayesian framework. The fully probabilistic approach allows us to obtain uncertainty information as well as inter-parameter trade-offs. The sampling is performed with a variant of the Hamiltonian Monte Carlo algorithm, which we developed previously (Fichtner and Simutė, 2017). We apply an L2 misfit on waveform data, and we work in the period band of 15–80 s.

We jointly infer three location parameters, timing and moment tensor components. We present two sets of source solutions: 1) full moment tensor solutions, where the trace is free to vary away from zero, and 2) moment tensor solutions with the isotropic part constrained to be zero. In particular, we study events with significant non-double-couple component. Preliminary results of ~Mw 5 shallow to intermediate depth events indicate that proper incorporation of 3-D Earth structure results in solutions becoming more double-couple like. We also find that improving the Global CMT solutions in terms of waveform fit requires a very good 3-D Earth model and is not trivial.

How to cite: Simutė, S., Krischer, L., Boehm, C., Vallée, M., and Fichtner, A.: Seismic source inversion using Hamiltonian Monte Carlo and a 3-D Earth model in the Japanese Islands, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9229, https://doi.org/10.5194/egusphere-egu2020-9229, 2020.

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A generalized slip-rate function for kinematic modeling

Sebastian von Specht, Kuo-Fong Ma, Yen-Yu Lin, and Fabrice Cotton

Over the last decades, many types of slip-rate functions (SRFs) have been introduced into kinematic rupture modeling. Commonly used SRFs are the Haskell-type rectangular pulse, cosine and trapezoidal windows and the Kostrov-/Yoffe functions. All these functions and many functional shapes inferred from multiwindow inversion techniques can be well described or are even identical to the functional form of the generalized beta distribution—a widely used and well studied probability density function (pdf) in statistics. The generalized beta pdf has three parameters, where one parameter relates to the SRF duration and two describe the shape of the pulse. The shape parameters have simple analytic expressions for their estimators. Using the generalized beta pdf with free shape parameters as SRF can effectively reduce the number of required free parameters in the inversion when compared to multiwindow SRF techniques. The generalized beta pdf provides us not only with analytic solutions of the derivative (slip-rate change) and antiderivative (slip) of the slip-rate function but also analytic expressions for their Fourier spectra. We apply the beta SRF for rupture modeling of two well studied earthquakes in Taiwan—the 2016 MW 6.4 Meinong earthquake and the MW 6.3 2018 Hualien earthquake—and compare results in terms of slip distribution and model uncertainties.

How to cite: von Specht, S., Ma, K.-F., Lin, Y.-Y., and Cotton, F.: A generalized slip-rate function for kinematic modeling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21170, https://doi.org/10.5194/egusphere-egu2020-21170, 2020.

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A new three-dimensional regularization for finite fault source inversions

Navid Kheirdast, Anooshiravan Ansari, and Susana Custódio

The earthquake rupture process is often represented by a source function that defines slip in space and time. If we assume slip to occur on a planar surface, then the source function becomes a function of three independent variables: two spatial dimensions (slip down-dip and slip along-strike) and one temporal dimension (source time function at each point on the fault). Finite fault inverse problems aim at exploring this model space in order to find the source function that generates synthetic ground motion that best fits the observed data. This inverse problem is severely ill-conditioned. In order both to ensure a regular solution and to avoid over-fitting the data, both physical and mathematical constraints can be imposed.  Common methods of finite fault source inversion typically apply a one-dimensional regularization in time, which gives preference to compacted-support source-time functions, like triangular or trapezoidal functions in time, or to two-dimensional regularizations that ensure smooth variations of slip over the fault plane (Mai et. al, 2016). In this work, we propose an innovative three-dimensional regularization for kinematic source inversions in the frequency domain, which simultaneously requires smooth variations of slip over space (2D) and frequency (1D, smooth spectra) . This new three-dimensional regularization selects the spatial slip distributions that are more similar to those of neighboring frequencies, thus effectively transferring knowledge from one frequency to another. In the framework of Tikhonov regularization, having more than one regularization condition requires more than one damping factor to be inserted in the inversion misfit. Additionally, no orthogonal decomposition (like Generalized Singular Value Decomposition) exists when more than one regularization conditions are imposed. Thus, we investigate a new 3D regularization method using a Bayesian approach with a Markov Chain Monte Carlo (MCMC) simulation. The new method is tested using the SIV-inv1 benchmark exercise. The proposed method is also preliminarily applied to study the rupture process of the 2019 M5.9 Torkamanchay, Iran, earthquake.

How to cite: Kheirdast, N., Ansari, A., and Custódio, S.: A new three-dimensional regularization for finite fault source inversions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-833, https://doi.org/10.5194/egusphere-egu2020-833, 2020.

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Simultaneous Bayesian Estimation of Complex Non-planar Earthquake Fault Geometry and Spatially-variable Slip from Geodetic Data

Rishabh Dutta, Sigurjón Jónsson, and Hannes Vasyura-Bathke

Earthquake fault ruptures are typically complex and can consist of en echelon segments, have bends, large step-overs, and be curved or warped at different spatial scales. Although surface fault ruptures can be mapped using a variety of geological and geophysical techniques, the subsurface topology of faults is challenging to estimate. One of the main options is to use geodetic data (InSAR and GPS) of coseismic surface deformation to estimate the subsurface earthquake fault geometry along with the distributed slip. The general practice is to assume a planar fault surface and estimate the strike and dip of a simple rectangular fault prior to the spatially-variable slip estimation. Using such simplistic fault geometry during source fault estimations of large earthquakes rarely captures all the crustal deformation details seen in the data and can cause biased estimation results of the fault slip. Here, we show how complex non-planar fault geometry can be estimated simultaneously with spatially-variable slip from geodetic data in a Bayesian framework, where our non-planar fault geometry parametrization approach allows for various undulations of the fault surface in both the along-strike and down-dip directions.

We exemplify this approach through synthetic tests considering a checkerboard-like slip pattern on a listric non-planar fault. The results show that fault slip can be over-estimated by about 50-100% when using pre-assumed planar fault geometry. In contrast, both the non-planar fault geometry and spatially-variable slip are better retrieved when using our estimation approach. We then apply this modeling approach to the 2011 MW9.1 megathrust Tohoku-Oki (Japan) earthquake. Here we use prior information like the location of the trench and earthquake hypocenters during the Bayesian estimation to reduce the extent of the model space. The resulting fault geometry shows variations in fault dip in both the along-strike and down-dip directions that compare well with Hayes’ slab1.0 model of the subduction interface. The estimated fault slip is also comparable to the results that pre-defined the fault geometry using the slab1.0 model. In the future, the proposed method could lead to more realistic source models of major earthquakes, aided by improving computational resources and spatial resolution of geodetic data.

How to cite: Dutta, R., Jónsson, S., and Vasyura-Bathke, H.: Simultaneous Bayesian Estimation of Complex Non-planar Earthquake Fault Geometry and Spatially-variable Slip from Geodetic Data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21277, https://doi.org/10.5194/egusphere-egu2020-21277, 2020.

EGU2020-12148 | Displays | SM2.1

Geodetic inversion of complex fault geometries for strong earthquakes

Yong Zhang, Yueyi Xu, and Rongjiang Wang

The fault geometry closely controls earthquake rupture process. Previous seismic inversion of the fault geometry is to derive the multiple-point moment tensor solutions. Because of the trade-off between the moment tensor and rupture velocity, the inversion has high instabilities. In contrast, geodetic inversion has less unknowns, since there is no need to solve for rupture velocity. But from the elastic dislocation theory, the relations between the surface deformation and sub-fault parameters (i.e. strike, dip and rake) are nonlinear. In this study, we develop a linear technique to invert geodetic data for sub-fault moment tensors. From the sub-fault moment tensor solutions, the strike, dip, rake, and their spatial variations can be estimated, which provide valuable information for assessing the complexities in fault geometry. We applied this technique to several significant earthquakes, i.e., the 2008 Mw7.9 Wenchuan earthquake, the 2015 Mw7.8 Gorkha earthquake, and the 2017 Mw6.5 Jiuzhaigou earthquake. The results of the 2008 Wenchuan earthquake suggest that the strike, dip and rake are all variable from southwest to northeast, which are well consistent with the aftershock distributions and mechanisms. The dip variations of the 2015 Gorkha earthquake suggest the earthquake has ruptured a listric fault (dep decreases with depth). Particularly, a dip anomaly appears in the northeast corner of the rupture area, indicating a geometric barrier accounting for the slip gap between the mainshock and largest Mw7.3 aftershock. For the 2017 Jiuzhaigou earthquake, two right-stepping and left-lateral strike-slip segments were distinguished. Accordingly, a compressional step-over was identified between the two segments.

How to cite: Zhang, Y., Xu, Y., and Wang, R.: Geodetic inversion of complex fault geometries for strong earthquakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12148, https://doi.org/10.5194/egusphere-egu2020-12148, 2020.

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Multi-Array Multi-Phase Back-Projection: Improving the imaging of earthquake rupture complexities

Felipe Vera, Frederik Tilmann, and Joachim Saul

We present a back-projection method capable of being parameterized with multiples arrays. The rupture imaging is weighted to restrict uncertainties induced by non-symmetric azimuthal coverage of seismic arrays. The strategy also exploits the differences in time delays between P and depth phase (pP) waveforms by assuming them as proxies of the rupture that can be simultaneously back-projected. Surprisingly, this helps to improve the final results, even when depth phases overlap with the direct arrivals due to the rupture time exceeding the pP-P delay. Thus, the approach heightens the spatiotemporal resolvability enough to image rupture complexities. The rupture image of two large events demonstrates its robustness. The first one is the 14 November 2007 Mw 7.7 Tocopilla earthquake in northern Chile. The high-frequency rupture (0.5 - 2.0 Hz) encircles two asperities while the short-period energy radiated predominates up-dip of the coseismic slip. We propose the contribution of asperity rupture complexities and along-dip barriers to high-frequency emissions beyond the megathrust frictional structure. The second one is the Mw 7.5 Palu strike-slip earthquake, which occurred on 28 September 2018 in Sulawesi island. The back-projection reveals a prominent supershear rupture at a speed of 4.5 km/s. The result correlates with space geodesy data highlighting the successful recovery of fault structures. Finally, we discuss the potential and challenges of automating this analysis for near-real-time applications, including near-source back-projection with strong-motion data.

How to cite: Vera, F., Tilmann, F., and Saul, J.: Multi-Array Multi-Phase Back-Projection: Improving the imaging of earthquake rupture complexities, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11233, https://doi.org/10.5194/egusphere-egu2020-11233, 2020.

Back-projection uses the time-reversal property of the seismic wavefield recorded at large aperture dense seismic arrays. Seismic energy radiation is imaged by applying array beam-forming techniques. The spatio-temporal rupture complexity of large earthquakes can be imaged simply and rapidly with a limited number of assumptions, which makes back-projection techniques an important tool of modern seismology. However, back-projection analyses exhibit frequency and array dependency (e.g. Wu et al., AGU19). In addition, the method relies on station network geometry and data quality and can suffer from imaging artifacts (e.g., Fan and Shearer, 2017) and back-projection results may not be consistently interpreted.

The Mw7.5 Palu, Sulawesi earthquake that occurred on September 28, 2018, ruptured a 180 km long section of the Palu-Koro fault. The earthquake triggered a localized but powerful tsunami within Palu Bay, which swept away houses and buildings. The supershear earthquake and unexpected tsunami led to more than 4000 fatalities. Ulrich et al. (2019) propose a physics-based, coupled earthquake-tsunami scenario of the event, tightly constrained by observations. The model matches key observed earthquake characteristics, including moment magnitude, rupture duration, fault plane solution, teleseismic waveforms, and inferred horizontal ground displacements. It suggests that time-dependent earthquake-induced uplift and subsidence could have sourced the observed tsunami within Palu Bay.

Back-projection has been used to track the rupture propagation of the Palu earthquake. Bao et al. (2019) image unilateral rupture traveling at a supershear rupture speed. Their results show array dependent ruptures, from a rather relatively linear rupture using the Australian array, to a spatio-temporally more scattered image using the seismic array in Turkey. In addition, they do not resolve any portion of the rupture as traveling at sub-Rayleigh speeds, while Wei et al. (AGU19) suggest a gradually accelerating rupture.

In this study, we build upon the dynamic rupture model of Ulrich et al. (2019) to investigate the reliability of standard back-projection techniques using a realistic and perfectly known earthquake model. In particular, we investigate whether or not rupture transfers across the segmented fault system, and the effect of specific geometric features of the fault system, such as fault bends, on rupture dynamics, leave a clear signal on the inferred beam power. Also, we investigate the effect of secondary phases, such as reflections from the free-surface or from fault segment boundaries, naturally captured by dynamic rupture modeling. In addition, we study the effect of small-scale source heterogeneities on the back-projection results by including different levels of fault roughness in the dynamic rupture simulations. Finally, we investigate the array dependence of back-projection results.

Overall, this study should help to better understand which features of rupture dynamics back-projection can capture. Our results are a first step towards fundamental analysis to better understand which features can be captured by back-projection and to provide guidelines for back-projection interpretation.

How to cite: Ulrich, T., Li, B., and Gabriel, A.-A.: Synthetic analysis of seismic back-projection using 3D dynamic rupture simulations of the 2018, Palu Sulawesi earthquake , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20703, https://doi.org/10.5194/egusphere-egu2020-20703, 2020.

EGU2020-10834 | Displays | SM2.1

Elastic Impact Consequences for High-Frequency Earthquake Ground Motions

Victor Tsai and Greg Hirth

A fundamental question of earthquake science is what produces damaging high-frequency ground motion, with the classic Brune-Haskell model postulating that abrupt initiation of fault slip causes it. However, even when amended with heterogeneous rupture, frictional slip models fail to explain observations of different sized repeating earthquakes, and have challenges explaining high-frequency radiation patterns as well as the dependence of stress drops on fault maturity and depth. We propose an additional cause for high-frequency earthquake spectra from elastic collisions of structures within a rupturing fault zone. The collision spectrum is set by an impact contact time that is proportional to the size of colliding structures, so that spectra depend on fundamentally different physical parameters compared with slip models. When added to standard frictional models, the collision model can reconcile the discrepant observations, since the size, shape and orientation of structures vary between different fault zones but remain constant within a given fault segment. High-frequency earthquake ground motions and damage may therefore be an outgrowth of fault-zone structure rather than sudden initiation of slip.

How to cite: Tsai, V. and Hirth, G.: Elastic Impact Consequences for High-Frequency Earthquake Ground Motions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10834, https://doi.org/10.5194/egusphere-egu2020-10834, 2020.

EGU2020-13327 | Displays | SM2.1

Dynamics of foreshocks and pre-slip during the nucleation of laboratory earthquakes

Alexandre Schubnel, Samson Marty, Blandine Gardonio, Harsha Bhat, Eiichi Fukuyama, and Raùl Madariaga

Over the past decades, an increasing number of seismological observations and improvement in data quality have allowed to better detect foreshock sequences prior to earthquakes. However, due to strong spatial and temporal variations of foreshock occurrence, their underlying physical processes and their links to earthquake nucleation are still under debate. Here we address these issues by looking at precursory acoustic activity during laboratory earthquakes (stick-slip instabilities).

Here, laboratory earthquake experiments were performed on saw-cut Indian metagabbro under upper crustal stress conditions ranging from 30 to 60 MPa confining pressure. Using a high-frequency monitoring system and calibrated piezoelectric acoustic sensors we continuously record particle velocity field at 10 MHz sampling rate during the experiments. Based on a trigger logic we identify acoustic emissions (AE) within continuous data. From P-wave arrival-time data and from spectral analysis we are able to estimate the following seismological parameters for each AE: location, absolute magnitude, stress-drop and size.

First, we show that the source parameters of AE (Mw -9.0 to Mw -7.0) follow the same scaling relationship as natural earthquakes justifying the use of acoustic precursors as proxy to foreshocks. We observe that foreshock triggering is systematically related to aseismic slip and that the dynamics of foreshocks mirrors the acceleration of slip-rate preceding failure. Experimental scalings demonstrate that : i- the nucleation evolves  from an aseismic process into a cascading one, and ii) the duration and magnitude of the pre-seismic moment correlates with the magnitude of the mainshock, at least at the scale of the laboratory. Finally, using Hertz contact theory, we find a scaling law between the seismic energy released by foreshocks, the fault roughness  and the normal stress acting on the fault interface.

How to cite: Schubnel, A., Marty, S., Gardonio, B., Bhat, H., Fukuyama, E., and Madariaga, R.: Dynamics of foreshocks and pre-slip during the nucleation of laboratory earthquakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13327, https://doi.org/10.5194/egusphere-egu2020-13327, 2020.

EGU2020-10142 | Displays | SM2.1

Kinematic constraining of the multi-fault rupture dynamics of the Norcia, Mw 6.5, 30 October 2016, Central Italy earthquake

Elisa Tinti, Emanuele Casarotti, Alice-Agnes Gabriel, Taufiqurrahman Taufiqurrahman, Thomas Ulrich, and Duo Li

The 2016 Central Italy sequence showed a remarkable complexity involving multiple faults. Highly heterogeneous slip distributions were inferred from kinematic finite source inversions. The coverage and quality of the geodetic and seismic data allow resolving high-resolution details of rupture kinematics of the largest event of the sequence, the Mw 6.5 30 October 2016 Norcia earthquake. Composite fault rupture models suggest that two fault planes may have slipped simultaneously. Nevertheless, kinematic modeling cannot assess the mechanic viability of such multiple fault plane models.

Using SeisSol, a software package for simulating wave propagation and dynamic rupture based on the arbitrary high-order accurate derivative discontinuous Galerkin method, we therefore try to generate spontaneous dynamic ruptures models compatible with the two fault planes constrained by kinematic inversions. To this end, we adopt a simple slip-weakening friction law with spatially variable dynamic friction and initial strength parameters along multiple faults, compatible with the slip distributions found in the literature. Although we do not to aim to explore the full parameter space, our approach allows testing the feasibility of kinematic models in conjunction with successfully generating spontaneous dynamic rupture scenarios matching seismic and geodetic observations with geological constraints. Such linking enhances and validates the physical implications of kinematic earthquake source inversion.

How to cite: Tinti, E., Casarotti, E., Gabriel, A.-A., Taufiqurrahman, T., Ulrich, T., and Li, D.: Kinematic constraining of the multi-fault rupture dynamics of the Norcia, Mw 6.5, 30 October 2016, Central Italy earthquake, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10142, https://doi.org/10.5194/egusphere-egu2020-10142, 2020.

The September 20, 1999 (UTC) Mw7.6 Chi-Chi earthquake in Taiwan was a devastating event of historic proportions. Although this event caused severe damage, it also provided a large data set of high-quality near-field strong motion acceleration records from the Taiwan Strong Motion Instrumentation Program. Despite ongoing advances in kinematic modeling in the last two decades, some questions remain unresolved.  One of those questions is the seismic energy partition in radiated energy and fracture energy. We address this question by investigating the dynamic rupture behavior of this event. We constructed a 3D dynamic rupture model which is constrained by the well resolved spatiotemporal slip distribution and in-situ stress measurements from fault-zone drilling. In our model, we consider the fault ruptures with both spatially uniform and non-uniform frictional behavior and perform a series of numerical experiments with different sets of input variables (e.g., slip-weakening distance, dc, and initial stress on fault plane) based on a slip-weakening friction law. We examined the parameters controlling the slip patterns as the result from kinematic modeling. For the constraints of the input variables, we first derived the constitutive relationship between slip and stress change on the subfaults from the temporal and spatial slip distribution of the kinematic models by Ji et al. (2003), and then determined the dynamic parameters (e.g., apparent slip-weakening distance, dc', and the ratio of strength excess and stress drop,  S). Our initial normal stress on the fault plane is based on the geophysical logging analysis of the Taiwan Chelungpu-fault Drilling Project. Our optimal model can simulate a rupture similar to the kinematic model  by Ji et al. (2003) and suggests that the final slip distribution is mainly controlled by the spatial distribution of the normal stress. We require a downscaling (α) of the apparent slip-weakening distance, αdc', from the derived constitutive law of the kinematic model to allow a dynamic rupture propagation with large slip velocity comparable to the observations. With the downscaled slip-weakening distance, αdc', and a heterogeneous stress distribution, the slip-weakening curves from our optimal model suggests the downscaling in radiated seismic energy and fracture energy accordingly .

How to cite: Liao, J., Ma, K.-F., Specht, S., and Oglesby, D.: Dynamic modeling of the 1999 Chi-Chi (Mw7.6) earthquake: New insights on energy partition in large earthquakes by incorporating in-situ stress measurements into the constitutive relationship from kinematic modeling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12423, https://doi.org/10.5194/egusphere-egu2020-12423, 2020.

EGU2020-9557 | Displays | SM2.1

Observations and modeling of the rupture development based on the analysis of Source Time Functions

Julien Renou, Martin Vallée, and Hideo Aochi

Our knowledge of earthquake source physics, giving rise to events of very different magnitudes, requires observations of a large population of earthquakes. The development of systematic analysis tools for the global seismicity meets these expectations, and allows us to extract the generic properties of earthquakes, which can then be integrated into models of the rupture process. Following this approach, the SCARDEC method is able to retrieve source time functions of events over a large range of magnitude (Mw > 5.7). The source time function (which describes the temporal evolution of the moment rate) is suitable for the analysis of transient rupture properties which provide insights into the generation of earthquakes of various sizes. Our study aims at observing the rupture development of such earthquakes in order to add better constraints on dynamic source models. We first focus on the development of earthquakes through the analysis of the SCARDEC catalog. The phase leading to the peak of the source time function ("development phase'') is extracted to characterize its evolution. From the computation of moment accelerations at prescribed moment rates, we observe that the evolution of the moment rate during the developement phase is independent of the final magnitude. A quantitative analysis of the moment rate increase as a function of time further indicates that this phase does not respect the steady t² self-similar growth. These observations are then compared with dynamic source models. We develop heterogeneous dynamic models which take into consideration rupture physics. Heterogeneous distributions of the friction parameter and the initial stress contribute to generate highly realistic rupture scenarios. Rupture propagation is strongly influenced by these two dynamic parameters which induce a clear preferential direction of propagation together with a local variability of the rupture velocity. Variability of the kinematic parameters also tends to correlate rupture velocity and slip velocity, which is a key feature for the transient behavior of the development phase previously observed. These findings are expected to put further constraints on future realistic dynamic rupture scenarios.

How to cite: Renou, J., Vallée, M., and Aochi, H.: Observations and modeling of the rupture development based on the analysis of Source Time Functions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9557, https://doi.org/10.5194/egusphere-egu2020-9557, 2020.

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Simultaneous optimisation of two sources of the 2012 Ahar earthquake doublet (Mw 6.4 and 6.3, Iran) based on InSAR data, GNSS data and seismic waveforms

Jan Ridderbusch, Henriette Sudhaus, Andreas Steinberg, Stefanie Donner, and Abdolreza Ghods

On August in 2012 a Mw6.4 earthquake hit the region near the town Ahar in NW Iran. With only 11 minutes delay it was followed by another large and close by Mw6.3 earthquake. The 2012 Ahar earthquakes have been unexpected in their large magnitudes and the activated faults are poorly studied. A mapped east-west striking surface rupture is attributed to the first earthquake, which shows a strike-slip mechanism. The second earthquake is reported to have a thrust mechanism and a deeper hypocenter, but is much more poorly constrained than the first earthquake. The short time interval between those two earthquakes made it impossible to distinguish their effects in the available static surface displacement data based on InSAR and GNSS, and difficult in global seismological records. Any source analysis using static displacement data and/or teleseismic waveforms therefore has to rely on the corresponding cumulative surface displacements and recorded waveforms of the first earthquake, respectively. In contrast, in regional waveform data, the seismic phase arrivals of both earthquakes are well separated in time. To tackle the coupling of the earthquakes we conducted a combined-data study that solves for the individual sources of the earthquake doublet simultaneously in a non-linear probabilistic finite-fault optimisation. In our combined-data study we improve the constraints on the doublet sources, particularly the second earthquake. We use InSAR data from RADARSAT-2 acquisitions and published co-seismic displacement vectors based on GNSS data. For the InSAR data, unfortunately, only measurements of an ascending orbit are available. The seismological data are teleseismic (distance larger than 1000 km) and regional waveform recordings (distances less than 1000 km). For the modelling we use Green’s functions of a layered regional velocity model and rectangular, constant-slip rupture models.

Our non-linear, finite-fault optimisation makes use of Bayesian bootstrap data weighting, which enables a very efficient estimation of model parameter uncertainties. This method accounts for modelling and data errors and can sample non-Gaussian posterior probabilities of model parameters. Our results show that the two earthquakes activated two different faults. The first earthquake ruptured a shallow east-west striking dextral fault extending from the surface vertically down to approximately 8 km depth (6 to 14 km confidence). The second earthquake ruptured a north to north-east striking fault with a dip of about 40 degree with an oblique rupture mechanism. The fault activated by the second earthquake seems to be located below the first one, at levels deeper than 9 km and a bit shifted to the west.We verify our results with model-independent seismic multi-array backprojection of the radiated seismic energy.

We used the python-based software toolbox Pyrocko for the data processing. The included module Grond implements the Bayesian bootstrap optimisation approach. Both are open-source under the GNU General Public License and available on pyrocko.org. The RADARSAT data used in this study have been provided through the RADARSAT-2 SOAR-EU loan agreement #16736. This research is further supported by the German Research Foundation DFG through an Emmy-Noether-Grant.

How to cite: Ridderbusch, J., Sudhaus, H., Steinberg, A., Donner, S., and Ghods, A.: Simultaneous optimisation of two sources of the 2012 Ahar earthquake doublet (Mw 6.4 and 6.3, Iran) based on InSAR data, GNSS data and seismic waveforms, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10432, https://doi.org/10.5194/egusphere-egu2020-10432, 2020.

EGU2020-18938 | Displays | SM2.1

Source Investigation of the 2018 Lombok (Indonesia) Earthquake Sequences

Dimas Salomo Januarianto Sianipar, Bor-Shouh Huang, Kuo-Fong Ma, Tio Azhar Prakoso Setiadi, Ming-Che Hsieh, Haekal Azief Haridhi, and Daryono Daryono

The western extension and deformation mechanism of Flores back-arc thrust in eastern Sunda-Banda Arc (Indonesia) are poorly investigated, and, thus, poorly constrained. From late July to August 2018, a sequence of large earthquakes (M6.4+) took place in the north of Lombok Island that marked the previously westernmost termination of the continuous zone of the back-arc thrusting. The 2018 Lombok earthquake sequences that began with Mw 6.4 (28 July 2018), and followed by Mw 6.9 (5 August 2018), and Mw 6.9 (19 August 2019) with massive subsequent aftershocks claimed on more than 500 casualties, nearly 500,000 people displaced and serious damages on Lombok Island. Here we relocate the aftershocks and perform the finite fault inversions of M6.4+ earthquake sequences constrained with teleseismic body and surface waves. Both refined hypocenters of aftershocks and rupture processes of large earthquakes provide detail kinematic descriptions of the source mechanisms of the sequences. The aftershocks distributions and slip model suggest that the earthquakes occurred on south-dipping low angle thrust faulting that striking to the east while it also activated aftershocks on surrounding complex shallow faulting with distinguishing distributions. The source inversions of large earthquakes over the entire of the western part of Flores back-arc thrust resulted as simple circular rupture propagations initiated from ~15 km depth for all events except the westernmost events (Mw 6.9 on 5 August 2019) that had a more complex rupture and initiated from shallower depth, and with slip distributed cross over the former identified westernmost termination of the Flores back-arc thrust. Our study suggests the further extension of back-arc thrusting and the possible structures revealed from the subsequence aftershocks. The source characterizations revealed in this study would be important for further seismic hazard analysis in this region.

How to cite: Sianipar, D. S. J., Huang, B.-S., Ma, K.-F., Setiadi, T. A. P., Hsieh, M.-C., Haridhi, H. A., and Daryono, D.: Source Investigation of the 2018 Lombok (Indonesia) Earthquake Sequences, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18938, https://doi.org/10.5194/egusphere-egu2020-18938, 2020.

EGU2020-12106 | Displays | SM2.1

Slip models of the 2016 Mw7.0 Kumamoto, Japan mainshock and its two foreshocks constrained by multi-mode InSAR data

Yongzhe Wang, Wanpeng Feng, Kun Chen, and Hailin Du

Interferometric Synthetic Aperture Radar (InSAR) data is of high spatial resolution and has been widely used in measuring surface deformation generated by earthquakes. However, the temporal resolution of InSAR data is relatively poor from an individual mode SAR sensor. A series of earthquakes hit Kumamoto, Japan in April 2016. These earthquakes were considered to be a sequence that started from two foreshocks (TFS) larger than Mw 6.0 and reached its climax for the largest earthquake of Mw7.0 only after 28 hours. To better reveal the source model characteristics of the TFS and the main shock, we firstly determined the geometrical parameters using the aftershock re-location data and the surface fault rupture data of field survey, and then applied multi-mode InSAR data only covering the TFS and the whole sequence, respectively, to carry out the joint inversion of respective source models based on the time correlation between the TFS and the main shock. The results show that both the source models determined here are well consistent with previous results constrained by seismological data. The strategy of inversion used in this study suggests that we may separate multiple seismic sources sequence from geodetic observations using the joint inversion based on the time correlation.

How to cite: Wang, Y., Feng, W., Chen, K., and Du, H.: Slip models of the 2016 Mw7.0 Kumamoto, Japan mainshock and its two foreshocks constrained by multi-mode InSAR data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12106, https://doi.org/10.5194/egusphere-egu2020-12106, 2020.

  Taiwan region is a seismically active region formed by the oblique convergence between Philippine Sea Plate and Eurasia Plate. Focal mechanisms of most small-moderate sized earthquakes can be well constrained by the local seismic array, except for those occurred offshore Taiwan where azimuthal coverage is limited. To better understand the tectonic structures, it is desirable to improve the focal mechanisms using better located hypocenters, reasonable velocity models, and the best available stations. In this study we focus on the shallow earthquakes in Taiwan Strait and the intermediate-depth earthquakes in southernmost Ryukyu. Both regions are less explored but large historic events had been reported.

  For earthquakes in Taiwan Strait, we systematically studied earthquakes from 1996 to 2019, including the Mw5.7 Taiwan Shoal sequence happened on 2018/11/25. A total of 22 new moment tensors (MTs) were resolved in the passive margin by combining Fujian and Taiwan seismic networks from either side of the strait. For events closer to Fujian, China, the velocity model with Moho depth of 35 km yields overall lower compensated linear vector dipole (CLVD) and acceptable misfit values; while as a 40 km thick crust is better for events closer to or on the shore of Taiwan. This Moho variation under the Taiwan Strait, although subtle, agrees well with the velocity structure constrained independently by previous studies. Earthquakes in the middle of the strait are dominant in strike-slip and normal slip within 30 km depth. Shallow thrusting events are found only in the Miaoli offshore area of Taiwan. As for the 2018 Taiwan Shoal earthquake sequence, it is located right on the region absence of known fault-plane solutions, therefore offers important new constraints. All events of the sequence show high angle strike-slips and shallow centroid depth of 11-21 km, more consistent with seismicity determined by Fujian seismic center. This event is far away from the M8 1604 Quanzhou earthquake, and is also clearly unrelated to the structure of 1994 Mw 6.7 normal-faulting event in Tainan Basin. The 2018 sequence is probably the reactivation of a pre-existing normal fault that was created by rifting during the Cenozoic.

  For future work, we will re-evaluate the MTs of M>5.5 intermediate-depth earthquakes of the Ryukyu subduction zone by including waveforms of stations YNG and IGK from Japan network in the inversion. We will also test different upper mantle velocities in the model for the computation of Green’s functions. We anticipate that our work can provide a set of parameters more suitable for the MT inversion, and the MT results can delineate the Ryukyu subduction zone properties better.

 

keywords : Taiwan Strait, focal mechanisms, moment tensor inversion

How to cite: Lee, C.-C., Tseng, T.-L., and Jian, P.-R.: Improving Focal Mechanisms for Earthquakes in Taiwan Strait and Ryukyu Subduction Zone with Broadband Waveforms of Combined Networks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6855, https://doi.org/10.5194/egusphere-egu2020-6855, 2020.

Spectra analysis is helpful to understand earthquake rupture processes and estimate source parameters like stress drop. Obtaining real source spectra and source time function isn’t easy, because the station recordings contain path effect and we usually can’t get precise path information. Empirical Green’s function (EGF) method is a popular way to cancel out the path effect, main two of which are the stacking spectra method (Prieto et al, 2006) and the spectral ratio method (Viegas et al, 2010; Imanishi et al, 2006). In our study, we apply the latter with multitaper spectral analysis method (Prieto et al, 2009) to calculate relative source spectra and relative source time function. Target event and EGFs must have similar focal mechanism and be collocated, so we combine correlation coefficient of wave at all stations and focal mechanism similarity to select proper EGFs.

The Bucaramanga nest has very high seismicity, so it’s suitable to calculate source spectra by using EGF method. We calculate the source spectra and source time function of about 1540 earthquakes (3-5.7ml, 135-160km depth) at Bucaramanga nest in Colombia. Simultaneously we also estimate corner frequency by fitting spectral source model (Brune, 1970; Boatwright, 1980) and stress drop using simple model (Eshelby, 1957) of earthquakes with multiple station recordings or EGFs. We obtain about 30000 events data with 12 stations from National Seismological Network of Colombia (RSNC).

The result show that the source spectra of most earthquakes fitted well by omega-square model are smooth, and the source spectra of some have obvious ‘holes’ near corner frequency, and the source time function of a few earthquakes appear two separate peeks. The first kind of earthquakes are style of self-arresting ruptures (Xu et al. 2015), which can be autonomously arrested by itself without any outside interference. Abercrombie (2014) and Wen et al. (2018) both researched the second kind of earthquakes and Wen think that this kind of earthquakes are style of the runaway ruptures including subshear and supershear ruptures. The last kind of earthquakes maybe be caused by simultaneous slip on two close rupture zone. Stress drop appear to slightly increase with depth and are very high (assuming rupture velocity/s wave velocity is 0.9). We also investigate the high-frequency falloff n, usually 2, of Brune model and Boatwright model by fitting all spectra, and find that the best value of n for Boatwright model is 2 and for Brune model is 3.5.

How to cite: Gong, W. and Chen, X.: The characteristic of source spectra and stress drop of earthquakes in the Bucaramanga nest, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3601, https://doi.org/10.5194/egusphere-egu2020-3601, 2020.

EGU2020-5019 | Displays | SM2.1

Stress Drop Mapping in the Northern Chilean Subduction Zone

Jonas Folesky, Joern Kummerow, and Serge A. Shapiro

The Northern Chilean subduction zone has been monitored by the IPOC network for more than ten years. During this time period two very large earthquakes occurred, the 2007 MW7.7  Tocopilla earthquake and the 2014 MW8.1 Iquique earthquake. Over the entire subduction zone a vast amount of seismic activity has been recorded and a huge catalog was compiled including over 100000 events (Sippl et al. 2018). With this exceptional data base we attempt a systematic analysis of the stress drops of as many events from the catalog as possible. We apply different estimation techniques, namely the spectral ratio type, the spectral stacking approach, and the lower bound method. A goal of our research is a comparison and possibly a combination of the techniques to obtain reliable and well constrained results.

The data set covers events at the interface, within the subducting plate, crustal events, and intermediate depth events. It therefore bears a great potential to better understand the stress drop distribution within a subduction zone. Also, the long observation interval allows to analyze temporal variations according to pre-, inter-, and post-seismic phases of megathrust earthquakes.   

We present preliminary results where a subset of 730 events with a magnitude range of ML2.7 - ML4.8  was used for analysis with the spectral ratio technique. For these events we show maps of spatial stress drop variation, and we analyze the time dependent stress drop variance. 

How to cite: Folesky, J., Kummerow, J., and Shapiro, S. A.: Stress Drop Mapping in the Northern Chilean Subduction Zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5019, https://doi.org/10.5194/egusphere-egu2020-5019, 2020.

EGU2020-17019 | Displays | SM2.1

Analysis of Source Parameters relationships for clusters of similar events recorded in Central Apennines

Daniele Spallarossa, Paola Morasca, Dino Bindi, Matteo Picozzi, and Kevin Mayeda

Aim of this study is to investigate the relationship between moment magnitude (Mw) and source duration (i.e. corner frequency) for moderate to small magnitude earthquakes recorded in Central Apennines, Italy, including the 2016-2017 Amatrice-Norcia-Visso sequence. A data-set of ~ 6000 events in the magnitude range ~1 and  6.5 was used to retrieve a reference data set of source parameters by applying spectral decomposition approach (Generalized Inversion Techniques). The large population of analyzed earthquakes allowed us to investigate the scaling of the source parameters with the earthquake size, their variability with hypocentral depth and to characterize the scaling between local and moment magnitudes in the magnitude range from 1 to 6.5 (Deichmann 2017). Analyzing the same data-set and taking advantage of the available high quality data for small events recorded in the area, we focus on the scaling properties of clustered events in the magnitude range between ~1 and  3.5. By applying different methodologies, relying on cross-correlation analysis, we detect a preliminary set of clusters. Then, events within 2 km from the geographic location of each cluster were extracted from a very large (more than 500000 events) high-resolution earthquake parametric catalog. New cross-correlation analyses were carried out on stations within 50 km from the centroid of each previously identified clusters to pad each ones with low magnitude events (below 2). This multi-steps procedure allowed us to identified 2933 events belonging to 45 clusters. For an in-deep analysis of source properties, we focus on three clusters selected on the basis of the number of events and different hypocentral depth distributions. For each cluster, the P-waves pulse duration (equivalent to corner frequency) of the events were compared each other on different stations. Results clearly show that below Ml ~ 2 the pulses duration remains nearly constant also for stations with low kappa values, showing a saturation effects. For a comparison with the GIT and cross-correlation results we also evaluate source parameters using a method based on coda-envelope amplitude measurements (Mayeda et al. 2003) applying site and path parameters previously calibrated for Central Apennines by Morasca et al. 2019. This comparison from independent and completely different methodologies applied on the same clusters well agrees with the saturation observed in pulse duration, strengthen the results and allowed us to define, for the given network geometry and earthquake distribution, the magnitude threshold below which we believe it is not possible to estimate source parameters. Moreover, our analysis of two clusters co-located on map but with different depth highlights a variation in stress drop with depth;

How to cite: Spallarossa, D., Morasca, P., Bindi, D., Picozzi, M., and Mayeda, K.: Analysis of Source Parameters relationships for clusters of similar events recorded in Central Apennines, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17019, https://doi.org/10.5194/egusphere-egu2020-17019, 2020.

EGU2020-4915 | Displays | SM2.1

Source Spectral studies using Lg wave in western Tibet

Sukanta Sarkar, Namrata Jaiswal, Chandrani Singh, Arun Kumar Dubey, and Arun Singh

The tectonic structure of western Tibet is complex and formed of several blocks, which are separated by distinct suture zones. This complexity makes the region very crucial for understanding the local tectonic settings. Here, we investigate the spectral characteristics of Lg wave from 420 waveforms recorded at 26 seismic stations located across Karakoram Fault (KKF) in western Tibet. We subdivide the study region into two parts across KKF. A frequency  dependent QLg is observed in both sides of KKF with strong attenuation in the crust. The moment magnitude of each earthquake is computed using displacement spectra and subsequently compared with the reported local magnitude. Variations of the corner frequency with magnitude and distance
are also studied, which show a decreasing nature due to the path dependency.

How to cite: Sarkar, S., Jaiswal, N., Singh, C., Dubey, A. K., and Singh, A.: Source Spectral studies using Lg wave in western Tibet, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4915, https://doi.org/10.5194/egusphere-egu2020-4915, 2020.

EGU2020-7075 | Displays | SM2.1

Multi-band imaging of seismic source rupture process

Hailin Du, Xu Zhang, and Yongzhe Wang

The standard method to image the source rupture process of a large earthquake is finite fault inversion, which uses the low-frequency signal to invert the slip distribution of the fault. However, in different stages of the source rupture process of a large earthquake, the seismic waves radiated by the source have different dominant frequencies, such as high frequency seismic waves excited by the rupture front. If we can analyze seismic waves in different frequency bands, it is expected to obtain a more detailed source rupture process of large earthquakes. Therefore, we respectively adopted the high frequency signal back-projection imaging method and the low frequency signal finite fault inversion method, and took the 2016 Kaikoura MW7.8 earthquake as an example to obtain the history of rupture propagation and fault slip distribution.The calculated results show that the high-frequency energy radiation of the earthquake can be divided into three stages, and the low-frequency energy radiation can be divided into two stages. The energy release process in different frequency bands is complementary in time and space. The rupture process of the whole source can be explained by the asperity model and the barrier model.

How to cite: Du, H., Zhang, X., and Wang, Y.: Multi-band imaging of seismic source rupture process, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7075, https://doi.org/10.5194/egusphere-egu2020-7075, 2020.

EGU2020-13283 | Displays | SM2.1

Imaging the rupture process of recent earthquakes using backprojection of local high frequency records

Ioannis Fountoulakis, Christos Evangelidis, and Olga-Joan Ktenidou

The seismic source spatio-temporal rupture processes of events in Japan, Greece and Turkey are imaged by backprojection of strong-motion waveforms. Normalized high-frequency (> 2Hz) S-waveforms from recordings on dense strong-motion networks are used to scan a predefined 3D source volume over time. 

Backprojection is an alternative novel approach to image the spatio-temporal earthquake rupture. The method was first applied for large earthquakes at teleseismic distances, but is nowadays also used at local distances and over higher frequencies. The greatest advantage of the method is that processing is done without any a-priori constraints on the geometry, or size of the source. Thus, the spatio-temporal imaging of the rupture is feasible at higher frequencies (> 1Hz) than conventional source inversion studies, even when the examined fault geometry is complex. This high-frequency energy emitted during an earthquake is of great importance in seismic hazard assessment for certain critical infrastructures. The actual challenge in using high-frequency local recordings is to distinguish the local site effects from the true earthquake source content - otherwise, mapping the former incorrectly onto the latter limits the resolvability of the method. It is not straightforward to remove the site effect component or even to distinguish good reference stations from amid hard-soil and rock sites. In this study, the advantages and limitations of the method are explored using waveform data from well-recorded events in Japan (Kumamoto Mw7.1, 2016), Turkey (Marmara Mw6.4, 2019) and Greece (Antikythera Mw6.1, 2019). For each event and seismic array the resolution limits of the applied method are explored by performing various synthetic tests.

How to cite: Fountoulakis, I., Evangelidis, C., and Ktenidou, O.-J.: Imaging the rupture process of recent earthquakes using backprojection of local high frequency records, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13283, https://doi.org/10.5194/egusphere-egu2020-13283, 2020.

EGU2020-22441 | Displays | SM2.1

Rapid Detection of Earthquake Rupture Directivity Using Strong Ground Motion Data in Taiwan

Cheng-Feng Wu, Ting-Li Lin, and Ying-Chi Chen

In the past decade, there have been several disaster earthquakes occurred in Taiwan.
From the observed data of the disaster earthquakes, the stations located in the source
rupture direction have obvious directivity pulses, and the distribution of the earthquake
disaster is related to the peak ground velocity. Therefore, how to use a large and high-
dense seismic database to develop a near-real-time detection system on the earthquake
rupture directivity, which is a very important task in Taiwan. In this study, we determine
the earthquake rupture directivity using near-field velocity data from 1991 to 2018, which
were collected under the Taiwan Strong Motion Instrument Program (TSMIP). The used
method is mainly constructed in the interpolation of the peak-ground-velocity map and
the directional attenuation regression analysis. Through the analysis of moderate-to-large
magnitude (M L > 5.5) seismic events, the source rupture directivity can be detected
effectively and quickly by the applied method. The detection results are also comparable
with those from the previous source studies. We also find out a linear relationship between
the directivity effect and earthquake magnitude. Since the TSMIP station may provide
real-time services in the future, the detection system proposed by this research can quickly
provide disaster prediction information, which is of great importance for earthquake
emergency response and hazard mitigation.

How to cite: Wu, C.-F., Lin, T.-L., and Chen, Y.-C.: Rapid Detection of Earthquake Rupture Directivity Using Strong Ground Motion Data in Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22441, https://doi.org/10.5194/egusphere-egu2020-22441, 2020.

Phase unwrapping is the process of recovering the absolute phase from unambiguous wrapped phase values that are measured modulo 2pi rad. From a mathematical point of view, phase unwrapping is an inverse problem, however, it is ill-posed and notoriously difficult to solve in the presence of noise. Meanwhile, phase unwrapping errors severely impact the estimation of earthquake and volcano source parameters using interferometric observations, therefore avoiding phase unwrapping completely is desirable. 

A potential solution to avoid the unwrapping error issue completely, is to carry out a geophysical inversion directly on the wrapped phase observations. To overcome the need for phase unwrapping, we propose a novel approach that we can invert directly the interferometric wrapped phase, circumventing the ill-posed phase unwrapping processing step. This approach includes (1) a downsampling algorithm, (2) a method to estimate the covariance function of the wrapped phase, (3) an appropriate misfit function between the observed and the simulated wrapped phase. We also assess the uncertainties of source parameters within a Bayesian approach, and finally we test the robustness of the inversion methodology in multiple simulations including variable decorrelation and atmospheric noise simulations.

We demonstrate the proposed methodology on synthetic cases with variable noise and one real earthquake case. We show that the method is robust in challenging noise scenarios. We also show an improvement with the Bayesian approach in performance with respect to similar previous methods, resulting in avoiding any influence of seed starting models, and escaping local minima. We study the impact of a small percentage of incorrectly unwrapped phase observations in current state-of-the-art methods, and show that the presence of a small fraction of unwrapping errors affect strongly the estimation process. We conclude that in the cases where phase unwrapping is difficult or even impossible, the proposed inversion methodology with wrapped phase will provide an alternative approach to assess earthquake and volcano source model parameters.

How to cite: Jiang, Y. and González, P.: Bayesian Inversion of Wrapped Satellite Interferometric Phase to Estimate Fault and Volcano Surface Ground Deformation Models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5604, https://doi.org/10.5194/egusphere-egu2020-5604, 2020.

EGU2020-11916 | Displays | SM2.1

Hierarchical seismic sources model and recent observational evidence

Satoshi Ide and Hideo Aochi

Earthquakes are multiscale phenomena with several scale invariant parameters, such as stress drop, apparent stress, and rupture propagation velocity, and dynamic rupture grows almost self-similarly. While the rupture process is always complex, the location of sources is not completely random, but nearly predetermined, especially along a well-developed fault system like a plate interface. To explain such behavior, some hierarchical structure is required, and one candidate is the hierarchical circular patch (fractal asperity) model suggested by Ide and Aochi (JGR, 2005) and Aochi and Ide (JGR, 2009), in which fracture energy inside a patch is proportional to the patch radius. In this paper, we review the characteristics of the model and show some observational evidence, which has been recently discovered mainly for subduction-type earthquakes in the Tohoku-Oki, Japan, region. Some hierarchical patch-like structure has been identified for several repeating earthquakes of M~5 (Uchida et al., GRL, 2012; Okuda and Ide, EPS, 2018). Identical onsets of seismic waves were observed for many pairs of large (M>4.5) and small (M<4.0) earthquakes (Okuda and Ide, Nature Communications, 2018; Ide, Nature, 2019). We can also observe long-term increase of seismicity before the rupture of system-size events (Okuda et al., Zishin, 2018). These lines of evidence suggest the qualitative validity of the hierarchical model and will be useful to improve the quantitative aspects of the model, such as patch density and the slip-weakening rate, to numerically simulate realistic earthquakes and seismicity (e.g., Aochi and Ide, EPS, 2011; Ide and Aochi, Tectonophysics, 2013; Aochi and Twardzik, Pageoph, 2019).

How to cite: Ide, S. and Aochi, H.: Hierarchical seismic sources model and recent observational evidence, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11916, https://doi.org/10.5194/egusphere-egu2020-11916, 2020.

EGU2020-18422 | Displays | SM2.1

Dynamic source inversion of the 2014 Mw6 South Napa, California, earthquake

Jan Premus and Frantisek Gallovic

Dynamic rupture modeling coupled with strong motion data fitting (dynamic source inversion) offers an insight into the rupture physics, constraining and enriching information gained from standard kinematic slip inversions. We utilize the Bayesian Monte Carlo dynamic source inversion method introduced recently by Gallovič et al. (2019), which, in addition to finding a best-fitting model, allows assessing uncertainties of the inferred parameters by sampling the posterior probability density function. The Monte Carlo approach requires running a large number (millions) of dynamic simulations due to the nonlinearity of the inverse problem. It is achieved by using GPU accelerated dynamic rupture simulation code FD3D_TSN (Premus et al., submitted) as a forward solver. We apply the inversion to the 2014 Mw6 South Napa, California, earthquake, employing strong motion data (up to 0.5 Hz) from the 10 closest stations. As an output, we obtain samples of the spatial distributions of dynamic parameters (prestress and parameters of the slip-weakening friction law). Regarding the rupture geometry, we consider two, presently ambiguous, fault planes (Pollitz et al., 2019), showing considerable differences in fitting seismograms in very close vicinity of the fault. We investigate properties of the rupture, especially in the region close to the free surface, and the viability of the model samples to explain the observed data in a broader frequency range (up to 5Hz).

How to cite: Premus, J. and Gallovic, F.: Dynamic source inversion of the 2014 Mw6 South Napa, California, earthquake, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18422, https://doi.org/10.5194/egusphere-egu2020-18422, 2020.

EGU2020-20584 | Displays | SM2.1

Dynamic Rupture and Ground Motion Modeling of the 2016 M6.2 Amatrice and M6.5 Norcia, Central Italy, Earthquakes Constrained by Bayesian Dynamic Source Inversion

Taufiq Taufiqurrahman, Alice-Agnes Gabriel, Frantisek Gallovic, and Lubica Valentova

The complex evolution of earthquake rupture during the 2016 Central Italy sequence and the uniquely dense seismological observations provide an opportunity to better understand the processes controlling earthquake dynamics, strong ground motion, and earthquake interaction. 

We here use fault initial stress and friction conditions constrained by a novel Bayesian dynamic source inversion as a starting point for high-resolution dynamic rupture scenarios. The best-fitting forward models are chosen out of ~106 highly efficient simulations restricted to a simple planar dipping fault. Such constrained, highly heterogeneous dynamic models fit strong motion data well. Utilizing the open-source SeisSol software (www.seissol.org), we then take into account non-planar (e.g., listric) fault geometries, inelastic off-fault damage rheology, free surface effects and topography which cannot be accounted for in the highly efficient dynamic source inversion. SeisSol is based on the discontinuous Galerkin method on unstructured tetrahedral meshes optimized for modern supercomputers. 

We investigate the effects of including the realistic modeling ingredients on rupture dynamics and strong ground motions up to 5 Hz. Synthetic PGV mapping reveals that specifically fault listricity decreases ground motion amplitudes by  ~50 percent in the extreme near field on the foot-wall. On the hanging-wall shaking is increased by ~150 percent as a consequence of wave-focusing effects within 10 km away from the fault. Dynamic modeling thus suggests that geometrical fault complexity is important for seismic hazard assessment adjacent to dipping faults but difficult to identify by kinematic source inversions.

How to cite: Taufiqurrahman, T., Gabriel, A.-A., Gallovic, F., and Valentova, L.: Dynamic Rupture and Ground Motion Modeling of the 2016 M6.2 Amatrice and M6.5 Norcia, Central Italy, Earthquakes Constrained by Bayesian Dynamic Source Inversion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20584, https://doi.org/10.5194/egusphere-egu2020-20584, 2020.

SM2.2 – Earthquakes and active tectonics in regions of slow lithospheric deformation: towards a re-evaluation of the Stable Continental Region concept in seismic hazard assessment

EGU2020-3125 * | Displays | SM2.2 | Highlight

Earthquake crisis unveils the growth of an incipient continental fault system

Eulàlia Gràcia, Ingo Grevemeyer, Rafael Bartolomé, Héctor Perea, Sara Martínez-Loriente, Laura Gómez de la Peña, Antonio Villaseñor, Yann Klinger, Claudio Lo Iacono, Susana Diez, Alcinoe Calahorrano, Miquel Camafort, Sergio Costa, Elia d'Acremont, Alain Rabaute, and César R. Ranero

Large continental faults extend for thousands of kilometres and often form the tectonic boundaries between plates that are associated with prominent topographic features. In these active areas, well-defined faults produce large earthquakes, and thus imply a high seismic hazard. These paradigms are called into question in the Alboran Sea, which hosts an allegedly complex diffuse boundary between the Eurasia and Nubia plates, and we discovered one of the few examples worldwide of the initial stages of these key tectonic structures. On the 25th January 2016, a magnitude Mw6.4 submarine earthquake struck the north of the Moroccan coast, the largest event ever recorded in the Alboran Sea. The quake was preceded by an earthquake of magnitude Mw5.1 and was followed by numerous aftershocks whose locations mainly migrated to the south. The mainshock nucleated at a releasing bend of the poorly known Al-Idrissi Fault System (AIFS). According to slip inversion and aftershock distribution, we assume a rupture length of 18 km. Here we combine newly acquired multi-scale bathymetric and marine seismic reflection data with a resolution comparable to the studies on land, together with seismological data of the 2016 Mw 6.4 earthquake offshore Morocco – the largest event recorded in the area – to unveil the 3D geometry of the AIFS. We found that, despite its subdued relief, the AIFS is a crustal-scale boundary. We report evidence of left-lateral strike-slip displacement, characterize their fault segments and demonstrate that the AIFS is the source of the 2016 events. The occurrence of the Mw 6.4 earthquake and previous events of 1994 and 2004 supports that the AIFS is currently growing through propagation and linkage of its segments, which eventually might generate a greater rupture (up to Mw 7.6), increasing the potential hazard of the structure. The AIFS provides a unique model of the inception and growth of a young plate boundary system in the Alboran Sea (Western Mediterranean).

This work has been recently published in Nature Communications (IF:12.35), 10, 3482 (2019) doi:10.1038/s41467-019-11064-5. I would like to present our article recently published in NCOMM, so, please consider our work for an ORAL INVITED presentation. Many thanks!

How to cite: Gràcia, E., Grevemeyer, I., Bartolomé, R., Perea, H., Martínez-Loriente, S., Gómez de la Peña, L., Villaseñor, A., Klinger, Y., Lo Iacono, C., Diez, S., Calahorrano, A., Camafort, M., Costa, S., d'Acremont, E., Rabaute, A., and Ranero, C. R.: Earthquake crisis unveils the growth of an incipient continental fault system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3125, https://doi.org/10.5194/egusphere-egu2020-3125, 2020.

EGU2020-21749 | Displays | SM2.2

Large magnitude earthquakes of late Holocene age in the Precambrian of Finnmark, Northern Norway

Odleiv Olesen, Lars Olsen, Steven Gibbons, Tormod Kværna, Bent Ole Ruud, and Tor Arne Johansen

The 80 km long Stuoragurra postglacial fault occurs within the c. 5 km wide Precambrian Mironjavri-Sværholt Fault Zone in the northern Fennoscandian Shield. Deep seismic profiling and drilling show that the fault dips at an angle of 30-40° to the southeast. The reverse fault can be traced down to a depth of c. 2.5 km on the reflection seismic profile. A total of c. 100 earthquakes has been registered along the fault between 1991 and 2019. Recordings at the ARCES seismic array in Karasjok c. 40 km to the SE of the fault and other seismic stations in northern Norway and Finland have been utilized. The maximum moment magnitude is 4.0. The Stuoragurra fault constitutes the Norwegian part of the larger Lapland province of postglacial faults extending southwards into northern Finland and northern Sweden. The formation of these faults has previously been associated with the deglaciation of the last inland ice. Trenching of different sections of the fault and radiocarbon dating of buried and deformed organic material reveal, however, a late Holocene age (between c. 700 and 4000 years before present at three separate fault segments). The reverse displacement of c. 9 m and segment lengths of 9-12 km of the two southernmost fault segments indicate a moment magnitude of c. 7. The results from this study indicate that the maximum magnitude of future earthquakes in Fennoscandia can be significantly larger than the existing estimate of c. 6.

How to cite: Olesen, O., Olsen, L., Gibbons, S., Kværna, T., Ruud, B. O., and Johansen, T. A.: Large magnitude earthquakes of late Holocene age in the Precambrian of Finnmark, Northern Norway, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21749, https://doi.org/10.5194/egusphere-egu2020-21749, 2020.

EGU2020-8505 | Displays | SM2.2

Strike-slip fault reactivation in the Western Alps due to Glacial Isostatic Adjustment

Juliette Grosset, Stéphane Mazzotti, Philippe Vernant, Jean Chéry, and Kevin Manchuel

The Western Alps represent the zone of highest seismicity density in metropolitan France. The seismicity is mainly located along two NE-SW strike-slip fault systems: the right-lateral Belledonne Fault and the left-lateral Durance Fault. Glacial Isostatic Adjustment (GIA) is one of the most common processes given to explain intraplate seismicity (e.g., Scandinavia, North America) and is also proposed as a cause of present-day deformation in the Alps. In order to test the impact of deglaciation from the Last Glacial Maximum on pre-existing vertical strike-slip faults in the Western Alps (Belledonne and Durance Faults), we use a finite-element approach to model fault reactivation throughout the deglaciation period, from ca. 18 kyr up to today. The models are tuned to fit present-day deformation rates observed by geodesy (uplift rate up to 2 mm/yr and horizontal radial extension). Simplified models (homogeneous icecap and Earth rheology) show that, under optimum conditions, GIA stress perturbations can activate a NE-SW right-lateral strike-slip fault such as the Belledonne Fault, requiring the fault to have been pre-stressed up to near-failure equilibrium before the onset of deglaciation. The maximum effect of GIA is 1.7 meters of right-lateral slip over 20 kyr, with a peak of displacement between 20 and 10 ka. These models indicate that GIA can result in a maximum slip rate of 0.08 mm/yr averaged over the Holocene, in association with earthquakes up to Mw = 7 (if all displacement is taken in one event). These results are consistent with local paleoseismicity and geomorphology evidence on the Durance fault. However, the impact of GIA on the left-lateral Belledonne Fault is poorly constrained by these simple models. Additional models based on realistic Alpine icecap reconstructions and regional rheology structures will also be presented, that allow us to test the specific effects of GIA on Holocene deformation along both the Belledone and Durance Fault systems.

How to cite: Grosset, J., Mazzotti, S., Vernant, P., Chéry, J., and Manchuel, K.: Strike-slip fault reactivation in the Western Alps due to Glacial Isostatic Adjustment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8505, https://doi.org/10.5194/egusphere-egu2020-8505, 2020.

EGU2020-8409 * | Displays | SM2.2 | Highlight

The Mw4.9 Le Teil surface-rupturing earthquake in southern France: New insight on seismic hazard assessment in stable continental regions

Jean-François Ritz, Stéphane Baize, Matthieu Ferry, Christophe Larroque, Laurence Audin, Bertrand Delouis, and Emmanuel Mathot

On November 11th 2019, a Mw 4.9 earthquake shook the Rhone River Valley in southern France, a rather densely populated area with many industrial facilities including several nuclear power plants. The “Le Teil” earthquake was felt as far as Montpellier and Grenoble, 120 km from the epicenter. Seismological data promptly showed that the earthquake corresponded to a reverse faulting event along a NE-SW trending fault with a focus at a very shallow depth (~1 km). In parallel, satellite-based radar observations (InSAR) showed the uplift of the SE compartment (up to 10 centimeters) along a sharp NE-SW trending ~4.5-km-long discontinuity. Field investigations conducted in the following days and weeks in the epicentral area uncovered several evidences of surface ruptures across roads and paths where the InSAR discontinuity is mapped. We also carried out airborne LiDAR surveys to map the rupture below the dense forest cover. Characteristics of surface deformations are fully consistent with InSAR and seismological data, and allow concluding to the reactivation of an Oligocene normal fault segment (i.e. La Rouvière fault) that belongs to the Cévennes fault system, a 120 km long polyphased system bounding the southern rim of the Massif Central. The absence of clear cumulative compressional deformation along the fault rupture, which on the contrary displays inherited extensional deformation (most likely Oligocene in age), suggests that the fault has not moved significantly since millions of years. These observations relaunch the question of seismic hazard assessment in stable continental regions such as continental France and most of Western Europe, where strain rates are very low.

How to cite: Ritz, J.-F., Baize, S., Ferry, M., Larroque, C., Audin, L., Delouis, B., and Mathot, E.: The Mw4.9 Le Teil surface-rupturing earthquake in southern France: New insight on seismic hazard assessment in stable continental regions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8409, https://doi.org/10.5194/egusphere-egu2020-8409, 2020.

EGU2020-9785 | Displays | SM2.2 | Highlight

Full characterization of the ML 5.4 2019/11/11 Le Teil earthquake in France based on a multi-technology approach

Amaury Vallage, Laurent Bollinger, Yoann Cano, Johann Champenois, Clara Duverger, Bruno Hernandez, Pascal Herry, Alexis Le Pichon, Constantino Listowski, Gilles Mazet-Roux, Marine Menager, Sophie Merrer, Béatrice Pinel-Puyssegur, Roxanne Rusch, Olivier Sèbe, Julien Vergoz, and Aurélie Guilhem Trilla

Metropolitan France is a region of slow tectonic deformation rates with sparse historical and instrumental seismicity, and where geodesy is not able to reach the required resolution in order to resolve the tectonic loadings. The few faults recognized as potential active rely on rare neotectonic slip rates, often integrated over geological scales.

In this context, the ML 5.4 Le Teil 2019 earthquake is of particular interest because it is the largest seismic event recorded in metropolitan France in the last 16 years. The last regional earthquake with a larger magnitude was the Lambesc event that occurred in 1909 about 110 km away from Le Teil epicenter. This recent earthquake offers a noteworthy opportunity to combine different technologies: seismological observations (RESIF and CEA) with satellite InSAR data and infrasound measurements, to help characterizing this stable continental region.

The analysis shows that the focal mechanism determined from the full waveform inversion of long-period seismological data is consistent with the activation of a reverse fault with a strike around 45°N and is associated with a moment magnitude of 4.8. Moreover, this event produced infrasound signals recorded by the OHP Alpine array located 110 km away. The analysis of these signals provides evidence of ground-to-air coupling in the epicentral region as well as ground shaking information.

Despite the moderate magnitude of the event, the ground deformation is resolved by InSAR with Sentinel-1 data. The interferogram is consistent with the shallow depth inverted from seismology and confirmed by the presence of surface ruptures. The inversion of multiple InSAR tracks allows characterizing the displacement at depth and along strike on the fault plane. The results are consistent with the focal mechanism derived from seismology. The earthquake has ruptured a 5-km long by ~1.5-km deep fault. The displacement reaches a maximum at a shallow 1 km-depth. The source inverted from InSAR coincides with the Rouvière fault, a branch of the Cévennes fault system formerly known as a normal fault. This reverse earthquake might be an example of an inherited structure re-activation as it is often the case in intraplate regions with polyphased history.

How to cite: Vallage, A., Bollinger, L., Cano, Y., Champenois, J., Duverger, C., Hernandez, B., Herry, P., Le Pichon, A., Listowski, C., Mazet-Roux, G., Menager, M., Merrer, S., Pinel-Puyssegur, B., Rusch, R., Sèbe, O., Vergoz, J., and Guilhem Trilla, A.: Full characterization of the ML 5.4 2019/11/11 Le Teil earthquake in France based on a multi-technology approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9785, https://doi.org/10.5194/egusphere-egu2020-9785, 2020.

The 20th May 2016 moment magnitude (MW) 6.1 Petermann earthquake was the 2nd longest single-event historic Australian surface rupture (21 km) and largest MW on-shore earthquake in 28 years. Trench logs from two hand-dug trenches show no evidence of penultimate rupture of surface eolian sediments or underlying calcrete. Available dating of eolian dunes 140 to 500 km away from the Petermann fault indicated eolian deposition during either the last glacial maximum (approximately 20 ka) or a period of aridification at approximately 180 - 200 ka. Ten 10Be cosmogenic nuclide erosion rates of bedrock outcrops at 0 to 50 km from the surface rupture trace are within error of each other between 1.4 to 2.6 mMyr-1. These samples have approximate averaging times between 208 to 419 ka. Bedrock erosion rates, trenching results and interpretation of the landscape history suggest the 2016 event is the only surface rupturing earthquake on the Petermann fault in the last 200 to 400 kyrs, and possibly the first ever on this fault. This finding is consistent with a lack of evidence for penultimate rupture for all eleven historic Australian surface rupturing events, as described by either trenching and/or landscape analysis and bedrock erosion rates. These ‘one-off’ events within Precambrian cratonic Australian crust are not consistent with trenching results and geomorphology of paleo-scarps within the Flinders Ranges and Eastern Australia which indicate multiple recurrent fault offset. Variable fault recurrence behaviour highlights that uniform seismic hazard modelling approaches are not applicable across Stable Continental Regions.

How to cite: Quigley, M., King, T., and Clark, D.: The 2016 Mw 6.1 Petermann Ranges earthquake rupture, Australia: another “one-off” stable continental region earthquake, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12723, https://doi.org/10.5194/egusphere-egu2020-12723, 2020.

EGU2020-10030 | Displays | SM2.2

Complex spatiotemporal patterns of intracontinental earthquakes: a different game

Mian Liu, Yuxuan Chen, Seth Stein, Gang Luo, and Hui Wang

Intracontinental earthquakes show complex spatiotemporal patterns. In North China, no large (M>7) earthquakes ruptured the same fault segments in the past 2000 years; instead they roamed among widespread fault systems. In Australia, morphogenic evidence indicates clusters of earthquakes separated by tens of thousands of years of dormancy. In central and eastern United States, paleoseismic studies suggest that large Holocene earthquakes occurred in places that are seismically inactive today. Such seismicity does not fit existing earthquake models that assume steady tectonic loading and cyclic stress release on fault planes. Intracontinental fault systems are widespread and collectively accommodate slow tectonic loading. A major fault rupture both transfers stress to the neighboring faults and perturbs loading conditions on distant faults. Thus, the loading rate on each individual fault can be variable. Slow tectonic loading means that local stress variations from fault interaction or nontectonic processes, or changes of fault strength, could trigger an earthquake. Furthermore, large intracontinental earthquakes usually rupture multiple fault segments or faults, which vary for each event. For these earthquakes, commonly used concepts such as recurrence intervals and characteristic earthquakes, all based on earthquake models assuming cyclic elastic rebound, are inadequate or inapplicable. On the other hand, the general patterns of intracontinental earthquakes can be described by the theory of complex dynamic systems, in which all faults interact with each other. The rupture of individual fault or fault segment cannot be predetermined, but the system behavior can be studied based on the records of previous events. We found that large intracontinental earthquakes, either on a fault system or in a region, are usually clustered and separated by long but variable periods of quiescence. The lengths of the quiescence periods inversely correlate with tectonic loading rates, and the characteristics of earthquake clusters depend on fault geometry and crustal rheology, through fault interaction and viscoelastic relaxation. Spatially, large intracontinental earthquakes are not limited to faults that are active recently, although weak regions tend to have more earthquakes. Intracontinental earthquakes require a different approach, one that focuses on stress interactions between faults in a complex dynamic system rather than stress accumulation and release on individual faults.

How to cite: Liu, M., Chen, Y., Stein, S., Luo, G., and Wang, H.: Complex spatiotemporal patterns of intracontinental earthquakes: a different game, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10030, https://doi.org/10.5194/egusphere-egu2020-10030, 2020.

The left-lateral strike-slip Haiyuan fault system is a major boundary fault zone on the northeast margin of the Qinghai -Tibet Plateau,separating active Tibet block and stable Alaxan & rdos blocks, and accommodating the eastward motion of Tibet Plateau. It consists of several sections, including Lenglongling segment (LLL), the Jinqianghe segment (JQH), the Maomaoshan segment (MMS), the Laohushan segment (LHS) and the rupture of the Haiyuan earthquake in 1920 from the west to the east. In 1920, a M8.5 Haiyuan earthquake occurred in the eastern segment of the fault zone, resulting in a surface rupture zone of about 240 km, with a maximum left-lateral coseismic displacement of 10 m. In the past 100 years after the earthquake, Haiyan fault is in a state of calm, no destructive earthquake of M 6.0 or above occurred. It is worth studying that how the fault activity and seismic hazard of each section of Haiyuan fault zone are at present.

We use geodetic data (High density InSAR and wide scale GPS) to study the present slip rate and locking degree of Haiyuan fault zone. we first use the Envisat/ASAR long-strip data of five tracks and the PSInSAR time series processing technology based on high coherence point target to obtain the average deformation rate field of the fault system during 2003~2010, and transform the deformation rate from line-of-sight (LOS) direction to the parallel fault direction. Then,we use two-dimensional screw dislocation model to fit the cross-fault deformation rate profiles, and obtain the fault kinematic parameters such as the fault slip rate and the locking depth. At the same time, we adopt the three-dimensional block model to invert the distribution characteristics of fault locking degree and slip rate deficit along the Haiyuan fault zone. We compare the difference of inversion results of different data individually and jointly, including large-scale sparse GPS data, high-density InSAR data and the combination of them. Finally we get the continuous strain accumulation state of the fault zone. The results show that from west to east, the slip rate decreases gradually, while the locking depth changes along the fault. The Laohushan section shows shallow surface creep. The analysis of the high-density cross-fault deformation rate profile of the Laohushan segment indicates that the creep length is about 19 km. Other segments in a locked state. But in the middle of the 1920 erathquake fracture section, the locking degree is weaker and shallower than other segments. These results are helpful to understand the present activity and assess regional seismic risk of Haiyuan fault zone.

How to cite: Qu, C. and Qiao, X.: Joint inversion of InSAR and GPS for fine slip rate and locking degree distribution along the Haiyuan fault zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2539, https://doi.org/10.5194/egusphere-egu2020-2539, 2020.

EGU2020-710 | Displays | SM2.2

Palaeo-earthquake magnitudes on the Dzhungarian fault, N. Tien shan, and implications for the rupture processes of intraplate strike-slip faults

Chia-Hsin Tsai, Richard Walker, Simon Daout, Kanatbek Abdrakhmatov, Aidyn Mukambayev, Christoph Grützner, and Ed Rhodes

Long-term and present-day crustal deformation in the northern Tien Shan is poorly known, but is a key to understanding the mode of lithospheric deformation deep within the continental interiors, as well as the hazards posed by the slow-moving intraplate faults. Driven by the India-Asia collision, the NW-SE strike-slip faults and the E-W range-front thrust faults in the interior of Tien Shan together accommodate about 15-20 mm/yr of shortening. Here we focus on the NW-SE striking Dzhungarian fault (DZF) and the E-W striking Lepsy fault (LPF), which are large oblique strike-slip faults bounding the Dzhungarian Alatau, northern Tien Shan. Two large historical earthquakes in ~1716 and 1812 (Mw 8) were recorded in this region, and clear fault traces as well as scarps are visible from satellite images along some of the main faults. However, their geometries, slip rates, mode of deformation, expected earthquake magnitudes and recurrence interval have not been studied in details. A previous study suggested that the LPF ruptured in a seismic event around 400 yrBP that might be the 1716 earthquake known from historical records. Offsets of over 15 m were found over a fault length of 120 km, indicating a magnitude in the range Mw 7.5-8.2. The slip to length ratio for the LPF is unusally high, suggesting either that faults in this region are capable of generating very large earthquakes for a given fault length, or that the rupture length is underestimated.

Using a combination of high-resolution digital elevation models (DEMs) and orthophotos from High Mountain Asia (NASA), Pleiades optical imagery (CNES), drone photos and multi-temporal interferometric synthetic-aperture radar (InSAR) from the Sentinel-1 satellites, we identify the geomorphic signatures and quantify the long-term and short-term strain accumulation along the faults. The ~400 km DZF shows evidence for relatively ‘fresh’ rupturing along much of its length. We calculate an average lateral slip per event of 9.9 m from offset stacking analysis, which underlines the potential future large earthquakes on this fault. The proximity of the DZF and LPF ruptures and equivalent level of preservation opens the possibility that they were formed in a single earthquake event, with a moment-magnitude greater than 8. We also present estimates of long-term and short-term rates of slip across the DZF in order to estimate average recurrence intervals and to build a kinematic model of the faulting in the Northern Tien Shan.

How to cite: Tsai, C.-H., Walker, R., Daout, S., Abdrakhmatov, K., Mukambayev, A., Grützner, C., and Rhodes, E.: Palaeo-earthquake magnitudes on the Dzhungarian fault, N. Tien shan, and implications for the rupture processes of intraplate strike-slip faults, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-710, https://doi.org/10.5194/egusphere-egu2020-710, 2020.

EGU2020-12276 | Displays | SM2.2

Study on Seismicity and Its Geodynamic Genesis in Ngari Areas

Guangyin Xu, Qing Wu, and Suyun Wang

The Ngari area in Tibet is in the forefront of land-continent collisions. The area is accompanied by the polymerization of plates, forming complex structures such as the Tethys Himalayan pleat belt, the Yarlung Zangbo suture belt, and the Gangdese continental margin magma arc from the south to the north. The multi-period dive collision-inland convergence process, the geological structure is complex and the seismicity is very high. Based on the Chinese historical earthquake catalogue, the China Modern Earthquake Catalogue and the seismic data from the International Seismological Center (ISC), we analyzed the seismic activity, focal mechanism and modern tectonic stress field in the Ngari area, and then analyzed the seismicity and its source of geodynamics. The main conclusions are as follows:(1) The seismic activities in the Ngari area are mainly distributed in the Himalayan tectonic belt, the Bangong-Nujiang tectonic belt, the Alkin-East Kunlun tectonic belt, and some near north-south trending tectonic belts; (2) Earthquakes near the Himalayan tectonic belt is dominated by reverse faulting events. The seismic activity near the Bangong-Nujiang tectonic belt and the Alkin-East Kunlun tectonic belt is dominated by strike-slip earthquakes. Near the north-south extensional tectonic belt, the earthquakes show as the normal faulting events. (3) The main direction of the modern tectonic stress field in the study area is near north-south direction; (4) Seismic activity, focal mechanism and modern tectonic stress field show that the geodynamic source in the Ngari region is from Collision and squeezing the between the Eurasian plate and the Indian Ocean plate.

How to cite: Xu, G., Wu, Q., and Wang, S.: Study on Seismicity and Its Geodynamic Genesis in Ngari Areas, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12276, https://doi.org/10.5194/egusphere-egu2020-12276, 2020.

EGU2020-2788 | Displays | SM2.2

New insights into the complex surface faulting of the 1927 M8.0 Gulang earthquake, NE Tibetan Plateau

Peng Guo, Zhujun Han, Fan Gao, Chuanhua Zhu, and Hailong Gai

The rupture patterns of large earthquakes in transpressional systems can provide important information for understanding oblique motion and strain partitioning between tectonic blocks. The 1927 M8.0 Gulang earthquake occurred on the transpressional boundary between the Tibetan and Gobi-Alashan blocks. Combined with the results of previous studies, we find that the Lenglongling fault (LLLF) and Southern Wuwei Basin fault (SWBF) might have both ruptured during the Gulang earthquake, but they exhibited different motions. A ~120-km-long surface rupture zone formed along the LLLF, with a left-lateral strike-slip motion and a coseismic offset of ~2.4-7.5 m. Bending, bifurcation, and change of the slip sense occurs at both ends of the fault. The ~42-km-long rupture zone on the SWBF, with a coseismic vertical offset of ~0.6-2.8 m, can be divided into two segments. The eastern segment shows thrust motion, while the western shows thrust motion with a left-lateral strike-slip component. Thus, the Gulang earthquake may be a multifault rupture event where strike-slip and thrust faults ruptured simultaneously. Analysis of deep and shallow structures and three-dimensional finite-element modeling reveal that the north-dipping LLLF and the SWBF may converge downward to a low-angle decollement. This pattern of deformation partitioning is similar to some other earthquakes where oblique block convergence is partitioned into strike-slip motion on steeply dipping faults and vertical motion on gently dipping faults.

How to cite: Guo, P., Han, Z., Gao, F., Zhu, C., and Gai, H.: New insights into the complex surface faulting of the 1927 M8.0 Gulang earthquake, NE Tibetan Plateau, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2788, https://doi.org/10.5194/egusphere-egu2020-2788, 2020.

EGU2020-2119 | Displays | SM2.2

Did the Yangsan Fault in a stable continent region move continuously during the Holocene?

Sung-Ja Choi, Jinhyuk Choi, Kyoungtae Ko, Yirang Jang, and Jeong-Heong Choi

The Korean Peninsula is more than 400 km away from passive plate margins and considered a stable continent region. However, it has suffered from damaging earthquakes in the last 20 years. There are two major North–North East tectonic faults in the region: the Chugaryong fault running from Wonsan through Seoul to the west coast and the Yangsan fault in the southeastern part of the peninsula. The Yangsan fault extends for over 170 km and has been active since the late Cretaceous period. The fault has experienced many earthquakes in the last 2000 years, most recently  Mw 5.5 earthquake in its vicinity without any surface rupture. The fault has been studied by various disciplines, such as structural geology to determine the characteristics of the fault, geophysical exploration to determine the extension of the fault, and mineralogy to analyze fault gouges.[A1]  However, the last fault movement remains unknown. Trench studies on the Yangsan Fault undertaken in the central south of the Yangsan Fault to obtain its last movement revealed that the fault had been reactivated at least twice during the Holocene period, at approximately 2 ka and 4 ka. Before the Holocene, another fault movement occurred at approximately 50 ka, with a strike-slip motion creating a meter-wide fault damage zone. LIDAR and aerial photographs demonstrated that a higher terrace younger than 320 ka had moved by 1.5 km with a left-lateral-strike-slip motion. We now surmise that the Yangsan Fault has been continuously reactivated for more than 60 million years, and could potentially generate severe geohazards in the near future. Furthermore, even if the fault is inside an intraplate, we propose that it has continuously been reactivated from the Late Cretaceous to the present by plate tectonics.

How to cite: Choi, S.-J., Choi, J., Ko, K., Jang, Y., and Choi, J.-H.: Did the Yangsan Fault in a stable continent region move continuously during the Holocene?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2119, https://doi.org/10.5194/egusphere-egu2020-2119, 2020.

A seismic source can be a capable tectonic source or a seismogenic source. A capable tectonic source is a tectonic structure that can generate both vibratory ground motion and tectonic surface deformation at or near the earth's surface in the present seismotectonic regime. On the other hand, A seismogenic source generates vibratory ground motion but is assumed to not cause surface displacement, covering wide range of seismotectonic conditions, from a well-defined tectonic structure to simply a large region of diffuse seismicity.

The ML 5.8 Gyeongju earthquake on September 11, 2016 in South Korea is the largest instrumental one since 1978 that occurred in buried fault not exposed to the surface area. So to speak, there is no evidence of surface faulting till now. On the other hand, the geometry of the causative fault of the Gyeongju earthquake was revealed in detail from the distribution of foreshocks and aftershocks. Therefore, the causative fault of the Gyeongju earthquake can be treated as a seismogenic source corresponding to a well-defined tectonic structure as mentioned above.

What level of ground motions would occur at the site of interest if a larger earthquake would occur at the causative fault of the Gyeongju earthquake? To make a rough estimate of that question, we carried out a simple study of modeling the causative fault with the data available, and simulating strong ground motions with the stochastic and empirical Green’s function techniques. The magnitude of the maximum earthquake potential on the causative fault is in the range of 6.0 to 7.0 and increased by 0.5. We do not claim the possibility of such a large earthquake in the region, but have a goal to evaluate the seismic safety evaluation of the site of interest from such an earthquake potential. This type of study may help us elucidate the seismic hazard in a low seismicity area such as South Korea and review the seismic safety of the site of interest.

How to cite: Choi, H.: What if a larger earthquake would occur at the causative fault of the Gyeongju earthquake with ML 5.8 on September 11, 2016 in South Korea?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2257, https://doi.org/10.5194/egusphere-egu2020-2257, 2020.

EGU2020-15231 | Displays | SM2.2

Seismotectonic regions for Germany - Concept and results

Tim Hahn, Jonas Kley, Diethelm Kaiser, Thomas Spies, Jörg Schlittenhardt, and Claudia Geisler

Seismotectonic regions are a basic input in seismic hazard assessment. Several seismotectonic regionalizations for Germany were proposed in the past. We are presently developing a new regionalization based on the definition in the Safety Standard of the Nuclear Safety Standards Commission KTA 2201.1 (2011-11): “A seismotectonic unit is a region for which uniformity is assumed regarding seismic activity, geological structure and development and, in particular, regarding neotectonic conditions. A seismotectonic unit may also be an earthquake source region.” Our new concept focusses on a transparent implementation of the required geological criteria. Our approach is to initially analyze those separately from present-day seismicity. Compared to existing source area models we strive for a better documentation and justification of the geological elements used to delimit seismotectonic regions. This includes an analysis of the geological history of structures in six time slices from the Permian to the Present that will be considered in the regionalization. The time slices are (1) Permian, (2) Triassic, (3) Jurassic to Early Cretaceous, (4) Late Cretaceous, (5) Cenozoic > 20 Ma and (6) Recent (< 20 Ma). They were chosen because they are separated by marked changes of stress and kinematic regimes and were associated with the evolution of new fault systems or reactivation of existing ones. The tectonic characteristics of the time slices are briefly described.

The present-day observable fault network comprises faults from all time slices. For each time slice, a subset of active faults will be extracted based on geological evidence for fault activity at that time, e.g. syntectonic deposits. The uncertainties of these age assignments will be documented. The fault subset will be used to estimate overall kinematics, a paleo-stress field and to delimit little deformed or stable areas. Faults, kinematics, stress and stable areas can then be compared to present-day seismicity/active faults, slip directions, stress and undeformed areas as well as other parameters such as crustal and lithospheric thickness. These steps are repeated for each time slice. The superposition of active faults and stable regions across all time slices will identify faults prone to reactivation and regions that remained undeformed over geological time, potentially indicating areas of increased or reduced present-day seismic hazard.

A comparison with seismicity of the last 1000 years shows partial agreement between regions of strong (or repeated) deformation and regions of higher seismicity. On the other hand, stronger earthquakes occasionally cluster in regions appearing stable since Permian time, the Anglo-Brabant Massif being a prominent example of this type.

How to cite: Hahn, T., Kley, J., Kaiser, D., Spies, T., Schlittenhardt, J., and Geisler, C.: Seismotectonic regions for Germany - Concept and results, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15231, https://doi.org/10.5194/egusphere-egu2020-15231, 2020.

EGU2020-6187 | Displays | SM2.2

Paleoseismological trenching of the eastern Rhine Graben Boundary Fault: the Ettlingen segment

Klaus Reicherter, Stephane Baize, Jochen Hürtgen, Francesca Cinti, Tom K. Rockwell, Herve Jomard, Gordon Seitz, and Joachim Ritter

Paleoseismic data on the eastern central Rhine Graben Boundary Fault, as part of the Upper Rhine Graben (URG) fault system, revealed Holocene earthquake activity with surface rupturing faults. The URG is one of the most seismically active areas in the stable continental interiors of Central Europe north of the Alps. We opened the first paleoseismic trenches N of Basel and S of Frankfurt along the ca. 300 km long eastern Rhine Graben Boundary Fault (RGBF). After extensive shallow geophysical and morphotectonic investigations and analyses, we discovered that the eastern central RGBF consists of several parallel fault strands that are marked by topographic steps, by varying hydrogeologic conditions, moisture content and by geophysical anomalies in the subsurface (GPR and ERT data). Some of the scarps close to the alluvial plain of the river Rhine have been identified as erosional features. We opened six trenches perpendicular and parallel to the second topographic scarp and strand of the main RGBF in Ettlingen area. Trenching the main RGBF was precluded due to forest cover and the presence of big blocks of rock in the colluvium at the base of the slope (red Triassic sandstones). Trenches were up to 20 m in length and 2 m in width, and up to 3 m in depth. None of the trenches reached the Triassic Buntsandstein “basement”, and all exposed Pleistocene and Holocene strata. Some strata are interpreted as blocky/gravelly colluvium of the Glacial periods, Loess, redeposited gleyey Loess, soli-/gelifluction layers and deposits and organic paleosols. Most of these layers are clearly displaced by faults and downthrown to the west, although some strata appear to warp or fold over faults. Massive liquefaction and periglacial features have been found, the relation to the sedimentary sequences in the trenches need to be elaborated in future. The process is interpreted to be instantaneous, as massive colluvium is placed against clayey/silty Loess deposits, and therefore we attribute these displacements to earthquake-related faulting. Creep along the strand can be ruled out. The displacement on free faces is on the order of 30 – 50 cm per event vertically, and considerable horizontal offset (ca. 2 m), and we found evidence for two of such events. Applying the commonly used empirical relationships, these findings are interpreted as two events with a magnitude M larger than 6. These results show the bias between the seismogenic landforms (scarps, hanging valleys, triangular facets, etc.) in the eastern UGR margin and seismicity recorded by seismic stations in the area, as currently most of the activity is found in the southern URG near Freiburg. Our findings contribute significantly to the completeness of the earthquake history in the eastern central URG.

How to cite: Reicherter, K., Baize, S., Hürtgen, J., Cinti, F., Rockwell, T. K., Jomard, H., Seitz, G., and Ritter, J.: Paleoseismological trenching of the eastern Rhine Graben Boundary Fault: the Ettlingen segment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6187, https://doi.org/10.5194/egusphere-egu2020-6187, 2020.

EGU2020-19871 | Displays | SM2.2

Rupture dynamics and fault mechanics of intraplate earthquakes in Brazil

Elizabeth H. Madden, Alice-Agnes Gabriel, Lucas Barros, Juraci Carvalho, and George França

Mitigating intraplate earthquakes is necessary, as local populations are at risk both directly from strong shaking and indirectly from the threat to public infrastructure such as dams. However, the infrequency of these events and insufficient knowledge of how the ground will respond to passing seismic waves challenges mitigation. In Brazil, one M 5 earthquake occurs about every 5 years. M 4 earthquakes are more common and produce shaking intensities up to VI and VII on the Modified Mercalli scale. Brazilian earthquakes are shallower on average than events in other intraplate regions, which raises the possibility that fault mechanics and earthquake dynamics are different here. To work toward improving hazard mitigation and to better understand the physics of earthquakes in Brazil, we present 3D numerical models of the rupture process of two recent earthquakes using the open-source dynamic rupture and wave propagation software, SeisSol (www.seissol.org). Typically, sparse data prohibits the modeling of intraplate events. However, the 2010 Mara Rosa earthquake, the largest earthquake ever recorded in the Goiás-Tocantins Seismic Zone in central Brazil, and the 2017 Maranhão earthquake, which occurred in a previously aseismic region of northern Brazil, are relatively well studied and ample data is available. We report results within the range of uncertainty from the uncertainty in observations of stress drop, epicentral depth, fault geometry and regional stress state. The Mara Rosa earthquake occurred at an epicentral depth of ~2 km, while the Maranhão earthquake occurred between ~12-16 km depth. Modeling these two events allows us to contrast the influence of depth on the modeled earthquake source characteristics. We propose that fault cohesion dominates fault strength for the shallowest intraplate events, assuming a typical Mohr-Coulomb relationship.

How to cite: Madden, E. H., Gabriel, A.-A., Barros, L., Carvalho, J., and França, G.: Rupture dynamics and fault mechanics of intraplate earthquakes in Brazil, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19871, https://doi.org/10.5194/egusphere-egu2020-19871, 2020.

The mechanisms that lead to the localisation of stable continental region (SCR) seismicity, and strain more generally, remain poorly understood. Recent work has emphasised correlations between the historical record of earthquake epicentres and lateral changes in the thickness, composition and/or viscosity (thermal state) of the lithospheric mantle, as inferred from seismic velocity/attenuation constraints. Fluid flow and the distribution of heat production within the crust have also been cited as controls on the location of contemporary seismicity. The plate margin-centric hypothesis that the loading rate of crustal faults can been understood in terms of the strain rate of the underlying lithospheric mantle has been challenged in that a space-geodetic strain signal is yet to be measured in many SCRs. Alternatives involving the release of elastic energy from a pre-stressed lithosphere have been proposed.

The Australian SCR crust preserves a rich but largely unexplored record of seismogenic crustal deformation spanning a time period much greater than that provided by the historical record of seismicity. Variations in the distribution, cumulative displacement, and recurrence characteristics of neotectonic faults provide important constraint for models of strain localisation mechanisms within SCR crust, with global application. This paper presents two endmember case studies that illustrate the variation in deformation characteristics encountered within Australian SCR crust, and which demonstrate the range and nature of the constraint that might be imposed on models describing crustal deformation and seismic hazard.

The ~0.5 m high 2018 MW 5.3 Lake Muir earthquake scarp in southwest Western Australia is representative of a class of ruptures in the Precambrian SCR of Australia where the scarps are isolated from neighbouring scarps and there is little or no landscape evidence for recurrence of morphotectonic earthquakes, or of the construction of regional tectonic relief. In contrast, scarps in the Phanerozoic SCR of eastern Australia typically occur within a scarp-length of neighbouring scarps, and demonstrate extended histories of recurrence of morphotectonic events. For example, the ~75 km-long Lake George fault scarp is associated with a vertical displacement of ~250 m which accrued as the result of many morphotectonic earthquakes over the last ca. 4 Myr. The scarp links into neighbouring scarps, forming a belt-like arrangement that defines the topographic crest of the southeast Australian highlands. The limited data available indicates that recurrence is highly episodic, with periods of fault activity potentially coinciding with changes at the plate boundaries.

How to cite: Clark, D.: Neotectonic constraint on models of strain localisation within Australian Stable Continental Region (SCR) crust, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6279, https://doi.org/10.5194/egusphere-egu2020-6279, 2020.

EGU2020-7327 | Displays | SM2.2 | Highlight

Geodetic deformation rates and driving processes in metropolitan France and neighboring Western Europe

Stephane Mazzotti, Juliette Grosset, Christine Masson, and Philippe Vernant

We constrain present-day deformation rates and styles in metropolitan France and neighboring Western Europe using a dataset of ca. 1200 GNSS horizontal and vertical velocities from continuous and semi-continuous stations. The characterization and correction of network-scale common-mode noise, combined with two independent network analysis technics allow the resolution of very small horizontal velocities (resp. strain rates) with a 95% confidence ca. 0.1–0.2 mm/yr (resp. ca. 1 x 10-9 yr-9) on a spatial scale of 100–200 km. The resulting velocity and strain rate fields show regional coherent patterns that can be associated with features that have been previously identified (e.g., orogen-normal extension in the Pyrenees and Western Alps), but also with new deformation patterns such as North-South shortening in northeastern France - southwestern Germany north of the Alpine Front (Vosges - Rhine Graben - Black Forest). A joint analysis of these new geodetic data with seismicity and focal mechanism catalogs allows the definition of regional seismo-tectonic models that can be compared with the numerous models of deformation processes proposed for Western Europe, from plate tectonics to erosion or Glacial Isostatic Adjustment. We show that plate and micro-plate tectonics play a minor (probably negligible) role in present-day deformation in metropolitan France and that alternative non-tectonic processes must be considered to better understand the origin of recent moderate earthquakes such as the March 2019 Ml=4.9 Montendre earthquake in the Aquitaine Basin or the Nov. 2019 Mw=4.8 Teil earthquake in the Rhone Valley.

How to cite: Mazzotti, S., Grosset, J., Masson, C., and Vernant, P.: Geodetic deformation rates and driving processes in metropolitan France and neighboring Western Europe, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7327, https://doi.org/10.5194/egusphere-egu2020-7327, 2020.

EGU2020-20740 | Displays | SM2.2

Geomorphology of the cumulative deformation since Oligocene age on the Mw 4.9 Le Teil earthquake fault (South of France,11/11/19)

Estelle Hannouz, Christian Sue, Stéphane Baize, Jean-François Ritz, Matthieu Ferry, Laurence Audin, Andy Combey, Christophe Larroque, Andrea Walpersdorf, and Anne Lemoine

The Mw 4.9 earthquake that occurred near Montelimar on November 11, 2019 showed peculiar characteristics: a very shallow hypocenter (1km depth) with unexpected surface ruptures for such a moderate magnitude, and only few aftershocks showing low magnitudes (ML < 2.7). This event occurred in the industrialized Rhone Valley (including nuclear power plants and chemical industry) where several historical earthquakes with similar intensities and magnitudes took place (e.g. 1773, 1873, 1934).

The earthquake broke a ~5-km-long segment of the northern tip of the Cevennes fault system (La Rouvière Fault Segment). This ~100 km-long fault network has a NE-SW orientation trend and is inherited from the Variscan orogeny (~300 Ma). It first registered an extensive and transtensive tectonic phase ending at the Oligocene age (~30 Ma) before being inverted, as revealed by the reverse focal mechanism of the Le Teil event.

To date, this fault network has been poorly investigated in terms of seismic hazard, likely due to the low Mw expected on such short structures. Therefore, we started a new study to document its paleo-earthquake record in the framework of the new French RGF program (Alps and surrounding basins, BRGM).

Our first target was to map the cumulative trace of the fault. A first airborne LiDAR survey was acquired by helicopter and UAV (unmanned aerial vehicle) just after the earthquake. They allowed the identification of a continuous inherited scarp of 1 – 2 m in height over ~4 km along the preexisting Oligocene fault. In order characterize the post-Oligocene deformation along this fault, we performed a detailed analysis of geomorphological field observations, as well as a geophysical study by acquisition of seismic, electrical and ground-penetrating radar profiles. These profiles aimed to better understand how the 11/11/19 earthquake surface rupture is connected at depth to the Oligocene structure (La Rouvière Fault).

Each step of the analysis aims at eventually locating sites for further paleoseismological trenches, accounting for fault location, sediment preservation with favorable age determination potential and accessibility. This kind of investigation will provide information on the evolution over time of the seismic activity of this fault network, as well as relevant data on the current hazard they present in the specific context of the French Rhone Valley.

How to cite: Hannouz, E., Sue, C., Baize, S., Ritz, J.-F., Ferry, M., Audin, L., Combey, A., Larroque, C., Walpersdorf, A., and Lemoine, A.: Geomorphology of the cumulative deformation since Oligocene age on the Mw 4.9 Le Teil earthquake fault (South of France,11/11/19), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20740, https://doi.org/10.5194/egusphere-egu2020-20740, 2020.

EGU2020-10802 | Displays | SM2.2

Quantitative paleoseismology in Carinthia, Eastern Alps: Calibrating the lacustrine sedimentary record with historical earthquake data

Christoph Daxer, Christa Hammerl, Maria del Puy Papi-Isaba, Stefano Claudio Fabbri, Patrick Oswald, Jyh-Jaan Steven Huang, Michael Strasser, and Jasper Moernaut

In intraplate settings with moderate seismicity, recurrence intervals of strong earthquakes (Mw >6) typically exceed the short time span of instrumental and historical records. To assess the seismic hazard in such regions, lake sediments are increasingly used as earthquake archives: they can record strong seismic shaking as mass transport deposits (MTDs), turbidites or sediment deformations, preserved over several thousands of years. To provide information on paleo-earthquake size, however, the sedimentary imprints need to be thoroughly calibrated with independent information on seismic shaking strength.

In Carinthia (Eastern Alps, Austria), numerous lakes have experienced several devastating historical earthquakes with local seismic intensities (SI) ranging from V-XI (EMS-98 scale), although being located in an intraplate environment. Given that i) these events are well-spaced in time (AD1201, AD1348, AD1511, AD1690, AD1857 and AD1976), ii) due to historical earthquake research, an exceptional historical documentation exists, and iii) accurate shakemaps can be built based on a local Intensity Prediction Equation (IPE), we can examine the relationship between seismic intensity and the type, size and spatial distribution of sedimentary imprint in the lakes.

Here, we present investigations on two large lakes – Wörthersee and Millstätter See – by a dense grid of reflection seismic profiles (~640 km overall), 80 short (~1.5 m) sediment cores and multibeam bathymetry. The lakes consist of several sub-basins with potentially different intensity thresholds for the generation of sedimentary imprints. Mapping of MTDs, their scarps and associated turbidites as well as accurate dating (radiocarbon and varve counting on sediment thin sections) shows that the AD1348 earthquake (Mw ~7) led to extensive slope failures in both lakes. The AD1511 (Mw ~6.9) and AD1690 (Mw ~6.5) events, which exhibited lower local intensities (~VII) compared to those of AD1348 (VIII), are recorded as minor MTDs and turbidites. Quantitative description of earthquake-related event deposits (cumulative turbidite thickness, volume of mass transport deposits/megaturbidites) suggests a linear correlation with the respective local intensities in both Wörthersee and Millstätter See.

The AD1976 earthquake (Mw ~6.5; SI V-VI at the lakes) is not evidenced in the sedimentary record and therefore can be used for constraining the minimum threshold intensity for seismically-induced event deposits. By applying a grid-search approach using an empirical intensity-attenuation relationship, we can narrow down possible earthquake scenarios. Our data suggests that the highly debated epicentre of the AD1348 earthquake was much closer to the Austrian-Italian border than the epicentre of the AD1976 Friuli earthquake, possibly originating from the Periadriatic lineament. The AD1511 event probably had its epicentre southeast of our study area in Slovenia, and therefore further east than previous studies suggested. The AD1690 earthquake, however, is most likely of a local origin.

Our study reveals that investigating one lake, let alone one sediment core, is insufficient to reconstruct a region’s seismic history. Due to the exceptional setting of Carinthia, however, we can constrain the intensity pattern and localise the most likely epicentral region and fault source of past earthquakes. In an ongoing interdisciplinary study, we use this calibration to construct long calibrated lacustrine records for the last 14 ka.

How to cite: Daxer, C., Hammerl, C., del Puy Papi-Isaba, M., Fabbri, S. C., Oswald, P., Huang, J.-J. S., Strasser, M., and Moernaut, J.: Quantitative paleoseismology in Carinthia, Eastern Alps: Calibrating the lacustrine sedimentary record with historical earthquake data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10802, https://doi.org/10.5194/egusphere-egu2020-10802, 2020.

EGU2020-13302 | Displays | SM2.2

High-resolution catalog of the the Maurienne Swarm (French Alps) based on template matching and double-different relocation

Riccardo Minetto, Agnès Helmstetter, Philippe Guéguen, Mickael Langlais, Olivier Coutant, Stéphane Schwartz, Gaël Janex, Jérôme Nomade, and Thierry Dumont

Since 2017, the Maurienne Valley (French Alps) has been affected by an episode of seismic unrest. In this study we focused on the seismic swarm that occurred in 2017 and 2018, which was characterized by 8 events with ML > 3 and a maximum magnitude of 3.7. The goal was to extend the existing SISmalp catalog, and also to provide accurate locations and magnitude estimations.

The employed data was recorded by a local seismic network composed of 6 broadband stations. The use of template matching allowed us to detect more than 70000 events, increasing the detection rate by more than ten times compared to the original catalog. We obtained high resolution locations applying a double difference relocation method, providing as input differential times calculated by cross-correlating templates with their respective detections. Finally, we estimated magnitudes using template-family-based linear regression analysis, in order to include even the weakest events. The seismic locations will be discussed in the tectonic and geological setting of the Maurienne Valley.

How to cite: Minetto, R., Helmstetter, A., Guéguen, P., Langlais, M., Coutant, O., Schwartz, S., Janex, G., Nomade, J., and Dumont, T.: High-resolution catalog of the the Maurienne Swarm (French Alps) based on template matching and double-different relocation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13302, https://doi.org/10.5194/egusphere-egu2020-13302, 2020.

EGU2020-9241 | Displays | SM2.2

Integrated geophysical analysis of the April 2017 Moiyabana intra-plate earthquake, Botswana

Max Moorkamp, Estella Atekwana, Islam Fadel, Alice-Agnes Gabriel, Folarin Kolawole, Elisha Shemang, Calistus Ramotoroko, Mark van der Meijde, Kevin Mickus, Ame Selepeng, and Loago Molwalefhe

The 3 April 2017 Moiyabana intra-plate earthquake in central Botswana occurred in a region that, until then, had been assumed to be seismically quiet. Its location away from the East African Rift system in a Proterozoic mobile belt between Archean Cratons has raised questions on the triggering mechanism and sparked various studies investigating the crustal and mantle structure in the region, the focal mechanism and the displacement associated with the event. Aeromagnetic and magnetotelluric data indicate movement on a NW striking and NE dipping fault. However, the details of the fault geometry differ when analysing each dataset independently. The geophysical inversion results plus reconstructions of fault movement from InSAR data are all compatible with normal movement and reactivation of a previous thrust fault. An open question though is to which degree fluids are responsible for triggering the event. Here we present first results of reconciling the different available datasets in an integrated analysis. We will show an updated geophysical model of the region around the hypocenter. Such a model can help to shed light on the rupture processes during the earthquake and forms a first step to unravel the genesis of this intra-plate event.

How to cite: Moorkamp, M., Atekwana, E., Fadel, I., Gabriel, A.-A., Kolawole, F., Shemang, E., Ramotoroko, C., van der Meijde, M., Mickus, K., Selepeng, A., and Molwalefhe, L.: Integrated geophysical analysis of the April 2017 Moiyabana intra-plate earthquake, Botswana, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9241, https://doi.org/10.5194/egusphere-egu2020-9241, 2020.

EGU2020-10595 | Displays | SM2.2

Is there active tectonics at the Nile cataracts in Sudan? An archaeoseismological study

Miklos Kazmer, Nada Bushra El Tahir, Krzysztof Gaidzik, and Balázs Székely

The Nile is the longest river on earth, accordingly with huge drainage and major floods, regulated by the African monsoon. Significant amount of sediment is carried by the river; its deposition forms alluvial plains along most of its course. However, in Upper Egypt and northern Sudan there are six major and several minor cataracts totalling 327 km in length. There the river flows directly on bedrock, and a multitude of islands and rocks in the riverbed makes navigation hard or impossible throughout much of the year. Obviously, the Nile is unable to remove these obstacles from its flow (despite its ability to carve a deep canyon in the African continent during Messinian lowstand of the Mediterranean Sea). It has been suggested that the Cataract Nile is in a youthful stage, flows along structurally controlled turns and that earthquakes in southern Egypt prove that portions of the Nubian Swell are still tectonically active (Thurmond et al., 2004). However, the Sudan part of the river does not show any seismic activity. An archaeoseismological study is in progress to locate evidence of past earthquakes preserved in monumental architecture erected during the past 3500 years. Pyramids in Meroe display masonry shifted in plane of the wall: this was caused by one or more earthquakes of intensity I0 = 9 on the Archaeological Intensity Scale. We suggest that an ongoing systematic study of monumental stone and adobe buildings along the Nile in the region of the Nubian Swell will find further evidence of major earthquakes in the region, contributing to a better understanding of seismic hazard in Sudan.

Reference

Thurmond, A.K., Stern, R.J., Abselsalam, M.G., Nielsen, K.C., Abdeen, M.M., Hinz, E. (2004): The Nubian Swell. - Journal of African Earth Sciences 39, 401-407.

How to cite: Kazmer, M., El Tahir, N. B., Gaidzik, K., and Székely, B.: Is there active tectonics at the Nile cataracts in Sudan? An archaeoseismological study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10595, https://doi.org/10.5194/egusphere-egu2020-10595, 2020.

EGU2020-19597 | Displays | SM2.2

Evidence of recent activity in the Camorro Fault (Central Betics, Southern Spain)

Jorge P. Galve, Cristina Reyes-Carmona, Antonio Jabaloy, Patricia Ruano, José Vicente Pérez-Peña, José Miguel Azañón, and Guillermo Booth-Rea

The Camorro Fault is located at the foot of the northern slope of a limestone karstic massif that is called ‘Sierra de Las Chimeneas’, in the central sector of the Betic Cordillera (Southern Spain). The fault shows a well-marked surface expression. It is a 6 km-length strike-slip with extensional component fault that forms part of the Torcal Shear Zone. This fault can be continued 7 km eastward along the foot of northern slope of the ‘Torcal de Antequera’ (Málaga), World Heritage Site since 2016. The Camorro fault plane is well-exposed in some sectors while in others, the fault plane has been either affected by karstification processes or partially covered by talus deposits.

One of the most characteristic geomorphological features of the ‘Sierra de Las Chimeneas’ area is an impressive rock avalanche deposit, covering an area of 2.2 km2 and for which we estimated a volume of 0.48 Hm3. Given the characteristics of the rock avalanche deposit, we consider that it could be triggered by an earthquake on the Camorro Fault. This hypothesis is supported by other investigations that have already referred to quaternary paleoseismicity in this area. Previous archaeological research revealed a period of human occupation in a cave (‘Cueva del Toro’) located in the ‘Torcal de Antequera’ that point out evidences about the occurrence of a cataclysm in the late Copper Age (about 5000 years ago). Other studies have also suggested a possible connection between seismic events and megalith-building near Antequera. Beyond this, an archaeoseismic analysis in the megalithic site of Antequera (also World Heritage Site since 2016) found deformation structures probably linked to oscillations between the megalith orthostats during an earthquake. According to all of mentioned research, the Camorro Fault could be a good candidate to account for such prehistoric earthquake.

Further geochronological work remains to be done, specially focused on dating (e.g. by cosmogenic isotopes) the fault scarp of the Camorro Fault and the associated rock avalanche deposits. If cosmogenic and archaeological dates coincide, we could attribute all the mentioned observations to an earthquake of severe magnitude in an area where the population ignore that hazard. Thus, we could contribute not only to the history of human occupation of the World Heritage Site but also providing insights into the earthquake recurrence and seismic hazard of the region.

How to cite: Galve, J. P., Reyes-Carmona, C., Jabaloy, A., Ruano, P., Pérez-Peña, J. V., Azañón, J. M., and Booth-Rea, G.: Evidence of recent activity in the Camorro Fault (Central Betics, Southern Spain), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19597, https://doi.org/10.5194/egusphere-egu2020-19597, 2020.

SM2.5 – Understanding large subduction earthquakes and tsunamigenesis

EGU2020-1379 | Displays | SM2.5

The effect of multiple splay fault rupture on tsunamis

Iris van Zelst, Leonhard Rannabauer, Alice-Agnes Gabriel, and Ylona van Dinther

Earthquake rupture on splay faults in subduction zones could pose a significant tsunami hazard, as they could accommodate more vertical displacement and are situated closer to the coast. To better understand this tsunami hazard, we model splay fault rupture dynamics and tsunami propagation and inundation constrained by a geodynamic seismic cycle (SC) model; building on work presented in Van Zelst et al. (2019). This two-dimensional modelling framework considers geodynamics, seismic cycles, dynamic ruptures, and tsunamis together for the first time. The SC model provides six blind splay fault geometries, self-consistent stress and strength conditions, and heterogeneous material properties in the domain. We find that all six splay faults are activated when the megathrust ruptures. The largest splay fault closest to the nucleation region ruptures immediately when the main rupture front passes the branching point. The other splay faults are activated through dynamic stress transfer from the main megathrust rupture or reflected waves from the surface. Splay fault rupture results in distinct peaks in the vertical surface displacements with a smaller wavelength and larger amplitudes. The effect of the vertical surface displacements also translates into the resulting tsunami, which consists of one large wave for the megathrust-only model and seven waves for the model including splay faults. Here, six of the waves can be attributed to the splay faults and the seventh wave results from the shallow tip of the megathrust. The waves from the rupture including splay faults have larger amplitudes and result in two episodes of coastal flooding. The first episode is due to the large wave caused by rupture on the largest splay fault nearest to the coast. The second flooding episode results from the combination and interference of the waves caused by the rest of the splay faults and the shallow megathrust tip. In contrast, the tsunami caused by rupture on only the megathrust has only one episode of flooding. Our results suggest that larger-than-expected tsunamis could be attributed to rupture on large splay faults. When multiple smaller splay faults rupture their effect on the tsunami might be hard to distinguish from a pure megathrust rupture. Considering the significant effects splay fault rupture can have on a tsunami, it is important to understand splay fault activation and to consider them in hazard assessment.

References:

Van Zelst, I., Wollherr, S., Madden, E. H. , Gabriel, A.-A., and Van Dinther, Y. (2019). Modeling megathrust earthquakes across scales: one-way coupling from geodynamics and seismic cycles to dynamic rupture. Journal of Geophysical Research: Solid Earth, 124, https://doi.org/10.1029/2019JB017539

How to cite: van Zelst, I., Rannabauer, L., Gabriel, A.-A., and van Dinther, Y.: The effect of multiple splay fault rupture on tsunamis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1379, https://doi.org/10.5194/egusphere-egu2020-1379, 2020.

Great earthquakes repeatedly occurred with different rupture processes in the Nankai trough, southwestern Japan. The 1944 Tonankai and the 1946 Nankai earthquakes (M ~8) caused serious tsunami damage over many areas along the coastline. The greatest earthquake in this region is the 1707 Hoei earthquake (M 8.4) that is believed to have ruptured the whole region (~600 km) of the Nankai Trough. The purpose of this study is to theoretically assess the tsunami height along the coasts excited by great earthquakes that can possibly occur in future in this region and simulate observable tsunami records during the earthquakes.

This study employed a new method for making various rupture scenarios. Based on a shear-stress distribution along the plate boundary estimated by the GNSS data analyses (Noda et al. 2018 JGR), we calculated coseismic slip distributions to release the accumulated stress for possible multi-segment rupture scenarios. Then, we used the strain energy released by the rupture to evaluate the possibility of each event. The released strain energy should be larger than the energy dissipated on the fault. However, for some scenarios, the released strain energy was smaller than the dissipated energy under the assumptions of friction laws. Such rupture scenarios are not likely to occur in the viewpoint of earthquake mechanics. This approach can provide necessary conditions of the strain energy or the accumulated stress levels for multi-segment rupture processes, while methods based on empirical or kinematic approaches do not treat stress or interseimsmic stress-accumulation periods required for ruptures.

Another distinctive point in our approach is that we theoretically synthesize ocean-bottom pressure changes caused by both seismic waves and tsunamis using a simulation method based on elastic and fluid dynamics (Saito and Tsushima 2016 JGR; Saito et al. 2019 Tectonophysics). Seismic wave contributions to ocean-bottom pressure changes are critically important for the synthetics in near-field or inside rupture areas because the seismic waves overlap with tsunami signals and work as noise for real-time tsunami monitoring. The records simulated in this study can be used to examine the monitoring ability of a deep-ocean observation network for megathrust earthquakes and tsunamis in this region.

How to cite: Saito, T. and Noda, A.: Tsunami scenarios for megathrust earthquakes: mechanics of multi-segment ruptures and elastic-fluid dynamics of wave propagation , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12273, https://doi.org/10.5194/egusphere-egu2020-12273, 2020.

EGU2020-6939 | Displays | SM2.5

Tsunami Generation due to Supershear Earthquakes: A Case Study

Faisal Amlani and Harsha Bhat

The 28 September 2018 Mw 7.5 Sulawesi strike-slip earthquake generated an unexpected tsunami with devastating consequences. Since such strike-slip earthquakes are not expected to generate large tsunamis, the latter’s origin remains much debated. A key notable feature of this earthquake is that it ruptured at supershear speed, i.e., with a rupture speed greater than the shear wave speed of the host medium. Dunham and Bhat (2008) showed that such supershear ruptures, in half-space, produce two shock fronts (or Mach fronts) corresponding to an exceedance of shear and Rayleigh wave speeds. The Rayleigh Mach front carries significant vertical velocity along its front. We couple the ground motion produced by such a supershear earthquake to a 1D non-linear shallow water wave equation that accounts for both the time-dependent bathymetric displacement as well its velocity. We use an extension of Fourier-based PDE solvers called the Fourier Continuation (FC) method to numerically solve the system. The FC method enables high-order convergence of Fourier series approximations of non-periodic functions by resolving the well-known Gibbs “ringing” effect.  FC-based solvers offer limited numerical dispersion, high-order accuracy and mild CFL conditions—making them ideal to solve this system. Using the local bathymetric profile of Palu bay around the Pantoloan harbor tidal gauge, we have been able to clearly reproduce the observed tsunami with minimal tuning of parameters. We conclude that the Rayleigh Mach front, generated by a supershear earthquake combined with the Palu bay geometry, caused the tsunami.

How to cite: Amlani, F. and Bhat, H.: Tsunami Generation due to Supershear Earthquakes: A Case Study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6939, https://doi.org/10.5194/egusphere-egu2020-6939, 2020.

EGU2020-5809 | Displays | SM2.5 | Highlight

Earthquake rupture properties and tsunamigenesis in the shallowest megathrust

Valenti Sallares, Cesar R. Ranero, Manel Prada, and Alcinoe Calahorrano B.

Seismological data provide compelling evidence of a depth-dependent rupture behavior of megathrust earthquakes. Relative to deeper events of similar magnitude, shallow earthquake ruptures have larger slip and longer duration, radiate energy that is depleted in high frequencies and have a larger discrepancy between their surface wave and moment magnitudes (MS and MW, respectively). These source properties make them prone to generating devastating tsunamis without clear warning signs. The origin of the observed differences has been a long-lasting matter of debate. Here we first show that the overall depth trends of all these observations can be explained by worldwide average variations of the elastic properties of the rock body overriding the megathrust fault, which deforms by dynamic stress transfer during co-seismic slip, and we discuss some general implications for tsunami hazard assessment. Second, we test this conceptual model for the particular case of the 1992 Nicaragua tsunami earthquake (MS7.2 and MW7.8). This event nucleated at ~20 km-deep but it appears to have released most of its seismic moment near the trench. This earthquake caused mild shaking and little damage, so that tsunami hazard based on human perception was underestimated and the destructive tsunami hit the coast unexpectedly. We use a set of 2D seismic data to map the P-wave seismic velocity above the inter-plate boundary, and we combine it with previously estimated moment release distribution to calculate slip and stress drop distributions and moment-rate spectra that are compatible with both the seismological and the geophysical data. The models confirm that slip concentrated in the shallow megathrust, with two patches of maximum slip exceeding 10-12 m in the near-trench zone that can explain the observed tsunami run-up, while the average stress drop is ~3 MPa. The low rigidity of the upper plate in the zone of maximum slip explains the high frequency depletion and the resulting MW-MS discrepancy without need to consider anomalous rupture properties or fault mechanics

How to cite: Sallares, V., Ranero, C. R., Prada, M., and Calahorrano B., A.: Earthquake rupture properties and tsunamigenesis in the shallowest megathrust, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5809, https://doi.org/10.5194/egusphere-egu2020-5809, 2020.

EGU2020-6805 | Displays | SM2.5 | Highlight

Predictability of large subduction earthquakes: insights from analog models and machine learning

Fabio Corbi, Jonathan Bedford, Laura Sandri, Francesca Funiciello, Adriano Gualandi, and Matthias Rosenau

Despite the growing spatio-temporal density of geophysical observations, our understanding of the megathrust earthquake cycle continues to be limited by a series of factors, in particular the short observation time compared to mega-earthquake recurrence and the partial spatial coverage of geodetic data. Here, we attempt to compensate for these natural limitations by simulating dozens of seismic cycles in a laboratory-scale analogue model of subduction. The model creates analog earthquakes of magnitude Mw 6.2–8.3, with a coefficient of variation in recurrence intervals of 0.5, similar to real subduction megathrusts. Using a digital image correlation technique, we measure coseismic and interseismic deformation – this is akin to having a dense continuous geodetic network homogeneously distributed over the whole margin. We show how, by deciphering the spatially and temporally complex surface deformation history, machine learning can predict the timing and size of analog earthquakes. Then, we investigate data characteristics that maximize the performance of a machine learning binary classifier predicting slip-events imminence. We show how this framing can be used for designing an efficient geodetic network, and defining the minimum space-time coverage requirements for analog earthquake prediction. Converting the laboratory scale to the natural scale, we found that a 70-85 km wide coastal swath gives the most important information on slip imminence and that model performance is mainly 
influenced by the alarm duration, with density of stations and record length playing a secondary role. Under optimal monitoring conditions, about ten seismic cycles long record is enough to predict alarm periods in good agreement with those observed.

How to cite: Corbi, F., Bedford, J., Sandri, L., Funiciello, F., Gualandi, A., and Rosenau, M.: Predictability of large subduction earthquakes: insights from analog models and machine learning, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6805, https://doi.org/10.5194/egusphere-egu2020-6805, 2020.

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Improving the Co-seismic Slip Distributions of Synthetic Catalogs With Real Observations

Hafize Başak Bayraktar, Antonio Scala, Gaetano Festa, and Stefano Lorito

Subduction zones are the most seismically active regions on the globe and about 90% of historical events, including the largest ones with the magnitude M>9, occurred along these regions (Hayes et al., 2018). Most of these events were followed by devastating tsunamis with, in some cases, perhaps unexpected wave height distributions. Observation of events in the megathrust environment reveals that some earthquakes are characterized by slip concentration on the very shallow part of the subduction zone. This shallow slip phenomenon was repeatedly observed in the last two decades for both ordinary megathrust events (e.g. 2010 Maule and 2011 Tohoku) and tsunami earthquakes (2006 Java and 2010 Mentawai). Shallow ruptures feature depleted short–period energy release and very slow rupture velocity possibly due to the presence of (hydrated) sediments (Lay et al., 2011; Lay 2014; Polet and Kanamori, 2000). Associated long rupture durations have been explained with fault mechanics-related rigidity and stress drop variation with depth (Bilek and Lay, 1999) or, more recently, with lower rigidity of surrounding materials (Sallares and Ranero, 2019).

The characteristics of co-seismic slip distribution have an important impact on tsunami hazard. There are numerous methods that have been proposed to generate stochastic slip distributions, also including shallow slip amplification (Le Veque et al., 2016; Sepulveda et al., 2017; Scala et al., 2019). However, these models need to be calibrated against slip models estimated for real events.

Here, we investigate similarities and differences between the synthetic slip distributions provided by Scala et al. (2019) and a suite of 144 slip models of real events that occurred in different subduction zones (Ye et al.,2016). In particular, Scala et al. (2019) model features shallow slip amplification in single events, whose relative probabilities are balanced to restore cumulative slip homogeneity on the fault plane over multiple seismic cycles. This study also aims to improve and/or calibrate this model to account for the behavior observed from real events.

How to cite: Bayraktar, H. B., Scala, A., Festa, G., and Lorito, S.: Improving the Co-seismic Slip Distributions of Synthetic Catalogs With Real Observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1050, https://doi.org/10.5194/egusphere-egu2020-1050, 2020.

EGU2020-21113 | Displays | SM2.5

Tsunamigenesis Revisited

James Moore, Judith Hubbard, Raquel Felix, Karen Lythgoe, and Adam Switzer

When modelling tsunamis and assessing tsunami hazard, it is frequently necessary to make simplifying assumptions in order to reduce the problem to one which is computationally tractable within a reasonable period of time. In this paper, we examine the key factors controlling the generation of the initial sea surface wave and present a series of clear and simple guidelines for real-world problems. We also provide number of computational resources (a tsunami loader) which may be utilised with existing tsunami propagation codes (e.g. COMCOT) to modify the initial sea-surface way, where necessary.

 

Most tsunami modelling codes operate under the assumption that the initial sea surface wave is identical to the seafloor perturbation. Yet this is only true for large tsunami sources (Kajiura 1963). With our tsunami loader we model the tsunamigensis process and the formation of the initial sea-surface wave. Critically, the diffusive effect of the water column above the deforming seafloor is accurately addressed, which can result in a substantial decrease in the energy in the initial sea-surface wave.

 

For example, let us consider a rectangular uplifting patch on the seafloor, at a depth of 4km. For a 4x4km square patch, the diffusive effect will result in an energy reduction of 90%. Even if one of those dimensions is 100 times larger, such that we have a relatively large 400x4 km uplifting region, the energy reduction is still 70%. We find the shortest dimension of the uplifting patch provides a strong control on the energy of the initial sea-surface wave, and consequential tsunami. If we move to a 40x40 km square patch we find the reduction is now 20%, and 400x40 km patch is now a relatively modest, but non-negligible 12%.

 

We also include other effects such as the time-dependence of seafloor deformation, which also reduces the potential tsunami energy, and horizontal advection of topography, which conversely increases the potential tsunami energy, in our analysis of the tsunamigenesis process. Currently implemented for fault sources, we are working to include landslide and volcanic sources.

How to cite: Moore, J., Hubbard, J., Felix, R., Lythgoe, K., and Switzer, A.: Tsunamigenesis Revisited, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21113, https://doi.org/10.5194/egusphere-egu2020-21113, 2020.

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Spatial variations of coda wave attenuation in Andaman-Nicobar subduction zone

Chandrani Singh, Rahul Biswas, Namrata Jaiswal, and M. Ravi Kumar

We investigate the spatial variations of coda attenuation (Qc) structure in the tectonically complex Andaman–Nicobar subduction zone (ANSZ), which is one of the most seismically active subduction zones on the Earth. The region constitutes the northernmost part of the Sunda subduction zone, where the Indian plate disappears beneath the Burmese plate along the Burma and Andaman arcs to the east. This is probably the first attempt to map the Qc variations across the whole ANSZ. In a seismically active area, the spatial distribution of Qc is important to evaluate the seismic hazard in relation to tectonics and seismicity.

A total of 289 high-quality events recorded at a network of broad-band stations operational since 2009 are considered for the analysis. The variations in attenuation characteristics at different frequencies reveal a marked contrast from the northern to the southern Andaman region, consistent with the geotectonic diversity of the region. At low frequencies, low Qc values are observed in the northern part of ANSZ in the vicinity of the Narcondum volcanic island, which does not appear in the high-frequency image. The low values are in agreement with the 3-D tomogram, which suggests a distinct low-velocity structure below this volcanic island. The Andaman trench also exhibits a relatively low Qc, which is well correlated with the low-Vp zone. The spatial distributions of Q0 (Qc at 1 Hz) structure of the region are further projected onto three east–west profiles to capture the detailed attenuation characteristics from north to south. Results show that the northernmost part of ANSZ is more attenuative than the southern part, which may be indicative of the changes in physical properties of the crust. The frequency relation parameter (n) shows an inverse correlation with the observed Q0 values. Furthermore, we have observed a good correlation between the Q0 variation and the seismicity pattern of the area that enables us to enhance our understanding about the role of crustal heterogeneity in the earthquake occurrence in this area.



How to cite: Singh, C., Biswas, R., Jaiswal, N., and Kumar, M. R.: Spatial variations of coda wave attenuation in Andaman-Nicobar subduction zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-32, https://doi.org/10.5194/egusphere-egu2020-32, 2020.

EGU2020-1675 | Displays | SM2.5

Linking geodynamic subduction models to self-consistent 3D dynamic earthquake rupture and tsunami simulations

Sara Aniko Wirp, Alice-Agnes Gabriel, Elizabeth H. Madden, Iris van Zelst, Lukas Krenz, and Ylona van Dinther

3D imaging reveals striking along-trench structural variations of subduction zones world-wide (e.g., Han et al, JGR 2018). Subduction zones include basins, sediments, splay and back-thrusting faults that evolve over a large time span due to tectonic processes, and may crucially affect earthquake dynamics and tsunami genesis. Such features should be taken into account for realistic hazard assessment. Numerical modeling bridges time scales of millions of years of subduction evolution to seconds governing dynamic earthquake rupture, as well as spatial scales of hundreds of kilometers of megathrust geometry to meters of an earthquake rupture front.

Recently, an innovative framework linking long-term geodynamic subduction and seismic cycle models to dynamic rupture models of the earthquake process and seismic wave propagation at coseismic timescales was presented (van Zelst et al., JGR 2019). This workflow was extended in a simple test case to link the 2D seismic cycle model to a three-dimensional earthquake rupture mode, which was then linked to a tsunami model  (Madden et al., EarthArxiv, doi:10.31223/osf.io/rzvn2). Here, we couple a 2D seismic cycle model to 3D earthquake and tsunami models and assess the geophysical aspects of this coupling. We extract all 2D material properties, stresses and the strength of the megathrust, and its geometry, from the seismic cycling model at a time step right before a typical megathrust event to use as initial conditions for the 3D dynamic rupture models. We explore the effects of along-arc variations of megathrust curvature, sediment content, and closeness to failure of the wedge on earthquake dynamics by studying the effects on slip, rupture velocity, stress drop and seafloor deformation.

In a next step, the dynamic seafloor displacements are linked to tsunami simulations that use depth-integrated (hydrostatic) shallow water equations. This approach efficiently models wave propagations and large-scale horizontal flows. We also present novel, fully coupled 3D dynamic rupture-tsunami simulations (Krenz et al., AGU19; Abrahams et al., AGU19; Lotto and Dunham et al., 2015, Computational Geosciences) which solve simultaneously for the solid earth and ocean response, taking gravity into account via a modified free surface boundary condition.

Earthquake rupture modeling and the fully-coupled tsunami modeling utilize SeisSol (www.seissol.org), a flagship code of the ChEESE project (www.cheese-coe.eu). SeisSol is an open source software package using unstructured tetrahedral meshes that are optimally suited for the complex geometries of subduction zones. The here presented links between geodynamic subduction and seismic cycling model with earthquake dynamics and tsunami models better account for the complexity of subduction zones and help evaluate the effects of along arc heterogeneities on earthquake and tsunami behavior and advance physics-based assessments of earthquake-tsunami hazards.

How to cite: Wirp, S. A., Gabriel, A.-A., Madden, E. H., van Zelst, I., Krenz, L., and van Dinther, Y.: Linking geodynamic subduction models to self-consistent 3D dynamic earthquake rupture and tsunami simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1675, https://doi.org/10.5194/egusphere-egu2020-1675, 2020.

EGU2020-2940 | Displays | SM2.5

Influence of incoming plate relief on overriding plate deformation and earthquake nucleation: Cocos Ridge subduction (Costa Rica)

Sara Martínez-Loriente, Valentí Sallarès, César R. Ranero, Jonas B. Ruh, Udo Barckhausen, Ingo Grevemeyer, and Nathan Nangs

We present a 2D p-wave velocity (Vp) model and a coincident multichannel seismic reflection profile mapping the structure of the southern Costa Rica margin and incoming Cocos Ridge. The seismic profiles image the ocean and overriding plates from the trench across the entire offshore margin, including the structures involved in the 2002 Mw6.4 Osa earthquake. The overriding plate consists of three domains: Domain I at the margin front displays thin-skinned deformation of an imbricated-thrust system composed of fractured rocks with relatively low Vp. Domain II under the middle continental shelf is comparatively less fractured, showing ~15 km long landward-dipping reflection packages and discrete active deformation of the shelf sediment and seafloor. Domain III in the inner shelf is little fractured and appears to be dominated by elastic deformation, with inactive structures of an extensional basin consisting of tilted blocks overlain by ~2 km-thick gently landward-dipping strata. The velocity structure supports the argument that the bulk of the margin is highly consolidated rock possibly similar to outcrops in the Osa Peninsula. Thick-skinned tectonics probably causes the uplift of Domains II and III. The incoming oceanic plate shows crustal thickness variations from ~14 km at the trench (Cocos Ridge) to 6-7 km beneath the continental shelf. We combine (1) inter-plate geometry and velocity-derived fracturing degree at the base of the overriding plate, (2) tectonic stresses and brittle strain above the inter-plate boundary extracted from 3D numerical models, and (3) earthquake locations, to investigate potential relationships between structure and earthquake generation. The 2002 Osa earthquake and its aftershocks appear to have nucleated at the leading flank of two subducting seamounts, coinciding with the area of highest tectonic overpressure in numerical models. Both estimated rock fracturing and modelled brittle strain, steadily increase from the leading flank of the subducting seamounts to their top, which we interpret to reflect the progressive damage caused by the incoming plate relief. Therefore, the analysis supports a spatial and temporal relationship between subducting seamount location, upper plate fracturing, brittle strain, tectonic overpressure, and earthquake nucleation.

How to cite: Martínez-Loriente, S., Sallarès, V., R. Ranero, C., B. Ruh, J., Barckhausen, U., Grevemeyer, I., and Nangs, N.: Influence of incoming plate relief on overriding plate deformation and earthquake nucleation: Cocos Ridge subduction (Costa Rica), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2940, https://doi.org/10.5194/egusphere-egu2020-2940, 2020.

EGU2020-3593 | Displays | SM2.5

Interaction between interplate fault topography and tsunamigenic structures at the subduction zone offshore West Mexico

Rafael Bartolome, Manel Prada, Claudia Gras, Slaven Begovic, William Bandy, and Juan José Dañobeitia

The megathrust topography is key in conditioning the structural integrity of the overriding plate, and thus, the generation of tsunamigenic structures. Our objective is to investigate the Rivera subduction zone, offshore the Mexican Pacific coast, known for hosting large megathrust tsunamigenic earthquakes (Mw > 7.5), and where little is known regarding the distribution of tsunamigenic structures along the margin. Our working hypothesis is that there is an interaction between the megathrust relief at the surface of the subducted slab (Rivera Plate) and the existence of tsunamigenic structures in the above unsubducted plate (North America). To investigate this interaction, we used seismic methods to characterize the variations of the physical properties of the overriding plate, generally related to tectonic (faults) structures that are sources of tsunamis, with the reliefs of the deeper subducted plate obtained with the same method. Here, we use spatially coincident 2D multichannel seismic (MCS, 5.85 km long-streamer) and active marine wide-angle seismic (WAS) data acquired during the TSUJAL survey in 2014 offshore west of Mexico to measure structural variations of the overriding plate and the megathrust interface. We have jointly inverted refracted and reflected travel-times (TT) from both MCS and WAS data to constrain the P-wave velocity (Vp) structure of the overriding plate and the geometry of the megathrust. Before the inversion and to increase the amount of refracted TT we have applied the downward continuation technique to MCS field data allowing to better image the refracted waves in the records. MCS data has a higher spatial sampling than OBS data, which translates into a higher density sampling of the refracted waves and hence the tomographic resolution. Therefore, the resulting tomographic model displays small-scale velocity structure variations of the overriding plate and the megathrust relief that would not be resolved with TT from OBS data only. We used further refracted and reflected TT from OBS data to constrain the Vp structure of the subducting oceanic plate and the geometry of the oceanic Moho. The inverted megathrust interface obtained with the tomography shows clear topographic features in its shallow portion (<~10 km from the trench). Such topographic variations are smaller than the average size of seamounts of the Rivera plate, but they are similar to the seafloor fabric generated by a relict East Pacific Rise segment identified west of the trench in the bathymetry map of the region. Time-migrated images were also obtained after processing the MCS data to constrain the tectonic framework of the shallow subduction zone regardless of the tomographic models. The seismic sections reveal the lack of an extensive accretionary prism, implying that subduction-erosion dominates the structure of the margin in this region. Integrating all the data results, we find that megathrust highs correlate with low-velocity anomalies, suggesting the presence of fluids, and correlate with the presence of extensional faults in the overriding plate as well. This correlation demonstrates the control that megathrust topography exerts on the formation of tsunamigenic structures along the Rivera plate boundary.

How to cite: Bartolome, R., Prada, M., Gras, C., Begovic, S., Bandy, W., and Dañobeitia, J. J.: Interaction between interplate fault topography and tsunamigenic structures at the subduction zone offshore West Mexico, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3593, https://doi.org/10.5194/egusphere-egu2020-3593, 2020.

EGU2020-5457 | Displays | SM2.5

Slip-dependent weakening revealed for a shallow megasplay fault in the Nankai subduction zone

Alexander Roesner, Matt Ikari, Andre Huepers, and Achim Kopf

The Nankai Trough megasplay fault likely hosts different modes of fault slip, from slow to megathrust earthquakes, and is responsible for related phenomena such as tsunamis and submarine landslides. All types of slip events require some kind of frictional weakening process (e.g. slip and/or velocity weakening) in order to nucleate and propagate. Most frictional earthquake studies analyze the velocity dependence of friction but disregard the slip dependence of friction observed in experimental friction studies.

We tested fluid-saturated powdered megasplay fault samples from Integrated Ocean Drilling Program Site C0004 in a direct shear apparatus under effective normal stresses from 2 – 18 MPa to investigate the velocity- and slip-dependence of friction of the megasplay fault. For every tested effective normal stress, we performed one velocity-step experiment and two constant velocity experiments (no velocity step). In the velocity-step experiments the samples were sheared to a total displacement of 10 mm, with an initial sliding velocity V0 = 0.1 µm/s for the first ~5 mm (run-in) followed by a velocity step increase to V = 1.0 µm/s over the last 5 mm. During the constant velocity experiments, the shearing velocity (0.1 and 1.0 µm/s respectively) was held constant for 10 mm of displacement.

The velocity-stepping tests showed an evolution from velocity weakening at low effective normal stresses to velocity strengthening at high effective normal stresses. All experiments revealed strong slip-weakening behavior, with the slip dependence having a much larger effect on friction than the velocity dependence. The friction slip dependence is also controlled by the effective normal stress, showing large weakening rate at low effective normal stresses and smaller weakening rate at higher effective normal stresses. Therefore, both frictional weakening mechanisms on the megasplay fault become more effective at shallow depths. This may amplify seafloor deformation by shallow coseismic slip and could increase the tsunamigenic potential of the fault zone.

How to cite: Roesner, A., Ikari, M., Huepers, A., and Kopf, A.: Slip-dependent weakening revealed for a shallow megasplay fault in the Nankai subduction zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5457, https://doi.org/10.5194/egusphere-egu2020-5457, 2020.

EGU2020-8603 | Displays | SM2.5

The heterogeneous distribution of elastic properties in the tsunamigenic region of subduction zones

Manel Prada, Valenti Sallares, Rafael Bartolome, Adria Melendez, Alcinoe Calahorrano, Slaven Begovic, Claudia Gras, and Cesar Ranero

In the shallow region of subduction zones, topographic variations of the interplate interface condition the structural integrity of the upper plate, and thus the distribution of elastic properties in this region, which determines its tsunamigenic potential. Yet, we know little about the distribution of elastic properties in these shallow regions, which yields large uncertainty during tsunami hazard assessment.

Here we assess topographic variations of the interplate boundary as well as the distribution of elastic properties of the upper plate in two tsunamigenic regions of the Middle American Trench. We focus on the rupture area of three tsunami earthquakes, the 1992 Nicaragua event, and the 1932 and 1995 Jalisco-Colima earthquakes (Pacific Mexican coast).

We use 2D coincident wide-angle (WAS) and multichannel seismic (MCS) lines acquired across the rupture area of each event to jointly invert refracted and reflected travel-times (TT) and obtain the P-wave velocity (Vp) structure of the tsunamigenic region of the upper plate, and the geometry of the interplate boundary. Mixing both types of seismic data allowed for the first time to retrieve small-scale local topographic variations of the interplate that would have been omitted with the classical inversion of WAS TT. From Vp, we derive other elastic parameters namely, density, S-wave velocity, and rigidity using well-established empirical relationships.

The results show that the heterogeneous distribution of the elastic properties of the upper plate in the shallow tsunamigenic region correlates with topographic variations of the interplate in both margins. These results not only sustain the direct relationship between the interplate relief and the tectonic structure of the overriding plate as it has been already stated by previous authors, but they also allow to quantify the relationship between topographic highs of the subducted plate with low rigidity regions in the upper plate. This quantification is of paramount importance in these shallow regions of the subduction, because low rigidity implies high slip during coseismic deformation, and therefore, high tsunamigenic potential. The heterogeneous distribution of elastic properties inferred for the upper plate in this study should be considered during tsunami modeling, tsunami hazard assessment and tsunami early warning systems.

How to cite: Prada, M., Sallares, V., Bartolome, R., Melendez, A., Calahorrano, A., Begovic, S., Gras, C., and Ranero, C.: The heterogeneous distribution of elastic properties in the tsunamigenic region of subduction zones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8603, https://doi.org/10.5194/egusphere-egu2020-8603, 2020.

EGU2020-10762 | Displays | SM2.5

Assessing future uncertainties: earthquake tsunami hazard in the Java trench, Indonesia.

Dimitra Salmanidou, Mohammad Heidarzadeh, and Serge Guillas

Historical earthquakes in the Java subduction zone have given genesis to tsunami affecting the southwest coasts of the island of Java, in Indonesia. The most recent earthquake on the 17th of July 2006, has given rise to a tsunami that killed more than 600 people. The tsunami was difficult to escape due to the small amount of ground shaking, which could have acted as an early warning, and due to the epicentre being very close to the shorelines, giving insufficient time for response. Historical data and scientific studies give little evidence for mega-thrust events in the Java trench, however such possibilities are not excluded and could have a devastating impact in the region. This work aims to assess the tsunami hazard occurring from a range of earthquake scenarios in the subduction zone. Taking as a benchmark the 2006 event, we initially validate our modelling approach against the wave observations recorded at three tide gauges. We then expand our work to account for future earthquake scenarios and their tsunamigenic consequences in the southern coasts of Java island. Bathymetry displacement is computed using the Okada elastic dislocation model. The nonlinear shallow water equation solver JAGURS is employed for the modelling of wave propagation. Our objective is to quantify the uncertainty of such events by using statistical surrogates: fast stochastic approximations of the model that can explore the likelihood of thousands of tsunami scenarios in a few moments of time. Gaussian process emulators are utilised to predict maximum wave amplification occurring from varying parameter distributions such as the moment magnitude of an earthquake. The resulting tsunami hazard footprints can be used in conjunction with existing socio-demographic information to assess tsunami risk in vulnerable areas. The end-data can eventually be used to inform policy making for better disaster mitigation planning.

How to cite: Salmanidou, D., Heidarzadeh, M., and Guillas, S.: Assessing future uncertainties: earthquake tsunami hazard in the Java trench, Indonesia., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10762, https://doi.org/10.5194/egusphere-egu2020-10762, 2020.

EGU2020-11165 | Displays | SM2.5

On the relationship between offshore geodetic coverage and slip model uncertainty

Ehsan Kosari, Matthias Rosenau, Jonathan Bedford, Michael Rudolf, and Onno Oncken

Characterizing the time-dependent slip evolution along subduction megathrusts during seismic cycles is a key step to unfold the recurrence patterns of and hazard imposed by great earthquakes. However, having adequate geodetic observations in the appropriate locations is essential for reliably modelling both interseismic coupling and coseismic slip. The 2011 Tohoku-oki earthquake as a well-known trench breaking and tsunamigenic megathrust seismic event clearly demonstrated the limitations of the distributed slip models using land-limited geodetic instruments. In this study, we have set up a scaled analogue megathrust model to produce analogue earthquakes while Digital Image Correlation (DIC) and the Analogue Geodetic Slip Inversion Technique (AGSIT) have been applied to retrieve the model surface velocities (incremental displacement) and model coseismic slip distribution, respectively. We generated more than 20 slip models for a series of events by sequentially disregarding trenchward rows of virtual GPS stations (vGPSs) for slip modelling thereby systematically reducing simulated offshore coverage. The analogue earthquakes have been categorized to two different sets as non-trench-breaking and trench-breaking ruptures. Here we show how slip models of analogue earthquakes change as a function of offshore coverage quantitatively and qualitatively. The sensitivities with respect to a potential bimodality of slip distribution and up-dip limit of the slip distribution model have also been assessed.

 

How to cite: Kosari, E., Rosenau, M., Bedford, J., Rudolf, M., and Oncken, O.: On the relationship between offshore geodetic coverage and slip model uncertainty , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11165, https://doi.org/10.5194/egusphere-egu2020-11165, 2020.

EGU2020-11267 | Displays | SM2.5

Application of stochastic fractal surface rupture on non-planar faults in tsunami simulation

Shane Murphy, Andrè Herrero, Fabrizio Romano, and Stefano Lorito

Non-planar faults and surface reached rupture are seldom considered in the source modelling of subduction zone earthquakes. Here we present a new method for accounting for both phenomena in the generation of stochastic slip distribution while still maintaining self similar properties. To do this, we use the composite source model, which involves the placement of numerous circular dislocations on the fault plane. The fault plane is described by an unstructured mesh allowing for a non-planar surface while surface rupture is correctly accounted for by reflecting the slip from circular dislocations that intersect with the fault trace.

In a case study we demonstrate that the inclusion of rupture at the surface alters the ground or seafloor deformation both in terms of the magnitude (between 60%-20% in 5km zone near the fault trace) and the orientation of the deformation vectors (i.e. by up to 5 degrees). Such changes can have a significant effect on tsunami source and subsequent wave.

Additionally, with a prescribed rupture velocity model, complex source time functions can also be calculated for each element on the fault plane. Generally, rise time is assumed to be instantaneous in tsunami simulation.

We will also present preliminary results focused on comparing the tsunami wave height observed along nearby coastlines generated by the different source models (i.e. with/without surface reached rupture and variable source time functions).    

How to cite: Murphy, S., Herrero, A., Romano, F., and Lorito, S.: Application of stochastic fractal surface rupture on non-planar faults in tsunami simulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11267, https://doi.org/10.5194/egusphere-egu2020-11267, 2020.

EGU2020-11535 | Displays | SM2.5

Study of the tsunami source in the Palu Bay following the Mw7.5 2018 Sulawesi earthquake

Fabrizio Romano, Haider Hasan, Stefano Lorito, Finn Løvholt, Beatriz Brizuela, Cristiano Tolomei, and Alessio Piatanesi

On 28 September 2018 a Mw 7.5 strike-slip earthquake occurred on the Palu-Koro fault system in the Sulawesi Island. Immediately after the earthquake a powerful tsunami hit the Palu Bay causing large damages and numerous fatalities.

Several works, inverting seismic or geodetic data, clearly estimated the slip distribution of this event, but the causative source of the tsunami is still not completely understood; indeed, the strike-slip mechanism of the seismic source alone might not be sufficient to explain the large runups observed (> 6 m) along the coast of the Palu Bay, and thus one or more additional non-seismic sources like a landslide could have contributed to generate the big tsunami. An insight of that can be found in an extraordinary collection of amateur videos, and on the only available tide gauge in the Bay, at Pantoloan, that showed evidence for a short period wave of at least 2-3 minutes, compatible with a landslide.

In this study, we attempt to discriminate the contribution in the tsunami generation of both the seismic source and  some supposed landslides distributed along the coast of the Bay.

In particular, we attempt to estimate the causative source of the tsunami by means of a nonlinear joint inversion of geodetic (InSAR) and runup data. We use a fault geometry consistent with the Sentinel-2 optical analysis results and analytically compute the geodetic Green’s functions. The same fault model is used to compute the initial condition for the seismic tsunami Green’s functions, including the contribution of the horizontal deformation due to the gradient of the bathymetry (10 m spatial resolution); the landslide tsunami Green’s functions are computed the software BingClaw by placing several hypothetical sources in the Bay. In both the cases the tsunami propagation is modelled by numerically solving the nonlinear shallow water equations.

In this work we also attempt to address the validity of Green’s functions approach (linearity) for earthquake and landslide sources as well as the wave amplitude offshore as predictor of nearby runup.

How to cite: Romano, F., Hasan, H., Lorito, S., Løvholt, F., Brizuela, B., Tolomei, C., and Piatanesi, A.: Study of the tsunami source in the Palu Bay following the Mw7.5 2018 Sulawesi earthquake, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11535, https://doi.org/10.5194/egusphere-egu2020-11535, 2020.

EGU2020-4035 | Displays | SM2.5

The Chile subduction zone : Nature of the lithosphere between alternating regions of slow earthquakes versus non slow earthquakes

Pousali Mukherjee, Yoshihiro Ito, Emmanuel S. Garcia, Raymundo Plata-Martinez, and Takuo Shibutani

Subduction zones host some of the greatest megathrust earthquakes in the world. Slow earthquakes have been discovered around the subduction zones of the Pacific rim very close to megathrust earthquakes. Investigating the lithosphere of the slow earthquake area versus non slow-earthquake area in subduction zones is crucial in understanding the role of the internal structure to control slow earthquakes. In this study, we investigate the lithospheric structure of stations in the slow earthquake area and non slow-earthquake areas in Chile using receiver function analysis and inversion method using teleseismic earthquakes. Here we focus on, especially the Vp/Vs ratios from both slow and non-slow earthquake areas, because the Vp/Vs ratio is sensitive to the fluid distribution in the lithosphere; the fluid distribution possibly controls the potential occurrence of slow earthquakes. Additionally, the nature of the slab can also play a crucial factor. The Vp/Vs ratio results across depth shows significantly higher value in the deeper oceanic slab region beneath the stations in the slow earthquake areas with higher contrast at the boundary.

How to cite: Mukherjee, P., Ito, Y., S. Garcia, E., Plata-Martinez, R., and Shibutani, T.: The Chile subduction zone : Nature of the lithosphere between alternating regions of slow earthquakes versus non slow earthquakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4035, https://doi.org/10.5194/egusphere-egu2020-4035, 2020.

EGU2020-4921 | Displays | SM2.5

A New Approach to Clarify Slow Earthquake Source Regions: Multi-band Receiver Function Analysis Including Local Deep-focus Events

Yasunori Sawaki, Yoshihiro Ito, Kazuaki Ohta, Takuo Shibutani, and Tomotaka Iwata

Slow earthquakes play important roles in the occurrence of megathrust earthquakes in subduction zones. An increasing number of seismic networks have contributed to significant findings and the detection of slow earthquake activities; however, it is still unclear what sort of seismological structures exhibits each slow earthquake activity. We have developed the multi-band receiver function (RF) method, in which the RFs are composed of different higher-frequency contents. We, here, reveal smaller-scale structures from the RFs from local deep-focus earthquakes around the Philippine Sea plate boundary in Southwestern Japan, where numerous slow earthquakes have been detected (e.g., Obara, 2002; Ito et al., 2007; Nishimura et al., 2013).

Deep-focus earthquakes, frequently occurring in the Pacific slab below Southwestern Japan, can be applicable to the multi-band RF analysis because the local deep events and teleseismic events are similar in the slowness of the first-arrival phases. Local deep-focus events, however, have different variations in back azimuths from teleseismic events, which enables us to estimate seismological structures in a wider range of azimuths by stacking traces from both events. We carefully select the deep-focus events with longer S-P time than 40 sec and exclude triplication phases from mantle transition zones. Here we apply this method to short-period 3-component seismograms of Hi-net (NIED, Japan) in the Northeastern Kii Peninsula, where short-term slow slip events (SSEs) and episodic tremors are very active (e.g., Obara et al., 2010; Nishimura et al., 2013; Yabe & Ide, 2014).

Cross-sections of higher-frequency RFs (up to 2 Hz) show sharp and strong negative phases from the plate interface shallower than 35 km depth, which is one of the most active regions of episodic tremors (Obara et al., 2010). At the deeper portion, the higher-frequency RFs exhibit the mantle wedge structure with obscure phases of the plate interface, where minor and continuous tremor activities have been reported (Obara et al., 2010). These results suggest that episodic tremors accompanied by short-term SSEs occur on the interface between the continental crust and the oceanic crust, whereas the source regions of minor tremors are between the oceanic crust and the mantle wedge as indicated in Kato et al. (2010).

How to cite: Sawaki, Y., Ito, Y., Ohta, K., Shibutani, T., and Iwata, T.: A New Approach to Clarify Slow Earthquake Source Regions: Multi-band Receiver Function Analysis Including Local Deep-focus Events, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4921, https://doi.org/10.5194/egusphere-egu2020-4921, 2020.

EGU2020-5842 | Displays | SM2.5

3D distribution of elastic properties and subduction inter-plate relief in NW Ecuador from joint refraction and inter-plate reflection travel-time tomography

Valenti Sallares, Adrià Melendez, Domagoj Terzic, Pedro Buinheira, Philippe Charvis, Audrey Galve, Jean-Yves Collot, and Alcinoe Calahorrano B.

Great subduction earthquakes occur along the seismogenic zone of the megathrust, a fault segment that is mechanically coupled so that seismic rupture can propagate. Numerous factors, including the rheology and structure of the plates, shear stress distribution, fluids pressure, or thermal structure, size and width of the coupled zone, have been proposed to play a role to determine and dynamic behavior of the rupture. These factors are conditioned, in turn, by the properties of the rocks undergoing deformation during seismic rupture and by the fault geometry and roughness. While the information on the 3D velocity field and on inter-plate geometry can potentially be extracted from travel-times of active seismic data, the experiments that are appropriate to define these parameters are scarce. Additionally, most 3D travel-time tomography codes use only first arrivals to define the velocity field, so that they do not provide information on the megathrust geometry. Here we combine for the first time ever wide-angle seismic data with a joint refraction and reflection travel-time tomography code (tomo3d), to retrieve a 3D velocity model of margin as well the geometry of the inter-plate boundary with unprecedented detail. In particular, we use data acquired in the French project “Esmeraldas” offshore NE Ecuador/SE Colombia in 2005. Our model, which builds on a previous one obtained by first arrival tomography, goes from the surface to 18-20 km depth, covering a substantial part of the seismogenic zone. This region, where the Nazca plate plunges beneath South America, has produced remarkable examples of variable earthquake rupture behavior. The entire ~500 km-long segment ruptured during the great tsunamigenic earthquake of 1906 (Mw = 8.8), and it was ruptured again by three smaller events, directly adjacent to one another, in 1942 (Mw = 7.8), 1958 (Mw = 7.7) and 1979 (Mw = 8.2). According to our results, the inter-plate boundary where these earthquakes took place is of variable dip and rough, spotted by 2-3 km-high and 10-15 km-wide features that resemble subducting seamounts. The velocity of the overriding plate just above the inter-plate boundary is strongly heterogeneous, showing velocity-derived rock rigidity variations of up to 30-40% both along- and across-strike. The presence of inter-plate relief and velocity changes is confirmed by parameter uncertainty analysis and data sensitivity tests. Interestingly, the sharpest velocity contrasts appear to follow the limits between crustal blocks of different origin and composition that, according to previous work, could correspond to crustal-scale faults acting as barriers during earthquake propagation. The combined effect of a rough inter-plate boundary and heterogeneous elastic properties on earthquake rupture and tsunamigenesis remains to be tested by dynamic rupture models

How to cite: Sallares, V., Melendez, A., Terzic, D., Buinheira, P., Charvis, P., Galve, A., Collot, J.-Y., and Calahorrano B., A.: 3D distribution of elastic properties and subduction inter-plate relief in NW Ecuador from joint refraction and inter-plate reflection travel-time tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5842, https://doi.org/10.5194/egusphere-egu2020-5842, 2020.

Large and destructive earthquakes (Mw≥ 7.5) occur worldwide particularly along the major subduction zones causing extensive damage and loss of life in the hinterland of epicentral region. Source models and rupture characteristics of these earthquakes (i.e. faulting geometry, focal depth, non-uniform finite-fault slip distributions) can be precisely determined by using seismological data and multidisciplinary earth-science observations. It is also known that earthquake source parameters play key roles in the modelling of secondary events such as earthquake-induced tsunamis. There are many studies emphasizing the importance of using heterogonous slip distribution models of earthquakes in mathematical tsunami simulations to predict synthetic tsunami waves more consistent with the observed ones. In this study, we obtained double-couple source mechanisms and slip distribution models of complex large earthquakes (Mw≥ 7.5) lately occurred at different parts of the Earth. For this purpose, we used point-source teleseismic P- and SH- body waveform inversion and kinematic slip distribution inversion techniques. Besides, azimuthal distributions of P- wave first motion polarities, which are recorded by near-field and regional seismic stations, are checked to approve obtained minimum misfit source mechanism parameters of earthquakes. We essentially observed that tsunamigenic earthquakes occurred at shallow focal depths (h ≤ 70 km) with dip-slip source mechanisms and rather complex slip distributions along the fault planes. However, in some cases, tsunami waves may be unexpectedly triggered due to the secondary effects of large strike-slip earthquakes (e.g., September 28, 2018 Palu, Indonesia - Mw7.5). Here, we discuss our inversion results, which reveal the significant contributions of earthquake source studies on resolving the relationships between the faulting geometry, rupture characteristics and tsunami generation. Furthermore, the necessity of high-resolution bathymetry data in numerical tsunami simulations is highlighted for the modelling of tsunami waves, in particular, recorded at the near-field tide-gauge stations. This study is partially supported by the Turkish Academy of Sciences (TÜBA) through GEBIP program.

How to cite: Yolsal-Çevikbilen, S. and Taymaz, T.: Seismological Constraints on Fault-Slip Source Models and Rupture Characteristics of Global Large Earthquakes (Mw ≥ 7.5) and Associated Tsunamis , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7922, https://doi.org/10.5194/egusphere-egu2020-7922, 2020.

EGU2020-7688 | Displays | SM2.5

Seismogenic behaviour in the Lesser Antilles: Insights from geodetic observations

Elenora van Rijsingen, Eric Calais, Romain Jolivet, Jean-Bernard de Chabalier, Jorge Jara, Steeve Symithe, Richard Robertson, and Graham Ryan

The Lesser Antilles subduction zone is a challenging region when it comes to unravelling its seismogenic behaviour. Over the last century, it has been seismically quiet, with no large thrust events recorded, leading to the question whether this subduction zone is able to produce large interplate earthquakes or not. The slow subduction velocity of ~20 mm/yr complicates this even further, as mega-earthquake recurrence times would be up to many hundreds of years in the case of a fully locked subduction interface, and up to several thousands of years for a partially locked interface. The record of two large historical earthquakes, a M ~8 in 1839 and M ~8.5 in 1843, is often referred to as evidence supporting the seismic character of the Lesser Antilles subduction zone. It remains, however, questionable whether these events actually occurred along the subduction interface.

Here we use GPS data acquired on various islands within the Antilles to infer interseismic coupling along the Lesser Antilles Arc. Previous block models have suggested low coupling of the subduction interface, making the occurrence of large megathrust earthquakes less likely. However, the non-uniqueness of these inversions, as well as uncertainties related to the distance between GPS stations and the subduction trench, cast doubts on how well the inferred coupling represents the actual degree of locking along the subduction interface. In this study, we attempt to improve these estimates, by using a Bayesian approach to derive a meaningful set of uncertainties on the distribution of interseismic coupling. By exploring the entire range of model parameters, we are able to provide a probabilistic estimate of interseismic coupling. To further improve our analysis with respect to previous models, we incorporate a layered elastic structure, as well as a more realistic fault geometry, testing two different slab models.

Our results suggest that the subduction interface of the Lesser Antilles subduction zone is most likely to be uncoupled. A sensitivity analysis highlights the deeper part of the interface (i.e., 30-60 km depth) as the region with higher sensitivity, since the GPS stations are distributed mostly above that portion of the subduction. A test regarding the proposed 1843 rupture contour reveals that this area is very unlikely to be locked. This apparent aseismic character of the Lesser Antilles raises questions about the role of slow slip along the interface. We therefore also analyse GPS time series to assess the spatial and temporal distribution of transient deformation signals in the region.

How to cite: van Rijsingen, E., Calais, E., Jolivet, R., de Chabalier, J.-B., Jara, J., Symithe, S., Robertson, R., and Ryan, G.: Seismogenic behaviour in the Lesser Antilles: Insights from geodetic observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7688, https://doi.org/10.5194/egusphere-egu2020-7688, 2020.

Tsunamis observed by offshore ocean-bottom pressure gauges have been used to infer fault models and stress drops for major (M > 7) offshore earthquakes, to understand the earthquake and tsunamigenesis (e.g., Satake et al. 2013). However, it is challenging to observe tsunamis due to moderate (M ~6) earthquakes with reasonable quality by those, previous, few and remote pressure gauge arrays. Recently, a new, dense and wide pressure gauge network, the Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench (S-net), was constructed off eastern Japan (Kanazawa et al. 2016). This array observed tsunamis associated with a moderate (M~6) earthquake which occurred inside the array, with amplitudes of less than one cm. We analyzed these millimeter-scale tsunami records to infer the finite fault model and stress drop, and to examine its relationship with other interplate earthquake phenomena.

We analyzed the pressure data associated with an Mw 6.0 earthquake off Sanriku on August 20, 2016. This earthquake was located at the shallowest part of the plate boundary off Sanriku, Japan, near the northern edge of the rupture area of the 1896 Sanriku tsunami earthquake (Kanamori, 1972). Although the signal-to-noise ratio is not high, the westward tsunami propagation with the velocity of ~0.1 km/s could be recognized when the waveforms were aligned according to the station locations. Using these data, we constrained the rectangular fault model with a uniform slip across the fault. As a result, the fault model was located ~10 km to the west of the Global CMT centroid (a seismic moment M0= 1.4 × 1018 Nm, Mw 6.0, and a stress drop of Δσ = 1.5 MPa). The stress drop seems not so small as expected in tsunami earthquakes such as the 1896 Sanriku tsunami earthquake (≪ ~1 MPa, e.g., Kanamori 1972) even if the uncertainty of the stress drop estimation is considered (Δσ > ~ 0.7 MPa). We also found the rupture area was unlikely to overlap with regions where slow earthquakes are active, such as low-frequency-tremors and very-low-frequency-earthquakes (e.g., Matsuzawa et al. 2015; Nishikawa et al. 2019; Tanaka et al. 2019).

This result demonstrates that the S-net new dense and wide pressure gauge array dramatically increases the detectability of a millimeter-scale tsunami and the constraints on earthquake source parameters of moderate earthquakes off eastern Japan. It is expected that more tsunamis due to minor-to-moderate offshore earthquakes are recorded by this new array, which will reveal the spatial variation of the stress drops, or mechanical properties, along the plate interface with much higher resolution than previously possible.

How to cite: Tatsuya, K., Saito, T., and Suzuki, W.: Fault modeling and stress drop estimation based on millimeter-scale tsunami records of an M6 earthquake detected by the dense and wide pressure gauge array, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11999, https://doi.org/10.5194/egusphere-egu2020-11999, 2020.

SM2.11 – Active tectonics of the Mediterranean as seen by recent seismic sequences

EGU2020-20019 | Displays | SM2.11 | Highlight

Multiphase tectonic interaction of Tyrrhenian - Tunisia Margin - Ionian systems: Implications for regional seismogenesis

César R. Ranero, Eulalia Gracia, Valenti Sallares, Ingo Grevemeyer, and Nevio Zitellini

The region at the transition from the west to the east Mediterranean is a complex puzzle of terrains spanning in age from the Mesozoic Ionian lithosphere to the Pleistocene arc and back arc domains of the Tyrrhenian system. Although the region has had a complicated evolutionary history, the current configuration of terrains fundamentally denotes Miocene to recent kinematics.

In this contribution we present new data from Tunisia Margin showing the evolution from its formation in early Miocene to recent, the tectonic interaction with the opening of the Tyrrhenian system and its current inversion, and discuss the implications for the regional kinematics evolution.  

The Tyrrhenian is no longer extending, but all basin borders indicate currently active large-scale thrusting  to strike slip tectonics. Tunisia margins formed by a well-know contractional tectonic phase in early Miocene expressed in large-scale tectonics with a clearly imaged thrust  and fold belt, cut by Messinian to Pliocene extensional faulting. However, high resolution multibeam bathymetry and images of the shallowest layers indicates ongoing inversion tectonics.

We compare the tectonic evolution of north Tunisia and Tyrrhenian with the patterns of deformation of the Ionian tectonic wedge observed in new and reprocessed seismic images. We interpret the current deformation of the Ionian tectonic wedge based on the integration of evolution of the kinematics from the data sets of observations from the three systems.

We conclude that the entire region is currently under collision of the Africa Plate with the Adria Plate and the Neogene terrains of the Tyrrhenian Domain.  The corollary is the subduction of the Ionian lithosphere is fundamentally stalled so that the megathrust fault is possibly not any longer accumulating significant shortening and most deformation is currently occurring in steeper faults re-activation or cutting the previous structural framework.

How to cite: Ranero, C. R., Gracia, E., Sallares, V., Grevemeyer, I., and Zitellini, N.: Multiphase tectonic interaction of Tyrrhenian - Tunisia Margin - Ionian systems: Implications for regional seismogenesis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20019, https://doi.org/10.5194/egusphere-egu2020-20019, 2020.

EGU2020-6978 | Displays | SM2.11

Seismic sources at intermediate depth in the Alboran Sea

Elisa Buforn, Lucía Lozano, Simone Cesca, Juan Vicente Cantavella, Maurizio Mattesini, and Agustín Udias

The occurrence of moderate magnitude earthquakes in intermediate depth (40<h<150 km) is a characteristic of the seismicity of the Ibero-Magrebian region. The most important concentration of this activity is in the western part of the Alboran Sea, with the epicenters following an N-S direction. In order to improve the knowledge of the geometry of these seismogenic structures, we have carried out a study of the hypocenters distribution and focal mechanisms for earthquakes that occurred in the period 2000-2020 (M>4.0). For the hypocentral location, we have used a non-linear probabilistic approach (NonLinLoc algorithm) jointly with 3-D lithospheric velocity tomography models recently developed for the Alboran-Betic-Rif zone. Focal mechanisms have been obtained from moment tensor inversion of stations at regional distances (Kiwi tools). Maximum likelihood hypocentres confirm a near vertical N-S distribution in a depth range between 50 and 100 km. Focal mechanisms show a different stress pattern, changing from a vertical tension axis for earthquakes located off-shore and western of 4.5ºW to vertical pressure axis for earthquakes inland and at eastern of 4.5ºW.

How to cite: Buforn, E., Lozano, L., Cesca, S., Cantavella, J. V., Mattesini, M., and Udias, A.: Seismic sources at intermediate depth in the Alboran Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6978, https://doi.org/10.5194/egusphere-egu2020-6978, 2020.

EGU2020-19316 | Displays | SM2.11

The upper crustal geological and structural setting in the area of the 2016-2018 Central Apennines seismic sequence. From subsurface modeling to seismotectonics.

Francesco Emanuele Maesano, Mauro Buttinelli, Lorenzo Petracchini, Chiara D'Ambrogi, Davide Scrocca, Daniela Di Bucci, Maurizio Marino, Franco Capotorti, Gian Paolo Cavinato, Sabina Bigi, Lorenzo Bonini, Maria Teresa Mariucci, Paola Montone, Pietro Tizzani, Raffaele Castaldo, Susi Pepe, and Giuseppe Solaro

Central Apennines (Italy) is a young and tectonically active mountain chain characterized by a high structural complexity where structures related to various tectonic phases are interacting with each other leading to the reactivation of inherited structures and/or to the segmentation of newly formed ones with a strong impact on the current seismotectonics of the area.

In this context, the surface geological and coseismic observations cannot always be extrapolated straightforward to depth and need to be interpreted in the context of the general upper crustal deformation history.

These considerations apply also to the area struck by the 2016-2018 Central Apennines seismic sequence where the activation of both single faults and complex fault systems has been observed.

In the framework of the RETRACE-3D project, we present a comprehensive 3D geological model derived from the interpretation of a large set of underground data acquired for hydrocarbons explorations and we discuss the implication of this geological reconstruction for the seismotectonics of the area by comparing our results with the coseismic observation.

Our results primarily show that, although the area is currently affected by an extensional tectonic regime, the main architecture of this portion of the chain is still dominated by previous compressional large-scale structures with widespread evidence of segmentation, reactivation and even inversion of various sets of inherited faults.

These results pose new points of discussion on information and input data needed to understand the seismogenesis in young and complex mountain chains, such as the Central Apennines, and strongly impact on the consequent seismic hazard assessment study.

How to cite: Maesano, F. E., Buttinelli, M., Petracchini, L., D'Ambrogi, C., Scrocca, D., Di Bucci, D., Marino, M., Capotorti, F., Cavinato, G. P., Bigi, S., Bonini, L., Mariucci, M. T., Montone, P., Tizzani, P., Castaldo, R., Pepe, S., and Solaro, G.: The upper crustal geological and structural setting in the area of the 2016-2018 Central Apennines seismic sequence. From subsurface modeling to seismotectonics., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19316, https://doi.org/10.5194/egusphere-egu2020-19316, 2020.

EGU2020-10607 | Displays | SM2.11

Lithological and structural control on the seismicity distribution in Central Italy

Massimiliano R. Barchi, Lauro Chiaraluce, and Cristiano Collettini

In the seismically active region of Central Italy, national (permanent) and local (not-permanent) seismic networks provided very accurate location of the seismicity recorded during the major seismic sequences occurred in the last 25 years (e.g. 1997-1998; 2009; 2016-2017), as well as of the background seismicity registered in the intervening periods.  In the same region, a network of seismic reflection profiles, originally acquired for oil exploration purposes, is also available, effectively imaging the geological structure at depth, to be compared with the seismicity distribution. 

This comparison reveals that, if the position of the brittle/ductile transition exerts a role at regional scale for the occurrence of crustal seismicity, at a more local scale the depth and thickness of the seismogenic layer is mostly controlled by the contrasting rheological properties of the different lithological groups involved in the upper crust. 

The upper crust stratigraphy, including the sedimentary cover and the uppermost part of the basement, consists of alternated strong (rigid, e.g. carbonates and dolostones) end weak (not-rigid, e.g. shales, sandstones, and phyllites) layers. This mechanically complex multilayer is involved in a belt of imbricated thrusts (Late Miocene-Early Pliocene), displaced by subsequent extensional (normal) faults (Late Pliocene-present), responsible for the observed regional seismicity. The top of the basement s.l. (composed of clastic sedimentary and slightly metamorphosed rocks) is involved in major thrusts.  For these different lithological units, combined field and lab studies of fault rock properties have documented localized and potentially unstable deformation occurring in granular mineral phases (carbonates) and distributed and stable slip within phyllosilicate-rich shear zones (shales and phyllites).

By comparing the geological structure with the seismicity distribution, we observed that:

-     The seismicity cut-off (i.e. the bottom of the seismogenic layer) is structurally (not thermally) controlled, and grossly corresponds to the top basement; the upper boundary of the seismogenic layer corresponds to the top of carbonates.

-     Most seismicity occurs within the rigid layers (carbonates and evaporites), and do not penetrate the turbidites and basements rocks.

-      Close to the axial region of the mountain range, where the larger amount of shortening is observed, the presence thrust sheets from the previous compressional phase, significantly affect the seismicity distribution and propagation.

-     Major east-dipping extensional detachments, recognized in the seismic profiles, are also marked by distinctive seismicity alignments.

How to cite: Barchi, M. R., Chiaraluce, L., and Collettini, C.: Lithological and structural control on the seismicity distribution in Central Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10607, https://doi.org/10.5194/egusphere-egu2020-10607, 2020.

EGU2020-11007 | Displays | SM2.11

The 2018 Mw 6.8 Zakynthos, Greece, earthquake – strike-slip and thrust faulting in shallow subduction

Efthimios Sokos, František Gallovič, Christos P. Evangelidis, Anna Serpetsidaki, Vladimír Plicka, Jan Kostelecký, and Jiří Zahradník

On October 25, 2018, at 22:54 UTC, an Mw 6.8 earthquake occurred southwest of Zakynthos island in the Ionian Sea. This is an area with different styles of faulting and the locus of strong events thus ideal for fault interaction studies. The 2018 Zakynthos earthquake was recorded by broad-band and strong-motion networks and provides an opportunity to resolve such faulting complexity. We used waveform inversion and backprojection of strong motion data, partly verified by co-seismic GNSS data, too. The aftershock sequence was relocated, and the moment tensors of the strongest events were evaluated. Stress inversion shows that the region is under sub-horizontal southwest-northeast compression, enabling mixed thrust- and strike-slip faulting. Based on detailed waveform inversion studies, we conclude that the 2018 mainshock consisted of two fault segments: a low-dip thrust, and a dominant, moderate-dip, right-lateral strike slip, both in the crust. This model explains the observed large negative CLVD component of the mainshock. Slip vectors of both ruptured segments, oriented to SW, are consistent with plate motion in the area. The sequence can be explained in terms of trench-orthogonal fractures in the subducting plate and reactivated faults in the upper plate. The 2018 event, and an Mw 6.6 event of 1997, occurred near three localized swarms of 2016 and 2017. Future numerical models of the slab deformation and ocean-bottom seismometer observations may illuminate possible relations between earthquakes, swarms and fluid paths in the region.

How to cite: Sokos, E., Gallovič, F., Evangelidis, C. P., Serpetsidaki, A., Plicka, V., Kostelecký, J., and Zahradník, J.: The 2018 Mw 6.8 Zakynthos, Greece, earthquake – strike-slip and thrust faulting in shallow subduction, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11007, https://doi.org/10.5194/egusphere-egu2020-11007, 2020.

EGU2020-20749 | Displays | SM2.11

The Destructive Earthquake (Mw6.4) of 26 November 2019 in Albania: a First Report

Gerasimos Papadopoulos, Apostolos Agalos, Panayotis Carydis, Efthimios Lekkas, Spyridon Mavroulis, and Ioanna Triantafyllou

On 26 November 2019 an Mw6.4 earthquake ruptured along the Adriatic coast of Albania causing extensive destruction, mainly in Durrës city and Thumanë town, claiming 51 human lives. Maximum intensity in both areas reached the level of VIII-IX. On 21 September 2019 a strong foreshock (Mw5.6) occurred at nearly the same epicenter causing considerable damage. Fault-plane solutions of the main shock showed reverse faulting striking NW-SE and likely dipping to east indicated by the regional tectonics. In the long-term sense the earthquake has not been a surprise since the area experienced destructive earthquakes in the past as well. We present observations collected during a post-event field survey as well as a seismic source model of the main shock. Apart from the strong motion and amplification phenomena due to loose soil conditions and soil liquefaction, the severe building damage is attributed to the synergy of several other factors including poor workmanship and construction quality, ageing of materials, impact of the September 21st earthquake and pre-existing stress on buildings suffering differential displacements due to soft soil conditions in their foundations. A finite fault model was developed from the inversion of 30 teleseismic P-wave records following the methodology by Hartzell and Heaton (1983, 1986). The hypocenter was manually located offshore but close to Durrës at depth of ~22 km using the NLLoc procedure and the Ak135 velocity model based on 71 seismic phases at distances ≤550 km. Based on this solution a rectangular fault was assumed of 29 km in length with a depth ranging from 15 to 27 km which is large enough to describe successfully the slip for an earthquake of Mw=6.4. A kinematic parameterization of the earthquake fault was used to identify the space-time distribution of slip. Synthetics were calculated for each cell in which the fault is divided and are compared with the recorded data during an inversion producing the solution vector, i.e. the slip for each cell. Strike of 345° and dip of 22° were found to better fit the data. The rake vector was allowed to vary within the range from 65° to 155° permitting up-dip and possible left lateral strike-slip movement of the hanging wall domain relatively to the footwall. Rupture velocity was allowed to vary from 2.4 km/s to 3.6 km/s, the best fit found for 2.6 km/s. The heterogeneous spatial slip distribution obtained shows a complex rupture process with maximum slip at the source of ~1.5 m with the rake vector at that point being of 115° but an arithmetic mean of the rake for the cells with significant slip over 10 cm is 99°. This implies that the thrust component is the one that played the important role in the rupture process. Significant values of slip were also found in a second patch in the northern area of the fault at depths from 19 to 26 km. The total duration of rupture was nearly 16 s, while the total seismic moment released was Mo=5.0x1018 N*m, corresponding to Mw=6.4.

How to cite: Papadopoulos, G., Agalos, A., Carydis, P., Lekkas, E., Mavroulis, S., and Triantafyllou, I.: The Destructive Earthquake (Mw6.4) of 26 November 2019 in Albania: a First Report, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20749, https://doi.org/10.5194/egusphere-egu2020-20749, 2020.

EGU2020-12848 | Displays | SM2.11

Connecting seismicity, gravity-driven erosion and deposition at submarine normal faults: Insights from mapping of the 2004 Mw 6.3 Les Saintes earthquake rupture (French Antilles)

Javier Escartin, Jeremy Billant, Frédérique Leclerc, Jean-Arthur Olive, Klemen Istenic, Nuno Gracias, Rafael Garcia, Aurelien Arnaubec, Christine Deplus, and Nathalie Feuillet and the SUBSAINTES Science Party

During the ODEMAR 2013 and SUBSAINTES 2017 cruises we mapped the full extent of the seafloor rupture associated with the 2004 Mw 6.3 Les Saintes extensional earthquake. Near-bottom bathymetry acquired both with ROVs and AUVs along the Roseau Fault reveal a normal fault scarp developing in an extensional graben within the Caribbean volcanic arc, between the islands of Guadeloupe and Dominica. Optical inspection during ROV dives along the scarp’s base, where fault mirrors are well-preserved, allowed us to identify and characterize the coseismic fault rupture, and measure the coseismic displacements using both laser calipers and measurements performed on video-derived, textured 3D models, with accuracies better than 1 cm.

 

The 2004 rupture extends ~20 km along the Roseau Fault, with a vertical displacement exceeding 2.5 m at its center, and tapering towards its ends. Local variations in apparent fault slip within a single 3D model (fault lengths of ~10 to 300 m) document local deposition of gravity debris cones at the base of the scarp, extending laterally between a few to tens of m, and covering the coseismic markers. Gullies eroding the footwall and depositing debris cones on the hanging wall do not show any significant displacement. Fault scarps on either side of the gully mouth instead record significant displacements, suggesting that either erosion or deposition along the gully bottom efficiently obliterated markers of coseismic deformation.

 

We inspected all overlapping seafloor imagery acquired in December 2013 and April 2017, >10 years after the 2004 Les Saintes earthquake, extending laterally over >3 km of the Roseau Fault rupture.  Neither the bed of gullies crossing the rupture, nor the debris and rubble at the base of the fault scarp show any noticeable seafloor change indicating mass wasting and transport, and only changes in mobile sediment (e.g., ripples) can be detected between both image sets.  We identified a single area, ~2m wide, with apparent deposition of pebbles during these 3.25 years period, and associated with a local mass-wasting event.

 

These observations point towards a systematic triggering of mass-wasting during seismic events, with deposition of rubble and rocks both at dejection cones at the mouth of gullies, or at the base of fault scarp sections displaying fault mirrors, covering or obliterating the coseismic markers. Therefore, long-term erosion and deposition processes here are gravity-driven and triggered by the history and magnitude of seismic events. Similar seismic controls may enable denudation of exposed oceanic lithosphere at fault scarps developing along and flanking mid-ocean ridges.

How to cite: Escartin, J., Billant, J., Leclerc, F., Olive, J.-A., Istenic, K., Gracias, N., Garcia, R., Arnaubec, A., Deplus, C., and Feuillet, N. and the SUBSAINTES Science Party: Connecting seismicity, gravity-driven erosion and deposition at submarine normal faults: Insights from mapping of the 2004 Mw 6.3 Les Saintes earthquake rupture (French Antilles), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12848, https://doi.org/10.5194/egusphere-egu2020-12848, 2020.

EGU2020-21933 | Displays | SM2.11

A deep resistivity Full Waver survey unravels the 3D structure of the Castelluccio basin in relation to the source of the 2016 Mw 6.5 Norcia earthquake (central Italy)

Vincenzo Sapia, Fabio Villani, Federico Fischanger, Matteo Lupi, Paola Baccheschi, Carlo Alberto Brunori, Riccardo Civico, Igino Coco, Paolo Marco De Martini, Fabio Giannattasio, Luigi Improta, Valerio Materni, Federica Murgia, Daniela Pantosti, Luca Pizzimenti, Stefano Pucci, Tullio Ricci, Valentina Romano, Alessandra Sciarra, and Alessandra Smedile

The Castelluccio basin in the central Apennines (Italy) is a ~20-km2-wide intramontane Quaternary depression located in the hangingwall of the NW-trending and SW-dipping Vettore-Bove normal fault system (VBFS). This system is responsible for the 2016-2017 seismic sequence, culminated with the 30 October 2016 Mw 6.5 Norcia earthquake that caused widespread surface faulting affecting also the northern part of the Castelluccio basin. Available borehole and geophysical data are not enough to constrain the basin structure, infill architecture and their relations with the long-term activity of the VBFS. Therefore, we carried out an extensive 3D survey using the innovative Fullwaver (FW) technology, conceived to perform deep electrical resistivity tomography (DERT). We aimed at: a) mapping the geometry of the pre-Quaternary limestone basement and the basin infill thickness down to a depth of ~1 km; b) mapping the subsurface structure of known faults and their extent underneath the alluvial cover; c) mapping possible blind faults splays.

The 3D survey covered a 23 km2 area and it was designed with the aim to map the region as regularly as possible, taking into account the rugged topography and logistic issues. We used a series of independent 2-channels receivers connected each to three grounded steel electrodes, 200 m spaced, to record the electrical field generated by a five kilowatt current regulated Time Domain Induced Polarization transmitter. Data were modelled with ViewLab software via a regularized inversion with smoothness constraints to cope with the expected subsurface strong resistivity changes, and to obtain a robust 3D resistivity model.

The FW technology allowed us to constrain the geometry of the basin. The infill material is imaged as a wide, N-trending moderately resistive (< 300 Ωm) to conductive  (< 100 Ωm) region, likely made of silty sands and gravels, deepening down to 500 m b.g.l. in the southern sector, suggesting the occurrence of two main depocenters. All over the basin, we identify paired high-resistivity (> 500-1000 Ωm) and low-resistivity (< 400 Ωm) belts related to the limestone basement and to the basin infill, respectively. They display NNE and NNW dominant trends. We interpret the sharp boundaries of NNE-trending belts as related to early extensional faults promoting the basin inception. The NNW-trending belts suggest the occurrence of faults that locally cross-cut the previous ones, and that we interpret as splays of the VBFS buried under the basin sedimentary cover. The recognition of different systems of extensional faults is coherent with results of high-resolution seismic profiling carried out recently in the basin. A high-resolution 2D transect with 15 m-spaced electrodes across the 2016 surface ruptures shows details of the active VBFS splay down to 300 m depth. Moreover, in the eastern sector of the survey area, low-resistivity round-shaped anomalies in the Mesozoic substratum hints for deep Miocene compressional structures. Therefore, our DERT imaging suggests a complex tectonics in the subsurface of the Norcia earthquake fault. In particular, the currently active NNW-trending faults seem to overprint a pre-existing structural framework, promoting fault segmentation at different spatial scales

How to cite: Sapia, V., Villani, F., Fischanger, F., Lupi, M., Baccheschi, P., Brunori, C. A., Civico, R., Coco, I., De Martini, P. M., Giannattasio, F., Improta, L., Materni, V., Murgia, F., Pantosti, D., Pizzimenti, L., Pucci, S., Ricci, T., Romano, V., Sciarra, A., and Smedile, A.: A deep resistivity Full Waver survey unravels the 3D structure of the Castelluccio basin in relation to the source of the 2016 Mw 6.5 Norcia earthquake (central Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21933, https://doi.org/10.5194/egusphere-egu2020-21933, 2020.

EGU2020-7362 | Displays | SM2.11

Structure and segmentation of the Ovindoli – Piano di Pezza – Campo Felice fault system (Central Apennines, Italy): Evolution and reactivation of inherited faults

Matthieu Ferry, Stéphanie Gautier, Stéphane Mazzotti, Fabio Villani, Eric Stell, Marie Jacottin, Daniela Pantosti, Vincenzo Sapia, Tulio Ricci, Lucilla Benedetti, Giuseppe Di Giulio, and Maurizio Vassallo

Active deformation in the Central Apennines is mostly accommodated by NW-SE normal faults systems that produce moderate to large earthquakes at shallow depth. Recent examples include the 1915 Mw≈7 Avezzano earthquake (Fucino basin) and the 2009 Mw=6.1 L’Aquila earthquake (Aterno basin) which were both associated with major loss of life and massive damage to buildings and infrastructure. Here, we study the 40-km-long Ovindoli – Piano di Pezza – Campo Felice – Monti d’Ocre (OPCM) fault system, a major NNW-SSE system that potentially links the Fucino and the Aterno fault-systems. The OPCM exhibits linear and arcuate sections with four main segments and borders the eastern margin of the Aterno basin. Paleo-earthquake rupture data on the Piano di Pezza (PPF) and Campo Felice (CFF) faults exhibit remarkable synchronicity with the Fucino fault system, with the most recent surface-rupturing earthquake likely occurring in the XIVth century. In order to better understand the relationships between and earthquake rupture scenarios, we focus on the basin geometry and fault surface expression of the Piano di Pezza fault combining geomorphology and subsurface geophysics. We map the fault trace with unprecedented detail using terrestrial laser scanner surveys and quantify surface deformation affecting alluvial fans as well as glacial moraines. We obtain a mean vertical offset of 2.5 m +/- 0.3 m for the most recent features, well in agreement with paleoseismological data. Furthermore, we document slip distributions at different time scales along strike with a maximum value at the connection between the PPF and the OF. Beneath the scarp, geophysical data reveal a complex faulting geometry with several parallel strands and two minor blind splays. Electrical resistivity tomography images show a cumulative vertical offset of ~ 15 m affecting an interface attributed to the Last Glacial Maximum and confirm the high vertical slip across the fault zone. Gravimetric anomalies across the basin also indicate the sedimentary fill has recorded a maximum finite cumulative throw of the PdP fault system of 110-140 m. This suggests a maximum vertical slip rate of 0.2-0.3 mm/year since the Pleistocene, which contrasts with the high post-LGM slip rate estimated from trenches. Overall, our observations suggest that the arcuate PPF originally formed as a reverse fault during the Mio-Pliocene compressive stage and is now reactivated as an extensional horsetail-like feature by ruptures along a major strike-slip fault (OF). This finding points to the PPF as mostly built through ruptures along the OF leaking onto an inherited structure. The time-varying slip rates may also denote an episodic behavior marked by short periods of high seismic activity (a few centuries) and long intervals of seismic quiescence (a few millennia). Furthermore, possible earthquake rupture scenarios along the OPCM may encompass the whole OPCM fault system (cumulative length ca. 40 km) or rupture termination along the PPF (cumulative length ca. 15-20 km) with significantly different impacts over the populated Fucino and Aterno basins.

How to cite: Ferry, M., Gautier, S., Mazzotti, S., Villani, F., Stell, E., Jacottin, M., Pantosti, D., Sapia, V., Ricci, T., Benedetti, L., Di Giulio, G., and Vassallo, M.: Structure and segmentation of the Ovindoli – Piano di Pezza – Campo Felice fault system (Central Apennines, Italy): Evolution and reactivation of inherited faults, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7362, https://doi.org/10.5194/egusphere-egu2020-7362, 2020.

EGU2020-19590 | Displays | SM2.11

Faults distribution and seismogenic potential in the Calabrian back-arc domain, SE Tyrrhenian Sea

Camilla Palmiotto, Maria Filomena Loreto, Francesco Muto, Valentina Ferrante, Franco Pettenati, Denis Sandron, and Vincenzo Tripodi

The Western Calabrian margin (Italy) is the most active segment of the Apennine back-arc system, formed in response to the slow Africa – Eurasia convergence. The offshore area represents the transitional region between the arc and the back-arc: it is affected by several fault systems, most of them able to trigger highly destructive earthquakes. Indeed, the Calabria and its western offshore are characterized by the highest seismic moment release of the entire Apennines, also evidenced by historical seismicity catalogue, the most accurate over the world. During last decades, scientific community invested huge resources in assessment of seismic and tsunami hazards. Furthermore, during last years several local-scale works allowed of improving knowledge of the faults geometry, magmatism, seismogenic and tsunamigenic potential along the western offshore region (Loreto et al., 2017; Brutto et al., 2016; De Ritis et al., 2019). Some active faults, belonging to NE-SW-trending normal fault systems accommodating the inner-arc collapse related to slab-decupling, are also responsible of the most destructive historical sequences, still to be adequately characterized. Using vintage SPARKER 30 Kj acquired in the seventies and recent multichannel seismic profiles together with middle resolution morpho-bathymetric data we produced a new tectonic map of the Calabria back-arc system. Further, we characterized some before-unknown faults and linked them with shallow structures, as ridges and slumps / slides. This area seemingly less populated of faults compared to the peri-Tyrrhenian margin, where several faults belong to different systems, i.e. (i) the rifting system active that allowed the opening of the Tyrrhenian Basin and (ii) the slab-decupling related normal faults system currently active. The comparison with historical and instrumental seismicity allowed us to highlight possible seismic gaps that, if considered, could strongly improve the map of seismogenic potential of the Tyrrhenian back-arc system.

 

Bibliography

Brutto, F. et al. (2016). The Neogene-Quaternary geodynamic evolution of the central Calabrian Arc: A case study from the western Catanzaro Trough basin. Journal of Geodynamics, 102, 95-114.

Loreto, M. F. (2017). Reconstructed seismic and tsunami scenarios of the 1905 Calabria earthquake (SE Tyrrhenian sea) as a tool for geohazard assessment. Engineering geology, 224, 1-14.

Tripodi, V. et al. (2018). Neogene-Quaternary evolution of the forearc and backarc regions between the Serre and Aspromonte Massifs, Calabria (southern Italy). Marine and Petroleum Geology, 95, 328-343.

How to cite: Palmiotto, C., Loreto, M. F., Muto, F., Ferrante, V., Pettenati, F., Sandron, D., and Tripodi, V.: Faults distribution and seismogenic potential in the Calabrian back-arc domain, SE Tyrrhenian Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19590, https://doi.org/10.5194/egusphere-egu2020-19590, 2020.

EGU2020-13032 | Displays | SM2.11

Active tectonics and seismicity in the Calabrian Arc subduction complex (Ionian Sea)

Alina Polonia, Sgroi Tiziana, Artoni Andrea, Barberi Graziella, Billi Andrea, Gasperini Luca, and Torelli Luigi

The Calabria Arc (CA) is the narrowest subduction-rollback system on Earth, and it has been struck repeatedly by destructive historical earthquakes often associated with tsunamis. In spite of the detailed earthquake catalogue, the source parameters of most historic earthquakes are still debated, especially for earthquakes that may have been generated offshore.

The subduction system is characterized by an irregular plate boundary reflecting the presence of continental blocks, indenters, and different rates of continental collision. Convergence between Eurasia and Africa produces both compressive and transtensional deformation in the offshore accretionary complex. Shortening occurs along the outer deformation front and along splay faults accommodating differences in rheology and basal detachment depth. Two oppositely dipping strike-slip/transtensional fault systems, i.e., the Ionian (IF) and Alfeo-Etna (AEF) faults produce deep fragmentation of the subduction system and the collapse of the accretionary wedge, in agreement with geodetic models suggesting plate divergence in this region. Transtensional lithospheric faults segmenting the subduction system are punctuated by mantle-rooted diapirism driven by arc orthogonal rifting, collapse of the accretionary wedge, and deep fragmentation of the subduction system along pre-existing Mesozoic transform faults.

Seismological observations in the Western Ionian Sea highlight the presence of earthquake clusters along wide and deep-seated active tectonic structures, which were proposed as likely seismogenic sources for large magnitude historic earthquakes/tsunamis in the region. Low to moderate magnitude earthquakes occurring offshore were relocated using a new 1D velocity model for the Ionian Sea, constrained by geological and geophysical observations, which included data collected by NEMO-SN1 seafloor observatory. Seismological data from NEMO-SN1 were integrated with observations carried out by over 100 land stations of the INGV network, and led us to compile a map of 3D distribution for over 2600 events. 3D locations and focal mechanism analyses allowed us to highlight local lithospheric structure. Although seismicity appears scattered in a wide corridor of deformation within the subduction system, we observe alignments of events along main fault systems with strike-slip and extensional mechanisms. Moreover, results from seismological data analysis, i.e., misfits in the 3D distribution of hypocenters and tomographic maps, could be explained by the presence of an anomalous area between the two structures, characterized by thinned lithosphere probably caused by incipient rifting, as suggested by seismic reflection images and geodynamic interpretations.  

How to cite: Polonia, A., Tiziana, S., Andrea, A., Graziella, B., Andrea, B., Luca, G., and Luigi, T.: Active tectonics and seismicity in the Calabrian Arc subduction complex (Ionian Sea), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13032, https://doi.org/10.5194/egusphere-egu2020-13032, 2020.

EGU2020-6096 | Displays | SM2.11

The strong earthquake of 26 November 2019 (MW6.4) and its associate active tectonic of Durresi Region in Albania.

Rrapo Ormeni, Ismail Hoxha, Petraq Naco, and Dashi Gego

On November 26, 2019, a strong earthquake (Mw6.4) occurred about 16 km, north of the Durresi city in the Adriatic Sea, and 35 km NW from the capital city of Tirana, in the western part of Albania. The main shock of November 26 at 15:20 (UTC) was followed by a great number of aftershocks.
The main event is not a shallow one, with the hypocentral depth at 39 km. This fact explains the localized destruction, not only in the epicentral zone but in a larger zone. This earthquake expresses the increase of recently seismic activity of the Adriatic seismogenic zone. The main shock has caused cracking of the earth, especially in the region where the epicenter of the earthquake is located. The largest cracks are in the vicinity of the Erzen river estuary.
These cracks have widths ranging from few cms to 1m and extending from several hundred meters to 1 km. The depth of cracking in some cases reaches into 2 meters. Those cracks are numerous and often create parallel systems between them that follow the current river bed or traces of the old river beds (paleoalvei).
Liquefaction phenomena have been observed extensively in the area between the villages of Juba and Hamallaj. In this area, there have been observed outflows of pressure water associated with sand and clays. The height of the water has often reached up to 1 meter around the water wells. The phenomenon of liquefaction in these areas has been associated with soil cracks of several cms wide and several tens of meters long.
Based on the neotectonic mapping and the focal mechanism of the mainshock, strike 219°, dip 40°, rake -90°, it is considered that the seismotectonic source that generated thisearthquake is related to NW-SE longitudinal tectonic of the Adriatic Sea. Based on the focal plane solutions provided by the IGEWE website, the mainshock was generated by the activation of an NW-SE striking thrust fault with the compression axes in the NE-SW direction.

Sea Adriatic neotectonic extend from Dalmatic coast to Ionian coast is an ancient tectonic, a reverse fault thrust, thus activated during the Quaternary geologic period to the present day, occasionally with strong earthquakes. The seismic movement has also caused a 10 cm elevation of the terrain in the epicenter of the earthquake, which has been accompanied by a coastline retreat in this area (Hamallaj beach).
The 21 September and 26 November 2019 earthquake sequences, as well as the 1926 seismic event that took place in the Durresi region, exhibit a rough NW−SE-trending structure, which is an active seismotectonic zone in western Albania, therefore constituting a threat for nearby urban areas.

How to cite: Ormeni, R., Hoxha, I., Naco, P., and Gego, D.: The strong earthquake of 26 November 2019 (MW6.4) and its associate active tectonic of Durresi Region in Albania., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6096, https://doi.org/10.5194/egusphere-egu2020-6096, 2020.

EGU2020-18616 | Displays | SM2.11

Factors controlling building damage distribution of the November 26 Mw 6.4 Albania earthquake

Spyridon Mavroulis, Efthymios Lekkas, Panayiotis Carydis, and Dimitri Papa

On November 26, 2019, an earthquake struck the central western part of Albania. It was assessed as Mw 6.4. Its epicenter was located offshore northwestern Durrës, in a distance of about 7 km north of the city and 30 km west from the capital city of Tirana. Its focal depth was about 10 km. Based on the focal plane solutions provided by several seismological institutes and observations, the mainshock was generated by the activation of a NW-SE striking reverse fault. Unfortunately, the earthquake claimed the lives of 52 people. Few hours after the mainshock, the authors visited the earthquake affected areas in order to conduct a field macroseismic survey and geological reconnaissance for assessing the earthquake impact on the building stock. The dominant buildings in the affected area are buildings with load bearing solid brick walls and concrete floor slabs, precast concrete panel buildings and buildings with reinforced concrete (R/C) frame and infill and partition walls. The main characteristic in the majority of these structures is the presence of prefabricated concrete floor slabs with width of 0.7-1.0 m and no connections between them. Building damage was distributed along two ellipses, whose major axis is oriented generally NW-SE. The western ellipse of major damage was observed in Durrës city, located within the Periadriatic Depression, and the eastern one in Thumanë, Laç, Fushë-Krujë, Kamëz towns and Tirana city along the eastern margin of the Tirana Depression. This NW-SE orientation coincides with the strike of the seismogenic fault as it is derived from the fault plane solutions. The first building type presents slight non-structural and structural damage in Durrës city. However, buildings of this type in Thumanë suffered very heavy structural damage including partial collapse resulting in many fatalities. The second type did not suffer significant non-structural or structural damage. The majority of the observed R/C multistorey buildings in Durrës suffered damage to the lower three to four storeys, while the above storeys remained intact. Damage is attributed to the soft soils in the earthquake-affected areas, the undesired resonance phenomena in high buildings, the large duration of the earthquake shaking, the shallow water table in coastal and swamp areas, the pre-existing stress of buildings founded on soft soils characterized by differential settlements and possible liquefaction phenomena, the poor construction quality and workmanship of the affected buildings, the interventions made, the ageing of materials due to differential displacements of the foundation soil, the applicable antiseismic regulations of the time, if ever were applied, the lack of maintenance and inadequate repair after previous destructive earthquakes and the impact of the September 21, 2019 Mw 5.6 earthquake on the buildings of the affected area. The damage are considered typical of an earthquake of this magnitude. The effect of the previous September 21, 2019 Mw 5.6 earthquake in the same area should be also taken into account. Based on the seismic zonation map of Albania, it is concluded that the resulted intensities from the 2019 earthquake are within the limits specified in the Seismic Zonation Map.

How to cite: Mavroulis, S., Lekkas, E., Carydis, P., and Papa, D.: Factors controlling building damage distribution of the November 26 Mw 6.4 Albania earthquake , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18616, https://doi.org/10.5194/egusphere-egu2020-18616, 2020.

We identify the main source of the M6.4 earthquake that rocked north-central Albania on November 26, 2019 to be located within the frontal area of the basal thrust of the Dinaric-Hellenic orogen. This earthquake was easily felt some 400 km away and was the strongest to affect the eastern Adriatic coast since the M7.1 event that struck Montenegro in 1979. Already two months earlier on Sept 21-22, several M5 earthquakes hit the same area. According to the USGS and EMSC-CSEM, all 2019 events occurred within a c. 40km wide epicentral area extending along strike of the front of the northernmost Hellenides between the city of Durres to offshore of the town of Lezhë. A depth interval of 10-20 km was poorly constrained for these events, with hypocenters located below the Periadriatic Foredeep Basin made up of deformed and poorly consolidated Neogene and Pleisto-Holocene sediments. For the M6.4 event, InSAR images from the Sentinal-1 satellite indicate up to 7 cm of epicentral uplift and a centroid depth of c. 17 km for the M6.4 event.

 

By combining own onshore geologic mapping with previously published subsurface imaging of the top of the pre-Neogene carbonates across the convergent margin, we have identified structures associated with a large ENE-dipping blind thrust forming the base of the Neogene-to-Present accretionary wedge at the front of the northernmost Hellenides belt. The centroid depth for the M6.4 event is interpreted to lie within this basal thrust; the shallower Sep 21 M5.6 event is inferred to lie at the western end of this same thrust, which does not appear to break the surface in offshore sections. Taken together, these events point to seismic slip on a thrust plane dipping some 30° ENE.

 

According to geological data this thick frontal Neogene thrust wedge is not continuous across the Dinaride-Hellenides junction, but is dextrally offset in northern Albania by the Lezhë Transfer Fault that forms the northern boundary of the epicentral area activated in 2019. NE of the Lezhë Fault in northernmost Albania and Montenegro, Neogene shortening perpendicular to the orogenic front is minor (≤ 10 km) in industrial seismic sections, whereas to the SW, a Neogene displacement of ≥ 100 km is determined from offset Triassic salt layers in the footwall and hangingwall of the basal thrust. The Lezhë Transfer Fault is thus interpreted to have accommodated a sudden increase of Neogene shortening along the orogenic front to the SSW. Onshore mapping indicates that this fault also transfers onshore Neogene clockwise bending of the Hellenides with respect to the Dinarides of Montenegro. It kinematically links the offshore orogenic front in the SSW to thrusting and orogen-parallel extension along th Shkoder-Peja Normal Fault in the hinterland of the coastal area. Our work suggests that seismic rupture is segmented along the Dinaride-Hellenide front, with the M6.4 Albanian and earlier M7.1 Montenegrinian events occurring in structurally and kinematically separate domains.

How to cite: Handy, M. R., Schmid, S. M., and Briole, P.: The M 6.4 Albanian earthquake of Nov. 26, 2019 and its relation to structures at the Dinarides-Hellenides junctions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5409, https://doi.org/10.5194/egusphere-egu2020-5409, 2020.

At 3:54 night time on November the 26th the city of Durres in Albania was hit by an earthquake of Mw 6.2, followed in the next 4 hours by three additional earthquakes with Mb > 5.0. These earthquakes, part of a sequence that continued with much reduced intensity until mid December, caused severe damages in Durres and the adjacent region, counting a final human toll of about 50 casualties and over 2000 injuried. The historical catalogues show that Albania has been affected by over 10 relatively strong earthquakes (Mw> = 6.0) in the last 200 years (Kiratzi and Dimakis 2013), testifying to an important seismic history.

The focal mechanisms of the Durres earthquakes show compressive fault planes placed at ca. 10 km depth. These earthquakes are part of a belt of compressional earthquakes that borders to the east the southern Adriatic, including the strong Montenegro earthquake (Mw 7.1) of April 1979, indicating that shortening is currently ongoing at the front of the southern Dinarides and Hellenides.

The geological structure of Albania, at the junction between the Dinarides and the Hellenides, shows structural complexities that have their roots in the Mesozoic paleogeography of the region (Argnani, 2013). The front of the Albanian fold-and-thrust belt extends to the sea, where it has been studied thanks to some seismic acquisition campaigns aimed at investigating the geology of the Adriatic Sea (Argnani 2013). This sector of the thrust front is characterized by the presence of important back thrusts, which are correlated to the spatial distribution of the Mesozoic domains of carbonate platforms and pelagic basins. In the sector facing the southern Adriatic basin the presence of a large thickness of Oligocene-Quaternary clastic sediments filling the foredeep promotes the development of triangle zones and backtrusts. The basal thrust of the triangle zone system affects Mesozoic carbonates at an estimated depth of 10-15 km (Fantoni and Franciosi, 2010) and appears to be the source of the Durres earthquakes. A similar structural setting can be envisaged for the Montenegro earthquake of 1979, as the offshore structures show a continuity, although a substantial change in strike occurs across the trend of the Shkoder-Peja line. A large lateral displacement of the internal units occurs along the Shkoder-Peja transversal line, which marks the junction between the Hellenides and the Dinarides. The shallow water limestones of the more external Kruja domain, however, are not laterally offset. Palaeomagnetic results indicate that the Miocene-Pliocene clock-wise rotation of the western arm of the Aegean opening was accomplished just south of the Shkoder-Peja line; these rotations impose an overall change in strike of the outer thrusts, although the frontal structures are specifically affected by the nature of the Mesozoic domains entering the thrust system.

References

Argnani A. (2013) -. Ital. J. Geosci., 132, 175-185.

Fantoni R., Franciosi R. (2010) - Rend. Fis. Acc. Lincei 21, S197–S209.

Kiratzi A., Dimakis E. (2013) - Ital. J. Geosci., 132, 186-193.

How to cite: Argnani, A.: The Durres earthquakes of November 2019: A geological perspective from the Adriatic offshore, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17895, https://doi.org/10.5194/egusphere-egu2020-17895, 2020.

EGU2020-22189 | Displays | SM2.11

The 2019Durrës, Albania earthquake sequence – preliminary results from a post-seismic campaign

Edmond Dushi, Bernd Schurr, Ehsan Kosari, Hugo Soto, Olgert Gjuzi, Alicia Rohnacher, Christian Haberland, Andreas Rietbrock, Frederik Tilmann, Mark Handy, Rexhep Koci, Kamil Ustaszewski, Torsten Dahm, Thomas Meier, and Llambro Duni

On 26th of November 2019 an Mw 6.4 earthquake ruptured near the port town of Durrës, only 25 km from Tirana, the capital of Albania. It caused major damage and killed 51 people, making it the deadliest earthquake in 2019 worldwide.

The earthquake occurred on the eastern Adriatic margin, where the Adriatic micro-plate collides with Eurasia causing widespread distributed deformation and crustal shortening that built the peri-Adriatic orogenic belts. Convergence is accommodated in the external Dinarides/Albanides by thrust faulting, mostlyalong E-dipping low-angle detachments with subordinate W-dipping back-thrusts in the most external thrust belt segment. The deformation front, particularly along the southeastern Adriatic coast, is seismically highly active, manifested not only by this most recent event, but also, e.g., by one of the largest instrumentally recorded earthquakes in Europe, the 1979 M7.1 Montenegro event slightly further north and a number of disastrous historic earthquakes.

The 2019 Durrës mainshock was apparently relatively deep (~25 km) and of thrust type. It was preceded by significant foreshock activity starting in September 2019 with two Mw 5.6 and 5.1 earthquakes a few kilometres south of the mainshock that also had a thrust mechanism, however with nodal planes differing from the mainshock, indicating that these occurred on a different fault.

Approximately two weeks after the mainshock, we installed a 30-station short-period seismic network to densely cover the epicentral area. We will present a preliminary analysis of the mainshock and its aftershock sequencehopefully elucidating the fault network responsible for the earthquake sequence.

How to cite: Dushi, E., Schurr, B., Kosari, E., Soto, H., Gjuzi, O., Rohnacher, A., Haberland, C., Rietbrock, A., Tilmann, F., Handy, M., Koci, R., Ustaszewski, K., Dahm, T., Meier, T., and Duni, L.: The 2019Durrës, Albania earthquake sequence – preliminary results from a post-seismic campaign, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22189, https://doi.org/10.5194/egusphere-egu2020-22189, 2020.

EGU2020-21285 | Displays | SM2.11

Seismotectonics of the Ionian-Akarnania Block (IAB) and Western Greece deduced from a local seismic deployment.

Antoine Haddad, Athanassios Ganas, Ioannis Kassaras, and Matteo Lupi

From July 2016 to May 2017, we deployed a local seismic network composed of 15 short-period seismic stations to investigate the ongoing seismotectonic deformation of Western Greece with emphasis on the region between Ambrakikos Gulf (to the north) and Kyparissia (to the south). The network was deployed to investigate the behavior of key crustal blocks in western Greece, such as the Ionian-Akarnania Block (IAB).

After applying automatic P- and S- wave phase picking we located 1200 local earthquakes using HypoInverse and constrained five 1D velocity model by applying the error minimization technique. Events were relocated using HypoDD and 76  focal mechanisms were computed for events with magnitudes down to ML 2.3 using first motion polarities.

We combined the calculated focal mechanisms and the relocated seismicity to shed light on the IAB block boundaries. Three boundaries highlighted by previous studies were also evidenced :

-The north-west margin of the block, the Cephalonia Transform Fault, Europe‘s most active fault. NW-striking dextral strike-slip motion was recognized for this fault near the Gulf of Myrtos and the town of Fiskardo.

- The south-east margin is the Movri-Amaliada right-lateral Fault Zone, activated during the Movri Mt. Mw 6.4 earthquake sequence.

- The Ambrakikos Gulf (a young E-W rift) and the NW-striking left-lateral Katouna-Stamna Fault zone depict the north and north-eastern margins of the IAB block.

Seismicity lineaments and focal mechanisms define theKyllini-Cephalonia left-lateral fault, which is also highlighted by bathymetry data. We interpret this fault as the south-western margin of IAB separating an aseismic area observed between Cephalonia and Akarnania from a seismogenic zone north of Zakynthos Island and bridging NW Peloponnese with Cephalonia.

How to cite: Haddad, A., Ganas, A., Kassaras, I., and Lupi, M.: Seismotectonics of the Ionian-Akarnania Block (IAB) and Western Greece deduced from a local seismic deployment., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21285, https://doi.org/10.5194/egusphere-egu2020-21285, 2020.

EGU2020-19127 | Displays | SM2.11

The Aegean - Anatolian region: insights on seismic hazard from geodetic and seismological observations

Federica Sparacino, Bruno Giovanni Galuzzi, Mimmo Palano, Margarita Segou, and Claudio Chiarabba

Geodetic and seismic moment-rates comparison provides significant insights into the seismic hazard of regions subjected to relevant crustal deformation. We performed such a comparison for the Aegean-Anatolian region, marking the collision zone between the African, Arabian and Eurasian plates, and characterized by a complex tectonic evolution. First we provided an improved description of the ongoing crustal deformation field of the Aegean-Anatolian region, based on an extensive combination of novel observations rigorously integrated with the published GNSS-based geodetic velocities. Then, the geodetic velocity field is used as model input to estimate the 2D strain-rate and moment-rates fields over a geographic 1° x 1° grid. Second, we collected the historical and instrumental earthquake data in order to define the long-term moment release rate by adopting a truncated Gutenberg-Richter relation. Finally, the geodetic and seismic moment-rates comparison allowed to differentiate crustal deformation modality (seismic versus aseismic), as well as to highlight seismic cycle gaps over the investigated region.

How to cite: Sparacino, F., Galuzzi, B. G., Palano, M., Segou, M., and Chiarabba, C.: The Aegean - Anatolian region: insights on seismic hazard from geodetic and seismological observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19127, https://doi.org/10.5194/egusphere-egu2020-19127, 2020.

Paleomagnetic mapping has beenapplied mainly for investigation of two types of regions: (1) platform areas, and (2) World Ocean. Conventional paleomagnetic mapping has been methodologically formed as the problem of identifying the bedding conditions of magnetostratigraphic units of sedimentary strata of the predominantly platform regions. It is uses mainly paleomagnetic laboratory data analysis derived from field studies. Employing completely different methodological principles, paleomagnetic maps of the Earth's basaltic crust of the World Ocean were constructed. These maps have not only geophysical but also geodynamical and structural-tectonic significance.

The most complex regions of the Earth are the areas of transition from the ocean to the continent, as well as the spreading and collision zones of lithospheric plate joining. Here the most diverse manifestations of the structures and movements of the earth's crust and upper mantle and various and multiphase magmatic appearances are developed. The Eastern Mediterranean, which is a striking example of such regions, is located in the junction between the two largest Earth's lithospheric segments: Eurasia and Gondwana.

The paleomagnetic mapping of transition zones from the ocean to the continent was only sporadic and was not methodologically and rigorously developed as the mapping of continental and oceanic platforms. For more than 20 years of research experience in the Eastern Mediterranean region, we have been able to develop a comprehensive methodology for tectono-paleomagnetic mapping of transition zones from ocean to the continent (e.g., Eppelbaum and Katz, 2015). These reconstructions were utilized as a basis for identifying a variety of mapped bodies and structures. The methodology is based on the integration of the mapping techniques for both continental and oceanic platforms: paleomagnetic reconstructions, results of radiometric dating of magnetically active rocks, biogeographical studies, satellite data examination, plate tectonic reconstructions and utilization of results of various geophysical surveys. All these data are used for combined identifying mapped geological bodies and structures.

Tectonic-paleomagnetic mapping as a new type of geological and geophysical surveys contributed to an essential amendment in understanding the nature and structure of the Eastern Mediterranean. It turned out that this is not a passive, but an active continental margin, where the Mesozoic terrane belt is developed. This belt includes tectonic units of the thinned continental crust and parts of the Neotethys Ocean crust with a block of the ancient Kiama hyperzone (Early Permian) and a series of ophiolite bodies and sporadic mantle diapirs (Eppelbaum and Katz, 2015).

Tectonic-paleomagnetic mapping reveals not only the historical-geodynamic, but also the deep-geophysical nature of the Eastern Mediterranean evolution. In particular, it was established that in the formation of the Sinai plate, two zones with the deep mantle trap complexes were involved: the Late Mesozoic and the Late Cenozoic relating to the Jalal and Sogdiana paleomagnetic hyperzones, respectively.

Eppelbaum, L.V. and Katz, Yu.I., 2015. Paleomagnetic Mapping in Various Areas of the Easternmost Mediterranean Based on an Integrated Geological-Geophysical Analysis. In: (Eppelbaum L., Ed.), New Developments in Paleomagnetism Research, Ser.: Earth Sciences in the 21st Century, Nova Science Publisher, NY, 15-52.

 

How to cite: Eppelbaum, L. and Katz, Y.: Tectono-paleomagnetic mapping of transition zones from ocean to continent (on example of the Eastern Mediterranean), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2049, https://doi.org/10.5194/egusphere-egu2020-2049, 2020.

SM3.1 – Taking advantage of the exponential growth of data: toward a better assessment of ground-shaking, seismic hazard and seismic risk

EGU2020-8641 | Displays | SM3.1

Rate and State Seismicity Simulations for Large Earthquake Cycles in Western Taiwan

Hung-Yu Wu, Kuo-Fong Ma, and Bill Fry

The stress state variation during the fault rupturing is the key issue for the earthquake hazard. However, the modern seismic catalogs exist the huge gap of large earthquake recurrence records. To understand the occurrence, the probabilities and the dynamic processing of large earthquakes, we employed the multi-cycle earthquake simulator, RSQSim, to exam the fundamental aspects of seismicity distribution in spatial and time in western Taiwan. This 3D, boundary element software assembles the Rate and State Friction law (RSF) and initial stress state to simulate the earthquakes distributions in completely, complex seismogenic system. The heterogeneous initial stresses and recurrence seismic events would be estimated in the long sequences. In this research, we focus on the similarity comparison to the CWB earthquake catalog and Taiwan Earthquake Model (TEM) for the RSQSim simulations. Additionally, this information provides the near optimal nucleation locations and seismic events propagation at the stress evolution in Taiwan faulting systems. Through this process, we would like to examine the recurrence time of the significant earthquakes in western Taiwan. RSQsim results include the comprehensive large events in temporal series to understand the key discrepancy between models and simulators, which will bring the mutual input to TEM for update discussion on slip rate, stress accumulation, and fault system. These modifications provide the better understanding of faults slip and stress state evolution to support the fundamental aspects of earthquake cycles.

How to cite: Wu, H.-Y., Ma, K.-F., and Fry, B.: Rate and State Seismicity Simulations for Large Earthquake Cycles in Western Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8641, https://doi.org/10.5194/egusphere-egu2020-8641, 2020.

EGU2020-5187 | Displays | SM3.1

Spatial Variability of Source and Attenuation Characteristics in Large Ground-Motion Datasets

Sreeram Reddy Kotha, Graeme Weatherill, Dino Bindi, and Fabrice Cotton

Ground-Motion Models (GMMs) characterize the random distributions of ground-motions for a combination of earthquake source, wave travel-path, and the effected site’s geological properties. Typically, GMMs are regressed over a compendium of strong ground-motion recordings collected from several earthquakes recorded at multiple sites scattered across a variety of geographical regions. The necessity of compiling such large datasets is to expand the range of magnitude, distance, and site-types; in order to regress a GMM capable of predicting realistic ground-motions for rare earthquake scenarios, e.g. large magnitudes at short distances from a reference rock site. The European Strong-Motion (ESM) dataset is one such compendium of observations from a few hundred shallow crustal earthquakes recorded at a several hundred seismic stations in Europe and Middle-East.

We developed new GMMs from the ESM dataset, capable of predicting both the response spectra and Fourier spectra in a broadband of periods and frequencies, respectively. However, given the clear tectonic and geological diversity of the data, possible regional and site-specific differences in observed ground-motions needed to be quantified; whilst also considering the possible contamination of data from outliers. Quantified regional differences indicate that high-frequency ground-motions attenuate faster with distance in Italy compared to the rest of Europe, as well as systematically weaker ground-motions from central Italian earthquakes. In addition, residual analyses evidence anisotropic attenuation of low frequency ground-motions, imitating the pattern of shear-wave energy radiation. With increasing spatial variability of ground-motion data, the GMM prediction variability apparently increases. Hence, robust mixed-effects regressions and residual analyses are employed to relax the ergodic assumption.

Large datasets, such as the ESM, NGA-West2, and from KiK-Net, provide ample opportunity to identify and evaluate the previously hypothesized event-to-event, region-to-region, and site-to-site differences in ground-motions. With the appropriate statistical methods, these variabilities can be quantified and applied in seismic hazard and risk predictions. We intend to present the new GMMs: their development, performance and applicability, prospective improvements and research needs.

How to cite: Kotha, S. R., Weatherill, G., Bindi, D., and Cotton, F.: Spatial Variability of Source and Attenuation Characteristics in Large Ground-Motion Datasets, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5187, https://doi.org/10.5194/egusphere-egu2020-5187, 2020.

EGU2020-5832 | Displays | SM3.1

Non-ergodic FAS Ground-Motion Model for France

Chih Hsuan Sung, Norman Abrahamson, Nicolas Kuehn, Paola Traversa, and Irmela Zentner

In this study, we use an ergodic ground motion model (GMM) of California of Bayless and Abrahamson (2019) as a backbone and incorporate the varying-coefficient model (VCM) to develop a new French non-ergodic GMM based on the French RESIF data set (1996-2016). Most of the magnitudes of this database are small (Mw = 2.0 – 5.2), so we adopt the Fourier amplitude spectral GMM rather than the spectral acceleration model, which allows the use of small magnitude data to constrain path and site effects without the complication of the scaling being affected by differences in the response spectral shape. For the VCM, the coefficients of GMPE can vary by geographical location and they are estimated using Gaussian process regression. That is, there is a separate set of coefficients for each source and site coordinate, including both the mean coefficients and the epistemic uncertainty in the coefficients. Moreover, the epistemic uncertainty associated with the predicted ground motions also varies spatially: it is small in locations where there are many events or stations and it is large in sparse data regions. Finally, we modify the anelastic attenuation term of a GMM by the cell-specific approach of Kuehn et al. (2019) to allow for azimuth-dependent attenuation for each source which reduces the standard deviation of residuals at long distances. The results show that combining the above two methods (VCM & cell-specific) to lead an aleatory standard deviation of residuals for the GMM that is reduced by ~ 47%, which can have huge implications for seismic-hazard calculations.

How to cite: Sung, C. H., Abrahamson, N., Kuehn, N., Traversa, P., and Zentner, I.: Non-ergodic FAS Ground-Motion Model for France, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5832, https://doi.org/10.5194/egusphere-egu2020-5832, 2020.

EGU2020-21645 | Displays | SM3.1

A new approach for arriving at higher frequencies through stochastic modelling: application to site attenuation (kappa)

Erion-Vasilis Pikoulis, Olga-Joan Ktenidou, Emmanouil Psarakis, and Norman Abrahamson

We propose a framework for stochastically modelling the Fourier spectrum of the noisy seismic recording, considering that a seismic signal is a random rather than a deterministic quantity. We show that under this assumption, the noisy recording’s periodogram can be modelled as independent Exponential random variables with a frequency-dependent mean. With this model, estimating seismological parameters can be tackled through Maximum Likelihood (ML), allowing a fast, accurate and robust solution. This new approach constitutes a general estimation framework applicable to any parameter estimation that uses spectral analysis. Here we apply it to the high-frequency decay parameter kappa, which is particularly important for estimating and adjusting ground motion on rock. The improved ML performance is shown through a series of experiments on synthetic and recorded seismograms. The biggest advantage of the new method is its ability to account for the noise in the recording instead of just trying to avoid it, as is typically done when any ‘acceptable’ frequency range is isolated through signal-to-noise (SNR) criteria. As a result, our proposed technique can achieve acceptable results even for what would be typically considered very low and often unusable SNR, pushing the boundary of what is considered usable quality in seismic recordings.

How to cite: Pikoulis, E.-V., Ktenidou, O.-J., Psarakis, E., and Abrahamson, N.: A new approach for arriving at higher frequencies through stochastic modelling: application to site attenuation (kappa), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21645, https://doi.org/10.5194/egusphere-egu2020-21645, 2020.

EGU2020-8939 | Displays | SM3.1

On-site Earthquake Early Warning: Predictive Models for Acceleration Response Spectra Considering Site-Effects

Antonio Giovanni Iaccarino, Matteo Picozzi, Dino Bindi, and Daniele Spallarossa

Including site specific amplification factors in ground motion prediction models represented an advance for PSHA (Atkinson 2006; Rodríguez-Marek et al. 2013; Kotha et al. 2017) that has become nowadays a standard. However, this issue has only recently received attention by the seismological community of earthquake early warning (EEW) (Spallarossa et al., 2019; Zhao and Zhao, 2019), which applications require a real-time prediction of ground motion and the delivery of alert messages to users for mitigating their exposure to seismic risk. Indeed, all EEW systems are high-technological infrastructures devoted to the real-time and automatic detection of earthquakes, rapid assessment of the associated seismic hazard for targets and the prompt delivery of alerts trough fast telecommunication networks. Among them, the on-site approaches are based on seismic networks placed near to the target, indifferently by the location of seismic threats and they issue the alert predicting the ground motion at the target from P-wave parameter. This configuration cause that On-Site EEWS are generally highly affected by site conditions.

In this work, we calibrated ground motion prediction models for on-site EEW considering acceleration response spectra (RSA) and the P-waves EEW parameters Iv2 and Pd, and we investigated the role of site-effects. We considered a dataset of nearly 60 earthquakes belonging to the Central Italy 2016-17 sequence. The high density of stations near to the sequence has allowed us to use a non-ergodic random-effect regression approach to explore and to reduce the contribution of site-effects to the uncertainty of the On-site laws predictions. We grouped the records in two ways: by stations and by EC8 classification. Then, we validated the estimated models by the Leave One Out (L1Out) technique and applied a K-means analysis to assess the performance of the EC8 classification.

The residuals analysis proved that grouping by station provides a set of relations that improves the predictions at many stations. On the contrary, L1Out cross-validation proved that the regressions retrieved grouping by EC8 classification produce higher uncertainties on the predictions than the others. Furthermore, the cross-validation proved that Iv2 is more correlated to RSA than Pd. Finally, the analysis of the random effect vs period curves confirmed that EC8 classification is unrelated to the site effect on RSA even looking only at the trend of these curves.

In conclusion, non-ergodic random-effect regression can be used also in the EEW applications to predict site-specific ground motion. EEWS that use this approach are less dependent by site-effect and able to provide more precise and reliable alerts.

How to cite: Iaccarino, A. G., Picozzi, M., Bindi, D., and Spallarossa, D.: On-site Earthquake Early Warning: Predictive Models for Acceleration Response Spectra Considering Site-Effects, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8939, https://doi.org/10.5194/egusphere-egu2020-8939, 2020.

Over the last two decades, seismic ground motion prediction has been significantly improved thanks to the development of shared, open, worldwide databases (waveform and parametric values). Unlike seismic ground motion, earthquake data recorded in buildings are rarely shared. However, their contribution could be essential for evaluating the performance of structures. Increasing interest in deploying instrumentation in buildings gives hope for new observations, leading to better understanding of behavior. This manuscript presents a flat-file containing information on earthquake responses of instrumented buildings. Herein, we present the structure of the NDE1.0 flat-file containing site and earthquake characteristics (vs30, Magnitude, Distance...), structural response parameters (i.e. drift ratio, peak top values of acceleration, velocity and displacement, pre- and co-seismic fundamental frequencies) computed for several intensity measures characterizing ground motion (peak and spectral values, duration...). The data are from real earthquake recordings collected in buildings over the years. This 1.0 version contains 8,520 strong motion recordings that correspond to 118 buildings and 2,737 events, providing useful information for analyses related to seismic hazard, variability of building responses, structural health monitoring, nonlinear studies, damage prediction, etc. Some specific analysis will be presented concerning seismic structural health monitoring and damage prediction, with a special focus on the engineering demand parameter versus intensity measures variability.

How to cite: Guéguen, P., Astorga, A., and Ghimire, S.: NDE1.0 – A new database of earthquake data recordings from buildings for performance based earthquake engineering, vulnerability assessment and seismic structural health monitoring, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6948, https://doi.org/10.5194/egusphere-egu2020-6948, 2020.

EGU2020-19670 | Displays | SM3.1

The hazard and risk assessment of Taipei Metropolitan through earthquake scenario from open data

Ming-Kai Hsu, Kuo-Fong Ma, Chung-Han Chan, and Danijel Schorlemmer

    Modeling seismic hazard based on the ground motion scenario through numerical simulation could enhance the earthquake prevention strategy, especially for highly populated urban region. Taiwan as an earthquake prone country, it is important to provide the earthquake awareness through multiple risk (impact and loss) scenarios. These end-to-end hazard and risk scenarios will increase the resilience of society to extreme earthquake events by identifying the factors critical to society in the earthquake hazard and risk scenarios. The results will help to provide resilient urban development and future design by understanding and strengthening societal capacity for resilience.  Taking advantage on the open data policy, we collected the dense seismic data and open exposure data in buildings in the Taipei Metropolitan to develop the task of the end-to-end hazard and risk scenarios. The seismic hazard was made through earthquake scenario from the rupture of the Shanchiao fault, which is to the west of the Taipei basin. The topography and velocity structure of the basin were taken into account in the simulation to explore the long duration of shaking and basin effect, together with thorough evaluation on site amplification of densely populated seismic stations within the basin. We explore the assessment of scenario-based loss estimation with the exposure model of 500x500 meter grid-based data from National Science and Technology Center for Disaster Reduction (NCDR) and the governmental open data consisted of Taipei building user license information and open street map shape file data. For building damage estimation, we developed building damage based fragility curves including 1999 Chi-Chi and 2016 Meinong earthquakes for the ground motion in PGA, PGV and Intensity. We also considered the acceleration response spectrum (Sa) and velocity response spectrum (Sv) in different interval of period. Through the development of the end members, we hope to build the earthquake hazard and risk scenarios to ensure effective disaster response from up-to-date, open, transparent and reliable risk-data.

How to cite: Hsu, M.-K., Ma, K.-F., Chan, C.-H., and Schorlemmer, D.: The hazard and risk assessment of Taipei Metropolitan through earthquake scenario from open data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19670, https://doi.org/10.5194/egusphere-egu2020-19670, 2020.

EGU2020-9345 | Displays | SM3.1

Taiwan Conditional Prediction Equation for Horizontal PGD for Crustal Sources

Chih Hsuan Sung, Norman Abrahamson, and Jyun Yan Huang

A conditional ground-motion model (GMM) is developed for peak ground displacement (PGD)for Taiwan. The conditional GMM includes the observed pseudo-spectral acceleration (PSA(T)) as an input parameter in addition to magnitude and distance. The conditional PGD model can be combined with the traditional GMMs for PSA values to develop a GMM for PGD without the dependence on PSA. The main advantages of the conditional model approach are that it can be quickly developed, is easily understandable, can fully capture the magnitude, distance, and site scaling of the secondary parameters that are compatible with the design response spectral values, and also has much smaller aleatory variability than traditional GMMs. In this study, we use part of the database of Taiwan SSHAC Level 3 project (13691 strong-motion records from 158 crustal events occurred between 1992 and 2018 with 4.5 ≤ Mw ≤ 7.65) to develop a new conditional scaling model for horizontal PGD consisted from the suite period of the PSA, rupture distance and moment magnitude. Furthermore, we combine this conditional model with each of two SSHAC Level 3 models and NGA-West2 ground-motion models for PSA(T) to derived new GMMs for the median and standard deviation of PGD. The results show that the new PGD GMMs include the more complex ground-motion scaling which capture from the GMMs of PSA, such as hanging-wall effects, sediment-depth effects, soil nonlinearity effects, and regionalization effects.

How to cite: Sung, C. H., Abrahamson, N., and Huang, J. Y.: Taiwan Conditional Prediction Equation for Horizontal PGD for Crustal Sources, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9345, https://doi.org/10.5194/egusphere-egu2020-9345, 2020.

EGU2020-6911 | Displays | SM3.1

Exploring the dimensionality of ground motion data

Reza Dokht Dolatabadi Esfahani, Kristin Vogel, Fabrice Cotton, Matthias Ohrnberger, Frank Scherbaum, and Marius Kriegerowski

For years, engineering seismologists aim to reduce the epistemic uncertainty related to ground motion prediction. Assuming that simple models with few variables are not sufficient to describe the complex phenomena, there is a trend in present-day science to increase complexity of ground motion models. Therefore, some of the most recent ground motion prediction equations use more than 20 variables to improve the predictive power of the model. However, the legitimate question to ask is whether the inclusion of additional variables leads to an improved predictive power of the model. In other words, what is the smallest number of predictive variables needed to reconstruct the distribution of ground motion induced shaking observed in data? In this study, by taking advantage of the exponential growth of ground motion data and new machine learning methods, we present a data-driven approach to derive the dimensionality of ground motion data in the Fourier amplitude spectrum (FAS) metric. We apply an autoencoder architecture, which is commonly used for mapping high dimensional data to a lower dimensional space (bottleneck) and search for the lowest dimensionality (minimum number of nodes in the bottleneck) required to reconstruct the FAS input data. The approach is tested on synthetic ground motion data with known dimensionality (2D and 4D) and finally applied to the FAS of recorded ground motion data. A simple autoencoder with variable nodes in the bottleneck is used to explore the dimensionality of the ground motion data. We use the relation between the total residual of the network with the number of codes in the bottleneck as an indicator of dimensionality. Its numerical value is estimated based on the reduction of residuals by increasing the number of codes in the bottleneck layer. In addition, we use the low dimensional manifold of the ground motion data to predict the ground motion shaking for a given scenario. The residual analyses between observed and reconstructed data and observed and predicted data are used to validate the training and prediction steps. We applied the method on different scenarios in two synthetic data sets which are simulated by a stochastic simulation method and secondly the Pan-European engineering strong motion data (EMS) to show the performance of the proposed method. The results show that the statistical properties of ground motion data can be captured by using a limited number of three to five parameters. Especially for low frequency data the most dominant features are already captured by two parameters (codes), which roughly correspond to magnitude and distance. For higher frequencies additional parameters, e.g. corresponding to stress drop and kappa, become more relevant. The standard deviation of the residuals can be reduced to its lower bound in comparison with the standard deviations of conventional methods. Finally, we use a two-dimensional manifold to predict the FAS for given magnitude and distance values.

How to cite: Dokht Dolatabadi Esfahani, R., Vogel, K., Cotton, F., Ohrnberger, M., Scherbaum, F., and Kriegerowski, M.: Exploring the dimensionality of ground motion data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6911, https://doi.org/10.5194/egusphere-egu2020-6911, 2020.

EGU2020-21607 | Displays | SM3.1

When 1D response analysis fails: application of Earthquake HVSR in Site-Specific Amplification Estimation

Chuanbin Zhu, Marco Pilz, and Fabrice Cotton

Ground response analyses (GRA) model the vertical propagation of SH waves through flat-layered media (1DSH) and are widely carried out to evaluate local site effects in practice. Horizontal-to-vertical spectral ratio (HVSR) technique is a cost-effective approach to extract certain site-specific information, e.g., site resonant frequency, but HVSR values cannot be directly used to approximate the level of S-wave amplification. Motivated by the work of Kawase et al. (2019), we propose a procedure to correct earthquake HVSR amplitude for direct amplification estimation. The empirical correction, in essence, compensates HVSR by generic vertical amplifications grouped by vertical fundamental resonant frequency (f0v) and 30 m average shear-wave velocity (VS30) via k-mean clustering. In this investigation, we evaluate the effectiveness of the corrected HVSR in approximating observed amplification in comparison with 1DSH modelling. To the end, we select a total of 90 KiK-net surface-downhole recording sites which are found to have no velocity contrasts below downhole sensor and thus of which surface-to-borehole spectral ratio (SBSR) can be taken as its empirical transfer function (ETF). 1DSH-based theoretical transfer function (TFF) is computed in the linear domain considering the uncertainty in VS profile through randomization. Five goodness-of-fit metrics are adopted to gauge the closeness between observed (ETF) and predicted (i.e., TTF and corrected HVSR) amplifications in both amplitude and spectral shape. The major finding of this study is that the empirical correction procedure to HVSR is highly effective, and the corrected HVSR has a “good match” in both spectral shape (Pearson’s r > 0.6) and amplitude (Index of agreement d > 0.6) at 74% of the investigated sites, as opposed to 17% for 1DSH modelling. In addition, the HVSR-based empirical correction does not need a site model and thus has great potentials in site-specific seismic hazard assessments.

How to cite: Zhu, C., Pilz, M., and Cotton, F.: When 1D response analysis fails: application of Earthquake HVSR in Site-Specific Amplification Estimation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21607, https://doi.org/10.5194/egusphere-egu2020-21607, 2020.

EGU2020-21146 | Displays | SM3.1

Within and Between-Events Variability of Strong-Velocity Pulses

Ming-Hsuan Yen, Kuo-Fong Ma, Fabrice Cotton, Yen-Yu Lin, and Ya-Ting Lee

Ground motions with strong pulses often bring significant damage to structures. The period and the amplitude of the strong-velocity pulses are critical for structural engineering and seismic hazard assessment. The scaling of pulses periods with magnitudes and the within-event variability of pulses is however poorly understood. In this study, we analyze two moderate earthquakes, namely 2016 Meinong earthquake and 2018 Hualien earthquake, using Shahi and Baker’s criteria (2014) to detect pulses. The observations in this study show that the amplitudes of the pulse decay with the distance from the source to the stations, and is also associated with the rupture direction from the asperity instead of the direction from the hypocenter. In addition, we further perform simulations using a simple FK method to clarify the causes of the variability of the pulse periods within and between events. We test the effect of faults dipping angles and the impacts of the asperity location and size. Through our simulations, we reveal that the amplitudes of the pulses in the shallow dipping fault are larger on the hanging wall than on the foot wall, and that the asperity properties has a large impact on the pulses periods and the amplitudes at the nearby stations. The results show that the asperity characteristics are critical for the occurrence of the strong-velocity pulses. The complete understanding of the kinematics of the rupture is then important for clarifying the effects of the strong-velocity pulses and improving ground-motions predictions.

How to cite: Yen, M.-H., Ma, K.-F., Cotton, F., Lin, Y.-Y., and Lee, Y.-T.: Within and Between-Events Variability of Strong-Velocity Pulses , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21146, https://doi.org/10.5194/egusphere-egu2020-21146, 2020.

The problem is considered of unrealistic ground motion estimates, which arise when the Cornell–McGuire method is used to estimate the seismic hazard for extremely low annual probabilities of exceedance. This problem stems from using the normal distribution in the modelling of the variability of the logarithm of ground motion parameters. In this study, the statistical properties of the logarithm of peak ground acceleration (PGA) are analysed by using the database of the strong-motion seismograph networks of Japan. The normal distribution and the generalised extreme value distribution (GEVD) models were considered in the analysis, with the preferred model being selected based on statistical criteria. The results indicate that the GEVD was a more appropriate model in eleven out of twelve instances. The estimates of the shape parameter of the GEVD were negative in every instance, indicating the presence of a finite upper bound of PGA. Therefore, the GEVD provides a model that is more realistic for the scatter of the logarithm of PGA, and the application of this model leads to a bounded seismic hazard curve.

How to cite: Pavlenko, V.: Statistical properties of peak ground acceleration and their effect on results of probabilistic seismic hazard analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21741, https://doi.org/10.5194/egusphere-egu2020-21741, 2020.

EGU2020-19528 | Displays | SM3.1

Contribution for seismic hazard assessment with local scale focus on Durrës (Albania) and damage observation after the ML 5.4, 21st September 2019 earthquake

Marco Mancini, Iolanda Gaudiosi, Redi Muci, Maurizio Simionato, and Klodian Skrame

The city of Durrës was recentely struck by a Mw 6.2 mainshock event (http://cnt.rm.ingv.it/event/23487611) that caused considerable damage and 51 victims. The city is located on an actively seismotectonic belt where seismic catalogues report few past events with magnitude higher than 6.

Surface geology is generally considered to influence the ground motion recorded on site. The analysis of the influence of local effects on seismic response at ground surface appears relevant also considering that Durrës is a densely populated city prone to high seismic risk and is characterized by several important archeological and cultural heritage sites.

Preliminary results obtained from recent geophysical in-situ measurements and geological surveys, carried out in Durrës after the ML 5.4, 21st September 2019 event, are presented with the aim of providing new elements for the assessment of local seismic hazard and following a comprehensive approach to the modifications induced by the site.

Twenty-nine single-station noise measurements, processed through the HVSR technique, two MASW surveys and two 2D array measurements were performed. Results from noise measurements define a zone eastward of the historical centre, where the characteristics of surficial soil layers are responsible for modification to the seismic demand. In particular, HVSR curves in this area show amplification higher than 4 at a period higher than 1s. Moreover, on this location a surface waves-velocity profile obtained from a joint inversion of Rayleigh curves from MASW and 2D array with ellipticity individuates a class D soil, EC8 sensu, corresponding to marshy soil of very poor geotechnical quality. These data may be considered as key elements in the site-specific response analyses, i.e. realized according to the international codes (EC8, NEHRP), which allow to quantify the expected ground motion. These results are potentially useful for  correlating  construction typologies and period vibration of the buildings with the site amplification.

In addition, a damage survey was carried out in one of the most damaged zones after the 21st September 2019 earthquake. Because of the following stronger event of the 26th November 2019, we think that these preliminary results may provide useful information for the post-earthquake reconstruction and enhancement of the urban resilience.

                The activities are carried out wihin the framework of the CNR/MOES Joint research project “Seismic risk assessment in cultural heritage cities of Albania” in the biennium 2018-2019 (https://www.cnr.it/en/bilateral-agreements/agreement/60/moes-ministry-of-education-and-sport-of-the-republic-of-albania).

How to cite: Mancini, M., Gaudiosi, I., Muci, R., Simionato, M., and Skrame, K.: Contribution for seismic hazard assessment with local scale focus on Durrës (Albania) and damage observation after the ML 5.4, 21st September 2019 earthquake, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19528, https://doi.org/10.5194/egusphere-egu2020-19528, 2020.

EGU2020-6555 | Displays | SM3.1

Correlation between shear wave velocity, void ratio and effective stress: Mapping Vs30 in Taiwan

Jia Cian Gao, Jyun Liang Guo, Jia Jyun Dong, and Chyi Tyi Lee

Site effect is one of the critical factors influencing the seismic hazard evaluation. Among others, the average shear-wave velocity of the upper 30 meters of a soil profile (Vs30) has been widely used for assessing the ground-motion amplification. However, spatial resolution of shear wave velocity data is usually poor for reginal- or national-wise evaluation. Standard Penetration Test N-value, the most abundant geotechnical data, was then used to estimate the shear wave velocity (Vs) empirically and the uncertainty of the Vs30 map can be reduced. In this study, we use the state variables of soils (void ratio and effective stress) to evaluate the shear wave velocity and to map the Vs30 in Taiwan. Engineering Geological Database for TSMIP (EGDT) comprises soil profile, shear wave velocity measurements, groundwater table, and soil physical properties (such as void ratio, water content, specific gravity, and unit weight), was used to construct the correlation between Vs, void ratio, and effective stress. The drilling database of Taiwan CGS was then used to estimate the spatial distribution of Vs30, where the Vs is un-available. The results were compared with the previous version of Vs30 map of Taiwan. The uncertainty of the new Vs30 map was evaluated and the propagation of uncertainty to the seismic hazard can be evaluated accordingly.

 

How to cite: Gao, J. C., Guo, J. L., Dong, J. J., and Lee, C. T.: Correlation between shear wave velocity, void ratio and effective stress: Mapping Vs30 in Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6555, https://doi.org/10.5194/egusphere-egu2020-6555, 2020.

EGU2020-11988 | Displays | SM3.1

Identification of Engineering Bedrock in Taiwan based on Site Amplification and Velocity Structures of Strong-motion Stations

Che-Min Lin, Jyun-Yan Huang, Chun-Hsiang Kuo, and Kuo-Liang Wen

There are two kinds of bedrocks that are widely used in seismology and earthquake engineering respectively. The seismology field uses the “seismic bedrock” to define an interface that has a practically lateral extent. The strata deeper than this interface is much more homogeneous in comparison with the shallower one. It is common to set the seismic bedrock within the upper crust has 3000 m/sec of the shear wave velocity. In contrast, the earthquake engineering prefers the shallower interface which dominates the main seismic site amplification, especially the predominant frequency of ground motion. The interface is called “Engineering Bedrock”, which the underlying stratum has the shear wave velocity from 300 to 1000 m/sec for different purposes. But, the reference shear wave velocity of the engineering bedrock is mostly defined as 760 m/sec for ground motion prediction and simulation. In Taiwan, the Central Weather Bureau (CWB) constructed and operates a dense strong-motion network called TSMIP (Taiwan Strong Motion Instrument Program), which provides numerous ground motion data for seismology and earthquake engineering. In our previous studies, the shallow shear wave velocity profiles of over 700 TSMIP stations were estimated by the Receiver Function method. The velocity profiles are from the ground surface to the depth with the shear wave velocity of at least 2000 m/sec. It allows us to compare the theoretical site amplification of the velocity profile of TSMIP stations with their observed one from the seismic records. The variance of fitness between theoretical and observed amplifications through shear wave velocity is analyzed to evaluate which reference velocity can appropriately define the depth of engineering bedrock, where the most site amplification occur beneath, in all of Taiwan. The difference between local geology is also discussed. Finally, an engineering bedrock map is proposed for further applications in earthquake engineering.

How to cite: Lin, C.-M., Huang, J.-Y., Kuo, C.-H., and Wen, K.-L.: Identification of Engineering Bedrock in Taiwan based on Site Amplification and Velocity Structures of Strong-motion Stations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11988, https://doi.org/10.5194/egusphere-egu2020-11988, 2020.

Marmara region located on the western end of the North Anatolian Fault Zone is a tectonically active region in Turkey. There have been frequent severe earthquakes in the region and will continue to occur. There was no serious earthquake in the region after the 1999 Mw7.4 Kocaeli and Mw7.2 Düzce earthquakes. A Marmara Sea offshore earthquake Mw5.8 close to Silivri Town of Istanbul Metropolitan City has occurred on September 26, 2019 daytime at 13:59. The earthquake happened at the coordinate of 40.87N – 28.19E with a depth of 7.0km on the Kumburgaz segment of the North Anatolian Fault line. It was felt in almost all Marmara region. In some settlements in Istanbul City, slight to moderate damages were observed. A foreshock earthquake of Mw4.8 occurred on the same segment on 24 September, 2019. 150 aftershock events ranging from M1.0 to M4.1 have been recorded within the 24 hours after the mainshock. The ground motions have been recorded in the region by the several institutions including AFAD (Disaster and Emergency Management Presidency), KOERI (Kandilli Observatory and Earthquake Research Institute) and IGDAS (Istanbul Gas Distribution Industry and Trade Inc.). The ground motion records and selected parameters have been examined in this study. The ground motion parameters (MMI, PGA, PGV, Sa, Sv, Sd) distribution have been achieved and checked by the recent NGA-West2 ground motion prediction equations (GMPEs); ASK2014, CY2014 and BSSA2014. The compatibility of the GMPEs for a moderate size Marmara Sea earthquake has been examined.

How to cite: Okuyan Akcan, S. and Zulfikar, C.: SEPTEMBER 26, 2019 Mw5.8 MARMARA SEA-SILIVRI (ISTANBUL) EARTHQUAKE: ANALYSIS OF GROUND MOTION RECORDS, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7315, https://doi.org/10.5194/egusphere-egu2020-7315, 2020.

EGU2020-12333 | Displays | SM3.1

The synthetic spectra of potential earthquakes for Taipei basin

Ming-Wey Huang, Chi-Ling Chang, and Sheu-Yien Liu

Modeling the amplitude spectra based on the source term, the path one and site ones for 54 sites located in and around the Taipei basin is the aim of this study. The site term includes the amplification function varied with frequency and the site-specific parameter (k0). The amplification functions for Class-C, -D, and -E site are from Huang et al. (2007) for the central Taiwan. Meanwhile, the amplification function for Class-B site can be referred to Boore and Joyner (1997). The root-mean-squared spectral amplitudes of two horizontal shear waves after three-point smoothing from the observed seismograms are compared to the synthetic amplitude spectra. The goodness of fit coefficient (GFC) and the residual errors (ERR) are calculated for concluding the fitness of the modeling amplitude spectra. Results show both the GFC and ERR of stations are varied with the earthquake magnitude and hypo-central distance. The averaged GFC are larger than 0.8 for 42 stations. Meanwhile, there are 12 station with averaged GFC smaller than 0.8. Besides, the ERRs of 28 stations are less than 0.5. Meanwhile, there are 18 stations with ERRs in the range of 0.5-0.6. The obtained results may be used for modeling the amplitude spectra for the Taipei area. The more accurate amplitude spectra can be improved by updating the parameters utilized in the source-, the path- and the site terms.

How to cite: Huang, M.-W., Chang, C.-L., and Liu, S.-Y.: The synthetic spectra of potential earthquakes for Taipei basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12333, https://doi.org/10.5194/egusphere-egu2020-12333, 2020.

For seismic analysis of complex and non-linear structure system, a seismic code recommends a dynamic time history analysis. In these cases, the input earthquake seismograms should be needed, and these input earthquake seismograms must be compatible with design response spectrum and reflect the site seismic characteristics including information about a fault and wave travel path between the fault and the site. The foreshocks, main shock and aftershocks of earthquake measured in the target area can be assumed to be the output signals of the system consisting of the fault and the wave travel path between the fault and the site. Each earthquake seismogram is considered as a amplitude modulated (AM) signal defined by the magnitude (or energy) and phase function with time. The probability distribution function (PDF) of the magnitude and phase function can be evaluated through the statistical and harmonic wavelet analysis of the measured output signals and these magnitude and phase PDFs include sufficient information to generate the possible output earthquake seismograms of the fault and travel path system for a site, which mean the phase and magnitude PDFs represent a site seismic characteristics.

In this paper, the method to generate the possible design response spectrum compatible earthquake seismograms based on the measured foreshocks, main shock and aftershocks of earthquake is proposed. At first the proposed method generate possible earthquake signals reflecting the phase characteristic of a site, and then modify the magnitude of these earthquake seismograms to determine the response spectrum compatible earthquake motions.  

How to cite: Park, H.-C. and Oh, H.-J.: Generation of response spectrum compatible earthquake seismogram considering phase characteristics of possible earthquake motions of site, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12182, https://doi.org/10.5194/egusphere-egu2020-12182, 2020.

EGU2020-19404 | Displays | SM3.1

Design and implementation of a mobile device APP for network-based EEW systems: application to PRESTo EEWS in Southern Italy

Simona Colombelli, Francesco Carotenuto, Luca Elia, and Aldo Zollo

A fundamental feature of any Earthquake Early Warning System is the ability of rapidly broadcast earthquake information to a wide audience of potential end users and stakeholders, in an intuitive, customizable way. Smartphones and other mobile devices are nowadays continuously connected to the internet and represent the ideal tools for earthquake alerts dissemination, to inform a large number of users about the potential damaging shaking of an impending earthquake.

Here we present a mobile App (named ISNet EWApp) for Android devices which can receive the alerts generated by a network-based Early Warning system. Specifically, the app receives the earthquake alerts generated by the PRESTo EWS, which is currently running on the accelerometric stations of the Irpinia Seismic Network (ISNet) in Southern Italy. In the absence of alerts, the EWApp displays the standard bulletin of seismic events occurred within the network. In the event of a relevant earthquake, instead, the app has a dedicated module to predict the expected ground shaking intensity and the available lead-time at the user position and to provide customized messages to inform the user about the proper reaction during the alert.

We first present the architecture of both network-based system and EWApp, and then and describe its essential operational modes. The app is designed in a way that is easily exportable to any other network-based early warning system.

How to cite: Colombelli, S., Carotenuto, F., Elia, L., and Zollo, A.: Design and implementation of a mobile device APP for network-based EEW systems: application to PRESTo EEWS in Southern Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19404, https://doi.org/10.5194/egusphere-egu2020-19404, 2020.

EGU2020-21712 | Displays | SM3.1

Seismic Hazard Assessment and Numerical Modeling for Seismic Microzonation purpose of Dushanbe, Tajikistan

Farkhod Hakimov, Hans-Balder Havenith, Anatoly Ischuk, Marco Pilz, and Klaus Reicherter

Seismic hazard assessment of urban areas is an important and extremely challenging task. It is so important because without the knowledge of the influence of local soil conditions and properties, of the changing layer thickness in urban areas, and without considering multiple possible scenario earthquakes for this territory, engineers do not have enough information on how to design and construct seismically safe buildings. The particular challenge of this task is due to the great uncertainty affecting the prediction of the spatially (and sometimes even temporally) changing seismic properties of soils with respect to urban development.
Dushanbe is the capital of Tajikistan, a mountainous country marked by high to very high seismic hazard. The reason for the high seismic hazard specifically near Dushanbe is related to its location between two fault systems: South Gissar fault and Ilek-Vaksh fault.  Estimation of the seismic hazard of the urban areas in Tajikistan is very important because they had developed in a very short time and many high buildings are being constructed now Existing seismic action estimations are based on the old approaches when the main factors of the local soil conditions only consider general engineering-geological features of the territory as well as macro-seismic observations data. An additional problem is the building code in Tajikistan; it uses the estimation of the ground motions in terms of the MSK-64 scale, but does not enough take into account the variety of the soil conditions in the Dushanbe city area. Existing seismic hazard estimation of the area of Tajikistan is based on the so-called “The map of general seismic zoning of the territory of Tajikistan”, that was produced in 1978 in terms of MSK-64 scale. The seismic microzonation map of the Dushanbe city area was made in 1975 in terms of MSK-64 scale as well and was based on the engineering-geological approach mostly. This map does not represent the highly variable soil conditions of the Dushanbe city area which are partly due to the anthropogenic influence of the large city. Therefore, earlier seismic zonation maps assigned an intensity of IX to most districts of the city. However, those previous studies did not sufficiently quantify the local effects of soils on the seismic hazard, mainly the macro-seismic conditions (the relative distance of districts to fault lines) were considered for the zonation. 
This study describes and implements a number of new approaches to the evaluation of maximum seismic impact and site effect values. 

How to cite: Hakimov, F., Havenith, H.-B., Ischuk, A., Pilz, M., and Reicherter, K.: Seismic Hazard Assessment and Numerical Modeling for Seismic Microzonation purpose of Dushanbe, Tajikistan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21712, https://doi.org/10.5194/egusphere-egu2020-21712, 2020.

EGU2020-2935 | Displays | SM3.1

Empirical Site Amplification Modelling for Horizontal and Vertical Ground Motions in Taiwan

Chun-Hsiang Kuo, Shu-Hsien Chao, Che-Min Lin, Jyun-Yan Huang, and Kuo-Liang Wen

Site amplification behavior are important in ground motion prediction. Seismic waves were amplified and caused significant building damages in the Taipei Basin by the 1986 Hualien offshore (subduction interface) and the 1999 Chi-Chi earthquakes (crustal), for which both of the epicentral distances were nearly 100 km. To understand local site amplifications in Taiwan, empirical site amplification factors for both horizontal and vertical ground motions are studied using recently constructed strong motion and site databases for the free-field TSMIP stations. Records of large magnitude earthquakes of MW larger than 5.5 from 1991 to 2016 were selected for this study. Site amplification factors at site conditions with Vs30 between 120 m/s to 1600 m/s and bedrock accelerations up to 0.8 g were evaluated using ratios of spectral accelerations at different periods. The reference site condition, i.e. the engineering bedrock, is assumed as Vs30 of 760 m/s (B/C boundary) in this study. Our empirical site amplification form are borrowed from the site response function of ASK14 and CY14 ground motion models in NGA-West2 project with slight modification. Therefore our site amplification model includes a linear amplification term and a nonlinear deamplification term. The coefficients of the empirical models were obtained by a nonlinear regression analysis using the selected Taiwan data. Site amplification factor is a function of Vs30 and spectral intensity in the model. Similar linear site amplification factor to the NGA models is derived in our model; however, more significant soil nonlinearity behavior than the NGA models is likely captured from the empirical data. The amplification factor in vertical component is smaller than that in horizontal.

How to cite: Kuo, C.-H., Chao, S.-H., Lin, C.-M., Huang, J.-Y., and Wen, K.-L.: Empirical Site Amplification Modelling for Horizontal and Vertical Ground Motions in Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2935, https://doi.org/10.5194/egusphere-egu2020-2935, 2020.

EGU2020-6801 | Displays | SM3.1

Using Large Strong Motion Datasets to Model Regional Site Response in Seismic Risk Assessment: Examples from Japan and Europe

Graeme Weatherill, Fabrice Cotton, and Sreeram Reddy Kotha

Characterisation of seismic risk within a probabilistic framework is dependent upon well-constrained models of the seismic source, the ground motion scaling and the local site response, in addition to both their aleatory variability and epistemic uncertainty. When assessing risk as a large geographical scale such as that of a country or continent, however, complex models of site response that require detailed parameterization of the site conditions are seldom feasible to constrain. Instead, the use of simpler proxies, such as the well-known topographically inferred 30 m averaged shear-wave velocity (VS30), have become widely adopted for this purpose. In practice, the inference of VS30 from topographic and/or geological proxies have substantial limitations in terms of both the geological environments for which they are appropriate and the increased uncertainty in the prediction of site response; limitations that are not always accounted for in existing seismic risk models.

The volume of data reported by both new and well-established stations is increasing at an exponential rate, with hundreds of thousands of strong motion records now available from thousands of stations. Through this enormous and ever-expanding data set it is possible to constrain thousands of station-specific amplifications and utilize this dataset to calibrate the site amplification directly upon regionally mappable parameters, which can be applied across large spatial scales needed for regional seismic risk analysis. In doing so, it is possible not only to adapt the model of site amplification to different geological environments, but also to adjust the uncertainty in the ground motion characterization to ensure that this is captured appropriately in the seismic risk analysis when using the mappable site proxies. Applications of this approach have been made for two case study regions: i) Japan, where detailed station metadata are available and the relative increase in uncertainty from using regionally-mappable parameters instead of well-constrained site properties can be constrained, and ii) Europe, where station metadata more limited but a large number of stations with repeated observations are available. The implications for the estimates of seismic losses when adopting this new approach in place of the existing methodology are illustrated using examples from the 2020 European Seismic Risk model.

How to cite: Weatherill, G., Cotton, F., and Kotha, S. R.: Using Large Strong Motion Datasets to Model Regional Site Response in Seismic Risk Assessment: Examples from Japan and Europe, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6801, https://doi.org/10.5194/egusphere-egu2020-6801, 2020.

EGU2020-18920 | Displays | SM3.1

Global Dynamic Exposure and the OpenBuildingMap - A Big-Data and Crowd-Sourcing Approach to Exposure Modeling

Danijel Schorlemmer, Thomas Beutin, Fabrice Cotton, Nicolas Garcia Ospina, Naoshi Hirata, Kuo-Fong Ma, Cecilia Nievas, Karsten Prehn, and Max Wyss

The substantial reduction of disaster risk and loss of life, a major goal of the Sendai Framework by the United Nations Office for Disaster Risk Reduction (UNISDR), requires a clear understanding of the dynamics of the built environment and how they affect, in the case of natural disasters, the life of communities, represented by local governments and individuals. These dynamics can be best understood and captured by the local communities themselves, following two of the guiding principles formulated by the UNISDR: "empowerment of local authorities and communities" and "engagement from all of society". The two lead to societies increasing their understanding of efficient risk mitigation measures.

Our Global Dynamic Exposure model and its technical infrastructure build on the involvement of communities in a citizen-science approach. We are employing a crowd-sourced exposure capturing based on OpenStreetMap (OSM), an ideal foundation with already more than 375 million building footprints (growing daily by ~150,000), and a plethora of information about school, hospital, and other critical facilities. We are harvesting this dataset with our OpenBuildingMap system by processing the information associated with every building in near-real-time. We are enriching this dataset in a truly big-data approach by including built-up area detection from remote sensing with satellite and radar imagery combined with different sources of road networks, as well as various open datasets and aggregated exposure models that provide relevant additional information on, buildings and land use. 

A task of such a scale does not come without challenges, particularly in matters of data completeness, privacy and the merging and homogenizing of different datasets. We are thus investing a large effort on the development of strategies to tackle these in a transparent and consistent way.

We are fully automatically collecting exposure and vulnerability indicators from explicitly provided data (e.g., hospital locations), implicitly provided data (e.g., building shapes and positions), and semantically derived data, that is, interpretation applying expert knowledge. The latter allows for the translation of simple building properties as captured by OpenStreetMap users or taken from open datasets into vulnerability and exposure indicators and subsequently into building classifications as defined in the Building Taxonomy 2.0 developed by the Global Earthquake Model (GEM) and in the European Macroseismic Scale (EMS98). A task of such a scale does not come without challenges, particularly in matters of data completeness, privacy and the merging and homogenizing of different datasets. We are thus investing a large effort on the development of strategies to tackle these in a transparent and consistent way. With our open approach, we increase the resolution of existing exposure models minute by minute through data updates and step by step with each added building, as we move forward from aggregated to building-by-building descriptions of exposure. 

We expect the quality of near-real-time estimates of the extent of natural disasters to increase by an order of magnitude, based on the data we are collecting. We envision authorities and first responders greatly benefitting form maps pinpointing the greatest trouble spots in disasters and from detailed quantitative estimates of the likely damage and human losses.

How to cite: Schorlemmer, D., Beutin, T., Cotton, F., Garcia Ospina, N., Hirata, N., Ma, K.-F., Nievas, C., Prehn, K., and Wyss, M.: Global Dynamic Exposure and the OpenBuildingMap - A Big-Data and Crowd-Sourcing Approach to Exposure Modeling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18920, https://doi.org/10.5194/egusphere-egu2020-18920, 2020.

EGU2020-20735 | Displays | SM3.1

A Web Tool for Interactive Generation and Visualization of Synthetic ShakeMaps

Marius Kriegerowski, Danijel Schorlemmer, Thierry Goubier, and Fabrice Cotton

Synthetic shaking-intensity maps provide the necessary information about the detailed shaking distribution for scenario-based seismic risk assessment as well as post-disaster rapid loss estimates. These ShakeMaps allow to identify areas heavily affected by an earthquakes and are becoming, combined with an exposure/vulnerability model, the underlying data for a risk or loss model. Such computations deliver decision makers the data for informed policy decisions for precautionary measures for increasing resilience, or, in case of post-disaster analyses, rapid estimates for disaster mitigation.


We present a new web engine for synthetic ShakeMaps harnessing the OpenQuake engine of the Global Earthquake Model (GEM) foundation. The back-end asynchronously digests requests parameterizing earthquake sources in terms of source depth, epicentral location, moment magnitude and focal mechanism. The back-end returns shaking in user definable ground-motion measures (e.g. PGA or IMS) and can be retrieved in various formats such as ASCII, GeoJSON, among others. This tool implements an open and documented API that users and other services can query systematically and automatically. It integrates into the LEXIS framework, a Horizon 2020 funded project aiming at improving rapid loss assessments and emergency decision support systems.
An interactive interface allows to explore the expected shaking in the spatial domain by selecting locations of interest on a map and defining the earthquake source interactively within a web browser. Besides the interactive mode, this service now provides, through HTTP requests, a simple interface for any type of ShakeMap to be used in automated systems that require rapid ShakeMap computations without the need to run local instances of OpenQuake.

How to cite: Kriegerowski, M., Schorlemmer, D., Goubier, T., and Cotton, F.: A Web Tool for Interactive Generation and Visualization of Synthetic ShakeMaps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20735, https://doi.org/10.5194/egusphere-egu2020-20735, 2020.

EGU2020-18240 | Displays | SM3.1

Bayesian downscaling of building exposure models with remote sensing and ancillary information

Raquel Zafrir, Massimiliano Pittore, Juan Camilo Gomez- Zapata, Patrick Aravena, and Christian Geiß

Residential building exposure models for risk and loss estimations related to natural hazards are usually defined in terms of specific schemas describing mutually exclusive, collectively exhaustive (MECE) classes of buildings. These models are derived from: (1) the analysis of census data or (2) by means of individual observations in the field. In the first case, expert elicitation has been conventionally used to classify the building inventory into particular schemas, usually aggregated over geographical administrative units whose size area and shape are country-specific. In the second case, especially for large urban areas, performing a visual inspection of every building in order to assign a class according to the specific schema used is a highly time- and resource intensive task, often simply unfeasible.

Remote sensing data based on the analysis of satellite imagery has proved successful in integrating large-scale information on the built environment and as such can provide valuable vulnerability-related information, although often lacking the level of spatial and thematic resolution requested by multi-hazard applications. Volunteered Geo Information (VGI) data can also prove useful in this context, although in most cases only geometric attributes (shape of the building footprint) and some occupancy information are recorded thus leaving out most of the building attributes controlling the vulnerability of the structures to the different hazards. An additional drawback of VGI is the incompleteness of the information, which is based on the unstructured efforts of voluntary mappers.

Former efforts have been proposing a top-down/bottom-up approach moving from regional scale to neighbourhood and per-building scale, based on the analysis and integration of different data sources at increasing spatial resolutions and thematic detail. Following the same principle, this work focuses on the downscaling of already existing building exposure models based on census data making use of a probabilistic approach based on Bayesian updating. Different aggregation models can be taken into account to increase the spatial resolution of the building exposure model, also including variable-resolution models based on geostatistical approaches. Land-use masks are first generated after a supervised classification of Sentinel-2 images, in order to better relate the built- up area to meaningful geographical entities. Two independent statistical models are then created based on prior input information. Maximum likelihood estimations are obtained for each model. Two types of auxiliary data have been employed in order to constrain the downscaling via a specific likelihood term in the Bayesian updating: 1) building footprints area from the open-source-volunteered geo-information OpenStreetMaps  and 2) built-up height and density estimators based on remote sensing developed by the DLR (the German Aerospace Agency).

This approach, developed within the scope of the RIESGOS, was tested in Valparaiso and Viña del Mar (Chile) where the residential building exposure model proposed by the GEM-SARA project has been downscaled. The performance of the different auxiliary data were separately tested and compared. An independent building survey has also been carried out by experts from CIGIDEN (Chile) using a Rapid Remote Visual Screening Survey and used for preliminary validation of the approach.

How to cite: Zafrir, R., Pittore, M., Gomez- Zapata, J. C., Aravena, P., and Geiß, C.: Bayesian downscaling of building exposure models with remote sensing and ancillary information, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18240, https://doi.org/10.5194/egusphere-egu2020-18240, 2020.

SM3.2 – Analysis and modeling of spatiotemporal earthquake occurrence: challenges and perspectives

In the last few years, deep learning has solved seemingly intractable problems, boosting the hope to find approximate solutions to problems that now are considered unsolvable. Earthquake prediction, the Grail of Seismology, is, in this context of continuous exciting discoveries, an obvious choice for deep learning exploration. We reviewed the literature of artificial neural network (ANN) applications for earthquake prediction (77 articles, 1994-2019 period) and found two emerging trends: an increasing interest in this domain over time, and a complexification of ANN models towards deep learning. Despite the relatively positive results claimed in those studies, we verified that far simpler (and traditional) models seem to offer similar predictive powers, if not better ones. Those include an exponential law for magnitude prediction, and a power law (approximated by a logistic regression or one artificial neuron) for aftershock prediction in space. Due to the structured, tabulated nature of earthquake catalogues, and the limited number of features so far considered, simpler and more transparent machine learning models than ANNs seem preferable at the present stage of research. Those baseline models follow first physical principles and are consistent with the known empirical laws of Statistical Seismology (e.g. the Gutenberg-Richter law), which are already known to have minimal abilities to predict large earthquakes.

How to cite: Mignan, A. and Broccardo, M.: Neural Network Applications in Earthquake Prediction (1994-2019): Meta-Analytic & Statistical Insights on their Limitations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6851, https://doi.org/10.5194/egusphere-egu2020-6851, 2020.

EGU2020-7590 | Displays | SM3.2 | Highlight

Real-time discrimination of earthquake foreshocks and aftershocks

Laura Gulia and Stefan Wiemer

Immediately after a large earthquake, the main question asked by the public and decision-makers is whether it was the mainshock or a foreshock to an even stronger event yet to come. So far, scientists can only offer empirical evidence from statistical compilations of past sequences, arguing that normally the aftershock sequence will decay gradually whereas the occurrence of a forthcoming larger event has a probability of a few per cent.

We analyse the average size distribution of aftershocks of the 2016 Amatrice–Norcia (Italy) and Kumamoto (Japan) earthquake sequences and we suggest that in many cases it may be possible to discriminate whether an ongoing sequence represents a decaying aftershock sequence or foreshocks to an upcoming large event.

We propose a simple traffic light classification (FTLS, Foreshock Traffic Light System) to assess in real time the level of concern about a subsequent larger event and test it against 58 sequences, achieving a classification accuracy of 95 per cent.

We finally test, in near-real-time, the performance of the FTLS to the 2019 Ridgecrest sequence, California: a Mw6.4 followed, about 2 days later, by a Mw7.1. We find that in the hours after the first Ridgecrest event (Mw 6.4, the b-value drops by 23% on average, when compared to the background value, resulting in a ‘red’ foreshock traffic light.

Mapping in space the changes in b, we identify an area to the north of the rupture plane as the most likely location of a subsequent event. The second mainshock of magnitude 7.1 then indeed occurred in this location and after this event, the b-value increased by 26 percent over the background value, resulting in a green traffic light state.

How to cite: Gulia, L. and Wiemer, S.: Real-time discrimination of earthquake foreshocks and aftershocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7590, https://doi.org/10.5194/egusphere-egu2020-7590, 2020.

EGU2020-2444 | Displays | SM3.2

The overlap of aftershock coda waves and forecasting the first hour aftershocks

Eugenio Lippiello, Giuseppe Petrillo, Cataldo Godano, Lucilla de Arcangelis, Anna Tramelli, Eleftheria Papadimitrou, and Vassilis Karakostas

We show that short term post-seismic incompleteness can be interpreted in terms of the overlap of aftershock coda waves. We use this information to develop a novel procedure which gives accurate occurrence probabilities of post-seismic strong ground shaking within 30 minutes after the mainshock. This novel approach uses, as only information, the ground velocity recorded at a single station without requiring that signals are transferred and elaborated by operational units. We will also discuss how this information can be implemented in the Epidemic-Type Aftershock Sequence model in order to reproduce statistical features in time and magnitude of recorded aftershocks.

Main references

de Arcangelis L., Godano C. & Lippiello E. (2018) The Overlap of Aftershock Coda Waves and Short-Term Postseismic Forecasting. Journal of Geophysical Research: Solid Earth, 123: 5661-5674,doi:10.1029/2018JB015518

Lippiello E., Petrillo G. , Godano G. , Tramelli A., Papadimitriou E. &, Karakostas V. (2019) Forecasting of the first hour aftershocks by means of the perceived magnitude. Nature Communications , 10, 2953, doi:10.1038/s41467-019-10763-3

How to cite: Lippiello, E., Petrillo, G., Godano, C., de Arcangelis, L., Tramelli, A., Papadimitrou, E., and Karakostas, V.: The overlap of aftershock coda waves and forecasting the first hour aftershocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2444, https://doi.org/10.5194/egusphere-egu2020-2444, 2020.

The hypothesis that extrapolation of the Gutenberg-Richter (GR) relationship allows estimates of the probability of large earthquakes is incorrect. For nearly 200 faults for which the recurrence time, Tr (1/probability of occurrence), is known from trenching and geodetically measured deformation rates, it has been shown that Tr based on seismicity is overestimated typically by one order of magnitude or more. The reason for this is that there are not enough earthquakes along major faults. In some cases there are too few earthquakes for the fault to be mapped based on seismicity. Some examples are the following rupture segments of great faults: the 1717 Alpine Fault, the 1856 San Andreas, the 1906 San Andreas, the 2001 Denali earthquakes, for which geological Tr are 100 years to 300 years and seismicity Tr are 10,000 to 100,000 years. In addition, the hypothesis leads to impossible results when one considers the dependence of the b-value on stress. It has been shown that thrusts, strike-slip and normal faults have low, intermediate and high b-values, respectively. This implies that, regardless of local slip rates, the probability of large earthquakes predicted by the hypothesis is high, intermediate and low in thrust, strike-slip, and normal faulting, respectively. Measurements of recurrence probability show a different dependence: earthquake probability depends on slip rate. Finally, the hypothesis predicts different probabilities for large earthquakes, depending on the magnitude scale used. For the 1906 rupture segment, the difference in probability of an M8 earthquake is approximately a factor of 50, using the two available catalogs. Various countries measure earthquake magnitude on their own scale that is intended to agree with the ML scale of California or the MS scale of the USGS. However, it is not trivial to match a scale that is valid for a different region with different attenuation of seismic waves. As a result, some regional M-scales differ from the global MS scale, which yields different Tr for the same Mmax in the same region, depending on whether the global or local magnitude scale is used. Based on the aforementioned facts, the hypothesis that probabilities of large earthquakes can be estimated by extrapolating the GR relationship has to be abandoned.

How to cite: Wyss, M.: The probability of large earthquakes cannot be calculated from seismicity rates, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3637, https://doi.org/10.5194/egusphere-egu2020-3637, 2020.

EGU2020-12091 | Displays | SM3.2

Hyperbolic geometry of earthquake networks
not presented

Ilya Zaliapin, Karla Henricksen, and Konstantin Zuev

We examine the space-time-magnitude distribution of earthquakes using the Gromov hyperbolic property of metric spaces. The Gromov δ-hyperbolicity quantifies the curvature of a metric space via so-called four-point condition, which is a computationally convenient analog of the famous thin triangle property. We estimate the standard and scaled values of the δ-parameter for the observed earthquakes of Southern California during 1981 – 2017 according to the catalog of Hauksson et al. [2012], the global seismicity according to the NCEDC catalog during 2000 – 2015, and synthetic seismicity produced by the ETAS model with parameters fit for Southern California. In this analysis, a set of earthquakes is represented by a point field in space-time-energy domain D. The Baiesi-Paczuski asymmetric proximity η, which has been shown efficient in applied cluster analysis of natural and human-induced seismicity and acoustic emission experiments, is used to quantify the distances between the earthquakes. The analyses performed in the earthquake space (D,η) and in the corresponding proximity networks show that earthquake field is strongly hyperbolic, i.e. it is characterized by small values of δ. We show that the Baiesi-Paczuski proximity is a natural approximation to a proper hyperbolic metric in the space-time-magnitude domain of earthquakes, with the b-value related to the space curvature. We discuss the hyperbolic properties in terms of the examined earthquake field. The results provide a novel insight into the geometry and dynamics of seismicity and expand the list of natural processes characterized by underlying hyperbolicity.

How to cite: Zaliapin, I., Henricksen, K., and Zuev, K.: Hyperbolic geometry of earthquake networks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12091, https://doi.org/10.5194/egusphere-egu2020-12091, 2020.

The magnitudes of earthquakes are known to follow a power-law distribution, where the frequency of earthquake occurrence decreases with the magnitude. This decay is usually characterized by the power exponent, the so-called b-value. Typical observations report b-values in the range 0.5-2. The origin of b-value variations is however still debated. Seismological observations of natural seismicity indicate a dependence of the b-value with depth, and with faulting style, which could be interpreted as a signature of a stress dependence. Within creeping regions of major tectonic faults, the b-value of microseismicity increases with creep rate. Stress dependent b-value of acoustic emissions is also commonly reported during rock failure experiments in the laboratory. Natural and laboratory observations all support a decrease of b-value with increasing differential stress. I report here on the origin of b-value variations obtained in a fault model consisting in a planar 2D rate-and-state frictional fault embedded between 3D elastic slabs. This model assumes heterogeneous frictional properties in the form of overlapping asperities with size-dependent critical slip distance distributed on a creeping segment. This allows to get complex sequences of earthquakes characterized by realistic b-values. The role of frictional heterogeneity, normal stress, shear stress, and creep rate on the b-value variations is systematically explored. It is shown that the size distribution of asperities is not the only feature controlling the b-value, which indicates an important contribution from partial ruptures, and cascading events. In this model cascades of events (and thus b-value) is strongly influenced by frictional heterogeneity and normal stress through fracture energy distribution. If the decrease of b-value with differential stress is reproduced in these simulations, it is also shown that part of the b-value fluctuations could be attributed to changes of nucleation length and stress drop with normal stress. A slight increase of b-value with slip rate exists but remains an order of magnitude smaller than the observations.

How to cite: Dublanchet, P.: What controls b-value variations: insights from a physics based numerical model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11079, https://doi.org/10.5194/egusphere-egu2020-11079, 2020.

EGU2020-20443 | Displays | SM3.2

Operational Aftershock Forecasting for Mw7.3 Sarpol-e Zahab (2017) Earthquake in Western Iran

Hossein Ebrahimian, Fatemeh Jalayer, and Hamid Zafarani

Methodology:

The implementation of short-term forecasts for emergency response management in the immediate aftermath of a seismic event, and in the presence of an ongoing seismic sequence, requires two basic components: scientific advisories expressed in terms of risk assessment, and protocols that establish how the scientific results can be translated into decisions/actions for risk mitigation. The operational earthquake forecasting framework is geared towards providing scientific advisories in the form of time-dependent probabilities expressing seismicity, hazard and risk that can be practically translated into decisions. Considering the triggered sequence of aftershocks in the process of post-event decision-making and prioritization of emergency operations still seems to need and to deserve much more attention. To this end, the adopted novel and fully-probabilistic procedure succeeds in providing spatio-temporal predictions of aftershock occurrence in a prescribed forecasting time interval (in the order of hours or days). The procedure aims at exploiting the information provided by the ongoing seismic sequence in quasi-real time considering the time needed for registering and transmitting the data. The versatility of the Bayesian inference is exploited to adaptively update the forecasts based on the incoming information as it becomes available. The aftershock clustering in space and time is modelled based on an Epidemic Type Aftershock Sequence (ETAS). One of the main novelties of the proposed procedure is that it considers the uncertainties in the aftershock occurrence model and its model parameters. This is done by moving within a framework of robust reliability assessment which enables the treatment of uncertainties in an integrated manner. Pairing up the Bayesian robust reliability framework and the suitable simulation schemes (Markov Chain Monte Carlo Simulation) provides the possibility of performing the whole forecasting procedure with minimum (or no) need of human interference.

Application:

This procedure is demonstrated through a retrospective application to early forecasting of seismicity associated with the 2017 Sarpol-e Zahab seismic sequence activities. On Sunday November 12, 2017, at 18:18:16 UTC, (21:48:16 local time), a strong earthquake with Mw7.3 occurred in western Iran in the border region between Iran and Iraq in vicinity of the Sarpol-e Zahab town. Unfortunately, this catastrophic seismic event caused 572 causalities, thousands of injured and vast amounts of damage to the buildings, houses and infrastructures in the epicentral area. The mainshock of this seismic sequence was felt in the entire western and central provinces of Iran and surrounding areas. The main event was preceded by a foreshock with magnitude 4.5 about 43 minutes before the mainshock that warned the local residence to leave their home and possibly reduced the number of human casualties. More than 2500 aftershocks with magnitude greater than 2.5 have been reported up to January 2019 with the largest registered aftershock of Mw6.4. The fully simulation-based procedure is examined for both Bayesian model updating of ETAS spatio-temporal model and robust operational forecasting of the number of events of interest expected to happen in various time intervals after main events within the sequence. The seismicity is predicted within a confidence interval from the mean estimate.

How to cite: Ebrahimian, H., Jalayer, F., and Zafarani, H.: Operational Aftershock Forecasting for Mw7.3 Sarpol-e Zahab (2017) Earthquake in Western Iran, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20443, https://doi.org/10.5194/egusphere-egu2020-20443, 2020.

Geological faults may fail and produce earthquakes due to external stresses induced by hydrocarbon recovery, geothermal extraction, CO2 storage or subsurface energy storage. The associated hazard and risk critically depend on the spatiotemporal and size distribution of any induced seismicity. The observed statistics of induced seismicity within the Groningen gas field evolve as non-linear functions of the poroelastic stresses generated by pore pressure depletion since 1965. The rate of earthquake initiation per unit stress has systematically increased as an exponential-like function of cumulative incremental stress over at least the last 25 years of gas production. The expected size of these earthquakes also increased in a manner consistent with a stress-dependent tapering of the seismic moment power-law distribution. Aftershocks of these induced earthquakes are also observed, although evidence for any stress-dependent aftershock productivity or spatiotemporal clustering is inconclusive.

These observations are consistent with the reactivation of a mechanically disordered fault system characterized by a large, stochastic prestress distribution. If this prestress variability significantly exceeds the induced stress loads, as well as the earthquake stress drops, then the space-time-size distribution of induced earthquakes may be described by mean field theories within statistical fracture mechanics. A probabilistic seismological model based on these theories matches the history of induced seismicity within the Groningen region and correctly forecasts the seismicity response to reduced gas production rates designed to lower the associated seismic hazard and risk.

How to cite: Bourne, S. and Oates, S.: Statistical mechanics-based forecasting of induced seismicity within the Groningen gas field, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22047, https://doi.org/10.5194/egusphere-egu2020-22047, 2020.

The clock of an earthquake can be advanced due to dynamic and static changes when a triggering signal is applied to a stress-loading fault. While static effects decrease rapidly with distance, dynamic effects can reach thousands of kilometers away. Therefore, earthquake triggering is traditionally associated to static stress changes at local distances and to dynamic effects at greater scales. However, static and dynamic effects near the triggering signal are often nested, thus identifying which effect dominates, becomes unclear. So far, earthquake triggering has been tested using different rate-and-state friction (RSF) laws utilizing alternative views of friction without much comparison. In this study, the analogy of an earthquake is simulated using single degree of freedom spring-block systems governed with three different RSF laws, namely “Dieterich”, “Ruina” and “Perrin”. First, the fault systems are evolved until they reach a stable limit cycle and then static, dynamic and their combination are applied as triggering signals. During synthetic simulations, effects of the triggering signal parameters (onset time, size, duration and frequency) and the fault system parameters (fault stiffness, characteristic slip distance, direct velocity and time dependent state effects) are tested separately. Our results indicate that earthquake triggering is controlled mainly by the onset time, size and duration of the triggering signal but not much sensitive to the signal frequency. In terms of fault system parameters, the fault stiffness and the direct velocity effect are the critical parameters in triggering processes. Among the tested RSF laws, “Ruina” law is more sensitive than “Dieterich” law to both static and dynamic changes and “Perrin” is apparently the most sensitive law to dynamic changes. Especially, when the triggering onset time is close to an unperturbed failure time (future earthquake), dynamic changes result the largest clock advancement, otherwise, static stress changes are substantially more effective. In the next step, realistic models will be established to simulate the effect of the recent (26 September 2019) Marmara earthquake with Mw=5.7 on the locked Kumburgaz fault segment of the North Anatolian Fault Zone. The triggering earthquake will be simulated by combining the static stress change computed via Coulomb law and the dynamic effects using ground motions recorded at broadband seismic stations within similar distances. Outcomes will help us to better understand the effects of static and dynamic changes on the seismic cycle of the Kumburgaz fault segment, which is expected to break soon with a possibly big earthquake causing damage at the metropolitan area of Istanbul in Turkey.

How to cite: Sopaci, E. and Özacar, A. A.: Investigation of Dynamic and Static Effects on Earthquake Triggering Using Different Rate and State Friction Laws and Marmara Simulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-533, https://doi.org/10.5194/egusphere-egu2020-533, 2020.

EGU2020-1182 | Displays | SM3.2

Modeling earthquake numbers by Negative Binomial Hidden Markov models

Katerina Orfanogiannaki and Dimitris Karlis

Modeling seismicity data is challenging and it remains a subject of ongoing research. Assumptions about the distribution of earthquake numbers play an important role in seismic hazard and risk analysis. The most common distribution that has been widely used in modeling earthquake numbers is the Poisson distribution because of its simplicity and easy to use. However, the heterogeneity in earthquake data and temporal dependencies that are often present in many real earthquake sequences make the use of the Poisson distribution inadequate. So, we propose the use of a Hidden Markov model (HMM) with state-specific Negative Binomial distributions in which some states are allowed to approach the Poisson distribution. A HMM is a generalization of a mixture model where the different unobservable (hidden) states are related through a Markov process rather than being independent of each other. We parameterize the Negative Binomial distribution in terms of the mean and dispersion (clustering) parameter. Maximum likelihood estimates of the models’ parameters are obtained through an Expectation-Maximization algorithm (EM-algorithm).

We apply the model to real earthquake data. We have selected the area of Killini Western Greece to test the proposed hypothesis. The area of Killini has been selected based on the fact that in a time window of 17 years three clusters of seismicity associated with strong mainshocks are included in the catalog. Application of the model to the data resulted in three states, representing different levels of seismicity (low, medium, high). The state that corresponds to the low seismicity level approaches the Poisson distribution while the other two states (medium and high) are following the Negative Binomial distribution. This result complies with the nature of the data. The variation within each state that is introduced to the model by the Negative Binomial distribution is greater in the states of medium and high seismicity. 

How to cite: Orfanogiannaki, K. and Karlis, D.: Modeling earthquake numbers by Negative Binomial Hidden Markov models , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1182, https://doi.org/10.5194/egusphere-egu2020-1182, 2020.

EGU2020-1443 | Displays | SM3.2

Effects of secondary static stress triggering on the spatial distribution of aftershocks, a case study, 2003 Bam earthquake (SE Iran)
not presented

Behnam Maleki Asayesh, Hamid Zafarani, and Mohammad Tatar

Immediate after a large earthquake, accurate prediction of spatial and temporal distribution of aftershocks has a great importance for planning search and rescue activities. Currently, the most sophisticated approach to this goal is probabilistic aftershock hazard assessment (PASHA). Spatial distribution of the aftershocks fallowing moderate to large earthquakes correlate well with the imparted stress due to the mainshock. Furthermore the secondary static stress changes caused by smaller events (aftershocks) could have effect on the triggering of aftershocks and should be considered in the calculations. The 26 December 2003 (Mw 6.6) Bam earthquake with more than 26000 causalities is one of the most destructive events in the recorded history of Iran. This earthquake was an interesting event and was investigated in a majority of aspects. Good variable-slip fault model and precise aftershocks data enabled us to impart Coulomb stress changes due to mainshock and secondary static stress triggering on the nodal planes of aftershocks to learn whether they were brought closer to failure.

We used recently published high-quality focal mechanisms and hypocenters to reassess the role of small to moderate earthquakes for static stress triggering of aftershocks during the Bam earthquake. By imparting Coulomb stress changes due to the mainshock on the nodal planes of the 158 aftershocks we showed that 77.8% (123 from 158) of the aftershocks received positive stress changes at least in one nodal plane. We also calculated Coulomb stress changes imparted by the mainshock and aftershocks (1≤M≤4.1) onto subsequent aftershocks nodal planes and found that 81.6% (129 of 158) of aftershocks received positive stress changes at least in one nodal plane. In summary, 77.8% of aftershocks are encouraged by the main shocks, while adding secondary stress encourages 81.6%. Therefore, by adding secondary stress the Coulomb Index (CI), the fraction of events that received net positive Coulomb stress changes compared to the total number of events, increased from 0.778 to 0.816.

How to cite: Maleki Asayesh, B., Zafarani, H., and Tatar, M.: Effects of secondary static stress triggering on the spatial distribution of aftershocks, a case study, 2003 Bam earthquake (SE Iran) , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1443, https://doi.org/10.5194/egusphere-egu2020-1443, 2020.

EGU2020-1749 | Displays | SM3.2

Effect of Combining Catalogs with Different Completeness

Myunghyun Noh

In most seismic studies, we prefer the earthquake catalog that covers a larger region and/or a longer period. We usually combine two or more catalogs to achieve this goal. When combining catalogs, however, care must be taken because their completeness is not identical so that unexpected flaws may be caused.

We tested the effect of combining inhomogeneous catalogs using the catalog of Korea Meteorological Administration (KMA). In fact, KMA provides a single catalog containing the earthquakes occurred in and around the whole Korean Peninsula. Like the other seismic networks, however, the configuration of the KMA seismic network is not uniform over its target monitoring region, so is the earthquake detection capability. The network is denser in the land than in the off-shore. Moreover, there are no seismic information available from North Korea. Based on these, we divided the KMA catalog into three sub-catalogs; SL, NL, and AO catalogs. The SL catalog contains the earthquakes occurred in the land of South Korea while the NL catalog contains those in the land of North Korea. The AO catalog contains all earthquakes occurred in the off-shore surrounding the peninsula.

The completeness of a catalog is expressed in terms of mc, the minimum magnitude above which no earthquakes are missing. We used the Chi-square algorithm by Noh (2017) to estimate the mc. It turned out, as expected, that the mc of the SL is the smallest among the three. Those of NL and AO are comparable. The mc of the catalog combining the SL and AO is larger than those of individual catalogs before combining. The mc is largest when combining all the three. If one needs more complete catalog, he or she had better divide the catalog into smaller ones based on the spatiotemporal detectability of the seismic network. Or, one may combine several catalogs to cover a larger region or a longer period at the expense of catalog completeness.

How to cite: Noh, M.: Effect of Combining Catalogs with Different Completeness, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1749, https://doi.org/10.5194/egusphere-egu2020-1749, 2020.

EGU2020-4907 | Displays | SM3.2

Is Coulomb stress the best choice for aftershock forecasting?

Shubham Sharma, Sebastian Hainzl, Gert Zöller, and Matthias Holschneider

The Coulomb failure stress (CFS) criterion is the most commonly used method for predicting spatial distributions of aftershocks following large earthquakes. However, large uncertainties are always associated with the calculation of Coulomb stress change. The uncertainties arise due to non-unique slip inversions and unknown receiver fault mechanism. Especially for the latter, uncertainties are highly dependent on the choice of the assumed receiver mechanism. There are two ways of defining the receiver faults, either by predefining fault kinematics by geological constraints, or by calculating optimally oriented planes, both ways are pretty unrealistic as real aftershocks show variable rupture mechanisms. Recent studies have proposed an alternative method based on deep learning to forecast aftershocks. Using a binary test (aftershocks yes/no), it has been shown that their method as well as alternative stress values, such as maximum shear or the von-Mises criteria, are more accurate and reliable than the classical CFS criterion with predefined receiver mechanism.

Here we use 351 slip inversions from SRCMOD database to calculate Coulomb failure stress on a layered-half space using variable receiver mechanisms as well as proposed alternative stress metrics. We also perform tests for different magnitude cut-offs, grid size variation, and aftershock duration to verify the use of ROC analysis for ranking of stress metrics. The observations suggest that introducing a layered-half space does not improve the stress maps and ROC curves. However, magnitude cut-off and aftershock duration does effect the efficiency of stress metric in a way that larger magnitudes and shorter aftershock durations are forecasted efficiently. Two alternative statistics based tests i.e. log-likelihood and information gain tests using rate-based forecasts (non-binary) are also performed to compare the ability of metrics to discriminate the regions with and without aftershocks. The results suggest that simple methods of stress calculations perform better than the classic Coulomb failure stress calculations.

How to cite: Sharma, S., Hainzl, S., Zöller, G., and Holschneider, M.: Is Coulomb stress the best choice for aftershock forecasting?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4907, https://doi.org/10.5194/egusphere-egu2020-4907, 2020.

EGU2020-5150 | Displays | SM3.2

Stochastic generator of earthquakes in French territories

Corentin Gouache, Pierre Tinard, François Bonneau, and Jean-Marc Montel

Both French mainland and Lesser Antilles are characterized by sparse earthquake catalogues respectively due to the low-to-moderate seismic activity and the low recording historical depth. However, it is known that major earthquakes could strike French mainland (e.g. Ligure in 1887 or Basel in 1356) and even more French Lesser Antilles (e.g. Guadeloupe 1943 or Martinique 1839). Assessing seismic hazard in these territories is necessary to support building codes and prevention actions to population. One approach to estimate seismic hazard despite lack of data is to generate a set of plausible seismic scenarios over a large time span. A generator of earthquakes is thus presented in this paper. Its first step is to generate only main shocks. The second step consists of trigger aftershocks related to main shocks.
To draw the time occurrence of main shocks, original draw of frequencies and year-by-year summation of it is proceeded. The frequencies are drawn, for each magnitude step, in probability density functions computed through the inter event time method (Hainzl et al. 2006). By propagating magnitude uncertainties contained in the initial catalogue through a Monte Carlo Markov Chain, each magnitude step has not only one main shock frequency but a distribution of it. Once a main shock is temporally drawn, its 2D location is drawn thanks to the cumulative seismic moment recorded on each 5x5 km cell in the French territories. A seismotectonic zoning is used to limit both the spatial distribution and magnitude of large earthquakes. Finally, the other parameters (strike, dip, rake and depth) are drawn in ranges of values depending on the seismotectonic zone where the main shock is located. 
For purpose of trigger aftershocks from the main shocks, an approximation of the Bath law (Richter 1958; Båth 1965) is proceeded during the computation of the frequency – magnitude distributions. Thus, for each magnitude step, an α–value distribution is obtained in which, for each main shock an α–value is drawn. In this way, the maximal magnitude of triggered aftershocks is known.

How to cite: Gouache, C., Tinard, P., Bonneau, F., and Montel, J.-M.: Stochastic generator of earthquakes in French territories, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5150, https://doi.org/10.5194/egusphere-egu2020-5150, 2020.

EGU2020-5242 | Displays | SM3.2

Occurrence of earthquake doublets in the light of the ETAS model

Christian Grimm, Martin Käser, and Helmut Küchenhoff

While Probabilistic Seismic Hazard Assessment is commonly based on earthquake catalogues in a declustered form, ongoing seismicity in aftershock sequences is known to be able to add significant hazard, which can also increase the damage potential to already affected structures in risk assessment. Especially so-called earthquake doublets (multiplets), i.e. a cluster mainshock being followed or preceded by one (or more) events with a similarly strong magnitude occurring within pre-defined temporal and spatial limits, can cause loss multiplication effects to the insurance industry, which therefore has a pronounced interest in investigating the frequency of earthquake doublets to happen worldwide. A widely used method to analyse and simulate the triggering process of earthquake sequences is the Epidemic Type Aftershock Sequence (ETAS) model. We estimate the ETAS model parameters for some regional areas and produce synthetic catalogues, which are then analysed particularly with respect to the occurrence of earthquake doublets and compared to the observed history. Also, different seismic subduction-type regions in the world are pointed out to have shown differing relative frequencies of earthquake doublets. Regression models are used to study whether certain mainshock and local, geophysical properties such as magnitude, dip and rake angle, depth, distance to subduction plate interface and velocity of converging subduction plates nearby show explanatory power for the probability of a cluster containing an earthquake doublet.

How to cite: Grimm, C., Käser, M., and Küchenhoff, H.: Occurrence of earthquake doublets in the light of the ETAS model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5242, https://doi.org/10.5194/egusphere-egu2020-5242, 2020.

EGU2020-5827 | Displays | SM3.2

How ETAS Can Leverage Completeness of Modern Seismic Networks Without Renouncing Historical Data

Leila Mizrahi, Shyam Nandan, and Stefan Wiemer

The Epidemic-Type Aftershock Sequence (ETAS) model is often used to describe the spatio-temporal distribution of earthquakes. A fundamental requirement for parameter estimation of the ETAS model is the completeness of the catalog above a magnitude threshold mc. mc is known to vary with time for reasons such as gradual improvement of the seismic network, short term aftershock incompleteness and so on. For simplicity, nearly all applications of the ETAS model assume a global magnitude of completeness for the entirety of the training period. However, in order to be complete for the entire training period, the modeller is often forced to use very conservative estimates of mc, as a result completely ignoring abundant and high-quality data from the recent periods, which falls below the assumed mc. Alternatively, to benefit from the abundance of smaller magnitude earthquakes from the recent period in model training, the duration of the training period is often restricted. However, parameters estimated in this way may be dominated by one or two sequences and may not represent long term behavior.

We developed an alternative formulation of ETAS parameter inversion using expectation maximization, which accounts for a temporally variable magnitude of completeness. To test the adequacy of such a technique, we evaluate its forecasting power on an ETAS-simulated synthetic catalog, compared to the constant completeness magnitude ETAS base model. The synthetic dataset is designed to mimic the conditions in California, where mc since 1970 is estimated to be around 3.5, and where a general decreasing trend in the temporal evolution of mc can be observed. Both models are trained on the primary catalog with identical time horizon. While the reference model is solely based on information about earthquakes of magnitude 3.5 and above, our alternative represents completeness magnitude as a monotonically decreasing step-function, starting at 3.5 and assuming values down to 2.1 in more recent times.

To compare the two models, we issue forecasts by repeated probabilistic simulation of earthquake interaction scenarios, and evaluate those forecasts by assessing the likelihood of the actual occurrences under each of the alternatives. As a measure to quantify the difference in performance between the two models, we calculate the mean information gain due to model extension for different spatial resolutions, different temporal forecasting horizons, and different target magnitude ranges.

Preliminary results suggest that the parameter bias introduced by successive application of simulation and inversion decreases exponentially with an increasing fraction of data used in the inversion. It is therefore expected that also the forecasting power of such a model increases with the amount of data available, indicating substantial importance of the method for the future of probabilistic seismic hazard assessment.

How to cite: Mizrahi, L., Nandan, S., and Wiemer, S.: How ETAS Can Leverage Completeness of Modern Seismic Networks Without Renouncing Historical Data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5827, https://doi.org/10.5194/egusphere-egu2020-5827, 2020.

EGU2020-6334 | Displays | SM3.2

Seismic hazard due to fluid injections

Joern Davidsen, Cole Lord-May, Jordi Baro, and David Eaton

Earthquakes can be induced by natural and anthropogenic processes involving the injection or migration of fluids within rock formations. A variety of field observations has led to the formulation of three different and apparently contradicting paradigms in the estimation of the seismic hazard associated with fluid injections. Based on a unified conceptual model accounting for the non-homogeneous pore-pressure stimulation caused by fluid injection in a prestressed region, we show here that all three paradigms naturally coexist. The loading history and heterogeneity of the host medium determine which of the three paradigms prevails. This can be understood as a consequence of a superposition of two populations of events triggered at different pore-pressure levels with different Gutenberg-Richter b-values.

How to cite: Davidsen, J., Lord-May, C., Baro, J., and Eaton, D.: Seismic hazard due to fluid injections, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6334, https://doi.org/10.5194/egusphere-egu2020-6334, 2020.

EGU2020-8814 | Displays | SM3.2

Modelling Seismicity in California as a Spatio-Temporal Point Process Using inlabru: Insights for Earthquake Forecasting

Mark Naylor, Kirsty Bayliss, Finn Lindgren, Francesco Serafini, and Ian Main

Many earthquake forecasting approaches have developed bespokes codes to model and forecast the spatio-temporal eveolution of seismicity. At the same time, the statistics community have been working on a range of point process modelling codes. For example, motivated by ecological applications, inlabru models spatio-temporal point processes as a log-Gaussian Cox Process and is implemented in R. Here we present an initial implementation of inlabru to model seismicity. This fully Bayesian approach is computationally efficient because it uses a nested Laplace approximation such that posteriors are assumed to be Gaussian so that their means and standard deviations can be deterministically estimated rather than having to be constructed through sampling. Further, building on existing packages in R to handle spatial data, it can construct covariate maprs from diverse data-types, such as fault maps, in an intutitive and simple manner.

Here we present an initial application to the California earthqauke catalogue to determine the relative performance of different data-sets for describing the spatio-temporal evolution of seismicity.

How to cite: Naylor, M., Bayliss, K., Lindgren, F., Serafini, F., and Main, I.: Modelling Seismicity in California as a Spatio-Temporal Point Process Using inlabru: Insights for Earthquake Forecasting, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8814, https://doi.org/10.5194/egusphere-egu2020-8814, 2020.

EGU2020-11642 | Displays | SM3.2

Estimating b-values and biases in small earthquake catalogues

Gina-Maria Geffers, Ian Main, and Mark Naylor

The Gutenberg-Richter (GR) b-value represents the relative proportion of small to large earthquakes in a scale-free population. For tectonic seismicity, this is often close to unity, but some studies have shown the b-value to be elevated (>1) in both volcanic and induced seismicity. However, many of these studies have used relatively small datasets – in sample size and magnitude range, easily introducing biases. This leads to incomplete catalogues above the threshold above which all events are assumed to be recorded – the completeness magnitude Mc. At high magnitudes, the scale-free behaviour must break down because natural tectonic and volcano-tectonic processes are incapable of an infinite release of energy, which is difficult to estimate accurately. In particular, it can be challenging to distinguish between regions of unlimited scale-free behaviour and physical roll-off at larger magnitudes. The latter model is often referred to as the modified Gutenberg-Richter (MGR) distribution.

We use the MGR distribution to describe the breakdown of scale-free behaviour at large magnitudes, introducing the roll-off parameter (θ) to the incremental distribution. Applying a maximum likelihood method to estimate the b-value could violate the implicit assumption that the underlying model is GR. If this is the case, the methods used will return a biased b-value rather than indicate that the method used is inappropriate for the underlying model. Using synthetic data and testing it on various earthquake catalogues, we show that when we have little data and low bandwidth, it is statistically challenging to test whether the sample is representative of the scale-free GR behaviour or whether it is controlled primarily by the finite size roll-off seen in MGR.

How to cite: Geffers, G.-M., Main, I., and Naylor, M.: Estimating b-values and biases in small earthquake catalogues, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11642, https://doi.org/10.5194/egusphere-egu2020-11642, 2020.

EGU2020-11871 | Displays | SM3.2

Earthquake Recurrence Intervals in Complex Seismogenetic Systems
not presented

Andreas Tzanis and Angeliki Efstathiou

We examine the association of recurrence intervals and dynamic (entropic) states of shallow (crustal) and deep (sub-crustal) seismogenetic systems, simultaneously testing if earthquakes are generated by Poisson processes and are independent (uncorrelated), or by Complex processes and are dependent (correlated). To this effect, we apply the q-exponential distribution to the statistical description of interevent times, focusing on the temporal entropic index (measure of dynamic state), in connexion to the q-relaxation interval that constitutes a characteristic recurrence interval intrinsically dependent on the dynamic state. We examine systems in different geodynamic settings of the northern Circum-Pacific Belt: transformational plate boundaries and inland seismic regions of California, Alaska and Japan, convergent boundaries and Wadati-Benioff zones of the Aleutian, Ryukyu, Izu-Bonin and Honshū arcs and the divergent boundary of the Okinawa Trough.

Our results indicate that the q-exponential distribution is universal descriptor of interevent time statistics. The duration of q-relaxation intervals is reciprocal to the level of correlation and both may change with time and across boundaries so that neighbouring systems may co-exist in drastically different states. Crustal systems in transformational boundaries are generally correlated through short and long range interaction; very strong correlation is quasi-stationary and q-relaxation intervals very short and extremely slowly increasing with magnitude: this means that on occurrence of any event, such systems respond swiftly by generating any magnitude anywhere within their boundaries. These are attributes expected of SOC. Crustal systems in convergent and divergent margins are no more than moderately correlated and sub-crustal seismicity is definitely uncorrelated (quasi-Poissonian). In these cases q-relaxation intervals increase exponentially, but in Poissonian or weakly correlated systems their escalation is much faster than in moderately to strongly correlated ones. In consequence, moderate to strong correlation is interpreted to indicate Complexity that could be sub-critical or non-critical without a means of telling (for now). The blending of earthquake populations from dynamically different fault networks randomizes the statistics of the mixed catalogue.

A possible partial explanation of the observations is based on simulations of small-world fault networks and posits that free boundary conditions at the surface allow for self-organization and possibly criticality to develop, while fixed boundary conditions at depth do not; this applies particularly to crustal transformational systems. The information introduced by q-relaxation may help in improving the analysis of earthquake hazards but its utility remains to be clarified.

Acknowledgement: This presentation is financially supported by the Special Account for Research Grants of the National and Kapodistrian University of Athens

How to cite: Tzanis, A. and Efstathiou, A.: Earthquake Recurrence Intervals in Complex Seismogenetic Systems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11871, https://doi.org/10.5194/egusphere-egu2020-11871, 2020.

EGU2020-12534 | Displays | SM3.2

The Gutenberg-Richter law based on rupture dynamics

Zhenguo Zhang, Wenqiang Zhang, Jiankuan Xu, and Xiaofei Chen

Earthquakes recorded by instruments obey the Gutenberg-Richter law, which expresses the dependence of earthquake frequency on magnitude. The Gutenberg-Richter law reveals the physics of earthquake sources and is important for analyzing the seismicity of active fault systems and vulnerable areas. Based on rupture dynamics, for the first time, we obtain a power-law distribution for the relationship between earthquake frequency and magnitude. The weight of an earthquake relies on its rupture area and recurrence interval. Our derived frequency-magnitude distribution agrees with the Gutenberg-Richter law, which is summarized from global and regional earthquake catalogs. This work provides a new way to understand the Gutenberg-Richter law and the physics of earthquake sources.

How to cite: Zhang, Z., Zhang, W., Xu, J., and Chen, X.: The Gutenberg-Richter law based on rupture dynamics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12534, https://doi.org/10.5194/egusphere-egu2020-12534, 2020.

EGU2020-17890 | Displays | SM3.2

A physical constraint on smoothed-seismicity models and the stationary seismicity assumption in long-term forecasting

Pablo Iturrieta, Danijel Schorlemmer, Fabrice Cotton, José Bayona, and Karina Loviknes

In earthquake forecasting, smoothed-seismicity models (SSM) are based on the assumption that previous earthquakes serve as a guideline for future events. Different kernels are used to spatially extrapolate each moment tensor from a seismic catalog into a moment-rate density field. Nevertheless, governing mechanical principles remain absent through the model conception, even though crustal stress is responsible for moment release mainly in pre-existent faults. Furthermore, a lately developed SSM by Hiemer et al., 2013 (SEIFA) incorporates active-fault characterization and deformation rates stochastically, so that a geological estimate of moment release could also be taken into account. Motivated by this innovative approach, we address the question: How representative is the stochastic temporal/spatial averaging of SEIFA, of the long-term crustal deformation and stress? In this context, physics-based modeling provides insights about the energy, stress, and strain-rate fields within the crust due to discontinuities found therein. In this work, we aim to understand the required temporal window of SEIFA to satisfy mechanically its underlying assumption of stationarity. We build various SEIFA models within different spatio-temporal subsets of a catalog and confront them with a physics-based model of long-term seismic energy/moment rate. Following, we develop a method based on the moment-balance principle and information theory to compare the spatial similarity between these two types of models. These models are built from two spatially conforming layers of information: a complete seismic catalog and a computerized 3-D geometry of mapped faults along with their long-term slip rate. SEIFA uses both datasets to produce a moment-density rate field, from which later a forecast could be derived. A simple physics-based model is used as proof of concept, such as the steady-state Boundary Element Method (BEM). It uses the fault 3D geometry and slip rates to calculate the long-term interseismic energy rate and elastic stress and strain tensors, accumulated both along the faults and within the crust. The SHARE European Earthquake Catalog and the European Database of Seismogenic Faults are used as a case study, constrained to crustal faults and different spatio-temporal subsets of the Italy region in the 1000-2006 time window. The moment-balance principle is analyzed in terms of its spatial distribution calculating the spatial mutual information (SMI) between both models as a similarity measure. Finally, by using the SMI as a minimization function, we determine the catalog optimal time window for which the predicted moment rate by the SSM is closer to the geomechanical prediction. We emphasize that regardless of the stationarity assumption usefulness in seismicity forecasting, we determine a simple method that provides a physical boundary to data-driven seismicity models. This framework may be used in the future to combine seismicity data and geophysical modeling for earthquake forecasting.

How to cite: Iturrieta, P., Schorlemmer, D., Cotton, F., Bayona, J., and Loviknes, K.: A physical constraint on smoothed-seismicity models and the stationary seismicity assumption in long-term forecasting, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17890, https://doi.org/10.5194/egusphere-egu2020-17890, 2020.

EGU2020-20053 | Displays | SM3.2

Short-term retrospective forecasting of earthquakes based on temporal variations of the b-value of the magnitude-frequency distribution

Paolo Gasperini, Emanuele Biondini, Antonio Petruccelli, Barbara Lolli, and Gianfranco Vannucci

In some recent works it has been hypothesized that the slope (b-value) of the magnitude-frequency distribution of earthquakes may be related to the differential stress inside the crust.  In particular, it has been observed that low b-values are associated with high stress values and therefore with high probability of occurrence of strong seismic shocks. In this paper we formulate a predictive hypothesis based on temporal variations of the b-value. We tested and optimized such hypothesis retrospectively based on the homogenized Italian instrumental seismic catalog (HORUS) from 1995 to 2018. A comparison is also made with a similar predictive hypothesis based on the occurrence of strong foreshocks.

 

How to cite: Gasperini, P., Biondini, E., Petruccelli, A., Lolli, B., and Vannucci, G.: Short-term retrospective forecasting of earthquakes based on temporal variations of the b-value of the magnitude-frequency distribution, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20053, https://doi.org/10.5194/egusphere-egu2020-20053, 2020.

EGU2020-20235 | Displays | SM3.2

The role of afterslip in driving aftershock sequences

Robert Churchill, Maximilian Werner, Juliet Biggs, and Ake Fagereng

Aftershock sequences following large tectonic earthquakes exhibit considerable spatio-temporal complexity and suggest causative mechanisms beyond co-seismic, elasto-static Coulomb stress changes in the crust. Candidate mechanisms include dynamic triggering and postseismic processes such as viscoelastic relaxation, poroelastic rebound and aseismic afterslip, which has garnered particular interest recently. Aseismic afterslip – whereby localized frictional sliding within velocity-strengthening rheologies acts to redistribute lithospheric stresses in the postseismic phase – has been suggested by numerous studies to exert dominant control on aftershock sequence evolution, including productivity, spatial distribution and temporal decay.

As evidence is based overwhelmingly on individual case study analysis, we wish to systematically compare key metrics of aseismic afterslip and corresponding aftershock sequences to investigate this relationship. We specifically look for any empirical relationship between the seismic-equivalent moment of aseismic afterslip episodes and the corresponding aftershock sequence productivity. We first compile published afterslip models into a database containing moment estimates over varying time periods, as well as spatial distributions, temporal decays and modelling methodology as a supplementary resource. We then identify the corresponding aftershock sequence from the globally comparable USGS PDE catalog. As expected, coseismic moment exerts an obvious control on both afterslip moment and aftershock productivity – an effect we control for by normalising by mainshock moment and expected productivity (the Utsu-Seki law) respectively. Preliminary results suggest broad variability of both afterslip moment and aftershock productivity with no obvious control of afterslip on aftershocks beyond the scaling with mainshock size, including when separated by mainshock mechanism or region. As this study is insensitive to spatial and temporal distributions, we cannot rule out the potential influence afterslip exerts in these but find no evidence that afterslip drives overall productivity of aftershock sequences.

How to cite: Churchill, R., Werner, M., Biggs, J., and Fagereng, A.: The role of afterslip in driving aftershock sequences, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20235, https://doi.org/10.5194/egusphere-egu2020-20235, 2020.

SM4.2 – Geophysical imaging of near-surface structures and processes

EGU2020-7735 | Displays | SM4.2

Auto-tuning Hamiltonian Monte Carlo

Andreas Fichtner, Lars Gebraad, Christian Boehm, and Andrea Zunino

Hamiltonian Monte Carlo (HMC) is a Markov chain Monte Carlo method that exploits derivative information in order to enable long-distance moves to independent models, even when the model space dimension is high (Duane et al., 1987). This feature motivates recent research aiming to adapt HMC for the solution of geophysical inverse problems (e.g. Sen & Biswas, 2017; Fichtner et al., 2018; Gebraad et al., 2020).

Here we present applications of HMC to inverse problems at variable levels of complexity. At the lowest level, we study linear inverse problems, including, for instance, linear traveltime tomography. Though this is not the class of problems for which Monte Carlo methods have been developed, it allows us to understand the important role of HMC tuning parameters. We then demonstrate that HMC can be used to obtain probabilistic solutions for two important classes of inverse problems: 2D nonlinear traveltime tomography and 2D elastic full-waveform inversion. In both scenarios, no super-computing resources are needed for model space dimensions from several thousand to ten thousand.

By far the most important, but also most complex, tuning parameter in HMC is the mass matrix, the choice of which critically controls convergence. Since manual tuning of the mass matrix is impossible for high-dimensional problems, we develop a new HMC flavour that tunes itself during sampling. This rests on the combination of HMC with a variant of the L-BFGS method, well-known from nonlinear optimisation. L-BFGS employs a few Monte Carlo samples to compute a matrix factorisation LLTwhich dynamically approximates the local Hessian H, while the sampler traverses model space in a quasi-random fashion. The local curvature approximation is then used as mass matrix. Following an outline of the method, we present examples where the auto-tuning HMC produces almost perfectly uncorrelated samples for model space dimensions exceeding 105.

 

References

[1] Duane et al., 1987. "Hybrid Monte Carlo", Phys. Lett. B., 195, 216-222.

[2] Sen & Biswas, 2017. "Transdimensional seismic inversion using the reversible-jump Hamiltonian Monte Carlo algorithm", Geophysics, 82, R119-R134.

[3] Fichtner et al., 2018. "Hamiltonian Monte Carlo solution of tomographic inverse problems", Geophys. J. Int., 216, 1344-1363.

[4] Gebraad et al., 2020. "Bayesian elastic full-waveform inversion using Hamiltonian Monte Carlo", J. Geophys. Res., under review.

How to cite: Fichtner, A., Gebraad, L., Boehm, C., and Zunino, A.: Auto-tuning Hamiltonian Monte Carlo, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7735, https://doi.org/10.5194/egusphere-egu2020-7735, 2020.

EGU2020-9190 | Displays | SM4.2

BEL1D: 1D imaging using geophysical data in the framework of Bayesian Evidential Learning

Hadrien Michel, Frédéric Nguyen, and Thomas Hermans

BEL1D has been newly introduced to the community as a viable algorithm for the stochastic interpretation of geophysical data in the form of 1D geological models. It relies on a simplified version of the Bayesian problem in reduced space called Bayesian Evidential Learning. However, the method is closer to machine learning than classical McMC approaches since it can be separated into a learning process followed by a prediction part. The learning phase consists in constituting statistical relationships between models parameters and geophysical data from a training set of numerical models. The prediction phase then samples the previous relationships according to field data. Compared to other stochastic methods such as McMC, BEL1D as key advantages: 1) it converges easily as long as the prior is consistent with the unique input parameter being the size of the training set, 2) every model in the posterior is drawn independently, making it easy to trace back their origin, 3) the CPU times are similar to McMC, but the method can be fully parallelized and the learning process can be done before data acquisition, leading to quasi instantaneous prediction of the posterior. BEL1D already has led to successful applications on surface nuclear magnetic resonance data as well as dispersion curves from surface waves analysis. Nonetheless, the method is not limited to those two examples and can be implemented for any 1D geophysical method as long as a forward model is provided. Currently, the method is implemented for blocky imaging but will be extended to non-blocky models in the future. The open-source codes are readily available.

How to cite: Michel, H., Nguyen, F., and Hermans, T.: BEL1D: 1D imaging using geophysical data in the framework of Bayesian Evidential Learning, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9190, https://doi.org/10.5194/egusphere-egu2020-9190, 2020.

EGU2020-3910 | Displays | SM4.2

Random objective surface-wave waveform inversion

Yudi Pan, Lingli Gao, and Thomas Bohlen

The full-waveform inversion (FWI) of surface waves, including both Rayleigh and Love waves, is becoming increasingly popular for near-surface characterizations. Due to the high nonlinearity of the objective function and a huge amount of data, FWI may converge towards a local minimum and is usually computationally expensive. To overcome these problems, we reformulate FWI under a multi-objective framework and propose a random objective waveform inversion (ROWI) method for surface-wave characterization. We use three objective functions: the classical least-squares (l2) waveform difference, the envelope difference, and the difference in the FK spectra. At each iteration, we randomly choose one shot and randomly assign one of the three objective functions to this shot. We only update the model with one iteration using a preconditioned steepest descent algorithm to optimize the currently assigned objective function. Therefore, ROWI has high freedom in exploring the model and objective spaces.
We use a synthetic example to compare the performance of ROWI with conventional FWI approaches. ROWI converges to better result compared to the conventional FWI approaches, while some of the conventional FWI approaches are trapped at local minima and fail to reconstruct reasonable results. We also apply ROWI to a field data acquired in Rheinstetten, Germany. The main geological feature, a refilled trench, is successfully reconstructed in the ROWI result. The reliability of the ROWI result is also proven by a migrated GPR profile. Overall, both synthetic and field-data examples show that ROWI is computationally more efficient, less dependent on the initial model, and more robust compared to conventional FWI approaches.

How to cite: Pan, Y., Gao, L., and Bohlen, T.: Random objective surface-wave waveform inversion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3910, https://doi.org/10.5194/egusphere-egu2020-3910, 2020.

EGU2020-7601 | Displays | SM4.2

Seismic Surface Wave Tomography on dense 3D active data

Ilaria Barone, Emanuel Kästle, Claudio Strobbia, and Giorgio Cassiani

Surface Wave Tomography (SWT) is a well-established technique in global seismology: signals from strong earthquakes or seismic ambient noise are used to retrieve 3D shear-wave velocity models, both at regional and global scale. This study aims at applying the same methodology to controlled source data, with specific focus on 3D acquisition geometries for seismic exploration. For a specific frequency, travel times between all source-receiver couples are derived from phase differences. However, higher modes and heterogeneous spatial sampling make phase extraction challenging. The processing workflow includes different steps as (1) filtering in f-k domain to isolate the fundamental mode from higher order modes, (2) phase unwrapping in two spatial dimensions, (3) zero-offset phase estimation and (4) travel times computation. Surface wave tomography is then applied to retrieve a 2D phase velocity map. This procedure is repeated for different frequencies. Finally, individual dispersion curves obtained by the superposition of phase velocity maps at different frequencies are depth inverted to retrieve a 3D shear wave velocity model.

How to cite: Barone, I., Kästle, E., Strobbia, C., and Cassiani, G.: Seismic Surface Wave Tomography on dense 3D active data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7601, https://doi.org/10.5194/egusphere-egu2020-7601, 2020.

The Bangong-Nujiang suture zone, located in the central Tibet, is one of several important geological boundaries in Qinghai-Tibet plateau. Abundant researches have been performed and most of them focused on deep tectonic structure and its dynamic mechanism through recent geophysical projects such as INDEPTH-III, Hi-CLIMB, ANTILOPE, SinoProbe, etc. (Zhao Wenjin et al., 2008; N´abelek et al. 2009; Gao Rui, et al., 2013;Zhao Junmeng et al. 2014; He Rizheng et al., 2014; Xu Qiang et al., 2017; Shang Xuefeng et al., 2017; Davlatkhudzha et al.,2018). Near-surface velocity study can not only obtain the physical parameters such as Vp and Vs in the area, but also improve seismic image quality of deep structure (Zhao Lingzhi et al., 2018). However, the velocity information obtained from passive seismic stations using either receiver function or ambient noise tomography is not enough to elaborate the near surface velocity structure of the Bangong-Nujiang suture zone. Besides, the active-source seismic reflection data usually doesn’t have sufficient offset density at near surface which poses a challenge to conventional near-surface velocity analysis methods.

This study makes full use of surface waves and first breaks to obtain near-surface P- and S-wave velocities based on a 2D deep seismic reflection survey data which was acquired by SinoProbe project in 2009 . We adopt the method of superposition of surface waves in common receiver domain to generate high quality F-K spectrum which enables us to obtain fundamental-order and high-order dispersion curves. First, a 2D layered model with an irregular topography was built and the 2D elastic finite difference modeling was executed to generate 161 synthetic seismic shot gathers which mimicking the actual acquisition geometry. These gathers contain surface waves, refractions, reflections and multiples energy, and the maximum offset is about 18 km. It is shown that the F-K spectrum quality has been improved for each receiver station using superposition of surface waves in the F-K domain by adding more shots. The S-wave velocity inverted from dispersion curves showed good agreement with the synthetic model. Second, high quality F-K spectrum generated from the above method enabled us to pick both fundamental and 1st order dispersion curves from the SinoProbe field data. The S-wave velocity was generated using three methods: 1) empirical equations based on dispersion curves; 2) fundamental order dispersion curves inversion; and 3) both fundamental and 1st order dispersion curves inversion. Results show that using higher order dispersion curves can generate a more reliable near-surface model. Third, first breaks were picked up to 18 km offset and diving wave tomography was applied to derive near-surface P-wave velocity from abundant first break information. It is shown that there is an excellent correlation between P- and S-wave velocities, the bottom of basin is clearly revealed, and over-thrusts are identified accordingly which is consistent with field geological survey in the middle segment of Bangong-Nujiang suture zone.

This study was financially supported by the CAGS Research Fund (grant YWF201907), and the National Natural Science Foundation of China (grant 41761134094). Data sources: SinoProbe-02 Project.

How to cite: Zhang, H., He, R., and Liu, Z.: Near-surface velocity structure study using surface waves and first breaks in the middle segment of the Bangong-Nujiang suture zone, Tibetan Plateau, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6253, https://doi.org/10.5194/egusphere-egu2020-6253, 2020.

Unaweep Canyon (Western Colorado, US) is an enigmatic alpine landform and hypothesized to represent a partially exhumed paleo valley which was glacially over-deepened in the late Paleozoic. Processing and interpretation of recently acquired 2D seismic reflection and refraction data support the concept of glacial over-deepening and indicate maximum bedrock depths of about 550 meters. Additionally, pronounced reflectors are observed within the sedimentary infill. The seismic data have also been subjected to surface wave analysis revealing a significant increase of the Vp/Vs ratio below a shallow (50 – 150 m depth) intra-sedimentary reflector. A large Vp/Vs ratio can be caused by both saturation and poor consolidation of dry low-porosity materials (e.g. dry sands).

To investigate the potential occurrence of an aquifer associated with this interface, a high-density/long-offset electrical resistivity survey was conducted in fall 2019 along the seismic line. The maximum offset is 915 m at an electrode spacing of 5 meters, aiming at reaching depths of investigations between 150 and 200 meters. Inversion of the ERT data was initially conducted by means of smoothness-constrained algorithms. The imaging results revealed consistent structures with those resolved through seismic methods, at least within the required depth of investigation between 150 – 200 m. Furthermore, improvements in the resolution of the ERT imaging results was investigated after the inclusion of seismic interfaces as structural constraints in the inversion of the data. The comparison of the two approaches permitted to improve the interpretation of the ERT imaging results, which indicate low resistivities in the zone of high Vp/Vs ratios and thus strengthen the aquifer hypothesis. We present an integrated interpretation based on seismic structure, resistivity distribution, Vp and Vs velocities, and a distant well core. In a larger context, the results provide new insights on the subsurface hydrology in this arid part of the continental US as well as on the significance of multi-valued datasets for the interpretation and characterization of aquifers.   

How to cite: Behm, M., Flores-Orozco, A., Chwatal, W., and Soreghan, G. S.: Hydrologic characterization of an alpine valley infill through integration of ERT, active seismic and active/passive surface wave interferometry (Unaweep Canyon, US), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6199, https://doi.org/10.5194/egusphere-egu2020-6199, 2020.

EGU2020-11061 | Displays | SM4.2

Comparison of different regularization schemes for the 1D laterally constrained inversion of seismic surface wave data

Julien Guillemoteau, Giulio Vignoli, and Jennifer Barreto

The 1D layered inversion of surface wave dispersion data is a powerful tool to characterize the vertical distribution of S-wave velocity. Its applications span from seismology to geotechnical engineering, going through exploration geophysics. As many others, also this non-linear inverse problem is considerably ill-posed. Thus, in the Tikhonov’s regularization framework, the associated non-uniqueness and instability of the solution with respect to the data and their uncertainty can be tackled by including prior information in the inversion process. However, for the case of the gradient-based deterministic inversion problem, only constraints enforcing smooth spatial variations of the S-velocities have been used, even when blocky targets were expected. This, clearly, generates results that might fit the observed data, but that are often not compatible with other sources of information. On the other hand, probabilistic approaches can be used to properly map the model space; however, they are still very computationally expensive to be used routinely, or to be easily integrated in a multi-physical inversion procedure involving other geophysical methods.

Our goal is to combine computer efficiency, capability of integration with other geophysical methods, and some exhaustiveness regarding the non-uniqueness of the inverse problem. For this, we developed a coherent set of tools for the deterministic inversion of dispersion curves that is capable of applying a quite large spectrum of constraints. This includes, for example, vertically and laterally constrained inversions with different levels and kinds of regularization (sharpness and/or smoothness). In this study, we evaluate the capabilities and the possible limitations of the different regularization approaches on various datasets.

How to cite: Guillemoteau, J., Vignoli, G., and Barreto, J.: Comparison of different regularization schemes for the 1D laterally constrained inversion of seismic surface wave data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11061, https://doi.org/10.5194/egusphere-egu2020-11061, 2020.

EGU2020-17349 | Displays | SM4.2

Application of diffraction wavefront tomography to GPR data from a glacier

Alexander Bauer, Benjamin Schwarz, Richard Delf, and Dirk Gajewski

In the recent years, the diffracted wavefield has gained increasing attention in the field of applied seismics. While classical seismic imaging and inversion schemes mainly focus on high-amplitude reflected measurements, the faint and often masked diffracted wavefield is neglected or even treated as noise. In order to be able to extract depth-velocity models from seismic reflection data, sufficiently large source-receiver offsets are needed. However, the acquisition of such multi-channel seismic data is expensive and often only feasible for the hydrocarbon industry, while academia has to cope with low-fold or zero-offset data. The diffracted wavefield is the key for extracting depth velocities from such data, as the moveout of diffractions – in contrast to reflections – can be measured in the zero-offset domain. Recently, we have demonstrated on multi-channel, single-channel and passive seismic data that by means of wavefront tomography depth-velocity models can be retrieved solely based on diffractions or passive seismic events along with the localizations of these scatterers. The input for wavefront tomography are so-called wavefront attributes, which can be extracted from the data in an unsupervised fashion by means of coherence analysis. In order to obtain the required diffraction-only data, we use a recently proposed scheme that adaptively subtracts the high-amplitude reflected wavefield from the raw data. Due to their most common acquisition geometry, most ground-penetrating-radar (GPR) data inherently lack offsets. In addition, GPR data generally contain a rich diffracted wavefield, which in turn contains information about sought-after structures, as diffractions are caused by small-scale heterogeneities such as faults, tips or edges. In this work, we show an application of the suggested workflow – coherence analysis, diffraction separation and diffraction wavefront tomography – to GPR data acquired at a glacier, resulting in a depth-velocity model and the localizations of the scatterers, both obtained in a fully unsupervised fashion. While the resulting  velocity model may be used for depth migration of the raw data, the localizations of the scatterers may in addition provide important information on the inner structure of the glacier in order to, for instance, localize water intrusions or fractures.

How to cite: Bauer, A., Schwarz, B., Delf, R., and Gajewski, D.: Application of diffraction wavefront tomography to GPR data from a glacier, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17349, https://doi.org/10.5194/egusphere-egu2020-17349, 2020.

EGU2020-4493 | Displays | SM4.2

Subsurface water flow detection by time-lapse reflection GPR data

Hemin Yuan, Majken Caroline Looms Zibar, and Lars Nielsen

Understanding subsurface water flow is important as it e.g. controls contaminant transport, has an impact on the amount of aquifer recharge, and can be used for storm water management purposes. However, there do not exist many methods that can observe the water flow in the field. Furthermore, the flow patterns can be very diverse due to the complex geological conditions, e.g. faults, fractures, and heterogeneous permeability of the subsurface formations. In order to map the subsurface water flow in a chalk formation, we performed a water injection experiment in the Rørdal Quarry, Northeast Denmark. A total water volume of 700 liters was injected via a 50 cm deep hole within 8 hours. Around the injection hole, we conducted time-lapse GPR measurements along 6 inlines and 6 crosslines. Seven measurements campaigns were performed over an eight-hour time period. We analyze the time-lapse GPR reflection sections in order to investigate the variations of the different measurements. Initially, we subtract the repeated measurements and baseline measurements, which shows that some survey lines have clear changes after water injection, while others only show very small or no changes. To verify the differences, we pick travel times of selected horizons in the time-lapse data and compare them (cf. Truss et al., 2007; Allroggen et al., 2015). This analysis highlights the travel time variations imposed by the injected water. Moreover, we perform correlation analysis of the measurements before and after water injection. The correlation coefficients show relatively small values on the lines that exhibit clear differences, further confirming the differences caused by the water infiltration. Initial integrated analysis of the different results shows that the water mainly flows towards the southeast from the injection hole. This is consistent with the orientation of the fracture system observed in the reflection GPR profiles, indicating that the water flow is primarily controlled by the fractures.

 

[1] Truss, S., Grasmueck, M., Vega, S., and Viggiano, D. A. 2007, Imaging rainfall drainage within the Miami oolitic limestone using high-resolution time-lapse ground-penetrating radar, Water Recourses Research, 43, W03405.

[2] Allroggen, N., Schaik, N.V., and Tronicke, J. 2015, 4D ground-penetrating radar during a plot scale dye tracer experiment, Journal of Applied Geophysics, 118, 139-144.

How to cite: Yuan, H., Zibar, M. C. L., and Nielsen, L.: Subsurface water flow detection by time-lapse reflection GPR data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4493, https://doi.org/10.5194/egusphere-egu2020-4493, 2020.

EGU2020-8606 | Displays | SM4.2

Data-driven synthesis of primary plane-wave responses

Giovanni Angelo Meles, Lele Zhang, Jan Thorbecke, Kees Wapenaar, and Evert Slob

Seismic images provided by standard Reverse Time Migration are usually contaminated by artefacts associated with the migration of multiples.

Multiples can corrupt seismic images by producing both false negatives, i.e. by destructively interfering with primaries, and false positives, i.e. by focusing energy at unphysical interfaces. Free-surface multiples particularly affect seismic images resulting from marine data, while internal multiples strongly contaminate both land and marine data. Multiple prediction / primary synthesis methods are usually designed to operate on point source gathers, and can therefore be computationally  demanding when large problems, involving hundreds of gathers, are considered.

In this contribution, a new scheme for fully data-driven retrieval of primary responses of plane-wave sources is presented. The proposed scheme, based on convolutions and cross-correlations of the reflection response with itself,  extends a recently devised Marchenko point-sources primary retrieval method for to plane-wave source data. As a result, the presented algorithm allows fully data-driven synthesis of primary reflections associated with plane-wave source data. Once primary plane-wave responses are estimated, they are used for multiple-free imaging via standard reverse time migration. Numerical tests of increasing complexity demonstrate the potential of the proposed algorithm to produce multiple-free images only involving the migration of few datasets.

The plane-wave source primary synthesis algorithm discussed in this contribution could then be used as an initial and unexpensive processing step, potentially guiding more expensive target imaging techniques. Moreover, the method could be applied to large 3D problems for which standard methods are prohibitively expensive from a computational point of view.

How to cite: Meles, G. A., Zhang, L., Thorbecke, J., Wapenaar, K., and Slob, E.: Data-driven synthesis of primary plane-wave responses, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8606, https://doi.org/10.5194/egusphere-egu2020-8606, 2020.

EGU2020-16478 | Displays | SM4.2

The multi-2D seismic imaging of the Solfatara Volcano, Italy, inferred by seismic attributes.

Sergio Gammaldi, Amir Ismail, Teresa Chiuso, and Aldo Zollo

The imaging of seismic reflection data provides a powerful high-resolution method for studying volcano structure and fluids presence. The shallow structure of the Solfatara crater, a surface marker of deep magmatic activity inside Campi Flegrei caldera (Southern Italy), is characterized in terms of seismic profile and attributes. The main contribution of this work is to provide a detailed and improved seismic reflection image of the Solfatara crater and the identification of gas accumulation. The profiles are deployed along the NNE-SSW directions, the first, and the second orthogonal to the last. The two profiles are 400 m long acquired during the active experiment RICEN (Repeated Induced Earthquake and Noise) performed in the framework of the EU project MEDSUV between May and November 2014. Pre-stack processing of the seismic data has been performed in order to remove the noisy traces, low-frequency noise and reduce the ground roll phases. A very detailed velocity analysis for the NMO correction has been performed with the integration of information derived from the Vp velocity model previously obtained by the non-linear Bayesian technique. After having applied residual statics and DMO corrections, the CMP gathering, the post-stack Kirchhoff migration technique was performed to produce the final seismic profiles in time and depth. Once having obtained the post-stack migrated imaged, the energy, root mean square, envelope and sweetness attributes were computed for defining the maximum and minimum value of amplitude zones. In addition, other attributes as the time-gain attribute in order to interpret the deep reflectors and the variance attribute to define the faults, discontinuities, and chaotic zones have been evaluated. To enhance fluids identification the Amplitude Versus Offset (AVO) variation technique has been further applied to identify the gas zone in the explored sections. By integrating all information from the original seismic profile, seismic attributes and geophysical investigation relative to the Solfatara volcano, the multi-2D image presents the fluids trapped in the Solfatara crater at depths between 10 to 50 m below the surface of the crater and their migration pathways up to 150 meters depth.

How to cite: Gammaldi, S., Ismail, A., Chiuso, T., and Zollo, A.: The multi-2D seismic imaging of the Solfatara Volcano, Italy, inferred by seismic attributes., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16478, https://doi.org/10.5194/egusphere-egu2020-16478, 2020.

EGU2020-11031 | Displays | SM4.2

Water- and land-borne geophysical measurements before and after the sudden drainage of large karst lakes in southern Mexico

Matthias Bücker, Liseth Pérez, Adrián Flores Orozco, Jakob Gallistl, Matthias Steiner, Lukas Aigner, Johannes Hoppenbrock, Wendy Morales Barrera, Carlos Pita de la Paz, Emilio García García, José Alberto Razo Pérez, Johannes Buckel, Andreas Hördt, and Antje Schwalb

The karst lakes of the sparsely-populated Lacandon Forest in Chiapas, southern Mexico, and their associated sediment infill are attracting increasing attention as high-resolution and continuous environmental and climate archives. To evaluate the information stored in the sediments, paleolimnologists retrieve sediment cores and analyze multiple biological and non-biological indicators. Our geophysical measurements presented here were motivated by the need to determine coring locations providing continuous sediments records from a total of four lakes of the Lacandon Forest. Therefore, we mapped the sediment thickness on the lake floor by applying seismic, electrical, and electromagnetic methods. The measurements were carried out with floating devices – and, after the sudden drainage of two of the studied lakes, complemented by measurements on the exposed lake floor.

During a first campaign in March 2018 when lakes were filled, we collected seismic data with a sub-bottom profiler (SBP). Furthermore, we collected transient electromagnetic (TEM) data with a floating measuring device to investigate the potential of the method for the determination of sediment thicknesses as an alternative to seismic methods. After the lake-level maximum that coincided with the first campaign, the water levels of two of the studied lakes dropped dramatically by July 2019, leaving lake Metzabok (maximum depth ~15 m) dry and lake Tzibaná (~70 m) with a water level decreased by approx. 30 m. In October 2019, when lake levels were still low, we conducted a second survey covering the dry lake floor of lake Metzabok and some dry parts of lake Tzibaná. During this second campaign, we collected electrical resistivity tomography (ERT), induced polarization (IP), and seismic refraction tomography (SRT) data along selected lines of the 2018 survey.

Our 2018 results from the water-borne survey show that sediment thickness estimates from seismic (SBP) and electrical (TEM) data agree well for water depths up to 20 m and sediment thicknesses ranging from 2 m to 10 m. The 2019 data collected on the dry lake floor confirms the findings of the first campaign and – due to the smaller distance between measuring devices and target – results in a more detailed picture of sediments and the underlying limestone bedrock.

How to cite: Bücker, M., Pérez, L., Flores Orozco, A., Gallistl, J., Steiner, M., Aigner, L., Hoppenbrock, J., Morales Barrera, W., Pita de la Paz, C., García García, E., Razo Pérez, J. A., Buckel, J., Hördt, A., and Schwalb, A.: Water- and land-borne geophysical measurements before and after the sudden drainage of large karst lakes in southern Mexico, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11031, https://doi.org/10.5194/egusphere-egu2020-11031, 2020.

EGU2020-4319 | Displays | SM4.2

The Study of Crustal Velocity Structure Characteristics in Yangjiang Area of South China

Xiaona Wang, Zhihui Deng, Xiuwei Ye, and Liwei Wang

This paper collects 43,225 absolute first arrival P wave arrival times and 422,956 high quality relative P arrival times of 6,390 events occurred in Yangjiang and its adjacent area from Jan, 1990 to Aug, 2019, these seismic data is recorded by 49 stations from Guangdong seismic network, Guangxi seismic network and Hainan seismic network. Based on the seismic data above, we simultaneously determine the crustal 3D P wave velocity structure and the hypocenter parameters of 6255 events in Yangjiang and its adjacent area by applying Double-Difference seismic tomography. The result shows that, shallow P wave velocity in Yangjiang area is higher due to the thinner sedimentary layer and widely exposed Yanshanian granite, Indosinian granite and Cambrian metamorphic rocks. There are obvious correspondences between the distribution of shallow velocity and fault structure as well as geological structure. A wide range of low velocity anomaly exists in 20km depth, which verifies the low velocity layer in the middle crust at Yangjiang area of South China continent. The velocity image from land to ocean in 30km depth shows low velocity in NW side and high velocity in SE side, which verifies the characteristic of crust thinning in South China coastal continent. The NEE seismic belt from Yangbianhai to Pinggang is speculated to locate in a buried fault of southwest segment of Pinggang fault. The buried thrust fault is a N78°E strike fault, dip to NW with a dip angle of 85 °. In addition, the buried fault locates in the abnormal junction of high velocity on the NW side and low velocity on the SE side, which reflects the tectonic activity characteristic of NW plate uplifting and SE plate declining from Miocene period. The characteristic of activity in the buried fault shows thrust movement with a small strike slip component, which is consistent with the focal mechanism of M4.9 earthquake occurred in 2004.

How to cite: Wang, X., Deng, Z., Ye, X., and Wang, L.: The Study of Crustal Velocity Structure Characteristics in Yangjiang Area of South China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4319, https://doi.org/10.5194/egusphere-egu2020-4319, 2020.

EGU2020-21588 | Displays | SM4.2

Active seismic monitoring of CO2-saturated brine injection into a fault (CS-D experiment in the Mont Terri Rock Laboratory)

Melchior Grab, Alba Zappone, Antonio P. Rinaldi, Sebastian Hellmann, Quinn Wenning, Clément Roques, Anne Obermann, Claudio Madonna, Yves Guglielmi, Christophe Nussbaum, Hansruedi Maurer, and Stefan Wiemer

Confirming the permanent containment is a key challenge for the storage of CO2 in deep underground reservoirs. Faults in the cap rock of such reservoirs are potential flow paths for the CO2 to escape. Our decametre-scale experiment at the Mont Terri Rock Laboratory aims to better understand mechanisms of CO2 leakage trough a fault, and to test strategies to monitor the propagation of CO2-saturated water through faults.

Two boreholes were drilled through the main fault in Mont Terri with packer-intervals dedicated to fluid-injection and hydraulic/geochemical monitoring. Another five boreholes in the close surrounding were equipped with various instruments for geotechnical and geophysical observations. During the first phase of the experiment, the hydraulic response of the fault was characterized with injections of formation water in a step-up mode at pressures up to 6.0 MPa. The second phase, which was still on-going at the time of the abstract submission, consists of a long-term (several months) injection of CO2-saturated formation water at a constant head of 4.5 MPa, which is below the fault opening pressure. All injection activities were monitored with active seismic measurements, along with a comprehensive set of hydraulic-, mechanical-, geochemical- and other geophysical surveys. We will present the active seismic imaging results from the step-up injection test and compare them with the other surveys. Additionally, preliminary results will be shown acquired during the long-term injection of CO2-saturated formation water into the fault.

How to cite: Grab, M., Zappone, A., Rinaldi, A. P., Hellmann, S., Wenning, Q., Roques, C., Obermann, A., Madonna, C., Guglielmi, Y., Nussbaum, C., Maurer, H., and Wiemer, S.: Active seismic monitoring of CO2-saturated brine injection into a fault (CS-D experiment in the Mont Terri Rock Laboratory), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21588, https://doi.org/10.5194/egusphere-egu2020-21588, 2020.

EGU2020-19576 | Displays | SM4.2

Constraining gradient-based inversion with a variational autoencoder to reproduce geological patterns

Jorge Lopez-Alvis, Eric Laloy, Thomas Hermans, and Frédéric Nguyen

Given the sparsity of geophysical data it is useful to rely on prior information on the expected geological patterns to constrain the inverse problem and obtain a realistic image of the subsurface. By using several examples of such patterns (e.g. those obtained from a training image), deep generative models learn a low-dimensional latent space that can be seen as a reparameterization of the original high-dimensional parameters and then inversion can be done in this latent space. Examples of such generative models are the variational autoencoder (VAE) and the generative adversarial network (GAN). Both usually include deep neural networks within their architecture and have shown good performance in reproducing high-dimensional structured subsurface models. However, they both use a highly nonlinear function to map from latent space to the original high-dimensional parameter space which hinders the optimization of the objective function during inversion. Particularly, such nonlinearity may give rise to local minima where gradient-based inversion gets trapped and therefore fails to reach the global minimum. GAN has been previously used with gradient-based inversion in a linear traveltime tomography synthetic test where it was shown to often fail in reaching a consistent RMSE (compared to the added noise) because optimization converges to local minima. On the other hand, inversion with MCMC and GAN was shown to reach acceptable RMSE values. When applicable, however, a gradient-based inversion is preferred because of its lower computational demand. We propose using VAE together with gradient-based inversion and show that optimization reaches lower RMSE values on average compared to GAN in a linear traveltime tomography synthetic case. We also compare the subsurface models that are generated during the iterations of the optimization to explore the effect of the different latent spaces used by GAN and VAE. We identify a trade-off between a strict following of the patterns and getting trapped in local minima during optimization, i.e. VAE seems to be able to break some continuous channels in order to not get trapped in local minima whereas GAN does not break channels. Finally, we perform some synthetic tests with nonlinear traveltime tomography and show that gradient-based inversion with VAE is able to recover a similar global structure to the true model but its final RMSE values are still far from the added noise level.

How to cite: Lopez-Alvis, J., Laloy, E., Hermans, T., and Nguyen, F.: Constraining gradient-based inversion with a variational autoencoder to reproduce geological patterns, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19576, https://doi.org/10.5194/egusphere-egu2020-19576, 2020.

Despite strong evidences that are visible at the surface that suggests the presence of buried structures, sometimes, both the GPR and magnetic data do not allow to clearly about the presence of these structures. Usually, this lack of perceptibility is due to the physical and chemical conditions of the medium that produces an increasing of background noise and masks the useful information. This causes a decrease in the signal-to-noise ratio of the data, preventing a good assessment about the existence of buried structures at subsurface.

Nevertheless, we believe that the recorded signal of both methods has the useful part of the signal hidden. Data fusion techniques are widely used in brain tumour detection in medicine by combining data from different clinical exams, both with low perceptibility.

This work presents an approach that allows using advanced fusion algorithms to combine geophysical data from GPR-3D and magnetics. This creates an enhanced image from both datasets with better quality than the individual images from each method.

The data fusion approach is performed through the combined use of 2D Discrete Wavelet Transform, Multiresolution Singular Value Decomposition and Image Gradient. This scheme allows us to select the useful information to obtain a higher quality and sharper fused image using the best of input datasets. The geophysical data fusion was successfully tested on three datasets, with different levels of perceptibility: high, intermediate and low.

 

Acknowledgment: This work is co-funded by the ICT Project (UID/GEO/04683/2019) with the reference POCI-01-0145-FEDER-007690, by the Project SFRH/BSAB/143063/2018 (FCT) and by the INTERREG 2014-2020 Program, through the "Innovación abierta e inteligente en la EUROACE" Project, with the reference 0049_INNOACE_4_E.

How to cite: Oliveira, R. J., Caldeira, B., Teixidó, T., and Borges, J. F.: Advanced fusion of geophysical data through combined use of 2D Discrete Wavelet Transform and Multiresolution Singular Value Decomposition applied to GPR-3D and magnetic data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20500, https://doi.org/10.5194/egusphere-egu2020-20500, 2020.

EGU2020-3233 | Displays | SM4.2

Target-oriented optimized survey design and quantitative comparison for 3D electrical resistivity tomography

Lincheng Jiang, Gang Tian, Bangbing Wang, and Amr Abd El-Raouf

In recent decades, geoelectrical methods have played a very important role in near-surface investigation. The most widely used of these methods is electrical resistivity tomography (ERT). Regardless of the forward and inversion algorithms used, the original data collected from a survey is the most important factor for quality of the resulted model. However, 3D electrical resistivity survey design continues to be based on data sets recorded using one or more of the standard electrode arrays. There is a recognized need for the 3D survey design to get better resolution using fewer data. Choosing suitable data from the comprehensive data set is a great approach. By reasonable selecting, better resolution can be obtained with fewer electrodes and measurements than conventional arrays. Previous research has demonstrated that the optimized survey design using the 'Compare R' method can give a nice performance.

This paper adds target-oriented selection and modified the original 'Compare R' method. The survey design should be focused on specific target areas, which need a priori information about the subsurface properties. We select electrodes and configurations as the target set by the comprehensive set firstly which meets the requirements of the target area. The number of measurements and electrodes is much less than the comprehensive set and the model resolution matrix takes less time to calculate. At the next step for rank, we calculate the sensitivity matrix of the target set only once and then calculate the contribution degree of each measurement separately from it. The time of iterative calculation of the resolution matrix when measurements set changing is less than the original method.

The traditional method of evaluating RMS is not appropriate for comparing the quality of collected data by different survey designs. SSIM (structural similarity index) gives more reliable measures of image similarity better than the RMS. The curves of SSIM values in three dimensions and the average SSIM are given as quantitative comparisons. Besides, the frequency of electrodes utilized given to guides on selecting the highest used electrodes. Finally, the curves of the average relative resolution S and the number of electrodes as the number of measurements increase are given, which proves the method works effectively.

The results show the significance of using target-oriented optimized survey design, as it selects fewer electrodes and arrays than the original CR method. Also, it produces better resolution than conventional arrays and takes less calculation time. 3D SSIM, frequency of electrodes used, the relationship between average relative resolution, number of electrodes and number of measurements, these quantitative comparison methods can effectively evaluate the data collected in various survey designs.

How to cite: Jiang, L., Tian, G., Wang, B., and Abd El-Raouf, A.: Target-oriented optimized survey design and quantitative comparison for 3D electrical resistivity tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3233, https://doi.org/10.5194/egusphere-egu2020-3233, 2020.

EGU2020-17489 | Displays | SM4.2

Efficiency Of Integrated Seismic Methods Approach To Near-Surface Characterization

Irena Gjorgjeska, Vlatko Sheshov, Kemal Edip, and Dragi Dojchinovski

Surface seismic methods are among the most popular, widely accepted, geophysical methods for near-surface characterization. The most practical and effective way to perform in-situ measurements and data processing using different seismic methods as are seismic refraction, seismic reflection and MASW method in an integrated approach is presented in this paper. Each method has some advantages and limitations, but their application in an integrated approach provides higher accuracy in subsurface modeling. The same seismic equipment and, in most of the cases, the same acquisition parameters were used, enabling time and cost effective survey for subsurface characterization. The choice of these parameters was not random. Experimental research by use of the above-mentioned seismic methods was carried out in a long period in order to define the optimal parameters for successful application of an integrated technique in future research. During this survey, particular attention was paid to the influence of the acquisition parameters on the dispersion image resolution in the MASW surveys and extraction of an effective dispersion curve.

The results of the performed surveys at characteristic locations in R. North Macedonia are presented to show the efficiency of the combined methods approach.

How to cite: Gjorgjeska, I., Sheshov, V., Edip, K., and Dojchinovski, D.: Efficiency Of Integrated Seismic Methods Approach To Near-Surface Characterization, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17489, https://doi.org/10.5194/egusphere-egu2020-17489, 2020.

The present work consists in imaging salt bodies from earth subsoil in the context of Reverse Time Migration (RTM) algorithm. The study of salt domes is economically important because they form a natural trap for hydrocarbons. For instance, more than a half of the hydrocarbon reserves that still exist today are related to salt bodies.

However, seismic images coming from strong salt tectonics area, are contaminated with spurious signal, like multiple events. Therefore, it is important to know how to treat and filter multiples in order to have seismic images that are geologically interpretable.

For this purpose, we solved the forward 3D elastic seismic wave equations using high order finite differences. The earth parameters come from 3D velocity and density models in a salt tectonic region in the North Gulf of Mexico. To obtain the imaging condition we compute the sensitivity kernels by using the adjoint solution of wave equation and by applying checkpointing. We tested this algorithm with simultaneous and separated sources. Fluid - solid interfaces at the ocean bottom is introduced, interfaces are well retrieved at large offsets.

Furthermore, we applied CPML absorbing boundaries, and replace also free surface conditions for absorbing boundaries to attenuate free surface multiples. The images we obtained from sensitivity kernels are easily interpretable. The calculations were performed on CALMIP supercomputing platforms in Toulouse France. 

How to cite: Abreu, J., Martin, R., and Darrozes, J.: 3D high order seismic imaging in Gulf of Mexico in the context of RTM algorithm using adjoint-based methods., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-90, https://doi.org/10.5194/egusphere-egu2020-90, 2020.

EGU2020-91 | Displays | SM4.2

Numerical modeling of attenuation and non linear effects in computational seismology

Kassem Asfour, Roland Martin, and Didier El Baz

We solve elastic waves equations in 2D/3D using a fourth order in time and space finite volume method based on exact Riemann solver and wave limiters and designed particularly to capture shock waves.  We validate our code by comparing our results with spectral elements solutions (SPECFEM). 

 

The goal is to detect the levels of fluid saturations in complex rheological media at different scales (near surface or crustal scale) by homogenizing the rheological laws for different frequency contents of the signal sources. For this purpose, at high frequencies, attenuation and/or non linear effects must be added to the stress-strain relations that affect the apparent/effective seismic wave velocities due to unconsolidated, granular or damaged and fractured nature of solid media.

 

In our present studies, we show first how we include the viscoelastic models using auxiliary differential equation approach as in Martin et al. 2019. Then, we show at two different scales (laboratory and natural near surface) how we are able to reproduce real seismic data for granular and porous media in the range of 300-4000 Hz for a 1m configuration experimental setup and how the attenuation and homogeneized seismic velocities of the porous matrix correlate to gravity laws and can explain the recorded signals. Recorded signals are compared to the numerical solutions for three different vertical pore pressure gradients due to the injection of gas.  

 

After validating the code on this controlled experiment we extend our methods at the scale of a 100m natural site in the Orgeval basin in the Parisian region/ France where we want to model the variations of the seismic velocities in the first tens of meters close to the near surface due to the presence of water contents. The idea is to be able to monitor the fluid contents of the ground in the neighborhood of the rivers and the fluid exchanges between nappes, rivers and the underground at the different periods of time (weekly, monthly and annualy). The Vp/Vs ratios reaching high values around 2 due to the water presence, and the Vp and Vs models being obtained by ray-tracing based first arrival time inversions.

The data are provided by the PIREN-Seine program in the last three years.

 

Complex non-linearity responses of the medium  can be modelled with our codes and also be extended to  damaged faults that can be activated and trigger earthquakes due to bulk and shear moduli decrease.

 

How to cite: Asfour, K., Martin, R., and El Baz, D.: Numerical modeling of attenuation and non linear effects in computational seismology, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-91, https://doi.org/10.5194/egusphere-egu2020-91, 2020.

EGU2020-12029 | Displays | SM4.2

High Resolution Imaging of the South Hikurangi Subduction Zone, New Zealand, Using 2‐D Full‐Waveform Inversion

Adnan Djeffal, Ingo Pecher, Satish Singh, and Jari Kaipio

Large quantities of fluids are predicted to be expelled from compacting sediments on subduction margins. Fluid expulsion is thought to be focussed, but its exact locations are usually constrained on very small scales and rarely can be resolved using velocity images obtained from traditional velocity analysis and ray-based tomography because of their resolution and accuracy limitation. However, with recent advancement in computing power, the full waveform inversion (FWI) is a powerful alternative to those traditional approaches as it uses phase and amplitude information contained in seismic data to yield a high-resolution velocity model of the subsurface.

Here, we applied elastic FWI along an 85 Km long 2D multichannel seismic profile on the southern Hikurangi margin, New Zealand. Our processing sequence includes: (1) downward continuation, (2) 2D traveltime tomography, and (3) full waveform inversion of wide-angle seismic data. We will present the final high-resolution velocity model and our interpretation of the fluid flow regimes associated with both the deforming overriding plate and the subducting plate.

How to cite: Djeffal, A., Pecher, I., Singh, S., and Kaipio, J.: High Resolution Imaging of the South Hikurangi Subduction Zone, New Zealand, Using 2‐D Full‐Waveform Inversion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12029, https://doi.org/10.5194/egusphere-egu2020-12029, 2020.

EGU2020-6434 | Displays | SM4.2

Multi-Cross-hole 3-D reverse time migration imaging

Fei Cheng and Jiangping Liu

Cross-well 2-D seismic CT imaging method has been widely used in many fields such as oil-gas exploration and engineering geological exploration, but for the real three-dimensional space, this traditional method can only obtain the two-dimensional velocity profile between the two wells, cannot obtain the lateral geological structure outside the profile; Besides, the seismic signal received from cross-well exploration is the response of geologic body in three-dimensional space, which may be influenced by the geologic body outside the two-well profile, and that will give a result of image distortion and having an effect on geological interpretation. Based on the theory of three-dimensional acoustic wave equation, this paper implements a three-dimensional cross-well reverse-time migration imaging method to obtain the cross-well 3-D geological structure with the observed value from multiple wells by using the first-order velocity-stress acoustic wave equation and firing time imaging conditions. Calculation results of the typical theoretical models show that: The multi-well three-dimensional imaging method adopted in this paper can accurately and effectively realize the cross-well 3-D geological imaging with high resolution and reliable results. Multi-well three-dimensional imaging method can effectively obtain the cross-well three-dimensional structure distribution, which can solve the issue of hard to obtain the transverse structure change by 2-D imaging. It also can solve the imaging problems of big dip angle interface in CT imaging and obtains the true cross-well 3-D geological structure with the multiple well data, which can provide the basis for cross-well 3-D seismic exploration.

How to cite: Cheng, F. and Liu, J.: Multi-Cross-hole 3-D reverse time migration imaging, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6434, https://doi.org/10.5194/egusphere-egu2020-6434, 2020.

EGU2020-2343 | Displays | SM4.2

Crustal Structure and tectonic attribute Revealed by a Deep Seismic Sounding Profile of Dinghu-Gaoming-Jinwan in the Pearl River Delta

Xiuwei Ye, Xiang Zhang, Jinshui Lv, Baofeng Liu, Xiaona Wang, LIwei Wang, and Zuoyong Lv

To find out the crustal structure and  tectonic attribute  of the Pearl river delta and offshore area(PRD), in 2015, the Guangdong Earthquake Agency collaboration with the other unit  carried out a three-dimensional joint onshore-offshore seismic detection  experiment in the PRD.  This paper processed the data of Dinghu-Gaoming-Jinwan L1 line on the west side of PDR. We utilized ray tracing and travel-time simulation  method to obtained a P-wave velocity model of the L1 profile.The study showed: Along the profile, The depth of the Moho gradually decreases from the northwestern inland 30.0km to the southwestern coastal 28.0km. Upheaval  of the Moho is between Dinghu and Gaoming. The low velocity layer in the mid-crustal  is  a heterogeneous continuum. The velocity of low velocity layer NW side is lower than the SE side, especially between Dinghu and Gaoming. The minimum velocity is 6.05km•s-1. The deep Wuchuan-Sihui fault and Guangzhou-Enping fault  may be one of the most important channels for deep material upwelling. It is the continuum upheaval  of the Moho which from Dinghu, Gaoming on the west side of PDR to  Qingyuan, Conghua on the east side of PDR delimited by Wuchuan-Sihui fault and Guangzhou-Enping fault.

How to cite: Ye, X., Zhang, X., Lv, J., Liu, B., Wang, X., Wang, L., and Lv, Z.: Crustal Structure and tectonic attribute Revealed by a Deep Seismic Sounding Profile of Dinghu-Gaoming-Jinwan in the Pearl River Delta, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2343, https://doi.org/10.5194/egusphere-egu2020-2343, 2020.

EGU2020-3380 | Displays | SM4.2

Crustal Structure Revealed by a Deep Seismic Sounding Profile of Baijing-Gaoming-Jinwan in the Pearl River Delta

Xiang Zhang, Xiuwei Ye, Jinshui Lv, and Xiaona Wang

The Pearl River estuary area, located in the middle part of the southern China coastal seismic belt, has long been considered a potential source of strong earthquakes above magnitude 7.0. To scientifically assess the potential strong earthquake risk in this area, a three-dimensional artificial seismic sounding experiment, consisting of a receiving array and seabed seismograph, was performed to reveal the deep crustal structure in this region. We used artificial ship-borne air-gun excitation shots as sources, and fixed and mobile stations as receivers to record seismic data from May to August 2015. This paper presents results along a line from the western side of the Pearl River estuary to the western side of the Baijian–Gaoming–Jinwan profile. A two-dimensional velocity structure was constructed using seismic travel-time tomography. The inversion results show that the Moho depth is 27 km in the coastal area and 30 km in the northwest of the Pearl River estuary area, indicating that the crust thins from land to sea. Two structural discontinuities and multiple low-velocity anomalies appear in the crustal section. Inside both discontinuity zones, a low-velocity layer, with a minimum velocity of 6.05kms−1, exists at a depth of about 15 km, and another, with a minimum velocity of 6.37kms−1, exists at a depth of about 21.5 km between the middle and lower crust. These low velocities suggest that the discontinuities may consist of partly molten material. Earthquakes with magnitudes higher than 5.0 occurred in the low-velocity layer along the profile. The deep Kaiping-Enping fault, rooted in the crust, may be one of the most important channels for deep material upwelling and is related to tectonic movement since the Cretaceous in the Pearl River Delta tectonic rift basin.

How to cite: Zhang, X., Ye, X., Lv, J., and Wang, X.: Crustal Structure Revealed by a Deep Seismic Sounding Profile of Baijing-Gaoming-Jinwan in the Pearl River Delta , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3380, https://doi.org/10.5194/egusphere-egu2020-3380, 2020.

EGU2020-7811 | Displays | SM4.2

Upper crustal velocity structure of Pearl River Delta, China, derived from dense-array observations of ambient noise

Zuoyong Lyu, Xiuwei Ye, Jinshui Lyu, Xiang Zhang, Liwei Wang, and Xiaona Wang

The Pearl River Delta, located in the middle of the southeast coast of south China, is a graben basin. Although this region is considered tectonically relatively inactive, many small earthquakes still occur, and multi groups of faults with different directions are well developed. To better understand the geological structures in this region, we use about 30 days of ambient noise data from 88 portable stations and 38 permanent broadband stations to obtain a high-resolution 3D upper crustal S-wave velocity model. Over 3700 Inter-station group-velocity curves were measured in the 1-10 s period range and tomographically inverted by a direct surface wave inversion method in a 0.05°×0.05°grid. The checkerboard test shows that the tomographic final resolution is 0.1°×0.1°. Our results show that in the shallow crust of the study area, the velocity distribution corresponds to surface geology and geological features. The Huizhou-Dongguan depression and the Pearl River mouth exhibit low S-wave velocity feature, while the high S-wave velocity zone corresponds to the distribution of Mesozoic granite. Some faults are almost between low velocity and high velocity zone, which may play an important role of the channel of magmatic activity. The upper crustal structure in this area is closely related to the intense magmatic tectonic activity and crustal extension since Mesozoic.

How to cite: Lyu, Z., Ye, X., Lyu, J., Zhang, X., Wang, L., and Wang, X.: Upper crustal velocity structure of Pearl River Delta, China, derived from dense-array observations of ambient noise, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7811, https://doi.org/10.5194/egusphere-egu2020-7811, 2020.

EGU2020-2677 | Displays | SM4.2

Gamma-ray spectrometry observations to monitor a presumed meteoritic signature at Maâdna crater (Talemzane, Algeria)

Atmane Lamali, Lamine Hamai, Sid Ahmed Mokhtar, Abdelkrim Yelles-chaouche, Abdeslam Abtout, Nacer-Eddine Merabet, Salah Eddine Bentridi, Leila Djadia, and Abdelmadjid Nadjemi

By measuring changes in radioelement concentrations, gamma-ray spectrometry is increasingly emerging as an efficient geophysical method that allows such changes to be geologically mapped according to lithology and soil type. At Maâdna crater in southern Algeria, this method has been used to monitor any changes in the composition of the target rocks that may be associated with the impact cratering process. For this purpose, several measurements were carried out in situ using a portable field gamma spectrometer. As a result, most predominantly calcareous surface lithologies, exposed on the rim and flanks of the crater, showed a very low emitted radiometric response over the three channels (K, Th, U). However, no more than 90 Cps were counted both inside and outside the crater. Such a rate is indeed expected in sedimentary rocks with low clay content, and this remains valid, as long as other exogenous mineralogical enrichments are excluded. On the other hand, the contoured radioelement concentrations maps, have demonstrated an anomalous enhanced gamma radiation levels of potassium-dominated peaks over the central part of the crater and in the surrounding wadis. Nevertheless, the central potassium anomaly is well correlated with the shallower magnetic one that has been described in previous studies (see e.g. Lamali et al., 2016). Therefore, either near the surrounding wadis or in the central part of this crater, this anomalously high level of radioactivity may be linked to an accumulation of later altered deposits. Consequently, there are no objective criteria to link these results to an impact event occurring at the Maâdna structure, similar to what was done at the Serra da Cangalha crater (Vasconcelos et al., 2012).

How to cite: Lamali, A., Hamai, L., Mokhtar, S. A., Yelles-chaouche, A., Abtout, A., Merabet, N.-E., Bentridi, S. E., Djadia, L., and Nadjemi, A.: Gamma-ray spectrometry observations to monitor a presumed meteoritic signature at Maâdna crater (Talemzane, Algeria), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2677, https://doi.org/10.5194/egusphere-egu2020-2677, 2020.

EGU2020-2667 | Displays | SM4.2

Georadar survey to explore a supposed ejecta layer around the Maâdna crater (Talemzane, Algeria)

Lamine Hamai, Atmane Lamali, Abdelkrim Yelles-chaouche, Abdeslam Abtout, Abdelmadjid Nadjemi, Nacer-Eddine Merabet, Salah-Eddine Bentridi, Leila Djadia, and Sid Ahmed mokhtar

Geophysics continues to play a critical role in the future discovery of terrestrial impact structures. While the signatures within these structures may not be unique, the application of geophysics can effectively characterize them, even when they are deeply eroded or completely buried underground. In the case of Maâdna crater (33°19' N, 4°19' E), among new performed geophysical surveys, a GPR technique has been especially used to explore a supposed ejecta layer. However, GPR survey results allowed the confirmation of nonexistence of such as melting materials at Maâdna crater. Nevertheless, our different scans were interpretative against the structural context of the Maâdna structure. Indeed, most of the analyzed profiles allowed us recognizing the typical deformation effects at this structure, which can also generally be encountered at any crater-like structured site. Consequently, in view to this new resulting GPR data, even we do not definitely reject an impact origin, we are still pleading for other caratering scenarios for this structure.

How to cite: Hamai, L., Lamali, A., Yelles-chaouche, A., Abtout, A., Nadjemi, A., Merabet, N.-E., Bentridi, S.-E., Djadia, L., and mokhtar, S. A.: Georadar survey to explore a supposed ejecta layer around the Maâdna crater (Talemzane, Algeria), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2667, https://doi.org/10.5194/egusphere-egu2020-2667, 2020.

EGU2020-18146 | Displays | SM4.2

Reflection seismic surveys to site the Drilling the Ivrea Verbano zonE (DIVE) proposed drill-holes, Val Sesia and Val d’Ossola, Italy.

Andrew Greenwood, Ludovic Baron, Yu Liu, György Hetényi, Klaus Holliger, Mattia Pistone, Alberto Zanetti, Luca Ziberna, and Othmar Müntener

The Ivrea-Verbano Zone in the Italian Alps represents one of the most complete and best-studied cross-sections of the continental crust. Here, geological and geophysical observations indicate the presence of the Moho transition zone at shallow depth, possibly as shallow as 3 km in the location of Balmuccia in Val Sesia. Correspondingly, the Ivrea-Verbano Zone is a primary target for assembling data on the deep continental crust as well as for testing several hypotheses regarding its formation and evolution.

            Within the context of a project submitted to the International Continental Scientific Drilling Program (ICDP), the Drilling the Ivrea-Verbano zonE (DIVE) team proposes to establish three drill holes across pertinent structures within the Ivrea-Verbano Zone. Two of the planned drill holes, each with a length of ~1000 m, are within Val d’Ossola and target the Pre-Permian lower and upper section of the lower crust. The third proposed drill hole, with a length of ~4000 m, is targeting the lower most crust of the Permian magmatic system of the Ivrea-Verbano Zone in the Val Sesia, close to the Insubric Line. Combined, the three drill holes will compose a complete section of the lower crust and the Moho transition zone, and will reveal the associated structural and composition characteristics at different scales.

To bridge across the range of spatial scales and to support the drilling proposal, we have carried out active seismic surveys using an EnviroVibe source in the Val d’Ossola. These surveys combined 2D transects (in-line) with the simultaneous collection of short cross-lines, and spatially varied source points, to collect sparse 3D data with a preferential CMP coverage across strike. This survey geometry was largely controlled by environmental considerations and access for the vibrator. Accordingly, 2D profiles, both in-line and cross-line, have been processed using crooked-line geometries, which include CMPs from the 3D infill.

The very high acoustic impedance contrast of the Quaternary valley infill sediments with respect to the predominant metapelitic and gabbroic lower crustal rocks, as well as the highly attenuative nature of the sediments, were both beneficial and problematic. The former enables mapping of the valley structure, while the latter largely prevents the detection of low-amplitude reflections from within the underlying lower crustal rocks.

Here, we present the latest results of these seismic reflection surveys and discuss the observations with respect to the prevailing structure and the planning of the drilling operations. Beyond the specific objectives pursued in this study, our results have important implications with regard to the acquisition and processing of high-resolution seismic reflection data in crystalline terranes and their capacity for resolving complex, steeply dipping structures.

How to cite: Greenwood, A., Baron, L., Liu, Y., Hetényi, G., Holliger, K., Pistone, M., Zanetti, A., Ziberna, L., and Müntener, O.: Reflection seismic surveys to site the Drilling the Ivrea Verbano zonE (DIVE) proposed drill-holes, Val Sesia and Val d’Ossola, Italy., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18146, https://doi.org/10.5194/egusphere-egu2020-18146, 2020.

EGU2020-18558 | Displays | SM4.2

Multiparametric data analysis for identifying active fault geometries in the Abruzzo and Molise regions (Central-Southern Appennines, Italy)

Germana Gaudiosi, Valeria Paoletti, Rosa Nappi, Paola Luiso, Federico Cella, Giovanni Florio, and Maurizio Fedi

The Central and Southern Apennines are characterized by the occurrence of intense and widely spread historical and recent seismic activity, mostly located along the chain.

In this paper, we present a multi-parametric data analysis in GIS environment (Geographic Information System) with the aim of identifying and constraining the geometry (strike, dip direction and dip angle) of the seismogenic faults in areas of Central-Southern Apennines characterized by outcropping/ buried and/or active/silent faults.

We use an integrated analysis of geo-structural, seismological and gravimetric data, for the identification and geometrical description of faults with density contrast, both at the surface and at depth. At the surface, the gravity lineaments inferred by Multiscale Derivative Analysis (MDA) were compared with the Quaternary faults mapped in the study areas and with the earthquakes’ epicentral distribution. The characterization of faults at depth was instead performed by the combination of the Depth from Extreme Points (DEXP) gravity imaging method with hypocentral sections.

We tested the effectiveness of this multi-method approach at Mt. Vettore-Mt. Bove, L’Aquila basin, Mt. Massico and San Giuliano di Puglia areas (Central and Southern Apennines).

Given the effectiveness of the obtained results, this multiparametric study has been applied to other three areas of the Abruzzo-Molise region: the south-western sector of Mt. Matese, the Fucino basin and the Sulmona basin.

The Matese area was hit by a seismic sequence in 2013-2014 (Mwmax= 5.1 on December 29, 2013). Our approach showed a correlation between the epicentral distribution of the 2013-2014 Matese seismic sequence (Mw=5.0) and the MDA lineaments from gravity data. The hypocentral distibution suggests that the fault rupture does not reach the surface. Therefore, the seismogenic fault responsible of 2013-2014 Matese seismic sequence is likely a buried fault.

The Fucino basin was struck by a Mw=7.0 earthquake on January 13, 1915, causing 30,000 causalities within a large area surrounding the basin. At present, the area is characterized by scarce instrumental seismicity with low magnitude.  Our analysis highlights a good correlation between NW-SE and NE-SW well-known faults and clear gravimetric MDA maxima bordering the plain. This area can be currently considered silent but, from historical seismological studies, it is one the highest seismic risk areas of Central Apennines.

Moreover, we investigated the area of the Sulmona basin, the southwards extension of the eastern system of Central Apennines developing from Mt. Vettore, Mt. Gorzano and Mt. Gran Sasso. In historical times, the faults of the most external extensional alignment, defined as silent and considered as probable seismic gaps, activated during the 2016 Amatrice–Visso–Norcia seismic sequence. Further to the southeast, two relatively large earthquakes occurred on the eastern flank of Mt. Maiella on November 3, 1706 (Mw=6.6) and on September 26, 1933 (Mw=5.7). The Sulmona area is presently characterized by poor and low magnitude instrumental seismicity. Our multi-parametric analysis highlighted a strong correlation between MDA maxima and the Mt. Morrone normal fault bordering the western side of Mt. Maiella and the eastern side of the Sulmona basin.

How to cite: Gaudiosi, G., Paoletti, V., Nappi, R., Luiso, P., Cella, F., Florio, G., and Fedi, M.: Multiparametric data analysis for identifying active fault geometries in the Abruzzo and Molise regions (Central-Southern Appennines, Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18558, https://doi.org/10.5194/egusphere-egu2020-18558, 2020.

EGU2020-12578 | Displays | SM4.2

Application of the migration velocity analysis to a long-offset seismic data in the Ulleung basin, offshore Korea

Woohyun Son and Byoung-Yeop Kim

  The 11 March 2011 Tohoku-oki earthquake (Mw = 9.0) ruptured a 500 km-long and 200 km-wide thrust of convergent boundary between the North American and Pacific plates. The earthquake caused crustal stress field changes and triggered widespread seismic activity in the northeast Japan. The southern Fukushima area was struck by many earthquakes. The largest normal faulting (Mw = 6.6) in the area ruptured the NW-trending Yunodake fault and the NNW-trending Itozawa fault on 11 April 2011. The coseismic surface ruptures were observed along active and presumed active faults identified previously. To investigate the near-surface structure of the Itozawa fault, we conducted ground penetrating radar (GPR) profiling across the fault, and we carried out two drilling surveys in hanging and foot walls of the fault. The survey line, which length was about 50 m, was located nearby a trench site (Toda and Tsutsumi, 2013). The GPR data were collected by common-offset modes using 50, 100, and 200 MHz GPR systems (pulseEKKO PRO made by Sensors and Software Inc.), and the station spacing was 0.05 m. Furthermore, we carried out wide-angle measurements, and acquired common mid-point (CMP) ensembles at the both sides of the surface rupture to estimate the electromagnetic wave velocity used in the depth conversion of the GPR sections. The GPR sections after careful data processing show detailed structure above a depth of about 10 m. We interpreted some horizons as an event showing coseismic deformation on 11 April 2011, the past seismic event reported by Toda and Tsutsumi (2013), that informing the former event, respectively. The horizons explain accumulation of vertical displacement on the Itozawa fault.

 

How to cite: Kimura, H., Tsutsumi, H., Higashimaru, N., and Taniguchi, K.: Near-surface structure revealed by ground penetrating radar profiling across an inland active fault ruptured one month after the 2011 Tohoku-oki earthquake, southern Fukushima, NE Japan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17608, https://doi.org/10.5194/egusphere-egu2020-17608, 2020.

Long-term monitoring of self-potential (SP) and electrical resistivity was conducted to examine the correlation between seismic activity and changes of these geo-electrical components in the Kyeongju area where the largest earthquake occurred in Korea. Resistivity monitoring was carried out in 2018 and 2019 but the data was not continuos occasionally because of some accidents in the field. The longest monitoring of resistivity was about 120 days and the resistivity data were acquired in 5 minutes interval. The transmitted electrical source current has 1 Hz square periodic pattern and the received voltage for the source signal was obtained in the sampling rate 10 Hz. SP data were measured in 2019 in the sampling rate of 1 kHz. The monitored resistivity and SP data are being analyzed by some graphic charts which show the variations of resistivity and SP with earthquakes of which magnitude higher than 1.0 that occurred within 4 km of the measuring site. Unfortunately, no clear correlation between the monitored geo-electrical data and seismic activity has yet been confirmed.

How to cite: Kim, H. S.: Long-term monitoring of self-potential and resistivity in Kyungju area where the largest earthquake occurred in Korea. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20794, https://doi.org/10.5194/egusphere-egu2020-20794, 2020.

EGU2020-17980 | Displays | SM4.2

Subsurface imaging with electrokinetically-induced seismoelectric signals generated by low-frequency seismic waves

Dmitry Alekseev, Mikhail Gokhberg, Aleksandra Pliss, Aleksey Goncharov, and Ilya Veklich

In this study we focus on the coupled macroscopic description of the second-kind seismo-electric (SE) effect in the subsurface structure arising due to low-frequency seismic waves. Starting with the Biot poroelasticity model, we derive the equations of the coupled geomechanical-electromagnetic problem assuming mechanical excitation in the form of seismic waves (primarily Rayleigh waves), and create code for seismoelectric field simulation. We present the results of the feasibility study showing the promising possibilities for determination of non-uniform subsurface structure parameters and allowing a subsurface imaging in terms of rock elastic constants, conductivity and permeability.

The study was supported by the Russian Foundation for Basic Research (Project No. 20-05-00691).

How to cite: Alekseev, D., Gokhberg, M., Pliss, A., Goncharov, A., and Veklich, I.: Subsurface imaging with electrokinetically-induced seismoelectric signals generated by low-frequency seismic waves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17980, https://doi.org/10.5194/egusphere-egu2020-17980, 2020.

The 2017 Ms 7.0 Sichuan Jiuzhaigou earthquake occurred at the intersection of the Tazang, Minjiang, and Huya faults on the eastern margin of the Tibetan Plateau. Since it occurred on an unmarked blind fault, it is still a controversial issue whether the fault, which triggered the earthquake, was the extension of the East Kunlun fault or the northern branch of the Huya fault. Therefore, the accurate source location is of great significance for studying the deep distribution of seismogenic faults and seismicity analysis.

We have not only collected seismic phase arrival data recorded by 24 permanent stations and 6 temporary stations, but also picked up the seismic waveform data recorded by partial permanent stations in this study. Using absolute seismic location method and relative seismic location method, we relocated the earthquake events with magnitude greater than or equal to 1.0 occurred in the Jiuzhaigou area from August to December 2017. In order to ensure reliable data quality, we selected 23422 P-wave absolute arrival times, 24734 S-wave absolute arrival times and 124519 high quality P-waveform cross correlation data of 3449 earthquake events for relocation research.

The mean value of root mean square residuals of travel time of all earthquakes decrease from 0.21s to 0.08s after relocation. The average location errors in the E-W, N-S, and vertical directions are 0.11km, 0.12km, and 0.16km, respectively. Ninety-nine percent of the earthquake events are distributed in the depth range of 1-25 km, and the dominant distribution range is 5-15 km. The result shows that the earthquakes are distributed along the strike of northwest and southeast, and the Jiuzhaigou mainshock divided these events into two clusters: northwest and southeast. From the parallel strike section, we conclude that the depth of the northwest seismic cluster is shallow with the depth range of 2-15 km, and the depth of the southeast seismic cluster is deeper with the depth range of 6-18 km. Moreover, the number of aftershocks in the northwest cluster is greater than that in the southeast cluster, but after an M 4.9 aftershock occurred in the northwest cluster on the ninety-first day after the Jiuzhaigou mainshock, the number of aftershocks in the northwest cluster began to decrease. The result provides a basis for studying the seismogenic background and seismicity of the Jiuzhaigou earthquake.

How to cite: Yu, X., Song, Q., and Deng, S.: An accurate relocation of the 2017 Ms 7.0 Sichuan Jiuzhaigou earthquake sequence and the seismicity analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1802, https://doi.org/10.5194/egusphere-egu2020-1802, 2020.

Recent fast developments of satellite magnetic observations facilitate global Lithospheric Magnetic Field (LMF) modelling and their applications to subsurface tectonics. Here, the vertical component (Bz) of LMF at an altitude of 200km in Mainland China and surroundings is calculated from two global LMF models NGDC-720 and EMM2017. Next, Bz is used to invert the Curie Point Depth (CPD) by Equivalent Source Dipole (ESD) forward and Nonlinear Conjugate Gradient Method (NCGM) inversion scheme. Then, the surficial Heat Flux (HF) is derived by a simple one-dimensional steady heat conduction equation from the CPD distribution. At last, the continental seismicity is compared statistically to Bz, CPD and HF. Our essential conclusions are as follow: 1) Histograms and boxplots show that most (81.8%) earthquakes (EQs, Ms≥5.0) occurred in negative Bz areas, and more than a half (53.2%) number of EQs (corresponding to an energy percent of 94.6%) occurred inside areas with Bz between -5 and -3nT, in a period between 2004 and 2007, which is the same with the satellite data collection. When the time span is extended (most to 110 years), these phenomena maintain while weaken; 2) Most (88%) EQs occurred in areas with CPD between 10 and 30km, while only a few (7% and 5%) occurred in shallow (<10km) and deep (>30km) CPD areas, in a period between 2000 and 2010; 3) EQs seldom occurred inside cold areas (HF<50mW/m2), and are prone to occur in warm areas (HF>120mW/m2). EQs are also prone to occur along the boundaries of warm or cold areas. The mechanism of the correlations between EQs and Bz, CPD and HF maybe the lithospheric strength jumps caused by the temperature variations at boundaries between blocks with different CPDs.

How to cite: Jiao, L. and Lei, Y.: Magnetic and thermal constraints on the spatial distribution of continental seismicity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4390, https://doi.org/10.5194/egusphere-egu2020-4390, 2020.

SM4.3 – Imaging, modelling and inversion to explore the Earth’s lithosphere

EGU2020-22586 | Displays | SM4.3 | Highlight

Electrical imaging of the Mohns Ridge in the Greenland Sea

Ståle Johansen, Martin Panzner, Rune Mittet, Hans Amundsen, Anna Lim, Eirik Vik, Martin Landrø, and Børge Arntsen

A detailed 120 km deep electromagnetic joint inversion model for the ultra-slow Mohns Ridge was constructed combining controlled source- and magnetotelluric data. About one third of mid-ocean ridges have a spreading rate less than 20 mm/yr1, but due to lack of deep imaging, factors controlling melting and mantle upwelling, depth to the lithosphere – asthenosphere boundary (LAB), crustal thickness and hydrothermal venting are not well understood for this class of ridges. Modern electromagnetic data have significantly improved understanding of fast-spreading ridges, but have not been available for the ultra-slow ridges. The new inversion images show mantle upwelling focused along a narrow, oblique and strongly asymmetric zone coinciding with asymmetric surface uplift. Though the upwelling pattern shows several of the characteristics of a dynamic system, instead it likely reflects passive upwelling controlled by slow and asymmetric plate movements.

Upwelling asthenosphere and melt are enveloped by the 100 Ωm contour denoted the electrical LAB (eLAB). This transition may represent a rheological boundary defined by a minimum melt content. We also find that a model where crustal thickness is directly controlled by the melt-producing rock volumes created by the separating plates can explain the thin crust below the ridge. Fluid convection extends for long lateral distances exploiting high porosity at mid crustal levels. The magnitude and long-lived nature of such plumbing systems could promote venting at ultra-slow ridges. Further, active melt emplacement into ca 3 km thick oceanic crust culminates in an inferred crustal magma chamber draped by fluid convection cells emanating at Loki´s Castle hydrothermal field.

How to cite: Johansen, S., Panzner, M., Mittet, R., Amundsen, H., Lim, A., Vik, E., Landrø, M., and Arntsen, B.: Electrical imaging of the Mohns Ridge in the Greenland Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22586, https://doi.org/10.5194/egusphere-egu2020-22586, 2020.

EGU2020-538 | Displays | SM4.3

Seismic diffraction imaging - a case study from the Southern Aegean Sea

Jonas Preine, Benjamin Schwarz, Alexander Bauer, Dirk Gajewski, and Christian Hübscher

The active seismic method is a standard tool for studying the Earth’s lithosphere. On scales from centimetres to kilometres, academic research is generally interested in highly complex geological targets such as volcanic edifices, crustal faults or salt environments. In order to properly image these structures, large and expensive multichannel acquisitions with a high offset-to-target depth ratio are required. In practice, however, these are often hardly affordable for academic institutions, with the result that reflections often only poorly illuminate laterally variable structures, which in turn compromises imaging and interpretation. As in common practice, most of the processing and interpretational steps are tailored to the reflected wavefield, faint diffracted contributions are typically considered as an unwanted by-product.

In recent works, however, it has been shown that diffractions possess unique properties which bear the potential to overcome the aforementioned limitations. Wave diffraction occurs at geodynamically important features like faults, pinch-outs, erosional surfaces or other small-scale scattering objects and encodes sub-wavelength information on the scattering geometry. Since diffracted waves do not obey Snell’s Law, they provide superior illumination compared to reflected waves. Moreover, due to their passive-source like radiation, they encode their full multichannel response in prominent data subsets like the zero-offset section. In order to explore what can be learned from the faint diffracted wavefield, we use academic seismic data from the Santorini-Amorgos Tectonic Zone (SATZ) in the Southern Aegean Sea. This is an area well known for its local complexity, indicated by the occurrence of extended fault systems and volcanic edifices as well as a complex acoustic basement. As the available seismic data in this region were acquired using a relatively short streamer, the SATZ represents a classical example for the need of innovative methods for seismic processing and interpretation.

By means of a robust and computationally efficient scheme for the extraction of diffractions that models and adaptively subtracts the reflected wavefield from the data, we reveal a rich diffracted wavefield from zero-offset data. On the one hand, we use the diffraction-only sections for analysing the small-scale structural complexity and demonstrate that the geological interpretation can benefit from these observations. On the other hand, we use the diffractions to estimate insightful wavefront attributes in the zero-offset domain. Based on these attributes, we perform wavefront tomography to obtain depth-velocity models. Compared to depth-velocity models derived from the reflected contributions, the diffraction-based velocity model fits the data significantly better. After refining this velocity model, we perform prestack depth migration and obtain highly valuable depth converted seismic sections. Concluding our results, we strongly encourage the incorporation of diffractions in standard processing and interpretational schemes.

How to cite: Preine, J., Schwarz, B., Bauer, A., Gajewski, D., and Hübscher, C.: Seismic diffraction imaging - a case study from the Southern Aegean Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-538, https://doi.org/10.5194/egusphere-egu2020-538, 2020.

Investigations of the deep lithosphere aiming at the reconstruction of the geological models remain one of the key sources of the knowledge about the processes shaping the outer shell of our planet. Among different methods, the active seismic Ocean-Bottom Seismometer (OBS) experiments conducted in wide-angle configuration are routinely employed to better understand these processes. Indeed, long-offset seismic data, combined with computationally efficient travetime tomographic methods, have a great potential to constrain the macro-scale subsurface velocity models at large depths. 

On the other hand, decades of development of acquisition systems, more and more efficient algorithms and high-performance computing resources make it now feasible to move beyond the regional raytracing-based traveltime tomography. In particular, the waveform inversion methods, such as Full-Waveform Inversion (FWI), are able to exhaustively exploit the rich information collected along the long-offset diving and refraction wavepaths, additionally enriched with the wide-angle reflection arrivals. So far however, only a few attempts have been conducted in the academic community to combine wide-angle seismic data with FWI for high-resolution crustal-scale velocity model reconstruction. This is partially due to the non-convexity of FWI misfit function, which increases with the complexity of the geological setting reflected by the seismograms. 

In its classical form FWI is a nonlinear least-squares problem, which is solved through the local optimization techniques. This imposes the strong constraint on the accuracy of the starting FWI model. To avoid cycle-skipping problem the initial model must predict synthetic data within the maximum error of half-period time-shift with respect to the observed data. The criterion is difficult to fulfil when facing the crustal-scale FWI, because the long-offset acquisition translates to the long time of wavefront propagation and therefore accumulation of the traveltime error along the wavepath simulated in the initial model. This in turns increases the possibility of the cycle-skipping taking into account large number of propagated wavelengths.

Searching to mitigate this difficulty, here we investigate FWI with a Graph-Space Optimal Transport (GSOT) misfit function. Comparing to the classical least-squares norm, GSOT is convex with respect to the patterns in the waveform which can be shifted in time for more than half-period. Therefore, with proper data selection strategy GSOT misfit-function has potential to reduce the risk of cycle-skipping. We demonstrate the robustness of this novel approach using 2D wide-angle OBS data-set generated in a GO_3D_OBS synthetic model of subduction zone (30 km x 175 km). We show that using GSOT cost-function combined with the multiscale FWI strategy, we reconstruct in details the highly complex geological structure starting from a simple 1D velocity model. We believe that further developments of OT-based misfit functions can significantly reduce the constraints on the starting model accuracy and reduce the overall risk of cycle-skipping during FWI of wide-angle OBS data.

How to cite: Górszczyk, A., Métivier, L., and Brossier, R.: Relaxing the initial model constraint for crustal-scale full-waveform inversion with graph-space optimal transport misfit function, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7743, https://doi.org/10.5194/egusphere-egu2020-7743, 2020.

EGU2020-18782 | Displays | SM4.3

Towards highly-sparse, autonomous imaging systems: high-resolution wavefield imaging for frontier exploration

Ivan Vasconcelos, Jingming Ruan, Dieuwertje Kuijpers, Matteo Ravasi, and Patrick Putzky

Can we image and monitor the internal ocean structure at global scales? Can we monitor in vast expanses of the Earth’s cryosphere subsurface with meter-length resolution? Can we characterize the interior structures of asteroids and comets out in space efficiently and with high confidence? At the core of these questions lies the understanding and development of wave-based imaging systems, based on seismic or radar, that rely on highly-sparse, high-quality data, but whose output image quality is comparable to that of densely sampled, wide aperture array-based data. Traditionally, exploration seismology has long relied on wide aperture, dense data sets together with high-end imaging such as reverse-time migration and full-waveform inversion to produce high resolution subsurface models. Given the recent rise of drone-like, autonomous systems, in this talk, we present approaches that can take highly-sparse data as would be recorded by autonomous platforms, into accurate high-resolution images as if they had been acquired by densely-sampled, wide aperture source and receiver arrays. We demonstrate two approaches that could achieve this goal. The first is the use of sparse multicomponent sources and receivers capable of exciting/recording fields and their spatial gradients, together with a gradient-based wavefield reconstruction approach and subsequent imaging. The second approach relies on a new deep learning architecture, the so-called Recurrent Inference Machine, designed specifically for inverse problems – showing that it can surpass the capabilities of deterministic approaches to data reconstruction and imaging. We illustrate these approaches using a numerical model for oceanic turbulence, where we show the compressive sensing potential of these acquisition, reconstruction and imaging methods for acoustic imaging of the ocean's internal structure – overcoming current limitations in data acquisition and processing for seismic oceanography. Finally, we postulate that these approaches, though still in their early days, will pave the way in enabling breakthrough imaging systems at the frontiers of geo-imaging, e.g., for oceanography at global scales, in imaging the Earth’s cryosphere or for planetary exploration.

How to cite: Vasconcelos, I., Ruan, J., Kuijpers, D., Ravasi, M., and Putzky, P.: Towards highly-sparse, autonomous imaging systems: high-resolution wavefield imaging for frontier exploration, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18782, https://doi.org/10.5194/egusphere-egu2020-18782, 2020.

EGU2020-20123 | Displays | SM4.3

Coda-wave interferometry and the Marchenko method

Kees Wapenaar and Johno van IJsseldijk

Coda-wave interferometry, introduced by Snieder and co-workers, employs the relative high sensitivity of the scattering coda in an acoustic or seismic response to time-lapse changes of the propagation velocity and/or structure. It has been successfully applied at many scales, ranging from inferring temperature changes in granite samples, via structural health monitoring of bridges, to monitoring the minute changes in the interior of a volcano prior to eruption. Whereas in most situations the velocity changes are assumed to take place in a large region, it has been shown that coda-wave interferometry can also be used to image a local perturbation of the propagation velocity or structure. The latter approach assumes diffuse waves and employs an array of receivers that surrounds the perturbation.

We investigate the application of coda-wave interferometry for monitoring of fluid-flow processes in aquifers, geothermal reservoirs, CO2-storage reservoirs and hydrocarbon reservoirs. In these applications the velocity perturbation is local, but the medium is probed with deterministic seismic body waves from the surface only. The location of the velocity perturbation is usually reasonably well known, but it is practically impossible to identify events in the coda that are directly related to the local perturbation. Recently we introduced the Marchenko method, which retrieves information about multiple scattering from reflection data at the surface in a data-driven way. Here we propose to use the Marchenko method to remove the response from the areas above and below the local velocity perturbation. In this way we isolate the scattering coda of the local velocity perturbation, which enables the application of coda-wave interferometry to monitor the fluid-flow process.

How to cite: Wapenaar, K. and van IJsseldijk, J.: Coda-wave interferometry and the Marchenko method , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20123, https://doi.org/10.5194/egusphere-egu2020-20123, 2020.

The goal of most seismic experiments is to use data readily available at the surface of the Earth to characterise the inaccessible interior. In order to solve this inverse problem, we generally make a number of assumptions about either the data or the Earth to simplify the physics. For example, we often assume that the Earth is an acoustic medium rather than an elastic medium, which for data without S-waves makes the problem far more tractable computationally than the full elastic problem.

One of the most common assumptions made about the data is the single-scattering assumption, widely known as the Born approximation. Clearly this is invalid in the presence of multiple scattering, which occurs in all seismic experiments. Despite this, the majority of imaging and inversion methods applied to seismic data are dependent on this assumption, including most full waveform inversion algorithms. As a consequence, seismic data processing requires a great deal of effort to remove multiply scattered waves from data.

A key justification for making this assumption is that a priori we can only estimate a relatively smooth Earth model that does not predict multiply scattered waves. However, with the recent emergence of so-called Marchenko methods, we now have access to full Green’s functions between sources and receivers at the Earth’s surface and virtual source or receiver locations inside the Earth’s interior, Green’s functions which can be estimated using only recorded reflection data and an estimate of the direct (non-scattered) wavefield travelling into the subsurface. As Marchenko methods become more commonplace, our justification for the single-scattering assumption diminishes, and hence we require new methods to use this information.

By iterating the Lippmann-Schwinger equation, we define a new compact form of the Frechét derivative of the Green’s function that involves all orders of scattering. In combination with Green’s functions obtained by a Marchenko method, these may be used for imaging and inversion of seismic data. We will describe an example of such a scheme, which we call “Marchenko Lippmann-Schwinger Full-Waveform Inversion”, to demonstrate how our redefined Green’s function derivative may be applied to solve seismic inverse problems for the Earth’s subsurface structure.

How to cite: Cummings, D. and Curtis, A.: Defining the Green’s function derivative for imaging & inversion without the Born approximation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10804, https://doi.org/10.5194/egusphere-egu2020-10804, 2020.

It has been widely recognized that the cross-correlation function (CCF) of ambient noise data recorded at two seismic stations approximates to the part of Green’s Function between these two stations. Theoretically, the CCF should include the higher modes, apart from the fundamental mode. However, currently well-known and mature methods that can extract dispersion curves are not pretty proficient in extracting higher modes. Fortunately, our newly proposing method, the Frequency-Bessel Transform Method (F-J Method), has presented its obvious advantage in extracting higher modes. This study applied F-J method to seismic ambient noise data for the east of South China, including Jiangnan Orogen and South China Fold System. We have acquired higher modes, not to mention the fundamental mode with wider frequency than previous studies. Combining both fundamental mode and higher modes, we used L-BFGS inversion method to inverse and acquire more accurate crustal and upper-mantle structure than previous studies only adopting fundamental mode for the east of South China. As shown in this study for the east of South China, we can use F-J method  to conveniently and precisely extract multimodes from ambient noise data and thus add more constrains for inversion results, which can significantly improve the preciseness of imaging crustal and upper-mantle structure.

How to cite: Chen, J. and Chen, X.: Extracting higher-mode dispersion curves from ambient noise data by using F-J method to acquire more accurate crustal and upper-mantle structure for the east of South China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5407, https://doi.org/10.5194/egusphere-egu2020-5407, 2020.

EGU2020-967 | Displays | SM4.3

Waveform tomography in the Mediterranean and Southeast Asia

Nienke Blom, Andreas Fichtner, Alexey Gokhberg, Nicholas Rawlinson, and Deborah Wehner

In this work, we present results from waveform tomography conducted in the Mediterranean and Southeast Asia. Whilst computationally more expensive than ray-based imaging methods, the advantage of waveform methods lies in their ability to incorporate in a consistent manner all the information contained in seismograms – not just the arrivals of certain, specified phases. We can therefore naturally and coherently exploit body and multimode surface waves, and take into account source effects, frequency-dependence, wavefront healing, anisotropy and attenuation.

Here, we look at applications of this method in two geologically complex regions: the Mediterranean and Southeast Asia. Both are characterised by broadscale convergence and a complicated pattern of interactions between larger and smaller-scale tectonic plates.

The Mediterranean is historically one of the best studied areas in the world, with an impressive density of seismic stations which greatly aids the detailed imaging of the region. We have been able to image the Central and Eastern Mediterranean down to the mantle transition zone, thereby illuminating the complex slab structures and geometries within the domain. We identify several main slabs that correspond to major current and former subduction zones.

In Southeast Asia, we work at a larger scale, with a model domain encompassing the Sunda arc (which gives rise to some of the world’s most significant natural hazards), the Banda arc with its spectacular 180° curvature and various smaller-scale features, such as the tectonically complex island of Sulawesi. To date, sparse instrument coverage in the region has led to a heterogeneous path coverage, in particular around Borneo which is located in an intra-plate setting. A recent series of temporary seismometer deployments in Sabah (North Borneo), Kalimantan, Sulawesi and the Celebes Sea allows us to fill the gaps in the publicly available data, thereby providing new opportunities to investigate the region's complexity using waveform tomography.

In this presentation, we will also discuss a number of features and “best practices” that can significantly influence waveform tomography results. In particular, we highlight how we can optimise sensitivity to deep structure by combining long-period data with a window selection approach that specifically targets body wave signals, and we discuss the effect of uncertainties in earthquake source parameters on the seismic inversion process.

How to cite: Blom, N., Fichtner, A., Gokhberg, A., Rawlinson, N., and Wehner, D.: Waveform tomography in the Mediterranean and Southeast Asia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-967, https://doi.org/10.5194/egusphere-egu2020-967, 2020.

Body-wave tomography with data from regional networks, also called ACH tomography, has provided a lot of information on the upper mantle structure, especially in continental settings. A key factor in this technique is the usage of relative residuals, not absolute ones. It is based on the assumption that travel times at stations in a regional network are affected in a similar way by errors in source location and origin time, as well as by large-scale heterogeneities in the lower mantle, and that demeaning the residuals strongly reduces the influence of these factors on the inverse regional model. This results in the well-known fact that the final velocity model is relative to an unknown vertically varying reference model. This also prevents combining data obtained with networks in the vicinity of each other but operating at different times, even though we may have stations, for example permanent stations, which are common to these networks. This is because the residuals at the two networks are measured with respect to different unknown averages and cannot be inverted together. This is very unfortunate as we have numerous examples of asynchronous network deployments which together would provide a much more useful station coverage and model than independently.

I will analyse how a simple change in the formulation of the direct problem allows to take into account that the residuals are demeaned, and, most importantly, how this can be used to remedy the limitations of regional body-wave tomographic methods in terms of asynchronous station deployment. I will first illustrate this with a very simple example and then present the results of a more realistic synthetic test combining data from two neighboring asynchronous networks in a single inversion.

How to cite: Maupin, V.: Combining asynchronous data sets in regional body-wave tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8571, https://doi.org/10.5194/egusphere-egu2020-8571, 2020.

The Receiver Function (RF) technique, that aims to isolate P to S teleseismic converted waves, is largely used to image seismic discontinuities at depth. In particular, in subduction zones, the subducting crust has often be identified on RF as a Low Velocity Layer (LVL) embedded between the mantle of the overriding plate and the mantle of the subducting lithosphere. In several subduction zones, a high Vp/Vs ratio inside this LVL has been estimated from the arrival times of the primary and backscattered P to S converted waves at the top and at the base of the LVL. However seismograms are filtered to enhance the signal over noise ratio and this processing step can dramatically reduce the resolution of the converted waves. In order to check if the signal periods associated to common filters could lead to an overestimation of the Vp/Vs ratio, a wavelet response in conversion for primary and backscattered converted waves is developed for a LVL typical of an oceanic crust. This multiscale analysis allows to illustrate that the LVL characteristics can be misinterpreted for the common frequency range due to interferences between the converted waves at the top and at the base of the LVL. For a dominant period of about 3s, the Vp/Vs of a typical oceanic crust can be largely overestimated (about Vp/Vs=2.8 instead of Vp/Vs=1.8) and its thickness underestimated (about 5 km instead of 7 km). The characteristics of a typical oceanic crust can be reliably retrieved only in the non interaction domain that corresponds to a constant spacing between the converted waves at the top and at the base of the LVL. This non-interaction domain corresponds to dominant signal period smaller than 1 s for the primary converted waves and 3 s for the backscattered. As the Vp/Vs is generally estimated based on the interpretation of both primary and backscattered waves, the period of 1 s is required for a reliable interpretation. The multiscale approach is applied to a real data example of teleseismic events recorded at a 3-component seismometer in order to reliably constrain the Vp/Vs ratio and the thickness of the oceanic crust at the top of the Hellenic subduction.

How to cite: Gesret, A.: How resolving are teleseismic forward and backscattered P to S converted waves?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14071, https://doi.org/10.5194/egusphere-egu2020-14071, 2020.

EGU2020-4468 | Displays | SM4.3

Moho Topography and Velocity/Density Model for the Hedmarken area, Eastern Norway, using Receiver Function Analysis and Rj-McMC

Claudia Pavez, Marco Brönner, Odleiv Olesen, and Arne Bjørlykke

A Receiver Function Analysis was carried out in the Mjøsa area, Eastern Norway, in order to better image this tectonically complex area, understand the crustal contrasts and complement geological analysis that were made previously in the area. For this, we used seismic traces received for seven broadband stations from the NORSAR permanent array. The H-K (depth vs Vp/Vs) stacking procedure and a Reversible jump Markov chain Monte Carlo (Rj-McMC) inversion were developed independently. The first analysis allows us to obtain a model with the Mohorovicic discontinuity values under each seismic station and the average Vp/Vs crustal ratio. With the inversion, it was possible to develop a 1D local velocity model. Applying the Nafe-Drake relationship, a 2D density model was obtained and tested against observed gravity. Results indicate the presence of a low anomalous density layer that is located to the NNW of the study area, which is probably related to low-density meta-sediments in the Åsta Basin located above the basement. A main crustal fault is also indicated from the density model, spatially coinciding with faults grown during the Sveconorwegian orogenic process.

 

How to cite: Pavez, C., Brönner, M., Olesen, O., and Bjørlykke, A.: Moho Topography and Velocity/Density Model for the Hedmarken area, Eastern Norway, using Receiver Function Analysis and Rj-McMC, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4468, https://doi.org/10.5194/egusphere-egu2020-4468, 2020.

EGU2020-5302 | Displays | SM4.3

Shear attenuation and anelastic mechanisms in the central Pacific upper mantle

Zhitu Ma, Colleen Dalton, Joshua Russell, James Gaherty, Greg Hirth, and Donald Forsyth

We determine the mantle attenuation (1/Q) structure beneath 70 Myr seafloor in the central Pacific. We use long-period (33-100 sec) Rayleigh waves recorded by the NoMelt array of broadband ocean-bottom seismometers. After the removal of tilt and compliance noise, we are able to measure Rayleigh wave phase and amplitude for 125 earthquakes. The compliance correction for ocean wave pressure on the seafloor is particularly important for improving signal-to-noise at periods longer than 55 sec. Attenuation and azimuthally anisotropic phase velocity in the study area are determined by approximating the wavefield as the interference of two plane waves. We find that the amplitude decay of Rayleigh waves across the NoMelt array can be adequately explained using a two-layer model: in the shallow layer, in the deeper layer, and a transition depth at 70 km, although the sharpness of the transition is not well resolved by the Rayleigh wave data. Notably, observed in the NoMelt lithosphere is significantly higher than values in this area from global attenuation models. When compared with lithospheric measured at higher frequency (~3 Hz), the frequency dependence of attenuation is very slight, revising previous interpretations. The effect of anelasticity on shear velocity (VS) is estimated from the ratio of observed velocity to the predicted anharmonic value. We use laboratory-based parameters to predict attenuation and velocity-dispersion spectra that result from the superposition of a weakly frequency dependent high-temperature background and an absorption peak. We test a large range of frequencies for the position of the absorption peak (fe) and determine, at each depth, which values of fe predict and VS that can fit the NoMelt and Vvalues simultaneously. We show that between depths of 60 and 80 km the seismic models require an increase in fe by at least 3-4 orders of magnitude. Under the assumption that the absorption peak is caused by elastically accommodated grain-boundary sliding, this increase in fe reflects a decrease in grain-boundary viscosity of 3-4 orders of magnitude. A likely explanation is an increase in the water content of the mantle, with the base of the dehydrated lid located at ~70-km depth.   

How to cite: Ma, Z., Dalton, C., Russell, J., Gaherty, J., Hirth, G., and Forsyth, D.: Shear attenuation and anelastic mechanisms in the central Pacific upper mantle , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5302, https://doi.org/10.5194/egusphere-egu2020-5302, 2020.

EGU2020-5754 | Displays | SM4.3

High temperature in the upper mantle beneath Cape Verde as a possible cause for the oceanic lithosphere rejuvenation inferred from Rayleigh-wave phase-velocity measurements.

Joana Carvalho, Raffaele Bonadio, Graça Silveira, Sergei Lebedev, Susana Custódio, João Mata, Pierre Arroucau, Thomas Meier, and Nicolas Celli

Cape Verde is an intraplate archipelago located in the Atlantic Ocean about 560 km west of Senegal, on top of a ~130 Ma sector of the African oceanic lithosphere. Until recently, due to the lack of broadband seismic stations, the upper-mantle structure beneath the islands was poorly known. In this study we used data from two temporary deployments across the archipelago, measuring the phase velocities of Rayleigh-waves fundamental-modes in a broad period range (8–250 s), by cross-correlating teleseismic earthquake data between pairs of stations. Deriving a robust average, phase-velocity curve for the Cape Verde region, we inverted it for a shear-wave velocity profile using non-linear gradient search.

Our results show anomalously low velocities of ∼4.2 km/s in the asthenosphere, indicating the presence of high temperatures and, eventually, partial melting. This temperature anomaly is probably responsible for the thermal rejuvenation of the oceanic lithosphere to an age as young as about 30 Ma, which we inferred from the comparison of seismic velocities beneath Cape Verde and the ones representing different ages in the Central Atlantic.

The present results, together with previously detected low-velocity anomalies in the lower mantle and relatively He-unradiogenic isotopic ratios, also suggest a hot plume deeply rooted in the lower mantle, as the origin of the Cape Verde hotspot.

The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020 – IDL and FIRE project Ref. PTDC/GEO- GEO/1123/2014.

How to cite: Carvalho, J., Bonadio, R., Silveira, G., Lebedev, S., Custódio, S., Mata, J., Arroucau, P., Meier, T., and Celli, N.: High temperature in the upper mantle beneath Cape Verde as a possible cause for the oceanic lithosphere rejuvenation inferred from Rayleigh-wave phase-velocity measurements., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5754, https://doi.org/10.5194/egusphere-egu2020-5754, 2020.

EGU2020-6466 | Displays | SM4.3

Anisotropic upper mantle structures in northeast Asia from Bayesian inversions of ambient noise data

Sang-Jun Lee, Seongryong Kim, and Junkee Rhie

The northeast Asia region exhibits complex tectonic settings caused by interactions between Eurasian, Pacific, and Philippine Sea plates. Distributed extensional basins, intraplate volcanoes and other heterogeneous features in the region marked results of the tectonic processes, and their mechanisms related to mantle dynamics can be well understood by estimating radial anisotropy in the lithospherie and asthenospherie. We constructed a three-dimensional radial anisotropy model in northeast Asia using hierarchical and transdimensional Bayesian joint inversion techniques with different types of dispersion data up to the depth of the upper mantle (~ 160 km). Thick and deep layers with positive radial anisotropy (VSH > VSV) were commonly found at depths between 70 and 150 km beneath the continental regions. On the other hand, depths and sizes of layers with positive radial anisotropy become shallower and thinner (30 ~ 60 km) respectively beneath regions where experienced the Cenozoic extension. These variations in positive radial anisotropy for different tectonic regions can be understood with the context of extensional geodynamic processes in back arc basins within the East Sea (Japan Sea). Interestingly, the most predominant positive radial anisotropy is imaged along areas with large gradient of the litheosphere-asthnosphere boundary beneath intraplate volcanoes. These observations favor the mechanism of edge-driven convection caused by the difference in lithosphere thickness and localized sublithospheric lateral flow from the continental region to back arc basins.

How to cite: Lee, S.-J., Kim, S., and Rhie, J.: Anisotropic upper mantle structures in northeast Asia from Bayesian inversions of ambient noise data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6466, https://doi.org/10.5194/egusphere-egu2020-6466, 2020.

Parker–Oldenburg algorithm was applied to gravity data in the western USA to map the Moho discontinuity. F-H Parasnis method was also applied to the gravity data to estimate the Bougeur reduction density for calculation of the Bougeur gravity anomaly. The inversion process uses the Oldenburg equation (Eq.2: Oldenburg, 1974), which is a rearrangement of Parker’s equation (Eq.1: Parker, 1973), to estimate the depth to the undulating interface from the gravity anomaly by means of an iterative process. These formulas are shown as follows,

where F(Δg) is the Fourier transform of the gravity anomaly,  G is the gravitational constant, ρ is the density contrast across the interface,  K is the wave number,  h(x) is the depth to the interface (positive downwards) and z0 is the mean depth of the horizontal interface.

The resulted Moho depth map shows depths ranging from 9 km to 50 km. Moho anomalies showed good spatial correlation with the major physiographic provinces in the study area. The subduction trench of the Farallon remnants (Juan de Fuca and Gorda) was mapped at the north of the Mendocino Triple Junction (MTJ). The subducting plates show north-east dipping direction with low dipping angle. The effect of the subduction appears in the structure at the northern part (i.e. Cascade Mountain, Walla-Walla plateau and Northern Rocky Mountains), whereas the southern part is affected by the transform movement of the Pacific Plate yielding a set of basins (Central Valley, Great Basin and Wyoming Basin). Results of this research, in conjunction with other information of the area, provide a new information for the analysis of the tectonic framework of the western North-America.

 

References

Oldenburg, D.W., 1974. The inversion and interpretation of gravity anomalies. Geophysics 39 (4), 526–536.

Parker, R.L., 1973. The rapid calculation of potential anomalies. Geophysical Journal of the Royal Astronomical Society 31, 447–455

How to cite: Shehata, M. and Mizunaga, H.: Application of Parker-Oldenburg Algorithm to map Moho Discontinuity using Gravity Data in Western USA , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-643, https://doi.org/10.5194/egusphere-egu2020-643, 2020.

EGU2020-19375 | Displays | SM4.3

Receiver function analyses and joint inversion with gravity data: new constraints on the Ivrea geophysical body along a high-resolution profile

Matteo Scarponi, György Hetényi, Jaroslava Plomerová, Stefano Solarino, and Ludovic Baron

We collected new seismological and gravity data in the Val Sesia and Lago Maggiore regions in NW Italy to constrain the geometry and properties of the Ivrea Geophysical Body. This piece of lower Adriatic lithosphere is known to be at anomalously shallow depth along the inner arc of the Western Alps, yet existing seismological constraints (vintage seismic refraction data, local earthquake tomography) are spatially sparse. With the aim to reach higher spatial resolution in imaging the structure of the IGB, we analyze the seismological data with various receiver function approaches to map the main velocity discontinuities, followed by joint inversion with gravity data to fill the bulk properties of bodies with densities.

The new data acquisition consisted of two type of campaigns. For seismology, we deployed 10 broadband seismic stations (MOBNET pool, IG CAS Prague) along a linear West-East profile at 5 km spacing along Val Sesia and across the Lago Maggiore. This network continuously recorded seismic data for 27 months at 100 Hz sampling rate. For gravimetry, we compiled existing datasets and then completed the spatial gaps by relative gravity surveys, tied to absolute reference points, to achieve 1 gravity point every 1-2 km along the profile.

The receiver function (RF) analyses aim at detecting velocity increases with depth: primarily the Moho and the shallow IGB interfaces and their crustal reverberations (multiples), together with their potential dip by analyzing the transverse component RFs. Furthermore, we aim at investigating the sharpness of the velocity gradient across the discontinuities by analyzing the frequency dependence of the corresponding RF peaks. We aim at reproducing the observations by simple synthetic models.

The 2D joint inversion combines S wave velocity VS and bulk density as physical parameters to match both the seismological and gravimetry data. The relationship between the two parameters is initially chosen from the literature, but depending on the first results the relation itself may be inverted for, considering the various high-grade metamorphic rocks observed at the surface in the area, whose properties may not align with classical VS–density equations. In conclusion, we propose new constraints on the IGB, demonstrating the advantage of using multi-disciplinary geophysical observations and improved data coverage across the study area.

How to cite: Scarponi, M., Hetényi, G., Plomerová, J., Solarino, S., and Baron, L.: Receiver function analyses and joint inversion with gravity data: new constraints on the Ivrea geophysical body along a high-resolution profile, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19375, https://doi.org/10.5194/egusphere-egu2020-19375, 2020.

EGU2020-1817 | Displays | SM4.3

Seismic Tomography of Macedonia

Cvetan Sinadinovski, Lazo Pekevski, Agus Abdullah, and Kevin McCue

A novel geotomography technique has been applied in and around Macedonia using selected earthquakes that occurred over a period of 40 years and were recorded on 47 seismograph stations. The aim was to test this new tomography method for the first time in investigation of the crustal shape and structures in that specific tectonic environment with an extensive dataset.

A three-dimensional velocity model and many cross-sections of the crust were produced by this methodology and compared with the previous models of Macedonia. They show the potential of the tomography application in revealing geological features on local and regional scale. 

The new images will contribute towards a better understanding of the seismicity and tectonics in that part of the Balkans and assist in the process of integrated seismic hazard assessment.

 

 

Keywords: Geotomography, Earthquakes, Seismic Imaging

How to cite: Sinadinovski, C., Pekevski, L., Abdullah, A., and McCue, K.: Seismic Tomography of Macedonia , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1817, https://doi.org/10.5194/egusphere-egu2020-1817, 2020.

The Ordos block is located on the west side of the North China Carton, adjacent to the northeastern part of the Tibetan Plateau. Affected by two tectonic movements, Ordos block internal structure remains relatively stable structure, but surrounded by active tectonic belts. With the development of the second and third part of the “China Seismological Science Array”, the distribution of seismic observation stations in Ordos region has been greatly improved. This study will use the new seismic observation data of "Array III", and combined with the phase observation data of "Array II" to form a more complete seismic phase travel time data set. The regional seismic body-wave travel time tomography will figure out a more reliable three-dimensional velocity structure of P waves in Ordos.

Our study area spans from 32°N to 42°N and 108°E to 114°E , which includes the Ordos block and its adjacent structures . The seismic data we used for inversion were recorded by 1244 stations including: 198 permanent stations and 1043 temporary stations (ChinArray II and III), from November 2013 to August 2017. After manual labeled the seismic phase, we select events with more than ten phase records of individual seismic events. The epicentral distance is less than 200km. Finally, we obtained about 22,500 phase records of 1882 local seismic events.

The preliminary results are consistent with previous studies and surface structures of a wide range of velocity distributions. However, in the middle-upper crust under the Liupan Mountain west, the low-speed anomaly extending downward is shown, which may be caused by the shallow crustal damage caused with the continuous eastward compression of asthenosphere in the northeastern margin of the Qinghai-Tibet Plateau during the Cenozoic. It is worth noting that there is an EW-trending low-velocity zone under the Dingbian-Suide fault beneath the Ordos Basin, with a depth form lower crust to 50 km in upper mantel. This low-velocity anomaly divides the high-speed disturbance in the Ordos block into two parts,indicate the depth of the fault can reach the upper mantel. In the Taihang Mountains in the west of the study area, low-velocity anomalies extending to the upper layer of the mantle are shown. We initially believe that this anomaly is related to the volcanic thermal motion that once existed on the area.

How to cite: Liu, Y. and Wu, J.: 3D Crustal P-Wave Velocity Structure for the Ordos block and Its Adjacent Area based on travel time tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21080, https://doi.org/10.5194/egusphere-egu2020-21080, 2020.

EGU2020-18510 | Displays | SM4.3

3D P-wave velocity model of Ireland's crust from controlled source tomography

Senad Subašić, Meysam Rezaeifar, Nicola Piana Agostinetti, Sergei Lebedev, and Christopher Bean

We present a 3D P-wave velocity model of the crust and uppermost mantle below Ireland. In the absence of local earthquakes, we used quarry and mining blasts recorded on permanent stations in the Irish National Seismic Network (INSN) and during various temporary deployments. We compiled a database of 1,100 events and around 20,000 P-wave arrivals, with each event associated with a known quarry. The source location uncertainty is therefore minimal. Both source and receiver locations are fixed in time and we used repeating events to estimate the travel time uncertainty for each source-receiver combination. We created a starting 1D velocity model from previously available data, and then used VELEST to calculate a preliminary minimum 1D velocity model. The 1D velocity model enabled us to remove outliers from the data set, and to calculate the final minimum 1D model used as the initial model in the 3D tomographic inversion. The resulting 3D P-wave velocity model will shed new light on the 3D crustal structure of Ireland.

How to cite: Subašić, S., Rezaeifar, M., Piana Agostinetti, N., Lebedev, S., and Bean, C.: 3D P-wave velocity model of Ireland's crust from controlled source tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18510, https://doi.org/10.5194/egusphere-egu2020-18510, 2020.

EGU2020-8538 | Displays | SM4.3

The structural architecture of the Whataroa Valley at the Alpine Fault (New Zealand) from first-arrival tomography and reflection imaging using an extended 3D VSP survey

Vera Lay, Stefan Buske, Sascha Barbara Bodenburg, Franz Kleine, John Townend, Richard Kellett, Martha Savage, Douglas Schmitt, Alexis Constantinou, Jennifer Eccles, Donald Lawton, Malcolm Bertram, Kevin Hall, Randolph Kofman, and Andrew Gorman

The Alpine Fault along the West Coast of the South Island (New Zealand) is a major plate boundary that is expected to rupture in the next 50 years, likely as a magnitude 8 earthquake. The Deep Fault Drilling Project (DFDP) aims to deliver insight into the geological structure of this fault zone and its evolution by drilling and sampling the Alpine Fault at depth.  

Here we present results from a 3D seismic survey around the DFDP-2 drill site in the Whataroa Valley where the drillhole penetrated almost down to the fault surface. Within the glacial valley, we collected 3D seismic data to constrain valley structures that were obscured in previous 2D seismic data. The new data consist of a 3D extended vertical seismic profiling (VSP) survey using three-component receivers and a fibre optic cable in the DFDP-2B borehole as well as a variety of receivers at the surface.

The data set enables us to derive a reliable 3D P-wave velocity model by first-arrival travel time tomography. We identify a 100-460 m thick sediment layer (average velocity 2200±400 m/s) above the basement (average velocity 4200±500 m/s). Particularly on the western valley side, a region of high velocities steeply rises to the surface and mimics the topography. We interpret this to be the infilled flank of the glacial valley that has been eroded into the basement. In general, the 3D structures implied by the velocity model on the upthrown (Pacific Plate) side of the Alpine Fault correlate well with the surface topography and borehole findings.

A reliable velocity model is not only valuable by itself but it is also required as input for prestack depth migration (PSDM). We performed PSDM with a part of the 3D data set to derive a structural image of the subsurface within the Whataroa Valley. The top of the basement identified in the P-wave velocity model coincides well with reflectors in the migrated images so that we can analyse the geometry of the basement in detail.

How to cite: Lay, V., Buske, S., Bodenburg, S. B., Kleine, F., Townend, J., Kellett, R., Savage, M., Schmitt, D., Constantinou, A., Eccles, J., Lawton, D., Bertram, M., Hall, K., Kofman, R., and Gorman, A.: The structural architecture of the Whataroa Valley at the Alpine Fault (New Zealand) from first-arrival tomography and reflection imaging using an extended 3D VSP survey, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8538, https://doi.org/10.5194/egusphere-egu2020-8538, 2020.

EGU2020-7916 | Displays | SM4.3

Shear-Wave Velocity Model of the Bohemian Massif Crust from Ambient Noise Tomography

Jiří Kvapil, Jaroslava Plomerová, Vladislav Babuška, Hana Kampfová Exnerová, Luděk Vecsey, AlpArray-EASI Working Group, and AlpArray Working Group

The current knowledge of the structure of the Bohemian Massif (BM) crust is mostly based on interpretation of refraction and reflection seismic experiments performed along 2D profiles. The recent development of ambient noise tomography, in combination with dense networks of permanent seismic stations and arrays of passive seismic experiments, provides unique opportunity to build the high-resolution 3D velocity model of the BM crust from long sequences of ambient seismic noise data.

The new 3D shear-wave velocity model is built from surface-wave group-velocity dispersion measurements derived from ambient seismic noise cross-correlations by conventional two-step inversion approach. First, the 2D fast marching travel time tomography is applied to regularise velocity dispersions. Second, the stochastic inversion is applied to compute 1D shear-wave velocity profiles beneath each location of the processing grid.

We processed continuous waveform data from 404 seismic stations (permanent and temporary stations of passive experiments BOHEMA I-IV, PASSEQ, EGER RIFT, ALPARRAY-EASI and ALPARRAY-AASN) in a broader region of the BM (in an area of 46-540 N 7-210 E). The overlapping period of each possible station-pair and cross-correlation quality review resulted in more than 21,000 dispersion curves, which further served as an input for surface-wave inversion at high-density grid with the cell size of 22 km.

We present the new high-resolution 3D shear-wave velocity model of the BM crust and uppermost mantle with preliminary tectonic interpretations. We compare this model with a compiled P-wave velocity model from the 2D seismic refraction and wide-angle reflection experiments and with the crustal thickness (Moho depth) extracted from P-wave receiver functions (see Kampfová Exnerová et al., EGU2020_SM4.3). 1D velocity profiles resulting from the stochastic inversions exhibit regional variations, which are characteristic for individual units of the BM. Velocities within the upper crust of the BM are ~0.2 km/s higher than those in its surroundings. The highest crustal velocities occur in its southern part (Moldanubian unit). The velocity model confirms, in accord with results from receiver functions and other seismic studies, a relatively thin crust in the Saxothuringian unit, whilst thickness of the Moldanubian crust is at least 36 km in its central and southern parts. The most distinct interface with a velocity inversion at the depth of about 20 to 25 km occurs in the Moldanubian unit. The velocity decrease in the lower crust reflects probably its transversely isotropic structure.

How to cite: Kvapil, J., Plomerová, J., Babuška, V., Kampfová Exnerová, H., Vecsey, L., Working Group, A.-E., and Working Group, A.: Shear-Wave Velocity Model of the Bohemian Massif Crust from Ambient Noise Tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7916, https://doi.org/10.5194/egusphere-egu2020-7916, 2020.

EGU2020-8404 | Displays | SM4.3

Imaging the Sicily Channel Rift Zone (Central Mediterranean) with seismic ambient noise tomography.

Matthew Agius, Fabio Cammarano, Fabrizio Magrini, Claudio Faccenna, Francesca Funiciello, and Mark van der Meijde

The tectonics of the Sicily Channel, located in the Central Mediterranean, are thought to be driven by the Calabrian back-arc system moving south-eastwards and the north moving African plate. The Channel is characterized by a seismically and volcanically active rift zone, which extends for more than 600 km in length offshore from the south of Sardinia to the south-east of Malta. Much of the observations we have today are either limited to the surface and the upper crust, or are broader and deeper from regional seismic tomography, missing important details about the lithospheric structure and dynamics. The project GEOMED () addresses this issue by processing all the seismic data available in the region in order to understand better the geodynamics of the Central Mediterranean.

We use seismic ambient noise recorded on more than 50 stations located on Algeria, Italy (Lampedusa, Linosa, Pantelleria, Sardinia (LISARD seismic network), Sicily), Libya, Malta, and Tunisia to generate high-resolution seismic tomography maps for the region at different depths. We measure Rayleigh-wave phase velocities with periods ranging from 5 to 100 seconds sampling through the entire lithosphere. We find that at short periods (<25 s), paths of station-pairs crossing across Africa and Italy have slower velocities than those crossing the Tyrrhenian and Ionian basins indicating that these paths are sampling thick continental crust. However, station pairs limited to the Sicily Channel Rift Zone (SCRZ) have faster phase velocities for periods > 20 s comparable to those beneath the basins suggesting that the SCRZ has a thinner crust. The seismic velocity maps are compared with the regional tectonics, seismicity, volcanic activity and other geophysical studies to present a more holistic understanding of the processes involved.

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 843696.

How to cite: Agius, M., Cammarano, F., Magrini, F., Faccenna, C., Funiciello, F., and van der Meijde, M.: Imaging the Sicily Channel Rift Zone (Central Mediterranean) with seismic ambient noise tomography., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8404, https://doi.org/10.5194/egusphere-egu2020-8404, 2020.

EGU2020-3506 | Displays | SM4.3

3D structure beneath Iranian plateau and Zagros using adjoint tomography

Abolfazl Komeazi, Farzam Yaminifard, Ayoub Kaviani, Georg Rümpker, Mohammad Tatar, Qinya Liu, and Kai Wang

We perform an adjoint waveform tomography using a combined data set consisting of regional earthquake waveforms and Rayleigh wave ambient-noise Green's function to construct a new 3-D wave velocity model of the crust and uppermost mantle beneath the Iranian plateau. The earthquake waveforms come from 250 regional events with magnitudes of 4.5-6.5 recorded by 136 broadband seismic stations. The EGFs are derived from cross correlations of more than three years of continuous seismic noise. The inversion starts with an initial model derived from the global Crust1 model locally modified using information from previous studies. Adjoint tomography refines the initial model by iteratively minimizing the frequency-dependent travel-time misfits between real and synthetic earthquake data and EGFs and synthetic Green’s functions measured in different period bands. Our new model covers the known tectonic units such as the Central Iranian Block, Zagros fold-and-thrust belt, Sanandaj-Sirjan metamorphic zone and Urumieh-Dokhtar magmatic arc. Overall, the adjoint tomography provides images with more real resolutions and amplitudes due to the finite-frequency consideration. Using the numerical spectral-element solver in adjoint tomography provides accurate structural sensitivity kernels, which help generate more robust images rather than those generated by ray-theory tomography. Our study also demonstrates improvement of lateral resolution and depth sensitivity using combined data set instead of only earthquake data.

How to cite: Komeazi, A., Yaminifard, F., Kaviani, A., Rümpker, G., Tatar, M., Liu, Q., and Wang, K.: 3D structure beneath Iranian plateau and Zagros using adjoint tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3506, https://doi.org/10.5194/egusphere-egu2020-3506, 2020.

EGU2020-979 | Displays | SM4.3

Seismic attenuation tomography of the North-Western Himalaya using Coda waves

Amit Bera, Himanshu Agrawal, Supriyo Mitra, and Shubham Sharma

We use 4695 local waveforms from 1206 earthquakes (epicentral distance < 350 km and 2.0 ≤ Mw ≤ 5.5) recorded by IISER Kolkata network (IK) at 22 stations (32°N to 35°N latitude and 74°E to 77°E longitude), located within the North-Western Himalaya (28°N to 39°N latitude and 68°E to 81°E longitude). We study the coda waves which are generally the tail of a seismogram and arrive after the main seismic waves. We use the temporal decay of coda amplitude to calculate the coda quality factor (Qc) from which we estimate the attenuation (Qc-1). We consider the single back-scattering model (Aki & Chouet, 1975) where both the scattering (Qsc-1) and intrinsic (Qi-1) component of the attenuation are included in the measurement. We use a lapse time of 2ts (ts is the S-wave arrival time) as the starting point of the coda window. Then, we consider multiple forward-scattering model, where the attenuation (Qc-1) is dominantly dependent on the intrinsic (Qi-1) component. In this model we use lapse time greater than 2ts so that the coda waves encounter multiple scatterers and enter the diffusive regime. We calculate the frequency dependent quality factor for each earthquake-receiver path at frequencies 1 to 14 Hz using the linear least squares approach on temporal decay of coda amplitude. We calculate Q0 (quality factor at a reference frequency f0 which is chosen to be 1 Hz for the analysis) and its frequency dependence (η) using weighted least squares approach on the power law dependence of Qc on frequency. To see the lateral variation of Q in our study area, we have produced 2-D maps by combining the Qc measurements together in a tomography. For single back-scattering model we use the back-projection algorithm which is based on the calculation of area overlap of ellipses with the gridded region. For multiple forward-scattering model, the same back-projection algorithm is modified to calculate the length overlap of traces with the gridded region. To understand the spatial resolution of the 2-D Qc maps, we use the point spreading function test which quantifies the recovery of Qc perturbation. In addition to this, we also perform a standard checkerboard resolution test to ensure simultaneous recovery of Qc perturbation. We observe low Q in the Kashmir basin and Lesser Himalaya and high Q in surrounding northeastern Higher Himalaya which clearly correspond to the coda wave attenuation signatures in the older Tethyan sedimentary rocks and crystalline igneous rocks in these regions respectively.

How to cite: Bera, A., Agrawal, H., Mitra, S., and Sharma, S.: Seismic attenuation tomography of the North-Western Himalaya using Coda waves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-979, https://doi.org/10.5194/egusphere-egu2020-979, 2020.

EGU2020-13371 | Displays | SM4.3

P-wave tomographic model from local bulletin data for improved seismic location in and around Israel

Lewis Schardong, Yochai Ben-Horin, Alon Ziv, Hillel Wust-Bloch, and Yael Radzyner

For the past 40 years, the Geophysical Institute of Israel has been in charge of the recording, monitoring and relocating of local earthquakes. Due to the variety of data analysts and data sources, as well as several network upgrades, the resulting bulletin data has to be completed and homogenised, and station metadata needs to be tracked down, and sometimes corrected. For those reasons, as well as because of the lack of consensus on an accurate model for seismic velocities in the area, published source locations are often poorly constrained. We present a homogenised Israeli bulletin, including natural and man-made explosion data. We extract sets of seismic sources with location accuracy greater than 5 km (GT5), as well as GT0 explosions.

We select a set of events with the highest network coverage, comprising (1) natural earthquakes, (2) man-made quarry or mine blasts, (3) GT5 earthquakes or explosions, and (4) GT0 explosions. We relocate them altogether using the BayesLoc package, a Bayesian, hierarchical, multi-event locator which produces, after source relocation, event-, station- and phase-specific correction terms. We put different a priori constraints on the different categories of seismic events, allowing poorly constrained origin parameters to improve thanks to the more accurate GT locations. BayesLoc also produces traveltime correction terms that can be used to correct systematic errors in the dataset, as well as error estimates.

Eventually, we invert this homogenised local traveltime dataset in order to invert for a P-wave crustal velocity model of Israel and its surroundings. To do so, we use the Fast Marching Tomography package, which allows the representation of a wide variety of input structures (starting model and geometry of layer boundaries) and can take many different types of input data. We show preliminary inversion tests and results that are in good agreement with past local studies.

This crustal model of Israel is ultimately to be used as a starting model in a larger tomographic study of the Eastern Mediterranean and Middle East region, where the Regional Seismic Travel Time approach is to be expanded, in order to improve the CTBT’s capabilities in monitoring the regional seismicity. Eventually, such a velocity model could also be used to relocate the whole earthquake catalogue more accurately, and improve the Earthquake Early Warning System currently in development in Israel.

How to cite: Schardong, L., Ben-Horin, Y., Ziv, A., Wust-Bloch, H., and Radzyner, Y.: P-wave tomographic model from local bulletin data for improved seismic location in and around Israel, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13371, https://doi.org/10.5194/egusphere-egu2020-13371, 2020.

EGU2020-9603 | Displays | SM4.3

Crustal structure of the Basque-Cantabrian Zone (N Spain) from seismic noise and receiver functions

Andrés Olivar-Castaño, Marco Pilz, David Pedreira, Javier A. Pulgar, Alba Díaz-González, and Juan Manuel González-Cortina

The Basque-Cantabrian Zone was one of the most subsident areas between the European plate and the Iberian sub-plate during the Mesozoic rifting process that gave birth to the Bay of Biscay. Since the latest Cretaceous and during the Cenozoic, a change to a contractional setting driven by the northward drift of the African Plate made this hyperextended rift basin to be inverted and incorporated into the Pyrenean-Cantabrian mountain belt. The resulting crustal structure shows a high complexity, as evidenced by the many existing geophysical observations pointing to the presence of intracrustal high-velocity bodies, deep transfer structures, and sharply-varying Moho depths across the area.

In this work, we use data provided by the dense SISCAN and MISTERIOS seismic networks, deployed in the region between 2014 and 2018, to obtain a detailed 3D shear-wave velocity model. We use the continuous recordings to compute seismic noise cross-correlation functions, from which we extract surface wave dispersion measurements. We use these measurements to obtain a set of phase velocity maps, and then perform a non-linear inversion at regularly spaced locations for the 1D shear-wave velocity structure. In the non-linear inversion, the forward modeling accounts for the presence of higher modes of surface waves, which have been shown to be more sensitive to velocity decreases with depth than the fundamental model. In order to better constrain the deeper layers of the model, we complement the seismic noise observations with an analysis of teleseismic receiver functions, which allows us to better constrain the depths of the major crustal discontinuities. Our results agree with previous geophysical studies, but significantly improve the availability of high-resolution information in the Basque-Cantabrian Zone.

How to cite: Olivar-Castaño, A., Pilz, M., Pedreira, D., Pulgar, J. A., Díaz-González, A., and González-Cortina, J. M.: Crustal structure of the Basque-Cantabrian Zone (N Spain) from seismic noise and receiver functions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9603, https://doi.org/10.5194/egusphere-egu2020-9603, 2020.

A receiver function analysis was carried out along two profiles located in north- and southwestern Norway. We selected and processed 801 teleseismic events registered by twelve seismic stations belonging to the 2002-2005 Geofon/Aarhus temporary network. The HK (depth vs Vp/Vs) stacking procedure and a Reversible jump Markov chain Monte Carlo (Rj-McMC) inversion were applied independently with the objective to reveal new crustal and crust-mantle transitional contrasts gaining a better understanding of the geology. In the southern profile, the most noticeable feature corresponds to a Moho offset of about ~5 km ca. 85 km to the east of the Norwegian coast: That feature was previously observed in several occasions and is also well-supported from this research. Furthermore, a very deep Moho discontinuity – at between 45 – 50 km depth - was found beneath the northern profile, approximately 70 km inland from the coast, and dipping about 30° to the northwest. Even when this deep structure was previously inferred through other methods, its presence was not certainly confirmed and so far, the origin of this feature is still disputed. We discuss two hypotheses, which are valid to explain the occurrence of the noticeable anomaly. First, a gradual and wide crust-mantle transition zone, which is also reflected in the velocity model or second, the presence of a paleo-slab of Fennoscandian basement subducted and deformed during the Caledonian Orogen (490-390 Ma).

How to cite: Brönner, M. and Pavez, C.: 2D velocity profiles along south- and northwestern Norway: An approach using receiver function analysis and Markov chains, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5537, https://doi.org/10.5194/egusphere-egu2020-5537, 2020.

Cratons are representative of the oldest cores of continental crusts. Study of cratons is important  as they preserve the pristine nature of continental crusts as well as they have economic significance as a major source of the world's mineral deposits. The crustal thickness, crustal composition, structure and physical properties of crust-mantle transition (the Moho) are the key parameters for understanding the formation and evolution of continental crust. The ratio of  seismic P-wave and S-wave velocity (Vp/Vs) is used as a parameter to understand the petrologic nature of the Earth's crust. Using these parameters, we address the crustal properties of all Archean cratons. The teleseismic P-wave receiver function analysis reveals that all the Eoarchean (4-3.6 Ga) cratons (Superior, North Atlantic Craton, North China Craton, Yilgarn, Zimbabwe, Kaapvaal) have crustal thickness ranges between 34-42 km and Vp/Vs ratio 1.68-1.79, the Paleoarchean (3.6-3.2 Ga) cratons (Baltic shield, Pilbara, Tanzania, Grunehogna) have 29-52 km crustal thickness and Vp/Vs ratio 1.7-1.85, the Mesoarchean (3.2-2.8 Ga) cratons (Sao Francisco, Guapore, Yangtze, Antananarivo) have 36-53 km thickness and Vp/Vs ratio 1.7-1.9, and Neoarchean (2.8-2.5 Ga) cratons (Guiana, Anabar, Gawler, Napier, Tarim) have 36-59 km thickness and Vp/Vs ratio 1.64-1.95. The nature of crust-mantle transition is overall sharp and flat.  We also found that the crusts which are stabilized earlier, are thinner compared to the later stabilized crusts. Our findings are well-correlated with the craton evolution process predicted by Durrheim and Mooney (1994), where older crusts are thin due to delamination process and relatively younger crusts are thick due to basaltic underplating. Our result of higher Vp/Vs ratio in the relatively younger crusts corroborates with the mafic nature of the crust whereas the older crusts are felsic-intermediate resulting lower Vp/Vs ratio. Our study is unique as it includes most of the global cratons and suggests a global model of continental crust formation and evolution process.

How to cite: Roy, P. and Borah, K.: Seismic P-wave receiver function modelling of Archean cratonic crust: A global perspective, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9666, https://doi.org/10.5194/egusphere-egu2020-9666, 2020.

EGU2020-7845 | Displays | SM4.3

Mapping the Moho in the Bohemian Massif with P-receiver functions

Hana Kampfová Exnerová, Jaroslava Plomerová, Jiří Kvapil, Vladislav Babuška, Luděk Vecsey, the AlpArray-EASI Working Group, and the AlpArray Working Group

We present a new detailed map of the Moho in the Bohemian Massif (BM) derived from P-to-S conversions calculated from broad-band waveforms of teleseismic events recorded at 325 temporary and permanent stations operating in a region framed in 10–19º E and 48–52º N during last two decades. We processed data collected from running AlpArray Seismic Network (2015 – 2019) (http://www.alparray.ethz.ch/) and its complementary experiment AlpArray-EASI (2014 – 2015), as well as from previous passive seismic experiments in the region – BOHEMA I-IV (2001 – 2014), PASSEQ (2006 – 2008) and EgerRift (2007 – 2013). The study aims at upgrading the current knowledge of structure of the BM crust and providing a homogeneous estimate of Moho depths, particularly for the use in deep Earth studies, e.g., the upper mantle tomography. Different velocity models, including the new one retrieved from the ambient-noise study (see Kvapil et al., EGU2020_SM4.3), are tested in the time-depth migration procedures. Regional variations of the Moho depth correlate with main tectonic units of the BM. The crust thickens significantly in the Moldanubian part of the BM and thins along the Eger Rift in the western part of the massif. Detailed variations of the Moho depth from the receiver functions along several profiles are compared with crustal sections retrieved from the ambient noise tomography.

How to cite: Kampfová Exnerová, H., Plomerová, J., Kvapil, J., Babuška, V., Vecsey, L., Working Group, T. A.-E., and Working Group, T. A.: Mapping the Moho in the Bohemian Massif with P-receiver functions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7845, https://doi.org/10.5194/egusphere-egu2020-7845, 2020.

EGU2020-13331 | Displays | SM4.3

Crustal structure of the Central Alborz, Iran from inter-event interferometry

Mahsa Afra, Taghi Shirzad, Jochen Braunmiller, Habib Rahimi, and Mojtaba Naghavi

Seismic interferometry can be used to turn earthquakes into virtual seismic sensors. Cross-correlation of seismic traces from two earthquakes recorded at the same actual sensor and summation of cross-correlations from many actual sensors result in the (strain) Green’s function between the two earthquakes. This simple concept provides an exciting new way for high-resolution imaging of subsurface structures in areas with poor instrument coverage. We test and apply this method to the Central Alborz region of Iran where we can compare results with regular local earthquake tomography. We first extracted the Rayleigh wave group velocities (U) from the Green’s functions and then inverted them for the upper crustal Rayleigh wave maps. We used vertical seismograms from 819 well-located Mw<4 earthquakes with vertical and horizontal location uncertainties of less than 2.5 km recorded between January 2006 and May 2019. The recordings are from seismic stations operated by the Iranian Seismological Center, the International Institute of Earthquake Engineering and Seismology, and the Tehran Disaster Management and Mitigation Organization. Pre-processing of each seismic trace consisted of removing the mean, detrending, instrument response correction, and 1-20 s band-pass filtering. We cross-correlated event-pairs at all available actual sensors and stacked the cross-correlations to obtain the inter-event empirical Green’s function (EGF); we used the phase weighted stacking procedure to enhance the signal-to-noise ratio of the EGF. We then applied the classical FTAN method to calculate the group velocities from the Rayleigh wave dispersion measurements. Cross-correlations, stacking and dispersion analysis were performed for all available event-pairs. Finally, we inverted the dispersion measurements to obtain group velocity maps using the Fast Marching Surface-Wave Tomography method. The resulting group velocity maps indicate a thick layer of low velocity material near the junction of the Mosha-North Tehran faults, a low velocity anomaly related to the Damavand Volcano, and abrupt transitions from high to low velocity anomalies associated with the Mosha fault.

How to cite: Afra, M., Shirzad, T., Braunmiller, J., Rahimi, H., and Naghavi, M.: Crustal structure of the Central Alborz, Iran from inter-event interferometry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13331, https://doi.org/10.5194/egusphere-egu2020-13331, 2020.

EGU2020-18907 | Displays | SM4.3

Lower crustal structures at Rockall Trough (west of Ireland) by Full waveform inversion

Gaurav Tomar, Christopher J. Bean, and Satish C. Singh

Rockall trough lies to the west of Ireland in NE Atlantic, it has a complex geology and has been debated for controversial geology for more than two decades. We have performed Full waveform inversion (FWI) on 2D seismic data set that is recorded in 2013-14 by using 10 km long streamer, this 2D seismic line is situated near the North-West margin in the Rockall Bank area. Full waveform inversion (FWI) is a powerful technique for obtaining elastic properties of the sub-surface from the seismic data. FWI provides properties of the sub-surface at the scale of the wavelength of the data set. We used travel time tomography on downward extrapolated data set to obtain a smooth starting velocity model for FWI. Downward continuation is a technique that enhances the first arrival and also reduces the computation time for forward modelling in FWI. The velocity model obtained from refraction travel time tomography, indicates the velocity from 1.6-4 km/s for the sediments and we have also observed very high velocity ~ 6-7.5 km/s just 3 km below sea-floor. We have performed FWI using these TTT velocity model as a starting model and inverted the refractions along with the wide angle reflections in the frequency range of 3-10 hz. FWI results gives the velocity of 6-7.2 km/s as well as defines geological structures that can be seen in the migrated seismic section. These high velocity structures could be a part of the continental crust and/or lower oceanic crustal igneous rocks like Gabbro.

How to cite: Tomar, G., J. Bean, C., and C. Singh, S.: Lower crustal structures at Rockall Trough (west of Ireland) by Full waveform inversion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18907, https://doi.org/10.5194/egusphere-egu2020-18907, 2020.

EGU2020-19836 | Displays | SM4.3

Full-waveform inversion at Santorini volcano

Kajetan Chrapkiewicz, Michele Paulatto, Joanna Morgan, Mike Warner, Benjamin Heath, Emilie Hooft, Brennah McVey, Douglas Toomey, Paraskevi Nomikou, and Constantinos Papazachos

Detailed knowledge about geometry and physical properties of magmatic systems at arc volcanoes promises to better constrain models of magma differentiation, transit and storage in the crust, and to help assess volcanic hazard.

Unfortunately, low-velocity zones associated with melt accumulation are particularly difficult to image by conventional travel-time tomography due to its limited resolving power, resulting in blurred boundaries and underestimated velocity contrasts.

Here we alleviate these issues by applying full-waveform inversion (FWI) to study a magmatic system of Santorini - an active, semi-submerged volcano with a known record of large, caldera-forming eruptions.

We use a 3D wide-angle, multi-azimuth seismic dataset from the recent PROTEUS experiment acquired with ca. 150 ocean-bottom/land seismic stations and ca. 14,000 air-gun shots. We implement a finite-difference immersed boundary method to simulate reflections off the caldera’s irregular topography, and pressure-velocity conversion to take full advantage of the multi-component data. We perform inversion with careful data-selection, increasing frequency up to 6 Hz, and extensive quality-control based on a phase spatial-continuity criterion.

A final P-wave velocity model of the upper crust offers a high-resolution image of Santorini magmatic and hydrothermal systems with pronounced low-velocity zones due to a high melt and water content respectively. The features are better resolved and the velocity contrasts distinctly sharper than in the starting model obtained with travel-time tomography. We also recover a previously undetected low velocity anomaly of >40% beneath Kolumbo - a submarine volcanic cone to the NE of Santorini caldera. We interpret this anomaly as a magmatic sill.

How to cite: Chrapkiewicz, K., Paulatto, M., Morgan, J., Warner, M., Heath, B., Hooft, E., McVey, B., Toomey, D., Nomikou, P., and Papazachos, C.: Full-waveform inversion at Santorini volcano, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19836, https://doi.org/10.5194/egusphere-egu2020-19836, 2020.

EGU2020-13470 | Displays | SM4.3

3-D seismic full-waveform inversion of the Taurus-Zagros region in Iran and Turkey

Neda Masouminia, Dirk-Philip van Herwaarden, Sölvi Thrastarson, Habib Rahimi, Lion Krischer, Michael Afanasiev, Christian Böhm, and Andreas Fichtner

We present an interpretation of a 3-D velocity model resulting from a regional analysis of earthquake waveforms. This model contains 3-D structure of the crust and upper mantle beneath the Arabian-Eurasian collision zone in eastern Turkey and Iran. We use full-waveform inversion (FWI) of three-component recordings from permanent networks. FWI can exploit all parts of a seismogram, including body and multi-mode surface waves in a broad range of frequencies. This allows us to constrain seismic structure of both the crust and the upper mantle.

In our method we simulate 3-D visco-elastic wavefields using a spectral-element method (Fichtner et al,2018). Our numerical mesh honors topography of the surface. We compare observed and synthetic waveforms using time-frequency phase misfits. Using adjoint techniques, we then compute sensitivity kernels with respect to the model parameters, which are VSV, VSH, VPV, and VPH. Finally, the kernels enable the iterative solution of the nonlinear inverse problem with the help of the L-BFGS algorithm and without a need for crustal corrections.

For this study we obtained seismic waveform data of 59 earthquakes within the magnitude range of Mw 4.5 to 6.3 that occurred in the region between 2012 and 2016. These events were recorded by 398 broadband seismic stations belonging to the two national Iranian networks and freely available seismic stations of the Turkish Network, made available by IRIS.

Starting from the first generation of the Collaborative Seismic Earth Model (Afanasiev et al.2019), we first constrained longer-wavelength structure. To this end, we considered 3-component recordings from a subset of 37 events in the period range from 50 to 80 s. This band was successively broadened by reducing the shorter period from 50 s to 40 s, and finally to 20 s. For each period band, the number and the length of measurement windows are increased; the number of events is also increased to 59 to use the complete dataset. After 46 iterations our model can explain recordings of events, which were not used in the inversion. The results provide to discuss about high-velocity anomaly beneath the Zagros and the shallow low velocities beneath Central Iran using cross-sections to investigate lateral variation of seismic velocity in the lithosphere.

REFERENCES

Afanasiev, M., Boehm, C., van Driel, M., Krischer, L., Rietmann, M., May, D. A., Knepley, M. G., Fichtner, A., 2019. Modular and flexible spectral-element waveform modelling in two and three dimensions. Geophysical Journal International 216, 1675-1692, doi: 10.1093/gji/ggy469.

Fichtner, A., van Herwaarden, D.-P., Afanasiev, M., Simute, S., Krischer, L., Cubuk-Sabuncu, Y., Taymaz, T., Colli, L., Saygin, E., Villasenor, A., Trampert, J., Cupillard, P., Bunge, H.-P., Igel, H., 2018. The Collaborative Seismic Earth Model: Generation I. Geophysical Research Letters 45, doi: 10.1029/2018GL077338.

How to cite: Masouminia, N., van Herwaarden, D.-P., Thrastarson, S., Rahimi, H., Krischer, L., Afanasiev, M., Böhm, C., and Fichtner, A.: 3-D seismic full-waveform inversion of the Taurus-Zagros region in Iran and Turkey, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13470, https://doi.org/10.5194/egusphere-egu2020-13470, 2020.

EGU2020-18042 | Displays | SM4.3

Prominent crustal discontinuities in Reykjanes Peninsula, Iceland

Pavla Hrubcová, Jana Doubravová, and Josef Horálek

Iceland, situated in the North Atlantic Ocean between Greenland and Norway, is located above the Mid-Atlantic Ridge. It is a part of the oceanic crust forming the floor of the Atlantic Ocean. Its tectonic structure is characterized by various seismically and volcanically active centers. We focused on active seismicity in the SW part, where the Reykjanes Ridge segment of the Mid-Atlantic Ridge is located in Reykjanes Peninsula as the landward ridge continuation connecting it to the Western Volcanic Zone. The seismicity in this area is monitored by REYKJANET seismic network stations operated by IGF CAS. The earthquakes are released in form of swarms and are largely confined to the upper few kilometers of the oceanic layer related to a large number of faults and fissures with the high seismic activity at depths of 2 to 6 km, however, some events may be as deep as 13 km.

Since knowledge of a detailed crustal structure is essential for all advanced studies of seismicity and focal parameters of the earthquakes, we concentrated on velocity model and prominent discontinuity depth retrieval in the area. We selected the best located events of the 2017 swarm in Reykjanes Peninsula and refined their locations by manual picking. This resulted in processing of waveforms from earthquakes with magnitudes >1 recorded at 15 REYKJANET seismic network stations. The waveforms typically displayed dominant direct P and S waves followed by converted and reflected waves secondarily generated at shallow and deeper subsurface structure. We tested a multi-azimuthal approach in data processing of Hrubcová et al. (2013; 2016) to increase resolution of these phases in the waveforms. We applied the waveform cross-correlation of the P and S waves, and rotated, aligned and stacked the seismograms to extract the reflected/converted phases. In the interpretation, we focused on the most prominent interface at the crust/mantle boundary, the Moho, and processed its reflected phases. These phases were inverted for laterally varying Moho depth by ray tracing and a grid search inversion algorithm and verified by modeling of full waveforms computed by the discrete wave number method.

How to cite: Hrubcová, P., Doubravová, J., and Horálek, J.: Prominent crustal discontinuities in Reykjanes Peninsula, Iceland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18042, https://doi.org/10.5194/egusphere-egu2020-18042, 2020.

EGU2020-12225 | Displays | SM4.3

Three dimensional crustal Vp structure beneath the Pearl River Estuary from joint onshore-offshore seismic experiment

Liwei Wang, Xiuwei Ye, Xiaona Wang, Zuoyong Lv, Yunpeng Zhang, and Baoshan Wang

The Pear River Estuary (PRE) area is located in the northern margin of South China Sea (SCS), which is a typical rifted passive continental margin between the South China Block and SCS Basin. The Littoral Fault Zone (LFZ) crossing the PRE is an important seismogenic and boundary fault. Strong earthquakes occurred in both the west and east segments. While, the middle segment of the LFZ in PRE area is lack of large earthquake and possibly a seismic gap. Imaging the fine structure of the PRE area is helpful to understand the background of the spatial heterogeneity of seismicity. To explore the crustal structure of the PRE area, we carried an active-source experiment in July 2015. During the experiment, an airgun array composed of four individual airguns with total volume of 6000 in3 mounted on SCSIO’s R/V Shiyan II was used as the seismic source. A total of 12200 shots were fired every 300 meters along 10 NW and 3 NE-trending shooting profiles. Six dynamite sources with a charge range from 1000 to 2500 kg were also shot on the land. During the experiment, 431 receivers including 29 ocean bottom seismographs (OBS), 256 short period seismometers, and 146 broadband seismometers were available. We manually picked Pg arrivals from the airgun and dynamite sources. We calculated a minimum 1D velocity model by VELEST. We then obtained the upper crust Vp structure using three-dimensional seismic tomography. Our preliminary result reveals that the Vp is consistent with local geological settings. There are low velocity anomaly beneath the LFZ and obvious velocity anomalies across the NW- and NE-trending active faults, which maybe potential threat to the Greater Bay Area.

How to cite: Wang, L., Ye, X., Wang, X., Lv, Z., Zhang, Y., and Wang, B.: Three dimensional crustal Vp structure beneath the Pearl River Estuary from joint onshore-offshore seismic experiment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12225, https://doi.org/10.5194/egusphere-egu2020-12225, 2020.

Fitting subduction zone guided waves with synthetics is an ideal choice for studying the velocity structure of the oceanic crust. After an earthquake occurs in subduction zones, seismic waves can be trapped in the low-velocity oceanic crust and propagated as guided waves. The arrival time and frequency characteristics of the guided waves can be used to image the velocity structure of the oceanic crust. The analysis and modeling based on guided wave observations provide a rare opportunity to understand the velocity structure of the oceanic crust and the variations in oceanic crustal materials during the subduction process.

High-frequency guided waves have been observed in the subduction zone of eastern Alaska. On several sections, observed seismograms recorded by seismic stations show low-frequency (<2Hz) onsets ahead of the main high-frequency (>2Hz) guided waves. Differences in the arrival times and dispersion characteristics of seismic phases are related to the velocity structure of the oceanic crust, and the characteristics of coda waves are related to the distribution of elongated scatters in the oceanic crust. Through fitting the observed broadband waveforms and synthetics modeled with the 2-D FDM (Finite Difference Method), we obtain the preferred oceanic crustal velocity models for several sections in the subduction zone of eastern Alaska. The preferred models can explain the seismic phase arrival times, dispersions, and coda characteristics in the observed waveforms. With the obtained P- and S- wave models of velocity structures on several sections, the material compositions they represent are deduced, and the variations of oceanic crustal materials during subducting can be understood. This provides new evidence for studying the details of the subduction process in the subduction zone of eastern Alaska.

How to cite: Guan, X., Zhou, Y., and Furumura, T.: Modeling guided waves to constrain the velocity structure of the oceanic crust in the subduction zone of eastern Alaska, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3268, https://doi.org/10.5194/egusphere-egu2020-3268, 2020.

EGU2020-7976 | Displays | SM4.3

Rayleigh wave ellipticity measurements in the North Tanzanian Divergence (Eastern African Rift)

Laura Parisi, Andrea Berbellini, and P. Martin Mai

Rayleigh wave ellipticity depends, in theory, only on the Earth structure below a seismic station, offering the advantage of a “single-station” method to infer crustal properties. Therefore, ellipticity measurements can be used to construct pseudo 3-D shear velocity models of the earth structure using even seismic stations that did not record simultaneously.  

Based on that, we carried-out ellipticity measurements by using teleseismic waveforms recorded by the OPS seismic network we deployed at the western flank of the North Tanzanian Divergence between June 2016 and May 2018, covering 17 sites. We then expanded our measurements on the waveforms recorded by the adjacent CRAFTI seismic network from January 2013 and December 2014, available on IRIS, which comprised more than 30 sites. 

While the OPS network covers the transition between the Tanzania Craton and North Tanzanian Divergence, the CRAFTI network is entirely contained in the North Tanzanian Divergence. Therefore, the imaging that can be obtained by integrating the two asynchronous passive seismology experiments will help to better understand the dynamics of this segment of the eastern branch of the Eastern African Rift.

Preliminary results show heterogeneity structure that are in agreement with previous tomographic studies based on ambient noise cross-correlation and body-waves arrival-times. In regions where previous seismological studies are not available, results match the known geological structure of the transition between the Tanzanian Craton and the North Tanzanian Divergence. This demonstrates that measurements of ellipticity can be a useful and integrative tool for earth structure imaging, especially at the edges of the active rifts where the seismicity is scarce.




How to cite: Parisi, L., Berbellini, A., and Mai, P. M.: Rayleigh wave ellipticity measurements in the North Tanzanian Divergence (Eastern African Rift), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7976, https://doi.org/10.5194/egusphere-egu2020-7976, 2020.

EGU2020-12007 | Displays | SM4.3

Seismic Structure and Tectonic Evolution of Borneo and Sulawesi

Harry Telajan Linang, Amy Gilligan, Jennifer Jenkins, Tim Greenfield, and Nick Rawlinson

Southeast Asia (SEA) is one of the most active tectonic regions on the planet due to its convergent setting, which accommodates rapid northward motion of the Indo-Australian plate and westward motion of the Philippines Sea plate. This activity gives rise to intense seismicity along the convergent plate margins in and around SEA, including the Sunda Arc, which wraps its way around the southern margin of the Indonesian archipelago.

Borneo is located at the centre of SEA, on the leading edge of the Sundaland block of the Eurasian plate, and exhibits lower rates of seismicity when compared to the surrounding regions due to its intraplate setting. Sulawesi, an island which lies adjacent to Borneo in the east, is characterised by intense seismicity due to multiple subduction zones in its vicinity. The tectonic relationship between the two islands is poorly understood, as is the provenance of their lithosphere, which may have Eurasian or East Gondwana origin.

The aim of this presentation is to showcase recent receiver function results from temporary and permanent broadband seismic stations in the region, which can be used to help improve our understanding of the structure of the crust and the mantle lithosphere beneath Borneo and Sulawesi. H-K stacking, receiver function migration and inversion are all applied in an effort to determine robust crustal thickness estimates and variations in shear wavespeed with depth. Our preliminary results from Borneo indicate that the crust beneath Sabah (northern Borneo), which is a post-subduction setting, appears to be much more complex than the rest of the island. Furthermore, we find that crustal thickness varies between different tectonic blocks defined from surface mapping, with the thinnest crust (24 km thick) occurring beneath Sarawak in the northwest.

How to cite: Linang, H. T., Gilligan, A., Jenkins, J., Greenfield, T., and Rawlinson, N.: Seismic Structure and Tectonic Evolution of Borneo and Sulawesi, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12007, https://doi.org/10.5194/egusphere-egu2020-12007, 2020.

EGU2020-694 | Displays | SM4.3

Towards 3D multiscale adjoint waveform tomography of the crust and upper mantle beneath Southeast Asia

Deborah Wehner, Nienke Blom, and Nick Rawlinson

Southeast Asia is one of the most complex tectonic regions on Earth. This is mainly a result of its location within the triple junction of the Australian, Eurasian and Philippine Sea plates which has created a complicated configuration of active plate tectonic boundaries. High plate velocities have generated thousands of kilometers of subducted material and ongoing subduction along the Sunda Arc represents a significant natural hazard (such as the 2004 Sumatra-Andaman earthquake, 2012 Indian Ocean earthquakes and 2018 Anak Krakatoa eruption). However, recent tectonic activity around Borneo may be related to postsubduction processes which could be the key to understanding how the tectonic subduction cycle terminates. Further east, the region is dominated by several minor tectonic plates and the spectacular 180-degree curvature of the Banda Arc. Our work aims to further improve the understanding of this area by providing detailed images of the upper mantle.

Adjoint waveform tomography is especially suitable for imaging such complex regions. By simulating the 3D wavefield, it is possible to directly compare observed and simulated seismograms, thereby taking into account both body and surface waves. The method can account for the effects of anisotropy, anelasticity, wavefront healing, interference and (de)focusing that can hamper other seismological methods, and is thus especially suitable for strongly heterogenous areas such as Southeast Asia.

To date, sparse instrument coverage in the region has contributed to a heterogeneous path coverage. In this project, we make use of publicly available data as well as our recently deployed networks of broadband seismometers on Borneo and Sulawesi. This, in addition to access to national permanent networks promises a significant improvement in data coverage around the Banda Arc, Borneo and Sulawesi, thereby providing new opportunities to untangle the region’s complexity.

We compiled a catalogue of well-constrained earthquakes, optimising for coverage, signal-to-noise ratio and data availability across a wide frequency band, and compared our observed data to synthetics generated from an initial model. In the first part of the inversion, we use long periods of 100 - 150 s to update our initial model using a gradient-based optimisation scheme. We use adjoint methods to obtain sensitivity kernels as the corresponding gradients and initial results will be documented in this presentation. In subsequent iterations, we permit increasingly shorter periods in order to progressively recover finer scales structure and avoid cycle skipping issues.

How to cite: Wehner, D., Blom, N., and Rawlinson, N.: Towards 3D multiscale adjoint waveform tomography of the crust and upper mantle beneath Southeast Asia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-694, https://doi.org/10.5194/egusphere-egu2020-694, 2020.

    The Middle-Lower Yangtze River Metallogenic Belt, known as the eastern “industrial corridor”, is an important polymetallic mineral resource base in eastern China. It has a significant influence on economic development in eastern China and its deep dynamic process has been the focus of debate for deposit scientists.

    Why does such a narrow zone contain so rich mineral resources? What is the deep dynamics? Many scholars put forward different explanations, such as “ continental extension mode ”, “ subduction mode ” and “ reverse L-shaped collision mode ”. Although these views reach a consensus in some aspects, they are different essentially. Thus, the deep structure information has become the key to distinguish such different views.

    In this study, we conduct teleseismic P wave receiver function, ambient noise tomography and teleseismic two-plane-wave tomography to probe the crustal and uppermost mantle structures in the Middle-Lower Yangtze River region. The data include (1) continuous seismic data from June 2012 to July 2013 recorded from Chinese provincial networks; (2) seismic event data from 2010 to 2011 recorded from Chinese provincial networks; (3) continuous seismic data from November 2009 to August 2010 deployed by Chinese Academy of Geological Sciences; (4) continuous seismic data from June 2012 to July 2013, from August 2014 to June 2015 and from July 2015 to November 2015 deployed by China University of Geosciences(Beijing). First, we apply receiver function to get the Moho depth below 191 seismic stations and the results are in good accordance with previous researches. We then conduct joint inversion of receiver function and surface wave dispersion to generate shear wave velocity structures below 191 seismic stations. The result shows that in the upper crust, the basin regions, including the JiangHan, HeHuai, SuBei, HeFei and NanYang basin, are all featured with low velocities, and the mountain regions with high velocities. In the northeastern of the Middle-Lower Yangtze River Metallogenic Belt, the result shows ore districts are clearly characterized with the strongest low velocity anomaly in the uppermost mantle at ~70km depth. The depth extent of the low-velocity zone becomes shallower and the amplitude of low velocity anomaly becomes larger from the northeast to southwest.

How to cite: Li, J.: Crustal and uppermost mantle velocity structure beneath the Mid-lower Yangtze metallogenic belt and its adjacent regions from joint inversion of receiver function and surface wave, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2007, https://doi.org/10.5194/egusphere-egu2020-2007, 2020.

EGU2020-13043 | Displays | SM4.3

Crustal and upper mantle Structure Beneath the Ordos Block by Multi-scale seismic tomography

Biao Guo, Jiuhui Chen, Xiaoshu Li, and Shuncheng Li

The Ordos Block located in the center of China mainland, which is one of the oldest and most stable cratons in Asia. It is contiguous to the Yinshan Block, the North China Craton, Alex Block, Yangze Block, and Northeast Tibet. Numerous geologic and geophysical studies engaged in the mechanics of the Ordos Block deformation and evolution, but the detail structure and deformation style of the Ordos Block remains uncertain due to poor geophysical data coverage. During 2013 and 2018, China Earthquake Administration developed XMLY Seismic Array in Ordos Block and adjacent area, which operated more than 1000 broadband seismic stations with an average station spacing of 35km. Using the P-wave Travel time data recorded by the array and multi-scale seismic traveltime tomography technique, we obtained a high-resolution P-wave velocity structure beneath Ordos Block. The seismic tomography algorithm employs sparsity constrains on the wavelet representation velocity model via the L1-norm regularization. This algorithm can efficiently deal with the uneven-sampled volume, and give multi-scale images of the model. Our preliminary results can be summarized as follows: 1, the crustal and upper mantle P-wave velocity structure is strongly inhomogeneous and consistent with the surface geological setting; 2, significant low-velocity anomalies exist beneath the northwestern margin of Ordos Block, which suggested that there exist upper mantle upwelling; 3, There have obvious boundary between Alex and Ordos Block along 104ºE at upper mantle;  4, Along 38ºN tectonic line, there exist different structure between south part and north part of Ordos upper mantle, the south part of Ordos show high-velocity feature and the upper mantle show low-velocity anomalies in north part of Ordos Block. This feature can be interpreted that the two parts of the Ordos Block undergone different Tectonic evolution processes.

How to cite: Guo, B., Chen, J., Li, X., and Li, S.: Crustal and upper mantle Structure Beneath the Ordos Block by Multi-scale seismic tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13043, https://doi.org/10.5194/egusphere-egu2020-13043, 2020.

EGU2020-21108 | Displays | SM4.3

Crust and upper mantle structure of North Borneo explored using surface waves from ambient noise

Omry Volk, Conor Bacon, Felix Tongkul, and Nick Rawlinson

South-East Asia is one of the most tectonically complex regions on Earth. North Borneo in particular is home to a number of intriguing features, the formation of which is not fully understood. These include the North-West Borneo trough, the rapidly uplifted 4000m high Mt Kinabalu and the uplifted circular sedimentary basins such as the Maliau Basin. To study North Borneo's tectonics in depth we deployed a new dense temporary network of 46 broadband seismometers across the regionin a semi-regular grid pattern with approximately 40km spacing. This closely spaced network, which operated for 22 months, allows a high-resolution seismic analysis of the crust and mantle under North Borneo. Here, we use ambient noise cross-correlations to measure phase velocities of surface waves. We then use the phase velocities to analyze the crustal and upper mantle structure.

How to cite: Volk, O., Bacon, C., Tongkul, F., and Rawlinson, N.: Crust and upper mantle structure of North Borneo explored using surface waves from ambient noise, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21108, https://doi.org/10.5194/egusphere-egu2020-21108, 2020.

EGU2020-4148 | Displays | SM4.3

Preliminary results from teleseismic tomography of the upper mantle beneath northern Borneo
not presented

Simone Pilia, Nick Rawlinson, Felix Tongkul, Amy Gilligan, and Dave Cornwell

We present preliminary P-wave tomographic images of the upper mantle beneath northern Borneo (Sabah) using teleseismic earthquake data. Sabah underwent diachronous double-polarity subduction, one dipping to the southeast (terminated in the early Miocene) and the other to the northwest (terminated 5-6 Ma). With the goal of better understanding post-subduction processes in Sabah, 24 permanent seismic stations of MetMalaysia were augmented by the deployment of 46 temporary stations of the nBOSS network, which ran from March 2018 to January 2020. Relative P-wave traveltime residuals from nearly a thousand teleseismic events have been extracted from the continuous records using an adaptive stacking technique, which uses the coherency of global phases across the entire network. Using a grid-based eikonal solver and a subspace inversion technique implemented in FMTOMO, relative arrival time residuals are mapped as 3-D P-wave perturbations.

The most intriguing feature of the final tomographic model is a north-east trending lithospheric structure running across northern Borneo and separating relatively low to high wavespeeds to the west and east, respectively. This structure possibly indicates the suture between pre-Cenozoic lithosphere to the east and the Cenozoic accreted material to the west.

Results from receiver function analysis (i.e., crustal thickness) and crustal velocities from ambient noise tomography will be in the future incorporated in the tomographic inversion in order to obtain an integrated view of the crust-mantle system beneath Sabah.

How to cite: Pilia, S., Rawlinson, N., Tongkul, F., Gilligan, A., and Cornwell, D.: Preliminary results from teleseismic tomography of the upper mantle beneath northern Borneo, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4148, https://doi.org/10.5194/egusphere-egu2020-4148, 2020.

EGU2020-12344 | Displays | SM4.3

Structure of the crust and upper mantle beneath the Bransfield Strait (Antarctica) using P-wave Receiver Functions

Joan Antoni Parera-Portell, Flor de Lis Mancilla, José Morales, and Javier Almendros

The Bransfield Strait is a tectonically active region located between the South Shetland archipelago (SSI) and the Antarctic Peninsula (AP), characterised by the presence of an incipient back-arc spreading ridge driven by on-going slab rollback of the Phoenix plate under the Antarctic and Shetland plates. Twelve broad-band seismic stations deployed in the region are used to obtain P-wave receiver functions from teleseismic earthquakes to improve the current understanding of the crust and upper mantle structures. This includes the depth and spatial variability of the Moho discontinuity, the average crustal Vp/Vs ratio and the thickness of the Mantle Transition Zone (MTZ). Results reveal a highly variable crustal thickness in the South Shetland block, ranging from ~30 km near the SW and NE ends of the South Shetland Trench to ~15 km in the central Bransfield Basin (Deception Island), where the highest Vp ⁄ Vs ratios in the region are reached (> 2). In contrast, the AP displays typical and homogeneous continental crust characteristics with an average crustal thickness of ~34 km and Vp/Vs ~1.77. A low velocity zone (LVZ) is identified under all stations suggesting partial melting in the upper mantle beneath the lithosphere, which is widespread throughout the region and not only confined to the mantle wedge above the subducted Phoenix oceanic slab. There is evidence of magmatic underplating under the SSB in accordance with the LVZ together with the active volcanism and the high crustal Vp/Vs ratio in the area. The Phoenix oceanic slab is inferred to subduct steeply, as the MTZ appears already thickened under the AP.

How to cite: Parera-Portell, J. A., Mancilla, F. D. L., Morales, J., and Almendros, J.: Structure of the crust and upper mantle beneath the Bransfield Strait (Antarctica) using P-wave Receiver Functions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12344, https://doi.org/10.5194/egusphere-egu2020-12344, 2020.

EGU2020-1102 | Displays | SM4.3

Diversity in the Indian lithosphere revealed from ambient noise and earthquake tomography

Gokul Kumar Saha and Shyam S. Rai

We present evidence of significant diversity in the Indian cratonic lithosphere mantle based on the analysis of 3-D shear wave velocity maps. These images are obtained through the inversion of 21600 fundamental mode Rayleigh wave group velocity dispersion data retrieved from ambient noise and from earthquake waveforms. The velocity model is constructed using two step approach-firstly generating group velocity maps at 1° square grid at time periods from 10s to 100s; and subsequently inversion of dispersion data at each grid node to a depth of 200 km in terms of velocity-depth model. Analysis of velocity images suggest a bipolar characteristics of lithospheric mantle. We observe a two layer-lithospheric mantle correlated with the Eastern Peninsular India comprising of Archean cratons like east Dharwar, Bastar, Singhbhum, Chotanagpur, Bundelkhand and Proterozoic Vindhyan Basin. The intra lithospheric mantle boundary is at a depth of ~90 km where Vs increases from 4.5 km/s to over 4.7 km/s. The positive velocity gradient continues to a depth of 140-180 km beyond which it reverses the trend and mapped as layer with lower velocity Vs of 4.3-4.4 km/s, as which could be possibly defined as the lithosphere-asthenosphere boundary. Geologically, the region correlates with the kimberlite fields with the xenoliths showing presence of eclogite in them. The other group of Precambrian terrains like 3.36 Ga western Dharwar, eastern Deccan Volcanics, southern Granulite terrane and the Marwar block in western India are characterized by an almost uniform mantle with shear wave velocity of 4.4-4.5 km/s, also supported by other seismological studies. We do not observe any low-velocity layer underlying these terrains. Presence of such a uniform lower than expected mantle velocity could be due to its fertilization through an early geodynamic process. The velocity imprint of Deccan volcanism is best preserved in term of the thinned lithosphere (100-120 km) restricted to the westernmost part of Deccan Volcanic Province (DVP). This suggests that the plume-Indian lithosphere interaction was primarily confined to the western most Deccan volcanic province and possibly extending into the Indian ocean.

How to cite: Saha, G. K. and Rai, S. S.: Diversity in the Indian lithosphere revealed from ambient noise and earthquake tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1102, https://doi.org/10.5194/egusphere-egu2020-1102, 2020.

EGU2020-3843 | Displays | SM4.3

Detection of S-wave reflectors beneath aftershock area of the 2016 Kaikoura earthquake

Satoshi Matsumoto, Yuta Kawamura, Tomomi Okada, Miu Matsuno, Yoshihisa Iio, Rick Sibson, Martha Savage, Kenny Graham, Manami Suzuki, and Stephen Bannister

S wave reflectors in the crust may be caused by strong heterogeneous structures such as ones containing  fluid. Especially, fluid around an earthquake fault could play an important role for initiation of the earthquake rupture as a mechanism for reducing fault strength. The location and geometry of the reflector can be determined from the travel time of the reflected phases. For detecting the reflections, we need to observe at stations located close to the hypocentre because of sufficient phase separation of the small lapse time of the reflected phases due to a reflector in the crust from direct S wave. In this study, we attempted to detect reflected waves in observed seismograms at the seismic stations in and around the 2016 Kaikoura earthquake (Mw7.6). Seismic records were obtained from the permanent GeoNet stations as well as from seismic stations deployed before the Kaikoura earthquake in the northern South Island. We applied reflection seismology techniques to the data obtained by the network. We used seismograms with smaller epicentral distance than 30 km and obtained dip move-out sections for each station. We detected several reflectors in the mid and lower crust from the sections. Strong reflected phases were observed at the southern edge of the focal area (from a reflector with depth about 20 km). Weak reflectors were detected in/beneath the aftershock area (in the mid- to lower-crust). In addition, the subducting slab might be imaged with dipping angle 20 degree.  Reflectors parallel to the slab were also found below the interface.

How to cite: Matsumoto, S., Kawamura, Y., Okada, T., Matsuno, M., Iio, Y., Sibson, R., Savage, M., Graham, K., Suzuki, M., and Bannister, S.: Detection of S-wave reflectors beneath aftershock area of the 2016 Kaikoura earthquake, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3843, https://doi.org/10.5194/egusphere-egu2020-3843, 2020.

EGU2020-4299 | Displays | SM4.3

P- and S-wave Velocity Structure beneath Central and East Java, Indonesia: Preliminary Result

Faiz Muttaqy, Andri Dian Nugraha, Nanang T Puspito, James J Mori, Daryono Daryono, Supriyanto Rohadi, and Pepen Supendi

The Central and East Java region is part of the Sunda Arc which has an important role in producing destructive earthquakes and volcanic complexes as a result of the subduction of the Indo-Australian plate under the Eurasian plate. Seismic tomography is one geophysical tool that is adaptable to understanding the mechanism process related to tectonic activity, seismicity, and volcanism. We collected a series of waveforms from 1,519 events in the period January 2009 to September 2017 and re-picked 11,192 phases for P- and S-waves at 34 stations of the BMKG network. We determined the 3-D P- and S-wave velocity structure beneath this high-risk region down to a depth of 200 km. In this study, we compare the tomographic images and relocated seismicity in order to represent the subducted slab geometry and the features in the seismic zones, i.e. the 2006 Yogyakarta earthquake zone (Opak fault), south of the mainland, and the 1994 Banyuwangi earthquake zone. Low-velocity anomalies beneath the volcanoes, i.e. Merapi, Merbabu, Kelud, Semeru, Bromo, and Ijen also imply the existence of fluid material and possible partial melting of the upper mantle which migrated from the subducted slab.

How to cite: Muttaqy, F., Nugraha, A. D., Puspito, N. T., Mori, J. J., Daryono, D., Rohadi, S., and Supendi, P.: P- and S-wave Velocity Structure beneath Central and East Java, Indonesia: Preliminary Result, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4299, https://doi.org/10.5194/egusphere-egu2020-4299, 2020.

EGU2020-5790 | Displays | SM4.3

Seismic scattering and absorption of oceanic lithospheric S waves in the Eastern North Atlantic

Katrin Hannemann, Tom Eulenfeld, Frank Krüger, and Torsten Dahm

Seismic scattering and absorption parameters provide valuable information about the propagation of seismic waves within the lithosphere. For the analysis of the seismic scattering and absorption parameters in the oceanic lithosphere of the Eastern North Atlantic, we use a seismological array which was installed in 5000 m water depth about 100 km North of the Gloria fault, defining the plate boundary between the Eurasian and African plate at this location. During our seismological experiment, more than 350 local and regional earthquakes were identified within 10 month for epicentral distances up to 900 km. The acquired regional earthquake recordings show up to 30 Hz P and S wave arrivals with long codas lasting tens to hundreds of seconds. Modelling results suggest that these long codas originate from scattering in the oceanic lithosphere. The waves travel with upper mantle apparent velocities and are therefore referred to as oceanic Pn (Po) and Sn (So) waves. We use direct So waves and their coda of pre-selected earthquakes to estimate frequency-dependent seismic scattering and intrinsic attenuation parameters. The results for the analysed events show that intrinsic attenuation is stronger than scattering attenuation and the estimated transport mean free path lengths between 30-800 km indicate that the So wave coda is weakly influenced by the oceanic crust. Furthermore, the calculated parameters show higher attenuation for western to northern event azimuths than for eastern to southern ones which might be related to differences in lithospheric ages for example of the Eurasian and African plates, or the influence of the Gloria fault itself.

How to cite: Hannemann, K., Eulenfeld, T., Krüger, F., and Dahm, T.: Seismic scattering and absorption of oceanic lithospheric S waves in the Eastern North Atlantic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5790, https://doi.org/10.5194/egusphere-egu2020-5790, 2020.

Lithosphere-asthenosphere boundary (LAB) has generally been estimated using surface tomography (Priestley and McKenzie, 2006; Auer et al., 2014), surface waves anisotropy (Plomerova et al, 2002; Burgos et al., 2014) and receiver function methods (Kumar and Kawakatsu, 2011; Rychert et al., 2012), indicating a change in the S-wave velocity. Recently, some limited studies have shown that LAB could be imaged using seismic reflection imaging method (Stern et al., 2015; Mehouachi and Singh, 2018), requiring a sharp P-wave velocity contrast. Here, we present a continuous seismic reflection image of the LAB from 2-to-75-Ma in the Equatorial Atlantic Ocean over the African plate along a 1400 km long profile using a 12 km long offset multi-channel multi-component (hydrophone and three component acceleration) seismic data. Optimal de-ghosting by summation of pressure (P) and vertical acceleration (Az) components has improved the bandwidth of the data by removing the notches (Vassallo et al., 2013). Further advanced processing consisted of noise removal in shot and receiver domains followed by low frequency boost-up. Trace interpolation followed by wavenumber filtering was performed in the common-midpoint domain to achieve a higher signal-to-noise ratio and flat events enhancement. Post-stack processing mainly consisted of frequency-space deconvolution, dip filtering and data-adaptive edge-preserving modified Kuwahara filter. We find two prominent reflections: The upper varies from ~12 km (3.5 s) two-way time (TWT) below the seafloor at 2 Myr to ~78 km (20 s) TWT below the seafloor at 75 Myr. For 27-to-47-Myr old lithosphere, we image a second almost flat continuous reflection from 21 s to 22 s TWT below the seafloor (~82 km). This event is not prominent for 49-to-75-Myr old lithosphere, probably due to the influence of the mantle thermal anomalies from St.Helena/Cameroon. There is also evidence of some other reflections between these two reflections, which also dip towards older age. The crust-mantle boundary or the Moho is also fairly imaged throughout the profile with crustal thickness ranging between 1.4 – 2.7 s TWT. We interpret the uppermost reflection as the base of lithosphere, the top of the LAB, and the lower reflection as the base of the LAB, while the reflections between these as sheared melt sheets. Our results provide the very first image of the whole LAB system continuously as it deepens and thins away from the ridge axis. In this presentation, we will discuss different implications of these images and provide a comprehensive model of the oceanic LAB over normal oceanic lithosphere.

How to cite: Audhkhasi, P. and Singh, S.: Continuous seismic reflection image of the Lithosphere-Asthenosphere Boundary (LAB) from 2-75 Ma on the African plate in the Equatorial Atlantic Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9622, https://doi.org/10.5194/egusphere-egu2020-9622, 2020.

EGU2020-11942 | Displays | SM4.3

Optimal resolution seismic tomography: Lithospheric thinning beneath the British Tertiary Igneous Province and other new observations

Raffaele Bonadio, Sergei Lebedev, Pierre Arroucau, Andrew Schaeffer, Andrea Licciardi, Matthew Agius, Claor Horan, Louise Collins, Brian O'Reilly, Peter Readman, and the Ireland Array Working Team

The maximum achievable resolution of a tomographic model varies spatially and depends on the data sampling and errors in the data. Adaptive parameterization schemes match the spatial variations in data sampling but do not address the effects of the errors. The propagation of systematic errors, however, is resistant to data redundancy and results in models dominated by noise if the target resolution is too high. This forces us to look for smoother models and thus limits the imaging resolution.

We develop a surface-wave tomography method that finds optimal lateral resolution at every point by means of error tracking. We first measure inter-station phase-velocities at simultaneously recording station pairs and compute phase-velocity maps at densely, logarithmically spaced periods. Unlike in the classical approach, multiple versions of the maps with varying smoothness constraints are computed, so that the maps range from very rough to very smooth. Phase-velocity curves extracted from the maps at every point can then be inverted for shear-velocity (Vs) profiles. As we show, errors in these phase-velocity curves increase nearly monotonically with the map roughness. Very smooth Vs models computed from very smooth phase-velocity maps will be the most robust, but at a cost of a loss of most structural information. At the other extreme, models that are too rough will be dominated by noise. We define the optimal resolution at a point such that the error of the local phase-velocity curve is below an empirical threshold. The error is estimated by isolating the roughness of the phase-velocity curve that cannot be explained by any Earth structure. A 3D Vs model is then computed by the inversion of the phase-velocity maps with the optimal resolution at every point. The estimated optimal resolution shows smooth lateral variations, confirming the robustness of the procedure. Importantly, optimal resolution does not scale with the density of the data coverage: some of the best-sampled locations require relatively low lateral resolution, probably due to systematic data errors. We apply the method to image the Ireland’s and Britain’s upper mantle, using our large, new regional dataset. We report a pronounced thinning of the lithosphere beneath the British Tertiary Igneous Province, with important implications for the Paleogene uplift and volcanism in the region.

How to cite: Bonadio, R., Lebedev, S., Arroucau, P., Schaeffer, A., Licciardi, A., Agius, M., Horan, C., Collins, L., O'Reilly, B., Readman, P., and Working Team, T. I. A.: Optimal resolution seismic tomography: Lithospheric thinning beneath the British Tertiary Igneous Province and other new observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11942, https://doi.org/10.5194/egusphere-egu2020-11942, 2020.

The distribution of magmatic structures within lithosphere beneath Mt. Baekdu is important to understand the origin, evolution history, and current status of the volcano. A previous study using ambient noise surface wave dispersions (Kim et al., 2017) suggested a feature of magma fractionation and cooling with layering of mafic and felsic structures in deeper crust. However, the existence of melt directly beneath the Mt. Baekdu and corresponding modification of lithosphere are still unclear. In this study, we additionally calculate P-wave receiver functions for stations in Mt. Baekdu and surrounding regions to confirm the previously defined structures and to check the existence of partial melting. We obtain data from three temporary arrays distributed over areas in Northeast China and DPRK. A harmonic decomposition approach is used to account for anisotropy and to obtain isotropic receiver functions. A series of joint inversions is performed for isotropic receiver functions and surface wave dispersion data using a hierarchical transdimensional Bayesian method. Retrieved isotropic radial receiver functions from stations near (<50 km) the Mt. Baekdu show a consistent negative signal between P and Ps conversion, which indicates low velocity layers in the crust. In addition, relatively high energy of tangential receiver functions with two- and four-lobe patterns indicate that effects of azimuthal anisotropy and/or tilted interfaces are also significant beneath the volcano. On the other hand, stations away from the volcano show features of more isotropic and homogeneous crustal structures. Inverted models show a clear pattern of thicker crust (35-40 km) with slower S-wave velocities (3.2-3.4 km/s in the crust and 4.0-4.2 km/s in the upper mantle) beneath the Mt. Baekdu compared to other regions with relatively shallow Moho (33-35 km) and high velocities (>3.4 km/s in the crust and >4.2 km/s in the upper mantle). The result indicates that a localized structure with elevated temperature and potentially partial melting is exist directly beneath the volcano. With further analysis, more detailed variations of whole lithosphere structures will be presented.

How to cite: Lim, Y. and Kim, S.: Variations of lithospheric structures around the Mt. Baekdu (Changbaishan) volcano from Bayesian joint inversions with receiver functions and surface wave dispersions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19226, https://doi.org/10.5194/egusphere-egu2020-19226, 2020.

EGU2020-6298 | Displays | SM4.3

Iterative P-wave velocity inversion using Markov chain Monte Carlo method

Hyunggu Jun, Hyeong-Tae Jou, Han-Joon Kim, and Sang Hoon Lee

Imaging the subsurface structure through seismic data needs various information and one of the most important information is the subsurface P-wave velocity. The P-wave velocity structure mainly influences on the location of the reflectors during the subsurface imaging, thus many algorithms has been developed to invert the accurate P-wave velocity such as conventional velocity analysis, traveltime tomography, migration velocity analysis (MVA) and full waveform inversion (FWI). Among those methods, conventional velocity analysis and MVA can be widely applied to the seismic data but generate the velocity with low resolution. On the other hands, the traveltime tomography and FWI can invert relatively accurate velocity structure, but they essentially need long offset seismic data containing sufficiently low frequency components. Recently, the stochastic method such as Markov chain Monte Carlo (McMC) inversion was applied to invert the accurate P-wave velocity with the seismic data without long offset or low frequency components. This method uses global optimization instead of local optimization and poststack seismic data instead of prestack seismic data. Therefore, it can avoid the problem of the local minima and limitation of the offset. However, the accuracy of the poststack seismic section directly affects the McMC inversion result. In this study, we tried to overcome the dependency of the McMC inversion on the poststack seismic section and iterative workflow was applied to the McMC inversion to invert the accurate P-wave velocity from the simple background velocity and inaccurate poststack seismic section. The numerical test showed that the suggested method could successfully invert the subsurface P-wave velocity.

How to cite: Jun, H., Jou, H.-T., Kim, H.-J., and Lee, S. H.: Iterative P-wave velocity inversion using Markov chain Monte Carlo method, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6298, https://doi.org/10.5194/egusphere-egu2020-6298, 2020.

EGU2020-7389 | Displays | SM4.3

Seismic Tomography Using Variational Inference Methods

Xin Zhang and Andrew Curtis

In a variety of geoscientific applications we require maps of subsurface properties together with the corresponding maps of uncertainties to assess their reliability. Seismic tomography is a method that is widely used to generate those maps. Since tomography is significantly nonlinear, Monte Carlo sampling methods are often used for this purpose, but they are generally computationally intractable for large data sets and high-dimensionality parameter spaces. To extend uncertainty analysis to larger systems, we introduce variational inference methods to conduct seismic tomography. In contrast to Monte Carlo sampling, variational methods solve the Bayesian inference problem as an optimization problem yet still provide fully nonlinear, probabilistic results. This is achieved by minimizing the Kullback-Leibler (KL) divergence between approximate and target probability distributions within a predefined family of probability distributions.

We introduce two variational inference methods: automatic differential variational inference (ADVI) and Stein variational gradient descent (SVGD). In ADVI a Gaussian probability distribution is assumed and optimized to approximate the posterior probability distribution. In SVGD a smooth transform is iteratively applied to an initial probability distribution to obtain an approximation to the posterior probability distribution. At each iteration the transform is determined by seeking the steepest descent direction that minimizes the KL-divergence.

We apply the two variational inference methods to 2D travel time tomography using both synthetic and real data, and compare the results to those obtained from two different Monte Carlo sampling methods: Metropolis-Hastings Markov chain Monte Carlo (MH-McMC) and reversible jump Markov chain Monte Carlo (rj-McMC). The results show that ADVI provides a biased approximation because of its Gaussian approximation, whereas SVGD produces more accurate approximations to the results of MH-McMC. In comparison rj-McMC produces smoother mean velocity models and lower standard deviations because the parameterization used in rj-McMC (Voronoi cells) imposes prior restrictions on the pixelated form of models: all pixels within each Voronoi cell have identical velocities. This suggests that the results of rj-McMC need to be interpreted in the light of the specific prior information imposed by the parameterization. Both variational methods estimate the posterior distribution at significantly lower computational cost, provided that gradients of parameters with respect to data can be calculated efficiently. We therefore expect that the methods can be applied fruitfully to many other types of geophysical inverse problems.

How to cite: Zhang, X. and Curtis, A.: Seismic Tomography Using Variational Inference Methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7389, https://doi.org/10.5194/egusphere-egu2020-7389, 2020.

EGU2020-19859 | Displays | SM4.3

Seismic attenuation tomography using body-wave energy normalised by the heterogeneous coda

Panayiota Sketsiou, Luca De Siena, Simona Gabrielli, and Ferdinando Napolitano

Seismic waves lose energy during propagation in heterogeneous Earth media. Their decrease of amplitude, defined as seismic attenuation, is central in the description of seismic wave propagation. The attenuation of coherent waves can be described by the total quality factor, Q, and it is defined as the fractional energy lost per cycle, controlling the decay of the energy density spectrum with lapse time. The coda normalization (CN) method is a method to measure the attenuation of P- or S-waves by taking the ratio of the direct wave energy and late coda wave energy in order to remove the source and site effects from P- and S-wave spectra. One of the main assumptions of the CN method is that coda attenuation, i.e. the decay of coda energy with lapse time measured by the coda quality factor Qc is constant. However, several studies showed that Qc is not uniform in the crust for the lapse times considered in most attenuation studies. In this work, we propose a method to overcome this assumption, measuring coda attenuation for each source-station path and evaluating the effect of different scattering regimes on the corresponding imaging. The data consists of passive waveforms from the fault network in the Pollino Area (Southern Italy) and Mount St. Helens volcano (USA).

How to cite: Sketsiou, P., De Siena, L., Gabrielli, S., and Napolitano, F.: Seismic attenuation tomography using body-wave energy normalised by the heterogeneous coda, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19859, https://doi.org/10.5194/egusphere-egu2020-19859, 2020.

EGU2020-9823 | Displays | SM4.3

Coda wave simulations across the Tyrrhenian Basin using radiative transfer

Chiara Nardoni, Luca De Siena, Fabio Cammarano, and Elisabetta Mattei

Lateral variations in crustal structure may affect the propagation of Lg phases, guided waves that propagate efficiently only in the continental crust. Seismic paths crossing continental-oceanic transitions are characterized by Lg blockage due to the drastic decrease in crustal thickness. Here, we investigate the effects of crustal thinning on wave propagation in the Tyrrhenian basin using radiative transfer theory. We first model regional coda envelopes (600-800km) using the software tool Radiative3D (Sanborn & Cormier 2018, GJI). It allows to synthesize seismograms envelopes produced by earthquakes by propagating energy packets through a deterministic structure, taking into account the crustal layers, including Moho transition depth, and parameters describing the medium heterogeneities.  Then, we approach the complex problem of meshing, including measured Moho depths, for simulations based on spectral elements (Komatitsch D. et al., 2012, SPECFEM3D, Computational Infrastructure for Geodynamics) and finite differences methods (Maeda et al., 2017, OpenSWPC). The results aim at understanding complex wave attenuation and leakage in the mantle, for future implementations into the Multi-Resolution Attenuation Tomography code (MuRAT – De Siena et al. 2014, JVGR)

How to cite: Nardoni, C., De Siena, L., Cammarano, F., and Mattei, E.: Coda wave simulations across the Tyrrhenian Basin using radiative transfer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9823, https://doi.org/10.5194/egusphere-egu2020-9823, 2020.

EGU2020-4220 | Displays | SM4.3

The Nature of Intrinsic Attenuation

Yu-Chang Wu and Cheng-Ju Wu

Intrinsic attenuation plays an important role in investigating the interior structure of Earth, especially for the Lithosphere-asthenosphere system, the best place to understand the physical mechanics of plate tectonic. The dissipation, the high attenuation of seismic waves in the low-velocity zones, and the frequency dependence are the characteristic of intrinsic attenuation. However, N. Takeuchi, et al. measured the Northwestern Pacific Ocean’s lithosphere-asthenosphere system, and state the attenuation of the asthenosphere is 50 times larger than the attenuation of lithosphere attenuation. The attenuation of the lithosphere shows strong frequency dependency, but the attenuation of the asthenosphere does not. Previous theories of attenuation failed to explain this phenomenon. Here we demonstrate an explicit attenuation formulation to explain the high attenuation of seismic waves in the low-velocity zones and to show the mechanisms of spectral of teleseismic body waves rapidly fall off as frequency bigger than 1 Hz by perturbing the wave equation with the novel method we proposed. The result also indicates that the difference between the attenuation of the lithosphere and asthenosphere is because their attenuation governs by different physics mechanisms and mathematical models. Moreover, we illustrate the explicit formulation of the relationship between apparent t*, wave velocity, and frequency.

How to cite: Wu, Y.-C. and Wu, C.-J.: The Nature of Intrinsic Attenuation , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4220, https://doi.org/10.5194/egusphere-egu2020-4220, 2020.

EGU2020-12921 | Displays | SM4.3

Passive seismic interferometry in XSoDEx experiment in northern Finland

Elena Kozlovskaya, Nikita Afonin, Jari Karjalainen, Suvi Heinonen, and Stefan Buske

There is the problem that application of controlled-source seismic exploration is not always possible in nature protected areas. As an alternative, application of passive seismic techniques in such areas can be proposed. In our study we show results of application of passive seismic interferometry for mapping the uppermost crust in the area of active mineral exploration in northern Finland using the data recorded during XSoDEx (eXperiment of SOdankylä Deep Exploration) project. The objectives of the project were to obtain a structural image of the upper crust in the Sodankylä area of Northern Finland in order to achieve a better understanding of the mineral system at depth. Within XSoDEx, a combined seismic reflection and refraction survey was organised by Geological Survey of Finland, University of Oulu, Finland (Oulu Mining School and Sodankylä Geophysical Observatory) and TU Bergakademie Freiberg, Germany. The vibrotrack of TU BAF was used as a source. The experiment was performed during July and August 2017 resulting in an approximately 80 km long seismic profile line. The seismic refraction data were simultaneously recorded by 60 vertical- and 40 three-component wireless autonomous receivers along an extended line around the reflection spread with maximum offsets of around 10 km. During night time, the receivers were recording passive seismic data. Thus the XSoDEx experiment provided a good opportunity to verify results of passive seismic interferometry with controlled-source seismic data, to identify limitations of this technique in areas of generally low level of high-frequency anthropogenic noise and to propose possible improvements of known techniques. Analysis of the data and theoretical modelling demonstrated that the dominating sources of ambient noise are non-stationary and have different origin in different parts of XSoDEx lines. In addition, the length of passive data for cross-correlation was limited to several hours and the long data recording period is usually considered as one of the main conditions for seismic interferometry applications. In order to obtain reliable Empirical Green Functions (EGF) from such short-term and non-stationary data, we applied a special technique (signal-to-noise ratio stacking). The calculated EGFs were inverted in order to obtain S-wave velocity models along XSodEx lines down to a depth of several hundreds metres. The obtained results are S-wave seismic velocity models of the upper crust in Northern Finland that agree well with geological data and complement the results of reflection seismic data interpretation.

How to cite: Kozlovskaya, E., Afonin, N., Karjalainen, J., Heinonen, S., and Buske, S.: Passive seismic interferometry in XSoDEx experiment in northern Finland , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12921, https://doi.org/10.5194/egusphere-egu2020-12921, 2020.

We developed a new method where teleseismic P-to-S converted waves are used to construct a fully 3-D shear-wave velocity model of the crust. The method differs from ambient noise and local earthquake tomography in its ray-paths being closer to vertical. Our approach requires a dense seismological network, and we first focus on the Central Alps considering the available permanent and temporary station datasets (e.g., Hetényi et al., 2018, Surv. Geophys.).

We implemented an accurate ray-propagator which respects Snell’s law in 3-D at any interface geometry. Following a teleseismic P ray propagator (Knapmeyer, 2004) from event to station which uses a 1-D global velocity model (iasp91), P-to-S conversion at the Moho is calculated for the crustal S ray considering the true local dip. The corresponding arrival to the surface is typically several km away from the station, which we then adjust by changing the ray-parameter. In the Central Alps, using the 3-D P-velocity structure of Diehl et al. (2009) and the local Moho geometry of Spada et al. (2013), the mean distance between the arriving S-wave and the station is about 150 m (median ca. 40 m).

For our approach we adopt a new model parameterization of velocities. It is rectangular in map view (nodes at 25x25 km in the Alps), while in depth we define a 2-layer model with separate velocities above and below each discontinuity. The introduction of this flexibility allows us to accommodate a velocity gradient within each layer and investigate velocity jumps across discontinuities.

The inversion proceeds iteratively, by visiting every node of the map following a Travelling Salesman Path. At each node, receiver function rays in the surrounding volume are considered for inversion, and bundled into sub-blocks and ranges of back-azimuth (5x5 km size, 45° or 60° bins for the Central Alps). The velocity model at the given node is inverted using the technique of Simulated Annealing, followed by a pattern search algorithm to avoid falling in a local minimum. During iterations of the Simulated Annealing, individual velocity model corresponding to each receiver function is extracted from the 3-D model along its ray path.

The inversion proceeds for 4 or 5 independent parameters: Moho and a hypothetical intra-crustal discontinuity depth, Vp/Vs ratio (either full crust, or separately for upper and lower crust) and the P-wave velocity jump at the intra-crustal discontinuity. Finally, the velocity structure is updated with the result obtained at the given node. We observe that a few rounds of Travelling Salesman Paths improve the overall misfit.

First results on the Central Alps show that the Moho depth generally reflects well the roots of the Alpine orogen. Resolving crustal Vp/Vs ratio is more stable when considering the full crust, instead of two separate layers. The Conrad discontinuity remains difficult to resolve. The obtained velocity structure is compared along profiles to recent Vs results from 3-D ambient noise tomography (Lu et al., 2018).

How to cite: Colavitti, L., Hetényi, G., and Working Group, A.: A new inversion method to construct a 3-D crustal shear-wave velocity model from P-to-S converted waves and application to the Central Alps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10386, https://doi.org/10.5194/egusphere-egu2020-10386, 2020.

EGU2020-5178 | Displays | SM4.3

Demultiple strategies in the submarine slope of Taiwan accretionary wedge

Feisal Dirgantara, Andrew Tien-Shun Lin, Char-Shine Liu, and Song-Chuen Chen

Reducing multiple contaminations in reflection seismic data remains one of the greatest challenges in seismic processing and its effectiveness is highly dependent on geologic settings. We undertook two-dimensional reflection seismic data crossing the upper and lower accretionary wedge slopes off SW Taiwan to test the efficiency of various multiple-attenuation scenarios. The area has resulted from an incipient arc-continent collision between the northern rifted margin of the South China Sea and the Luzon volcanic arcs. The wedge extends from shallow water to deep water bathymetries, hence promoting both short-period and long-period multiples within the seismic records. The multichannel seismic data were achieved under 468 hydrophones, 4-ms sampling rate, 12.5-m channel spacing, 50-m shot spacing and 15-second recording length. Preprocessing flow includes swell noise removal, direct wave mute, and missing channel and shot restoration. A subset of demultiple methods based on the periodicity nature and the spatial move-out behavior of multiples were explored to attenuate multiples energy under different geologic environments. The first step relies on the simultaneous subtraction of surface-related multiples, which combined wave-equation multiple attenuation (WEMA) and surface-related multiple elimination (SRME). WEMA is a shot domain multiple attenuations based on a combination of numerical wave extrapolation through the water layer and the water bottom reflectivity. This method was capable to partially suppress the water layer multiples. SRME was applied to attenuate the residual multiple energy at near-offset. This method assumes surface-related multiples can be kinematically predicted by convolution of prestack seismic traces at possible surface multiple reflection locations. Some primary reflections seem to be better retained after the combined subtraction process than using WEMA or SRME filtering independently. The second step lies on parabolic Radon transform to attenuate far-offset multiples by subtracting the noise energy in tau-p on input gathers that have been corrected for normal move-out and inverse transform the remaining primary energy back to CMP-offset domain. Predictive deconvolution in the x-t domain was performed to attenuate low-frequency reverberations in the upper wedge slope. A double-gap deconvolution operator was extended to predict reverberations with correct relative amplitudes, followed by time-variant bandpass filtering to reduce much of residual multiple energy. In general, WEMA and predictive deconvolution were more effective in attenuating the multiples energy at the upper wedge slope where the water depths are shallower; whereas SRME and parabolic Radon were capable of reducing the energy of multiples at the lower wedge slope. Nevertheless, multiples energy could not be fully eliminated due to several factors. The dependency of some demultiple methods (e.g. parabolic Radon, WEMA, SRME) on velocity function may perturb the forward multiple predictions before subtraction as primary velocities might not be present due to the highly tilted strata in the thrust belts domain. Furthermore, parabolic Radon may not perform well in shallow water and area with slowly increasing velocities with depth (e.g. the upper wedge slope). Since the reflection seismic dataset spans various tectonic environments and water depth, results suggest there was no single demultiple method capable to suppress multiples in all environments.

How to cite: Dirgantara, F., Lin, A. T.-S., Liu, C.-S., and Chen, S.-C.: Demultiple strategies in the submarine slope of Taiwan accretionary wedge, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5178, https://doi.org/10.5194/egusphere-egu2020-5178, 2020.

SM5.1 – Advances in fibre optics and ground sensing technologies - instrumentation, theory and applications

EGU2020-12594 * | Displays | SM5.1 | Highlight

Seafloor seismology with Distributed Acoustic Sensing in Monterey Bay

Nathaniel Lindsey, Jonathan Ajo-Franklin, Craig Dawe, Lise Retailleau, Biondo Biondi, and Lucia Gualtieri

Emerging distributed fiber-optic sensing technology coupled to existing subsea telecommunications cables enable access to meterscale, multi-kilometer aperture, broadband seismic array observations of ocean and solid earth phenomena. In this talk, we report on two multi-day Distributed Acoustic Sensing (DAS) campaigns conducted in 2018 and 2019 with the Monterey Accelerated Research System (MARS) observatory tether cable. In both experiments, a DAS instrument located on shore was connected to a fiber inside the buried MARS cable and recorded a ~10,000-component, 20-kilometer-long, strain-rate array. We use the 8 TB DAS dataset to address three questions:

1. How can seafloor DAS earthquake records inform offshore seismic hazard assessments? Offshore seismic hazards are poorly characterized despite dense coastal populations. The MARS DAS array captured multiple unaliased earthquake recordings, which document phase conversions and abrupt S-wave delays of 0.25 s at mapped (and unmapped) faults that transect the cable. Minor earthquakes in Northern California produce seismic waves in the range 0.5 - 50 Hz, which interact with submarine faults lying just offshore. Spectral ratios and wavefield synthetics are used to explore how seismic waves from well-characterized earthquakes interact with poorly-characterized subsea faults.

2. How are ocean microseisms and other coastal processes recorded by subsea DAS? Horizontal seabed ambient noise recorded with the MARS DAS array matches the expected dispersion of primary microseisms (f~0.05-0.15 Hz) induced by shoaling ocean surface waves, but at a higher band than onshore observations. Separation of incoming and outgoing waves recorded over the DAS array validates the Longuet-Higgins-Hasselmann theory that bi-directional ocean wind-waves undergo nonlinear wave interaction, producing secondary microseisms (f~0.4-1.5 Hz), even when the outgoing energy is observed to be <1% of the incoming energy. Continuous wavelet transforms of sea state observations from buoys, onshore broadband seismometers, and subsea DAS provide insight into the physics of microseism generation and ocean-solid earth coupling. Additionally, DAS provides observation of post-low-tide tidal bores (f~1-5 Hz), storm-induced sediment transport (f~0.8-10 Hz), infragravity waves (f~0.01-0.05 Hz), and breaking internal waves (f~0.001 Hz) consistent with previous point sensor observations in Monterey Bay. 

3. How is the coastal seafloor structure organized from shore to shelf break? The northern continental shelf of Monterey Bay is comprised of allochthonous Cretaceous granite overlain by marine sediments of varying thickness, and is crosscut by abandoned (and subsequently filled) paleochannels. Noise interferometry applied to the full MARS DAS dataset in the 0.25 - 5 Hz range retrieves Scholte waves, which are dispersive and coherent over 2 - 6 kilometers. We apply fundamental mode dispersion (1.5D) imaging to subarray noise correlations in order to understand the sediment thickness distribution across the shelf. Our model is compared with recent seismic reflection profiling conducted by the USGS California Seafloor Mapping Program.

How to cite: Lindsey, N., Ajo-Franklin, J., Dawe, C., Retailleau, L., Biondi, B., and Gualtieri, L.: Seafloor seismology with Distributed Acoustic Sensing in Monterey Bay, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12594, https://doi.org/10.5194/egusphere-egu2020-12594, 2020.

EGU2020-5369 | Displays | SM5.1

The FOCUS experiment 2020 (Fiber Optic Cable Use for Seafloor studies of earthquake hazard and deformation)

Marc-Andre Gutscher, Jean-Yves Royer, David Graindorge, Shane Murphy, Frauke Klingelhoefer, Chastity Aiken, Antonio Cattaneo, Giovanni Barreca, Lionel Quetel, Giorgio Riccobene, Salvatore Aurnia, Florian Petersen, Dietrich Lange, Morelia Urlaub, Seabstian Krastel, Felix Gross, Heidrun Kopp, Milena Moretti, Laura Beranzoli, and Nadia Lo Bue and the FOCUS Team

Laser reflectometry (BOTDR), commonly used for structural health monitoring (bridges, dams, etc.), for the first time is being tested to study movements of an active fault on the seafloor, 25 km offshore Catania Sicily (an urban area of 1 million people). Under ideal conditions, this technique can measure small strains (10E-6), across very large distances (10 - 200 km) and locate these strains with a spatial resolution of 10 - 50 m. As the first experiment of the European funded FOCUS project (ERC Advanced Grant), in late April 2020 we aimed to connect and deploy a dedicated 6-km long strain cable to the TSS (Test Site South) seafloor observatory in 2100 m water depth operated by INFN-LNS (Italian National Physics Institute). The work plan for the marine expedition FocusX1 onboard the research vessel PourquoiPas? is described here. First, microbathymetric mapping and a video camera survey are performed by the ROV Victor6000. Then, several intermediate junction frames and short connector cables (umbilicals) are connected. A cable-end module and 6-km long fiber-optic strain cable (manufactured by Nexans Norway) is then connected to the new junction box. Next, we use a deep-water cable-laying system with an integrated plow (updated Deep Sea Net design Ifremer, Toulon) to bury the cable 20 cm in the soft sediments in order to increase coupling between the cable and the seafloor. The targeted track for the cable crosses the North Alfeo Fault at three locations. Laser reflectometry measurements began April 2020 and will be calibrated by a three-year deployment of seafloor geodetic instruments (Canopus acoustic beacons manufactured by iXblue) also started April 2020, to quantify relative displacement across the fault. During a future marine expedition, tentatively scheduled for 2021 (FocusX2) a passive seismological experiment is planned to record regional seismicity. This will involve deployment of a temporary network of OBS (Ocean Bottom Seismometers) on the seafloor and seismic stations on land, supplemented by INGV permanent land stations. The simultaneous use of laser reflectometry, seafloor geodetic stations as well as seismological land and sea stations will provide an integrated system for monitoring a wide range of types of slipping events along the North Alfeo Fault (e.g. - creep, slow-slip, rupture). A long-term goal is the development of dual-use telecom cables with industry partners.

How to cite: Gutscher, M.-A., Royer, J.-Y., Graindorge, D., Murphy, S., Klingelhoefer, F., Aiken, C., Cattaneo, A., Barreca, G., Quetel, L., Riccobene, G., Aurnia, S., Petersen, F., Lange, D., Urlaub, M., Krastel, S., Gross, F., Kopp, H., Moretti, M., Beranzoli, L., and Lo Bue, N. and the FOCUS Team: The FOCUS experiment 2020 (Fiber Optic Cable Use for Seafloor studies of earthquake hazard and deformation), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5369, https://doi.org/10.5194/egusphere-egu2020-5369, 2020.

EGU2020-13988 | Displays | SM5.1

The contribution of the CAM fibre optic submarine cable telecom ring to the early warning of tsunami and earthquakes

Luis Matias, Yasser Omar, Fernando Carrilho, Vasco Sá, Rachid Omira, Carlos Corela, Rui A. P. Perdigão, and Afonso Loureiro

CAM is the acronym of the submarine telecommunication fibre optic cables that interconnect in a ring Portugal mainland, Azores and Madeira archipelagos. The current cables will cease their operation by 2024 (due to the end of cable lifetime), and the process of their replacement by a new set of cables is now under consideration by the Portuguese authorities with the technical requirements to be defined until mid-2020.

The CAM cables span along the plate boundary between Eurasia and Nubia, an offshore domain prone to generate destructive earthquakes and tsunamis. The impacts caused by these natural hazards can be mitigated by effective tsunami (TWS) and earthquake (EEWS) early warning systems that would benefit (but not only) Portugal, Spain and Morocco. In TWS, a confirmation that a tsunami was generated, and an evaluation of its amplitude, are only obtained after the recordings from the closest coastal tide-gauge are analysed. Hence, this information will not benefit large stretches of coastline. EEWS that rely on land station observations of strong motion will not profit the regions closest to the epicentre. Furthermore, the quality of the offshore earthquake parameters computed by the operational centres is less than optimal when land stations only are used.

All these difficulties can be overcome by deploying sensors integrated in the commercial telecom submarine cables to be installed in the future, without reduction of the reliability, lifetime, operational and functional requirements demanded by operators. Being closer to the tectonic sources, such sensors (and the cable itself) will record the geophysical parameters and transmit them to land much faster than the speed of destructive waves, providing the processing centres with critical lead time. This is the approach that has been advocated by the Joint Task Force led by three U.N. agencies (ITU, WMO and UNESCO-IOC) (Howe et al., Front. Mar. Sci. 6:424, 2019). Such initiative was given the name of SMART, for Science Monitoring and Reliable Telecommunications.

In this work we evaluate the contribution of a SMART CAM fibre optic ring of cables, with repeaters equipped with geophysical sensors, to three critical aspects of TWS and EEWS: 1) quality of fast earthquake parameters; 2) earthquake early warning lead time; 3) tsunami confirmation warning time. The performance parameters selected where: i) azimuthal gap between the minimum set of stations required for an earthquake location to be accepted; ii) size of the estimated location error ellipse; iii) error on the focal depth estimation; iv) gain in P-wave advance time for the minimum set of stations required by the EEWS; v) gain in tsunami travel time to the closest tide-gauge or pressure sensor.

The methodology and results obtained are valuable to encourage national authorities to implement the SMART cable concept in the technical specifications for future telecommunication submarine cables. This has been recently suggested by the ANACOM President (the Portuguese regulator for telecommunications advising the national authorities) for the CAM cables that must be operational in early 2024. The first author would like to acknowledge the financial support from FCT through project UIDB/50019/2020-IDL.

How to cite: Matias, L., Omar, Y., Carrilho, F., Sá, V., Omira, R., Corela, C., Perdigão, R. A. P., and Loureiro, A.: The contribution of the CAM fibre optic submarine cable telecom ring to the early warning of tsunami and earthquakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13988, https://doi.org/10.5194/egusphere-egu2020-13988, 2020.

EGU2020-12055 | Displays | SM5.1

Precise Distributed Acoustic Sensing measurements by using seafloor optical fiber cable system for seismic monitoring

Masanao Shinohara, Tomoaki Yamada, Takeshi Akuhara, KImihiro Mochizuki, and Shin'ichi Sakai

Distributed Acoustic Sensing (DAS) measurements which utilize an optical fiber itself as a sensor can be applied for various purposes. An observation of earthquakes using an optical fiber deployed on the seafloor with DAS technology is attractive because DAS measurements enable a dense seismic observation as a long linear array. Spatial resolution of the observation reaches a few meters. The length of the array is determined by the measurement range of the DAS interrogator deployed on the optical fiber, and a fine spatial sensor interval can be configured. DAS measurements have become increasingly accurate and the current state of technology exhibit high signal quality. Because DAS measurement is useful for earthquake observation, there were some trials for an observation of earthquakes using an optical fiber deployed on the land or the seafloor. However, There are few observations using DAS technology on seafloor until the present.

In 1996, a seafloor seismic tsunami observation system using an optical fiber cable was deployed off the coast of Sanriku by Earthquake Research Institute, the University of Tokyo. The system has three seismic stations and two tsunami-meters, and a length of the cable is approximately 115 km. The system has six spare (dark) optical fibers which are dispersion shifted single mode type, and have been incorporated for future extension of the observation system. We have started development of a seafloor seismic observation system utilizing DAS technology on the Sanriku cable observation system as a next generation of marine seismic observation system. In 2019, we performed DAS measurements using a dark fiber from Sanriku seafloor observation system three times. An interrogator was installed in the cable landing station temporarily. Data were recorded with various values of parameters, such as length of data collection (array aperture), gauge length, ping rate, acquisition offset, for evaluation of data quality and signal to noise ratios. The total recording period for three measurements was approximately three weeks. As a result, many earthquakes including micro-earthquakes were recorded. The obtained data will be used to develop data processing techniques for seismic observations utilizing DAS measurements.

 

How to cite: Shinohara, M., Yamada, T., Akuhara, T., Mochizuki, K., and Sakai, S.: Precise Distributed Acoustic Sensing measurements by using seafloor optical fiber cable system for seismic monitoring, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12055, https://doi.org/10.5194/egusphere-egu2020-12055, 2020.

EGU2020-8484 | Displays | SM5.1 | Highlight

Automatic Earthquake Detection and De-noising for Distributed Acoustic Sensing: Examples from On-land and Underwater Fibers

Itzhak Lior, Daniel Mata, Gauthier Guerin, Diane Rivet, Anthony Sladen, and Jean-Paul Ampuero

The use of underwater optical fibers, such as those currently traversing most of the world's oceans, for distributed acoustic sensing (DAS) holds great potential for seismic monitoring by complementing on-land seismic observations, especially near underwater faults. The analysis of underwater DAS records presents special challenges due to the noisy environment and the uneven cable-seafloor coupling. To fully exploit the potential of these records, automatically detecting and extracting seismic signals is imperative. To this end, a new automatic earthquake detection scheme is presented, based on waveform-similarity. Cross correlations between nearby records along the fiber are continuously calculated in short overlapping intervals. Earthquakes are detected as abrupt increases in cross correlation values over large segments of the cable. This procedure is applied to records of four existing fibers: one on land (Near Teil, south of France) and three underwater (one in Toulon, south of France, and two in Pylos, south-west Greece). Detected earthquakes are compared to earthquake catalogs and detection thresholds are obtained. That several of the detected earthquakes do not appear in any earthquake catalog demonstrates the proposed method's robustness. The cross correlation time shifts are then used to perform moveout corrections to the time series and phase weighted stacking (PWS) is applied to groups of neighboring traces. Unlike simple stacking approaches, PWS significantly enhances signal to noise ratios, allowing for more precise earthquake analysis and characterization. Further developing and applying such automatic techniques to ocean bottom fibers will enhance the performance of earthquake early warning systems, improving alert times for earthquakes occurring on underwater faults.

How to cite: Lior, I., Mata, D., Guerin, G., Rivet, D., Sladen, A., and Ampuero, J.-P.: Automatic Earthquake Detection and De-noising for Distributed Acoustic Sensing: Examples from On-land and Underwater Fibers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8484, https://doi.org/10.5194/egusphere-egu2020-8484, 2020.

EGU2020-11133 | Displays | SM5.1

Testing in sandbox experiments the potentialities of active-Distributed Temperature Sensing to quantify distributed groundwater fluxes in porous media

Olivier Bour, Nataline Simon, Nicolas Lavenant, Gilles Porel, Benoit Nauleau, Behzad Pouladi, and Laurent Longuevergne

Active-Distributed Temperature Sensing is a new method that has been recently developed for quantifying groundwater fluxes in the sub-surface along fibre-optic cables with a great spatial resolution. It consists in measuring and modelling the increase of temperature due to a heat source, dissipated through heat conduction and heat advection, depending on groundwater fluxes. Here, we propose to estimate the applicability and limitations of the method using sandbox experiments where flow rate and temperature are well controlled. For doing so, active-DTS experiments have been achieved under different flow rates and experimental conditions. In addition, we compare three different and complementary methods to estimate in practice the spatial resolution of DTS measurements. 

Active-DTS experiments have been conducted by deploying a fiber optic cable in a large PVC tank (1.6m long; 1.2 m width and 0.3 m height) and filled with 0.4-1.3 mm diameter sand. The height of water in water reservoirs on either side of the sandbox can be adjusted to control the head gradient and the flow rate through the sand. Heating was done by injecting during at least 8 hours for each experiment, a well-controlled electrical current along the steel armouring of the fiber optic cable. The three methods for estimating spatial resolution were applied and compared using FO-DTS measurements obtained on the same fiber-optic cable but with two different DTS units having different spatial resolution. Results show that a large range of groundwater fluxes may be estimated with a very good accuracy. Finally, we compare the advantages and complementarities of the different methods proposed for estimating the spatial resolution of measurements. In particular, the spatial resolution estimated using a temperature step change is both dependent on the effective spatial resolution of the DTS unit but also on heat conduction induced because of the high thermal conductivity of the cable. By showing the applicability of the method for a large range of flow rates and with an excellent spatial resolution, these experiments demonstrate the potentialities of the method for quantifying fluid fluxes in porous media for a large range of applications.

How to cite: Bour, O., Simon, N., Lavenant, N., Porel, G., Nauleau, B., Pouladi, B., and Longuevergne, L.: Testing in sandbox experiments the potentialities of active-Distributed Temperature Sensing to quantify distributed groundwater fluxes in porous media, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11133, https://doi.org/10.5194/egusphere-egu2020-11133, 2020.

EGU2020-22234 | Displays | SM5.1

Monitoring sediment coverage from Distributed Temperature Sensing measurements in the Port of Rotterdam, the Netherlands

Manos Pefkos, Pieter Doornenbal, Arjan Wijdeveld, Ebi Meshkati Shahmirzadi, and Pauline Kruiver

Distributed Temperature Sensing (DTS) measurements were conducted in the Port of Rotterdam as part of the INTERREG NWE SURICATES project. In the Port of Rotterdam a program is running to retain sediments in the harbor for river bank protection, and to lower the costs of transferring sediment from the port to the offshore dump locations. The aim of the DTS monitoring is to find spatial patterns in sediment deposition and erosion and thus determining the sediment balance before, during and after re-allocation. Fibre optic cables were installed in two layouts. Two fibre optic cables of lengths 1.2km and 750m were laid out flat parallel and perpendicular to the shore and they passively recorded temperature. Another cable was wrapped helically on a vertical pole condensing 150 m of length into 0.77m, increasing the spatial resolution. This cable was used for passive measurements and active heating experiments. The acquired data span the period from May to September 2019.

The active heating experiments showed that the water-sediment interface along the pole can be tracked from the difference in response between the time when the heating cable is switched on and off. The pole’s passive temperature analysis indicates that signals from the water phase exhibit high variability with time, whereas those from the sediment phase have low variability. Frequency domain analysis of the water phase shows clear peaks in the Fourier Amplitude Spectrum (FAS) at one day and half-day cycles, with the half-day cycle peak having the highest magnitude. The same peaks are present in the sediment phase’s FAS, but their magnitudes are about an order of magnitude lower.

The Fourier amplitude at frequencies corresponding to half-day periods was used for classification of the phases along the pole. The interface between water and sediment is defined as the maximum in the derivative of the Fourier amplitude with height. The interface’s height and thus the occurrence of erosion or deposition was tracked over time. The analysis shows that the sediment interface varied around 5cm over a period of 2.5 months between two dredging actions.

Representative signals from the Fourier amplitude at half-day cycles from the pole were used to derive sediment coverage over the flat passive cables. However, further research is required to establish the minimum horizontal distance over which coverage can be established.

We conclude that, by comparing the spectral properties of the temperature signal of water and sediment phases, sediment coverage over fibre optic cables can be monitored with DTS measurements. The finest time and spatial resolution over which this coverage can be found remains to be decided and can be the subject of future work.

How to cite: Pefkos, M., Doornenbal, P., Wijdeveld, A., Shahmirzadi, E. M., and Kruiver, P.: Monitoring sediment coverage from Distributed Temperature Sensing measurements in the Port of Rotterdam, the Netherlands, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22234, https://doi.org/10.5194/egusphere-egu2020-22234, 2020.

EGU2020-9825 | Displays | SM5.1

Effect of complex topography on the wavefield recorded by DAS and buried fiber optic cable at Azuma volcano, Northeast Japan

Kentaro Emoto, Takeshi Nishimura, Hisashi Nakahara, Satoshi Miura, Mare Yamamoto, Shunsuke Sugimura, Takahiro Ueda, Ayumu Ishikawa, and Tsunehisa Kimura

First DAS observation at Mt. Azuma, Japan was conducted in July 2019 using buried fiber optic cable along the road access to the volcano. Mt. Azuma is an active volcano located in the Tohoku region. Different from non-volcanic regions, wavefields in the volcano is more complex due to its topography and the strong heterogeneities beneath the volcanic edifice. The strength of the scattering of seismic waves due to small-scale velocity heterogeneities in the volcano is reported to be more than one order higher than that in the non-volcanic region. To estimate small-scale heterogeneities, a dense observation network is necessary. The high spatial resolution is one of the advantages of the DAS observation. Therefore, DAS observation in the volcano might be a good chance for the estimation of the small-scale heterogeneity.

 

We used 14km length of the fiber optic cable buried along to the access road to the observatory near the summit installed by the Ministry of Land, Infrastructure, Transport and Tourism to monitor the volcanic activities. The spatial and temporal samplings were 10m and 1000Hz, respectively. The observation period was for 3 weeks. In addition to regional and teleseismic earthquakes, volcanic earthquakes were also observed. A teleseismic P-wave was analyzed to investigate the effect of small-scale heterogeneities. Because the incident angle of the teleseismic P-wave is almost vertical to the portion of the fiber optic cable used for the DAS observation, a simple model can be used. We calculate the cross-correlation coefficient (CCC) of waveforms between channels and analyze its dependence on the distances between channel pairs. The recorded wavefield was fluctuated by scattering due to the small-scale heterogeneities and different waveforms were recorded even though the propagation distances are the same. Therefore, the spatial variation of the waveforms of teleseismic P-wave recorded at surface stations would be related to the small-scale heterogeneities beneath of the array.

 

The CCC decreases with increasing separation distance and converges to a constant value. This shape can be modeled by the Gaussian function and we defined the spatial scale of CCC by fitting the Gaussian function. The scale decreases with increasing frequency. The finite difference simulation of the wave propagation was performed by changing the velocity structure and compare the synthetic and observed CCCs. We found that the effect of the topography is most significant on the CCC. Because analyzed waveforms mainly consist of the converted surface wave from the teleseismic P-wave, the effect of subsurface small-scale heterogeneities is not significant. Our result shows that it is necessary to consider the effect of the topography in analyses of DAS data recorded in volcanoes.

How to cite: Emoto, K., Nishimura, T., Nakahara, H., Miura, S., Yamamoto, M., Sugimura, S., Ueda, T., Ishikawa, A., and Kimura, T.: Effect of complex topography on the wavefield recorded by DAS and buried fiber optic cable at Azuma volcano, Northeast Japan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9825, https://doi.org/10.5194/egusphere-egu2020-9825, 2020.

EGU2020-18084 | Displays | SM5.1

Four years of soil strain monitoring on Etna Volcano Mount by means of a Three-axial Fiber Bragg Grating Sensor

Umberto Giacomelli, Enrico Maccioni, Giorgio Carelli, Daniele Carbone, Salvatore Gambino, Massimo Orazi, Rosario Peluso, and Fiodor Sorrentino

Rock strains detection is one of the principal ways to monitor geohazards. Classic strainmeters are cumbersome, hard to install and very expensive. Opto-electronics devices based on fiber Bragg grating technology allow to realize strainmeters with high sensitivity, low-cost, small volume and high performance.
We present the long term result of continuous soil strain monitoring on the Etna mount by a three-axial fiber Bragg grating sensor. The sensor has been developed in the framework of European Project MED-SUV (MEDiterranean SUpersite Volcanos). The installation site is a 8.5 meters deep borehole at a distance of about 7 km South-West from the summit craters of the Etna mount, at an elevation of about 1740 meters. This kind of sensor has a resolution better than 100 nanostrains on a daily timescale. Despite it is only a prototype, the sensor has worked for four years with a duty-cycle higher than 90% detecting both fast event, as earthquakes, and slow event, as epochal rocks strain behavior.

How to cite: Giacomelli, U., Maccioni, E., Carelli, G., Carbone, D., Gambino, S., Orazi, M., Peluso, R., and Sorrentino, F.: Four years of soil strain monitoring on Etna Volcano Mount by means of a Three-axial Fiber Bragg Grating Sensor, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18084, https://doi.org/10.5194/egusphere-egu2020-18084, 2020.

EGU2020-14022 | Displays | SM5.1

Continuous source system and distributed acoustic sensing for reservoir to crust monitoring

Takeshi Tsuji, Tatsunori Ikeda, and Koshun Yamaoka

We have developed a permanent seismic monitoring system using a continuous seismic source and distributed acoustic sensing (DAS). The active seismic source system continuously generates waveforms with wide frequency range. By stacking the continuous waveforms, our monitoring system improves signal-to-noise ratio of the seismic signal. Thus, less-energy vibration using small-size source could be utilized for the exploration of deeper geological targets. Presently, we have deployed the small-size monitoring source system in the Kuju geothermal field in the northeast Kyushu Island, Japan. Although our monitoring source system is small and generates high frequency vibrations (10-20Hz), the signal propagated >80 km distance using two-month continuous source data. Our field experiments demonstrate that variation of seismic velocity of the crust could be identified with high accuracy (~0.01 %). To record the monitoring signal from continuous source system, we need to deploy seismometers. Deployment of many seismometers increase spatial resolution of the monitoring results. Recently, we have deployed the DAS system close to the continuous seismic source system. Using DAS, dense and long seismometer network can be realized, and we succeeded to identify the temporal variation of seismic velocity. By using both continuous source and DAS, we are able to monitor wide area with lower cost. Our monitoring system could accurately monitor the larger-scale crust and smaller-scale reservoir in high temporal resolution.

How to cite: Tsuji, T., Ikeda, T., and Yamaoka, K.: Continuous source system and distributed acoustic sensing for reservoir to crust monitoring, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14022, https://doi.org/10.5194/egusphere-egu2020-14022, 2020.

EGU2020-10096 | Displays | SM5.1

High precision and high resolution monitoring of subsurface changes with DAS and airgun

Baoshan Wang, Xiangfang Zeng, Jun Yang, Yuansheng Zhang, Zhenghong Song, Xiaobin Li, Rongbin Lin, Manzhong Qin, and Congxin Wei

Recently large-volume airgun arrays have been used to explore and monitor the subsurface structure. The airgun array can generate highly repeatable seismic signals, which can be traced to more than 200 km. And the airgun source can be ignited every 10 minutes. The airgun source makes it possible to precisely monitor subsurface changes at large scale. The spatial resolution of airgun monitoring is poor subjecting to the receiver distribution. The distributed acoustic sensing (DAS) technique provides a strategy for low-cost and high-density seismic observations. Two experiments combing DAS technique and airgun source were conducted at two sites with different settings. At the first site, a telecommunication fiber-optic cable in urban area was used. After moderate stacking, the airgun signal emerges on the 30-km DAS array at about 9 km epicentral distance. In the second experiment, a 5-km cable was deployed from the airgun source to about 2 km away. About 800-m cable was frozen into the ice above the air-gun, the rest cable was cemented on the road crossing through a fault. And the airgun has been fired continuously for more than 48 hours with one-hour interval. On the stacking multiple shots’ records, the wavefield in fault zone emerges too. These two experiments demonstrate the feasibility of using various fiber-optic cables as dense array to acquire air-gun signal in different environments and to monitor the subsurface changes.

How to cite: Wang, B., Zeng, X., Yang, J., Zhang, Y., Song, Z., Li, X., Lin, R., Qin, M., and Wei, C.: High precision and high resolution monitoring of subsurface changes with DAS and airgun, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10096, https://doi.org/10.5194/egusphere-egu2020-10096, 2020.

EGU2020-7495 | Displays | SM5.1

Integration of Machine Learning on Distributed Acoustic Sensing Surveys

Camille Jestin, Clément Hibert, Gaëtan Calbris, and Vincent Lanticq

Distributed Acoustic Sensing (DAS) is an innovative technique which has been recently employed for near-surface geophysics purposes. It involves the use of fibre-optic cable as a sensor. The fibre is analysed by sending a laser pulse from an interrogator unit. The phase of the backscattered signal contains the information on the strain on the cable, enabling the detection of a passing acoustic wave with enough energy for the cable excitation. Allowing the interrogation of long profiles and the generation of a dense spatial sampling, uneasy to obtain with classic geophysical techniques, DAS instrumentation then proved its relevance for seismic applications but also for infrastructure monitoring.

During DAS acquisition, and more precisely when closely looking at infrastructures integrity, it is necessary to clearly identify the source of the acoustic vibrations at the structure neighbourhood. Indeed, in the context of pipeline monitoring for example, it appears important to be able to classify events which generate seismic signals recorded by DAS systems and which can be related to a potential threat for the structure. In order to launch an alarm if necessary, the source identification must be fast, accurate and robust. Moreover, because DAS acquisition can generate traces every few meters along fibres of tens of kilometres, the used machine-learning algorithm must demonstrate its ability to handle a big amount of data.

In this study, we analyse the efficiency of the Random Forests (RF) machine-learning algorithm applied to data acquired with DAS system for the discrimination of event sources. RF algorithm has been selected because of its ability to handle large numbers of attributes related to signal characteristics and to enable a good reliability for the discrimination of sources. This algorithm has already proved its efficiency for automated classification of seismic waveforms (e.g. earthquakes, volcanic tremors, rock falls, avalanches, etc.).

We focus our study on tests lead along a gas pipeline instrumented with fibre-optic cable. Different third-party works have been conducted: excavation, saw sections, drill, jackhammer, etc. We work on the discrimination of six classes of seismic source. After running a detection phase based on a threshold on signal energy, we obtain several hundred of exploitable seismic traces to inject to the RF algorithm. We demonstrate the efficiency of the application of machine learning on DAS data to discriminate seismic waveforms from the correct class, with an overall precision on our test set of 99%.

How to cite: Jestin, C., Hibert, C., Calbris, G., and Lanticq, V.: Integration of Machine Learning on Distributed Acoustic Sensing Surveys , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7495, https://doi.org/10.5194/egusphere-egu2020-7495, 2020.

EGU2020-20516 | Displays | SM5.1

Rotation in civil engineering structures: analysis of the City-Hall (Grenoble) building using 3C and 6C sensors

Frédéric Guattari, Philippe Gueguen, Coralie Aubert, and Théo Laudat

Civil engineering structures are often modeled as single-degree-of-freedom systems, taking into account only horizontal translation forces. However, their response to seismic loading produces rotational forces that can in some cases generate considerable stresses and resultant damage. These rotational forces are essentially related to (1) rotational deformation about both horizontal axes (rocking), resulting from ground-structure interactions, considering the structure as a rigid body; (2) rotation about the vertical axis (torsion), essentially activated when the centre of mass (i.e. where the seismic inertial forces apply) is offset from the centre of rigidity (i.e. where the elastic forces apply). The simplified model including the rotations of the ground-structure interaction is based on modal decomposition, i.e. each component of the motion is assumed to be independent of the others. Thus, in structures, only translational sensors are usually installed and the rotational components are evaluated via the spatial derivatives of the horizontal and vertical components. However, there are combinations of translations and rotations and rotations can only be evaluated by measuring all 6 components of motion (3 translations and 3 rotations). In this presentation, a simple analysis is made to explain the rotations observed in the City Hall building in Grenoble (France), a 12-storey reinforced concrete building. This building has been continuously monitored for 10 years, with 3-component accelerometers located at the bottom and top. Modal decomposition is performed using ambient vibrations. A set of earthquake records is then used to evaluate rotations using derived functions and compared with the records of a 6C rotation sensor temporarily installed at the top of the building. The comparison between the direct rotation measurement and the spatially derived rotation is performed. 

How to cite: Guattari, F., Gueguen, P., Aubert, C., and Laudat, T.: Rotation in civil engineering structures: analysis of the City-Hall (Grenoble) building using 3C and 6C sensors, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20516, https://doi.org/10.5194/egusphere-egu2020-20516, 2020.

EGU2020-10742 | Displays | SM5.1

Fiber Optics foR Environmental SEnsE-ing (FORESEE) at Pennsylvania State University

Eileen Martin, Tieyuan Zhu, Junzhu Shen, Srikanth Jakkampudi, Weichen Li, and Ayush Dev

The FORESEE Distributed Acoustic Sensing (DAS) Array records roughly 1/3 terabyte of data per day along 5 kilometers of dark fiber optic telecommunications cable underneath the Pennsylvania State University campus. The campus sits in the Allegheny Mountain region of the US, and our aim is to understand urban hydrology and detection of geohazards (particularly karst features). We have verified a number of features of these data similar to prior urban seismic studies, both in ambient noise and in distant earthquake records, which builds further evidence that dark fiber can be a useful tool for seismology in cities.

These data also contain a number of new signals not observed on previous dark fiber arrays. We see a stronger response to air waves than prior experiments. For instance, musical bass lines are clearly observed in the 30-100 Hz range during a concert, and we can see the spatial decay of higher versus lower frequencies throughout the array. This is the first dark fiber array in the eastern US, where thunderstorms occur with some frequency, and we have observed clear recordings of ground motion due to thunder. Source inversion of the waveforms throughout the array leads to locations that show reasonable agreement compared to the National Lightning Detection Network. These thunderquake signals could be an important source of broadband energy for seismic imaging in an area with little earthquake seismicity.

We have performed ambient noise interferometry throughout the array with a variety of pre-processing workflows, but some subsets of the array are strongly affected by nearby sources. With the wide variety of natural and manmade signals in these data, we are working towards further efficient automation to detect repeatable signals that could be used for targeted interferometry, and methods to automate filtering of non-ideal noise sources. As one example of filtering a specific noise, we were surprised the array is able detect the paths of individuals walking along a sidewalk by the fiber. While this array records data on a public college campus, a likely future area of research may include urban areas with a mix of commercial and residential purposes, so we desire tools to remove individual signals as they are recorded. Thus, we have developed a neural network to detect and remove footsteps from data before those data are shared with researchers. To encourage others working on urban seismic acquisition to remove similar signals, we are generalizing these methods for footstep removal to different scales.

How to cite: Martin, E., Zhu, T., Shen, J., Jakkampudi, S., Li, W., and Dev, A.: Fiber Optics foR Environmental SEnsE-ing (FORESEE) at Pennsylvania State University, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10742, https://doi.org/10.5194/egusphere-egu2020-10742, 2020.

EGU2020-7343 | Displays | SM5.1

Distributed Acoustic Sensing from mHz to kHz: Empirical Investigations of DAS Instrument Response

Patrick Paitz, Pascal Edme, Cédric Schmelzbach, Joesph Doetsch, Dominik Gräff, Fabian Walter, Nathaniel Lindsey, Athena Chalari, and Andreas Fichtner

With the upside of high spatial and temporal sampling even in remote or urban areas using existing fiber-optic infrastructure, Distributed Acoustic Sensing (DAS) is in the process of revolutionising the way we look at seismological data acquisition. However, recent publications show variations of the quality of DAS measurements along a single cable. In addition to site- and orientation effects, data quality is strongly affected by the transfer function between the deforming medium and the fiber, which in turn depends on the fiber-ground coupling and the cable properties. Analyses of the DAS instrument response functions in a limited part of the seismological frequency band are typically based on comparisons with well-coupled conventional seismometers for which the instrument response is sufficiently well known to be removed from the signal.

In this study, we extend the common narrow-band analyses to DAS response analyses covering a frequency range of five orders of magnitude ranging from ~4000 s period to frequencies up to ~100 Hz. This is based on a series of experiments in Switzerland, including (1) active controlled-source experiments with co-located seismometers and geophones, (2) low-frequency strain induced by hydraulic injection in a borehole with co-located Fiber-Bragg-Grating (FBG) strain-meters, and (3) local to teleseismic ice- and earthquake recordings with  co-located broadband stations.

Initial results show a site-unspecific, approximately flat instrument response for all experiments.

The initial results suggest that the amplitude and phase information of DAS recordings are sufficient for conventional geophysical methods such as event localisation, full-waveform inversion, ambient noise tomography and even event magnitude estimation. Despite the promising initial results, further engagement by the DAS community is required to evaluate the DAS performance and repeatability among different interrogation units and study sites.

How to cite: Paitz, P., Edme, P., Schmelzbach, C., Doetsch, J., Gräff, D., Walter, F., Lindsey, N., Chalari, A., and Fichtner, A.: Distributed Acoustic Sensing from mHz to kHz: Empirical Investigations of DAS Instrument Response, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7343, https://doi.org/10.5194/egusphere-egu2020-7343, 2020.

EGU2020-18525 | Displays | SM5.1

Comparing high-sensitivity geophones to fiber-optic DAS technologies in a hard-rock VSP survey

Björn Lund, Anna Stork, Michael Roth, Ari David, Andy Clarke, Carl Nygren, and Sam Johansson

As part of the preparations for a microseismic network on the planned nuclear waste repository in Forsmark, Sweden, we carried out a suite of measurements for site characterisation and instrument testing using geophones and DAS fiber-optic technology. Three high-sensitivity 240 V/m/s geophones were grouted into a 200 m deep borehole together with a linear, a helical and a helical engineered fiber-optical cable. Two different interrogators were used for DAS acquisition. We performed a walk-away vertical seismic profile (VSP) survey with 10 m source spacing out to 1.1 km offset and compare the responses of the four different measurement systems. The complete transfer functions of the fiber-optic systems have not yet been determined, and depend on factors such as incidence angle, signal frequency content and the fiber gauge length. Preliminary results show that all systems record signals with high signal-to-noise ratio and that which system has highest performance depends on source-receiver distance, signal frequency content and wave incidence angle. Due to incomplete knowledge of the fiber transfer functions we cannot match the DAS velocity signal with the geophone signal. Investigation of the detection capabilities of the fiber and geophone systems is underway and will be presented together with a discussion of the relative merits of the various systems for microseismic monitoring.

How to cite: Lund, B., Stork, A., Roth, M., David, A., Clarke, A., Nygren, C., and Johansson, S.: Comparing high-sensitivity geophones to fiber-optic DAS technologies in a hard-rock VSP survey, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18525, https://doi.org/10.5194/egusphere-egu2020-18525, 2020.

EGU2020-12695 | Displays | SM5.1

Simulation of Rotational ground motion in the near field region

Raghukanth Stg and Varun K singla

 

In recent times, seismic rotational motions have received significant attention of seismologists as well earthquake engineers. The measurement of rotational motions is useful not only for the complete characterization of the ground motion, but also for estimating the additional risk to civil engineering structures posed by these motions. With the development of state-of-the-art instruments such as the ring-laser and fiber-optic gyroscopes, and mechanical and magneto-hydrodynamic devices, it is now possible to measure these motions over a large frequency bandwidth with accuracy. Nevertheless, nearly all earthquake-prone regions of the world lack the instrumentation to record these motions and the existing database of recorded rotational motions thus remains limited. In such situations analytical methods can provide estimates of rotational ground motions. It is thus common to simulate these motions and most analytical methods simulate rotations as curl of the displacement field. Because this relation (refer to as ‘curl-based’) is based on the ‘small deformations’ assumption, the ‘curl-based’ rotations are in a sense an approximation of the exact rotations, the latter being computed by using the rotational matrix directly extracted from the deformation gradient using the polar decomposition theorem. Despite this, no study has so far discussed the situations in which the ‘curl-based’ relation no longer holds. This paper therefore attempts to shed light on this issue by conducting two numerical studies. In the first study, a simple case of a point kinematic dislocation shear source buried in a homogeneous, elastic half-space is considered to get a qualitative idea about the combinations of the source and medium parameters that can lead to appreciable errors in the ‘curl-based’ rotations. The second study considers the finite-fault source model of Chi-Chi earthquake to illustrate these errors in a realistic scenario. Both these studies indicate that the ‘curl-based’ rotations can be in appreciable error when ‘exact’ rotations are in excess of  20 degrees and that this can happen in the near-field regions of surface-rupturing faults.

How to cite: Stg, R. and K singla, V.: Simulation of Rotational ground motion in the near field region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12695, https://doi.org/10.5194/egusphere-egu2020-12695, 2020.

EGU2020-7381 | Displays | SM5.1

The ROMY project: A 4-component ring laser for geophysics and geodesy

Heiner Igel, Felix Bernauer, Joachim Wassermann, Shihao Yuan, Andre Gebauer, and Ullrich Schreiber

The ROMY ring laser was constructed with 4 non-orthogonal triangular-shaped cavities of 12 m side length in the Geophysical Observatory outside Munich, Germany, in 2016. The large dimensions of the individual rings have the benefit of allowing high sensitivity surpassing in principle the sensitivity of the G-ring at the Fundamentalstation Wettzell. However, the concrete construction of ROMY is geometrically less stable than the G-ring that is built on a rigid Xerodur plate. Each of the four rings has its own Sagnac frequency. The horizontal triangular ring laser at the top of the inverted tetrahedral ROMY structure allows direct comparison of teleseismic signals and noise with the G-ring at a distance of 200km. It also serves as redundant component. In principle, three orthogonal components of rotational ground motion can be obtained by linear combination from any combination of three rings, that - due to the variable Sagnac frequency - have different noise characteristics. We report on the behavior and observations of ROMY from a seismological point of view. It is fair to say that ROMY provides the most accurate direct 3-component rotational ground motion seismic observations to date. In combination with a collocated broadband seismometer as well as a surrounding small-scale seismic array, we analyse regional, teleseismic events, and ocean-generated noise and compare with array-derived rotation.

How to cite: Igel, H., Bernauer, F., Wassermann, J., Yuan, S., Gebauer, A., and Schreiber, U.: The ROMY project: A 4-component ring laser for geophysics and geodesy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7381, https://doi.org/10.5194/egusphere-egu2020-7381, 2020.

EGU2020-8434 | Displays | SM5.1

Towards a two-axis cold-atom gyroscope for rotational seismology

Remi Geiger, romain gautier, leonid sidorenkov, and arnaud landragin

Cold-atom inertial sensors target several applications in navigation, prospection, geoscience and tests of fundamental physics. The operation of these sensors is based on atomic interferometry taking advantage of superpositions between quantum states of different momentum of an atom. These superposition states are obtained by means of optical transitions with two (or more) photons communicating momentum to the atom and acting as beam splitters and mirrors for the matter waves. The SYRTE cold-atom gyroscope currently represent the state of the art of atomic gyroscopes with a short-term sensitivity of 40 nrad/s/sqrt(Hz) limited by vibration noise (using a 4 Hz sampling rate), a long term stability of 3e-10 rad/s and an accuracy of 10 nrad/s. The detection noise limit of the sensor (quantum projection noise) is currently 5 nrad/s/sqrt(Hz) in the band DC-1 Hz, which already represents an interest to sense ground rotations in a typical frequency range between 1 mHz and 1 Hz. A second horizontal axis of measurement is currently being implemented. Moreover, we are designing a new experiment which aims a reaching a (quantum projection noise limited) sensitivity of 1 nrad/s/sqrt(Hz) in this frequency band along two axes of measurement, which represents an interesting perspective for the field of rotational seismology. This contribution will present the results recently achieved with the SYRTE gyroscope experiment to reach state-of-the-art performances and present the route to applications of this sensor in geosciences.

 

How to cite: Geiger, R., gautier, R., sidorenkov, L., and landragin, A.: Towards a two-axis cold-atom gyroscope for rotational seismology, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8434, https://doi.org/10.5194/egusphere-egu2020-8434, 2020.

EGU2020-13360 | Displays | SM5.1

Resolving dynamic ground motions with high-rate GNSS and implications for data fusion in broadband seismology and Earthquake Early Warning

Roland Hohensinn, Nikolaj Dahmen, John Clinton, Alain Geiger, and Markus Rothacher

In this paper we highlight the potential of geodetic high-precision and high-rate GNSS (Global Navigation Satellite System) sampling (1 to 100 Hz) for resolving seismic ground motions, of both the near and the far field of an earthquake. The analysis of the budget and characteristics of the error of high-rate GNSS displacement time series yields results, discussion, and conclusions on the sensitivity and waveform resolvability as well as on the derivation of a minimum detectable displacement (in the statistical sense).

Based on these analyses, we show how GNSS can contribute to optimal broadband displacement and velocity waveform products by means of data fusion by combining measurements taken from co-located sensors – e.g. accelerometers or gyroscopes – in real-time, near real-time and postprocessing mode. Concerning the inclusion of GNSS for such an analysis, we also briefly explore the ability of GNSS to record signals from different earthquake magnitudes and epicentral distances. We show that high-rate GNSS is sensitive to displacements down to the level of a few millimeters, and even below – an example also comes from the detection of very small vibrations from 100 Hz GNSS data.

We analyze measurements of synthetized signals obtained from experiments with a shake table, as well as from real data from strong earthquakes, namely the 6.5 Mw event of 2016 near the city of Norcia (Italy) and the 7.0 Mw Kumamoto earthquake of 2016 (Japan). Based on these data and our main findings, we finally discuss the role of GNSS in Earthquake Early Warning in terms of a fast hypocenter localization and reliable magnitude estimation.

How to cite: Hohensinn, R., Dahmen, N., Clinton, J., Geiger, A., and Rothacher, M.: Resolving dynamic ground motions with high-rate GNSS and implications for data fusion in broadband seismology and Earthquake Early Warning, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13360, https://doi.org/10.5194/egusphere-egu2020-13360, 2020.

EGU2020-21787 | Displays | SM5.1

The seismic broad-band antenna of the Low Noise Underground Laboratory (LSBB, Rustrel, France): a tool for measuring the rotation ground motion.

Olivier Sèbe, Stéphane Gaffet, Roxanne Rusch, Jean-Baptiste Decitre, Charly Lallemand, Daniel Boyer, Alain Cavaillou, Jean-Marc Koenig, Serge Olivier, and François Schindele

In the last 20 years, seismologists have recognized that a better sensing of the seismic wavefield is obtained by considering the rotational ground motions in addition to the translation measurements usually provided by seismometers. Even though recent technological developments have resulted in new portable rotation sensors with a sensitivity and a bandwidth suited to seismological applications requirements, the ground rotations have for a long time been estimated indirectly by dense seismic arrays.

The Low Noise Underground Laboratory (LSBB) includes a dense 3D seismic antenna composed of 6 STS2 broad-band seismometers since March 2005. From 2016, this array has been upgraded by the installation of about 10 new seismometers at the surface and inside the galleries of the laboratory. Thanks to these dense and small aperture seismic networks, the vertical and horizontal rotations of the ground motion have been estimated by finite difference approximation of the spatial derivatives of the local ground motions. These measurements provide the opportunity to conduct six degree of freedom (6DOF) analysis (3C translations and 3C rotations) to find out the direction of the wave propagation and to estimate the seismic wave local phase velocity.

The performance of this seismic array in deriving the local spatial gradient of the seismic wavefield, as well as the rotation tensor, will be illustrated by several selected seismic records such as the 2016 central Italy crisis (Amatrice and Norcia events) as well as the recent local Teil earthquake. In addition, the Array Derived Rotations (ADR) from the LSBB antenna are compared with the rotations measured by different kinds of rotation sensors including 2 prototypes of the new BlueSeis3A and a Lily Borehole Tiltmeter.

How to cite: Sèbe, O., Gaffet, S., Rusch, R., Decitre, J.-B., Lallemand, C., Boyer, D., Cavaillou, A., Koenig, J.-M., Olivier, S., and Schindele, F.: The seismic broad-band antenna of the Low Noise Underground Laboratory (LSBB, Rustrel, France): a tool for measuring the rotation ground motion., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21787, https://doi.org/10.5194/egusphere-egu2020-21787, 2020.

EGU2020-15258 | Displays | SM5.1

Direct and array-derived rotations in the Gran Sasso underground laboratory: application to earthquakes and seismic noise.

Andreino Simonelli, Matteo Desiderio, Umberto Giacomelli, Gaetano De Luca, Aladino Govoni, and Angela Di Virgilio

In the present work, we analyze rotational and translational ground motions, in order to retrieve the local wavefield properties. We apply the same method of analysis to both earthquake and ambient noise signals, in order to estimate the wave field direction and the phase velocity as a function of frequency.
For the first case, the vertical rotation rate is measured by a large ring laser gyroscope (named GINGERino). Translational motions, on the other hand, are recorded by a broad-band seismometer (a station of the INGV national seismic network IV, code-named GIGS). These instruments are colocated in a gallery inside the facilities of the Laboratorio Nazionale del Gran Sasso (LNGS) of the Istituto Nazionale di Fisica Nucleare (INFN), at 1km depth, and constitute a 4 Components (4C) station. We examine the data recorded in late November 2019 i.e. the earthquakes in Northern Albania, Balkan region and off the coast of Crete. An additional event that struck the Mugello Region (Italy) in early December 2019 is also analyzed. We focus on the rotational motions induced by S and Love waves. In the second case, we exploit a temporary array (named XG) of 3-component broad-band seismometers (code-named GIN*) installed in the facilities of the LNGS. Here, the rotation rate is derived trough a finite-difference scheme involving the stations of the array. First, we test the reliability of the ADR method XG with simulated earthquake data. In a second step we analyse the secondary microseism signal recorded during a sea storm that occurred in the Mediterranean basin in early January 2019. Such workflow is also compared to an f-k analysis, that is a common array data processing method. Finally, theoretical P body waves noise sources are computed and compared to the estimated BAZ.

How to cite: Simonelli, A., Desiderio, M., Giacomelli, U., De Luca, G., Govoni, A., and Di Virgilio, A.: Direct and array-derived rotations in the Gran Sasso underground laboratory: application to earthquakes and seismic noise. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15258, https://doi.org/10.5194/egusphere-egu2020-15258, 2020.

EGU2020-15252 | Displays | SM5.1

Monitoring volcanic and seismic activity with multiple fibre-optic Distributed Acoustic Sensing units at Etna volcano

Charlotte Krawczyk, Philippe Jousset, Gilda Currenti, Michael Weber, Rosalba Napoli, Thomas Reinsch, Giorgio Riccobene, Luciano Zuccarello, Athena Chalari, and Andy Clarke

Volcanic and seismic activities produce a variety of phenomena that put population at risk. Etna volcano provides an example where volcanic and tectonic processes are strongly coupled. Distributed Acoustic Sensing (DAS) technology has been for the first time tested both in 2018 and 2019 as a new tool for monitoring the complex tectonic and volcanic interactions at Etna volcano from summit to the sea floor. We connected up to 3 iDAS interrogators, sometimes simultaneously, to optical cables close to the summit, in urban areas and offshore. Each iDAS measured the dynamic strain rate along the whole length of the optical fibre, from the interferometric analysis of the back-scattered light.

In the summit area, we connect an iDAS interrogator inside the Volcanological Observatory of Pizzi Deneri (2800 m elevation close to Etna summit) to record dynamic strain signals along a 1.5 km-long fibre optic cable that we deployed in the scoria of Piano delle Concazze. We recorded signals associated with various volcanic events, local and distant earthquakes, thunderstorm, as well as many other anthropogenic signals (e.g., tourists). To validate the DAS signal we collocated along the fibre cable multi-parametric arrays (comprising geophones, broadband seismometers, infrasonic arrays). During the survey periods, Etna activity was mainly characterized by moderate but frequent explosive and/or effusive activity from summit craters. Our observations suggests that DAS technology can record volcano-related signals (in the order of tens nanostrain) with unprecedented spatial and temporal resolutions, opening new opportunities for the understanding of volcano processes.

In urban environments, taking advantage of the existence of fibre optic telecommunication infrastructures, we connected iDAS interrogator to fibre optic cables, known to cross active faults linked to the volcano eastern flank dynamics. We recorded dynamic strain rate along a 4 km cable for about 20 days in Zafferana village and along a 12 km-long cable running from Linera to Fleri. We also tested DAS recording along a 40 km-long fiber optic telecommunication cable on the western side of the volcano, at the border between the sedimentary layer and the volcano edifice.

On the sea floor, we connected an iDAS interrogator to a 30-km long fibre within a cable transmitting data from sub-marine instrumentation to INFN-LNS facility at the Catania harbour. We record dynamic strain signals from local and regional earthquakes and detect some previously unknown faults offsetting the sea floor below the eastern flank of the volcano.

Our results demonstrate that DAS technology is able to contribute to the monitoring system of earthquake and volcanic phenomena at Etna volcano, and thereby could improve the volcanic and seismic hazard assessment in the future.

How to cite: Krawczyk, C., Jousset, P., Currenti, G., Weber, M., Napoli, R., Reinsch, T., Riccobene, G., Zuccarello, L., Chalari, A., and Clarke, A.: Monitoring volcanic and seismic activity with multiple fibre-optic Distributed Acoustic Sensing units at Etna volcano, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15252, https://doi.org/10.5194/egusphere-egu2020-15252, 2020.

EGU2020-11641 | Displays | SM5.1

Fibre optic Distributed Acoustic Sensing of volcanic events at Mt Etna

Gilda Currenti, Philippe Jousset, Athena Chalari, Luciano Zuccarello, Rosalba Napoli, Thomas Reinsch, and Charlotte Krawczyk

We explore a unique dataset collected by Distributed Acoustic Sensing (DAS) technology at the summit of Etna volcano in September 2018. We set-up an iDAS interrogator (Silixa) inside the Observatory Pizzi Deneri to record strain rate signals along a 1.3 km-long fibre optic cable deployed in Piano delle Concazze. This area is affected by several North-South trending faults and fractures, that are originated to accommodate the extension of the nearby North-East Rift zone, where magmatic intrusions often occur. The field evidence of the segments of these faults and fractures is hidden by lava flows and volcano-clastic deposits (e.g. scoria and lapilli) produced by the effusive and explosive activity of Etna volcano.

We propose a new technological and methodological framework to validate, identify and characterize volcano-related dynamic strain changes at an unprecedented high spatial (2 m) and temporal (1 kHz) sampling over a broad frequency range. The DAS record analysis and the validation of the iDAS response is performed through comparisons with measurements from a dense network of conventional sensors (comprising 5 broadband seismometers, 15 short-period geophone and two arrays of 3 infrasound sensors) deployed along  the fibre optic cable.  Comparisons between iDAS signals and dynamic strain changes estimated from the broadband seismic array shows an excellent agreement, thus demonstrating for the first time the capability of DAS technology in sensing seismic waves generated by volcanic events.

The frequent and diverse Etna activity during the acquisition period (30 August - 16 September 2018) offers the great opportunity to record a wide variety of signals and, hence, to test the response of iDAS to several volcanic processes (e.g. volcanic tremor, explosions, strombolian activity, local seismic events). Here, we focus the analysis on the signals recorded during a small explosive event on 5 September 2018 from the New South-East Crater (NSEC). This explosive event generated both seismic waves (recorded by the seismometers) propagating in the ground, and acoustic pressure signals (recorded by the infra-sound arrays) propagating in the atmosphere. We show that the DAS records catch both, as confirmed by the conventional sensors records.

Spectrogram analyses of the DAS signals reveal that the frequency content is confined in two distinctive frequency bands in the ranges 0.5-10 Hz and 18-25 Hz, for the seismic and acoustic wave, respectively. The amplitude and frequency response of the ground to the arrival and propagation of the seismo-acoustic wave along the fibre reveal spatial characteristic patterns that reflect local geological structures. For example, the finer spatial sampling of the iDAS records allows catching details of the variability of dynamic strain amplitudes along the fibre. Amplified signals are found at localized narrow regions matching fracture zones and faults. There, a decrease in the propagation velocity of the seismo-acoustic waves is also clearly pinpointed. 

These preliminary findings demonstrate the DAS potentiality to revolutionize the study of volcanic process by discovering new signal features undetectable with traditional sensors and methodologies.

How to cite: Currenti, G., Jousset, P., Chalari, A., Zuccarello, L., Napoli, R., Reinsch, T., and Krawczyk, C.: Fibre optic Distributed Acoustic Sensing of volcanic events at Mt Etna , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11641, https://doi.org/10.5194/egusphere-egu2020-11641, 2020.

EGU2020-2321 | Displays | SM5.1

Long-Term Monitoring with Fiber Optics Distributed Temperature Sensing at Campi Flegrei: The Campi Flegrei Deep Drilling Project

Renato Somma, Claudia Troise, Luigi Zeni, Alessandro Minardo, Alessandro Fedele, Maurizio Mirabile, and Giuseppe De Natale

Monitoring volcanic phenomena is a key question, for both volcanological research and for civil protection purposes. This is particularly true in densely populated volcanic areas, like the Campi Flegrei caldera, including part of the large city of Naples (Italy). Borehole monitoring of volcanoes is the most promising way to improve classical methods of surface monitoring, although not commonly applied yet. Fiber Optics technology is the most practical and suitable way to operate in such high temperature and aggressive environmental conditions. In this paper, we describe a fiber optics DTS (Distributed Temperature Sensing) sensor, which has been designed to continuously measure temperature all along a 500 m. deep well drilled in the West side of Naples (Bagnoli area), lying in the Campi Flegrei volcanic area. It has been then installed as part of the international ‘Campi Flegrei Deep Drilling Project’, and is continuously operating, giving insight on the time variation of temperature along the whole borehole depth. Such continuous monitoring of temperature can in turn indicate volcanic processes linked to magma dynamics and/or to changes in the hydrothermal system. The developed monitoring system, working at bottom temperatures higher than 100 °C, demonstrates the feasibility and effectiveness of using DTS for borehole volcanic monitoring.

How to cite: Somma, R., Troise, C., Zeni, L., Minardo, A., Fedele, A., Mirabile, M., and De Natale, G.: Long-Term Monitoring with Fiber Optics Distributed Temperature Sensing at Campi Flegrei: The Campi Flegrei Deep Drilling Project, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2321, https://doi.org/10.5194/egusphere-egu2020-2321, 2020.

EGU2020-8442 | Displays | SM5.1

Danish earthquake recorded by distributed acoustic sensing (DAS)

Camilla Rasmussen, Peter H. Voss, and Trine Dahl-Jensen

On September 16th 2018 a Danish earthquake of local magnitude 3.7 was recorded by distributed acoustic sensing (DAS) in a ~23 km long fibre-optic cable. The data are used to study how well DAS can be used as a supplement to conventional seismological data in earthquake localisation. One of the goals in this study is extracting a small subset of traces with clear P and S phases to use in an earthquake localisation, from the 11144 traces the DAS system provide. The timing in the DAS data might not be reliable, and therefore differences in arrival times of S and P are used instead of the exact arrival times.
The DAS data set is generally noisy and with a low signal-to-noise ratio (SNR). It is examined whether stacking can be used to improve SNR. The SNR varies a lot along the fibre-optic cable, and at some distances, it is so small that the traces are useless. Stacking methods for improving SNR are presented.

A field test at two location sites of the fibre-optic cable was conducted with the purpose of comparing DAS data with seismometer data. At the field sites, hammer shots were recorded by a small array of three STS-2 sensors located in a line parallel to the fibre-optic cable. The recordings generally show good consistency between the two data sets. 
In addition, the field tests are used to get a better understanding of the noise sources in the DAS recording of the earthquake. There are many sources of noise in the data set. The most prominent are a line of windmills that cross the fibre-optic cable and people walking in the building where the detector is located. Also, the coupling between the fibre-optic cable and the ground varies along the cable length due to varying soil type and wrapping around the fibre-optic cable, which is also evident in field test data. Furthermore, the data from the field tests are used to calibrate the location of the fibre-optic cable, which is necessary for using the DAS data in an earthquake localisation.
Data processing is done in Matlab and SEISAN.

How to cite: Rasmussen, C., Voss, P. H., and Dahl-Jensen, T.: Danish earthquake recorded by distributed acoustic sensing (DAS), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8442, https://doi.org/10.5194/egusphere-egu2020-8442, 2020.

EGU2020-6767 | Displays | SM5.1

Catalog of 3C rotations measured at the LSBB’s antenna.

Roxanne Rusch, Olivier Sebe, Jean-Baptiste Décitre, and Stéphane Gaffet

Rotational seismology refers to the study of the 3 components of rotation that are part of the
seismic wave field equation (Aki and Richards, 2002). Using the Geodetic Method (GM)
[Spudich et al, 1995], that is, using spatial finite differences of the local ground motions, the
3C of rotations were calculated at the dense broadband seismic array of the LSBB (Low
background noise underground research Laboratory, France, http://lsbb.eu). A catalog of 3C
rotations was created by systematically applying this method to several seismic events. The
uncertainty of the rotation measurements has been estimated through a sensitivity analysis.
This catalog will be presented and illustrated using examples. The analysis of the rotational
motions relatively to the seismic events source’s properties will be discussed as well.

How to cite: Rusch, R., Sebe, O., Décitre, J.-B., and Gaffet, S.: Catalog of 3C rotations measured at the LSBB’s antenna., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6767, https://doi.org/10.5194/egusphere-egu2020-6767, 2020.

EGU2020-16191 | Displays | SM5.1

Towards field data applications of six-component polarization analysis

David Sollberger, Heiner Igel, Cedric Schmelzbach, Felix Bernauer, Shihao Yuan, Joachim Wassermann, André Gebauer, Ulrich Schreiber, and Johan Robertsson

The analysis of the relative amplitudes of a passing seismic wave recorded on a single seismometer measuring six degrees of freedom of ground motion (translation and rotation) theoretically allows one to extract information on the wave that can conventionally only be obtained from receiver arrays. In the past, it has been shown on numerical data that the extension of conventional three-component (3C) polarization analysis techniques to six-components, allows one to unambiguously identify the wave type of a passing wave and characterise it in terms of its propagation direction (without the 180° ambiguity inherent in 3C data) and local wave speed. Additionally, due to the increase in the dimensionality of the data, two waves arriving at a station at the same time can be simultaneously characterised under ideal conditions (low noise).

Attempts to apply such 6-C polarization analysis techniques to field data have so far been met with limited success. Varying noise levels on the individual components and complex wavefields (with more than two interfering waves arriving at the station at the same time) usually prevent the stable recovery of wave parameters using single-station 6-C polarization analysis.

Here we discuss first attempts to overcome these issues. We (1) test the robustness of different wave parameter estimators (maximum likelihood, MUSIC) towards high levels of noise and (2) we try to reduce the number of interfering events in the analysis window by performing 6-C polarization analysis on time-frequency decomposed seismograms (i.e. spectrograms) using the S-transform.

The new techniques are extensively tested on field data recorded on the high-performance ROMY ringlaser.

How to cite: Sollberger, D., Igel, H., Schmelzbach, C., Bernauer, F., Yuan, S., Wassermann, J., Gebauer, A., Schreiber, U., and Robertsson, J.: Towards field data applications of six-component polarization analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16191, https://doi.org/10.5194/egusphere-egu2020-16191, 2020.

EGU2020-11000 | Displays | SM5.1

Challenges of DAS measurements in seismic urban areas: case study at Etna volcano eastern flank

Rosalba Napoli, Gilda Currenti, Athena Chalari, Camille Jestin, Danilo Contrafatto, Philippe Jousset, Graziano Larocca, Daniele Pellegrino, Mario Pulvirenti, and Antonino Sicali

We present the use of distributed acoustic sensing of telecommunication fibre to perform seismic monitoring on the lower eastern flank of Etna volcano. Eastern flank of Etna is structurally characterized by the existence of many faults until under the sea. One of the clearest morphological feature is the Timpe Fault System (TFS) crossing highly populated urban areas. The TFS is formed by several main segments producing shallow seismicity with a dominant normal faulting style and a right-lateral component, related to WNW-ESE regional extension. This area is highly seismogenic, with occurrence of a very frequent seismic activity punctuated by destructive earthquakes with magnitude ranges 4.3≤ML≤5.1 and a mean recurrence time of about 20 years.

To monitor the seismic response of this area we deployed an “intelligent” Distributed Acoustic Sensing (iDAS) system (SILIXA) in order to interrogate a 12-km-long telecommunication fibre-optic cable, managed by TELECOM Italia internet provider. The telecom cable runs from Linera to Zafferana villages along two primary directions roughly N-S and E-W and crosses the Santa Venerina and the Fiandaca faults, both part of the TFS. The former was entirely hidden until the 2002 eruption when a ML 4.4 earthquake exposed the fault at the surface and heavily damaged Santa Venerina village. The latter has been reactivated during the 2018 Etna activity, when a ML4.8 earthquake strongly damaged the Fleri village.

The iDAS was in acquisition for three months (11 September - 9 December 2019) and recorded the strain rate from natural and anthropogenic sources at a sampling frequency of 1 kHz with 2-m spatial resolution and a gauge length of 10 m. A second fibre in the same cable, was interrogated simultaneously by a FEBUS A1 system (FEBUS OPTICS) from 2 to 9 December 2019 with a spatial resolution and a gauge length of 5 m at a sampling frequency of 200 Hz. To validate the DAS measurements, gathered by both systems, two broadband seismometers (Trillium Compact 120 s) were deployed in the vicinity of the cable. We located using hammer shots along the cable at key positions.

During the acquisition period more than 800 local seismic events occurred on Etna with ML ranging between 0.4 and 3.4. Several regional earthquakes from Greece and Albania also occurred up to ML6.1. These seismic sources allows for investigating the response of the fibre and the detectability thresholds of iDAS and FEBUS A1 in urban areas with heterogeneous installation conditions of the telecommunication cable (cased conduit, attached conduit, aerial track).  We perform data analysis to characterize DAS amplitude and frequency responses to better estimate the coupling of the fibre to the ground.

How to cite: Napoli, R., Currenti, G., Chalari, A., Jestin, C., Contrafatto, D., Jousset, P., Larocca, G., Pellegrino, D., Pulvirenti, M., and Sicali, A.: Challenges of DAS measurements in seismic urban areas: case study at Etna volcano eastern flank, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11000, https://doi.org/10.5194/egusphere-egu2020-11000, 2020.

EGU2020-8225 | Displays | SM5.1

Urban Distributed Acoustic Sensing Using In-Situ Fibre Beneath Bern, Switzerland

Krystyna Smolinski, Patrick Paitz, Daniel Bowden, Pascal Edme, Felix Kugler, and Andreas Fichtner

Anticipating the risks natural hazards pose to an urban environment requires an understanding of the shallow Earth structure of the region. While urban infrastructure often hinders the deployment of a traditional seismic array, Distributed Acoustic Sensing (DAS) technology facilitates the use of existing telecommunication fibre-optic cables for seismic observation, with spatial resolution down to the metre scale.

Through collaboration with the SWITCH foundation, we were able to use existing, in-situ fibres beneath Bern, Switzerland for seismic data acquisition over two weeks, covering a distance of 6 km with a spatial resolution of 2 m. This allowed for not only real-time visualisation of anthropogenic noise sources (e.g. road traffic), but also of the propagation of resulting seismic waves.

Data is analysed in the time and frequency domain to explore the range of signals captured and to assess the consistency of data quality along the cable. The local velocity structure can be constrained using both noise correlations and deterministic signals excited by traffic.

Initial results reveal the ability of DAS to capture signals over a wide range of frequencies and distances, and show promise for utilising urban DAS data to perform urban seismic tomography and hazard analysis.

How to cite: Smolinski, K., Paitz, P., Bowden, D., Edme, P., Kugler, F., and Fichtner, A.: Urban Distributed Acoustic Sensing Using In-Situ Fibre Beneath Bern, Switzerland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8225, https://doi.org/10.5194/egusphere-egu2020-8225, 2020.

EGU2020-20200 | Displays | SM5.1

New prototype of 6-component seismograph Rotaphone CY: laboratory testing and pilot measurements

Jiri Vackar, Jiri Malek, and Johana Brokesova

Rotaphones are seismic sensor systems consisting of parallel pairs of geophones attached to a rigid frame anchored to ground. Such an arrangement allows to measure both translational and rotational ground motions. Translations are measured by individual geophones while rotations are determined using differential records from the paired geophones. The individual geophones are calibrated simultaneously with each measurement utilizing overdetermined rotational components. A new prototype, Rotaphone CY has been recently developed. The design has been improved taking into account experience with field measurements performed using older prototypes. The device is optimized for recording weak ground motions from local microearthquakes, both natural or induced, in a high-frequency range. The instruments were carefully tested in laboratory conditions. Tests were followed by pilot field deployments in various places in the Czech Republic. A local network of six Rotaphones CY has been deployed in the scope of Litomerice geothermal project to investigate induced seismicity related to the production of geothermal energy. The instrument has also been recently deployed at the nuclear power plant Dukovany to monitor local seismicity with the aim to improve seismic hazard estimate. A small-aperture array of four these instruments was installed at the Geophysical Observatory Fürstenfeldbruck, Germany, in the frame of a comparative rotation sensors experiment. Examples of 6-component records from these pilot measurements are shown.

How to cite: Vackar, J., Malek, J., and Brokesova, J.: New prototype of 6-component seismograph Rotaphone CY: laboratory testing and pilot measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20200, https://doi.org/10.5194/egusphere-egu2020-20200, 2020.

EGU2020-19873 | Displays | SM5.1

Quantum sensors for gravimetry and gravity gradiometry

Christian Schubert, Waldemar Herr, Sven Abend, Naceur Gaaloul, Dennis Schlippert, Wolfgang Ertmer, and Ernst M. Rasel

Quantum sensors utilising atom interferometry enable absolute measurements of gravity (1) and gravity gradients (2). This contribution will introduce the operation principle of atom interferometers, discuss their current state of the art and limitations, and outline the key techniques for improvements. It will report on our developments for transportable (3) and stationary high-perfomance devices (4) and give a perspective for space-borne quantum sensors in the frame of geodetic missions (5).
The presented work is supported by the CRC 1227 DQmat within the project B07, the German Space Agency (DLR) with funds provided by the Federal Ministry of Economic Affairs and Energy (BMWi) due to an enactment of the German Bundestag under Grant No. DLR 50WP1700, 50WM1952, by "Niedersächsisches Vorab" through "Förderung von Wissenschaft und Technik in Forschung und Lehre" for the initial funding of research in the new DLR-SI Institute, and through the "Quantum and Nano- Metrology (QUANOMET)" initiative within the project QT3.

(1) V. Ménoret et al., Scientific Reports 8, 12300, 2018; A. Trimeche et al., Phys. Rev. Appl. 7, 034016, 2017; C. Freier et al., J. of Phys.: Conf. Series 723, 012050, 2016; A. Louchet-Chauvet et al., New J. Phys. 13, 065026, 2011; A. Peters et al., Nature 400, 849, 1999.
(2) P. Asenbaum et al., Phys. Rev. Lett. 118, 183602, 2017; G. Rosi et al., Nature 510, 518, 2014; J. M. McGuirk et al., Phys. Rev. A 65, 033608, 2002.
(3) S. Abend et al., Proceedings of the International School of Physics "Enrico Fermi" 197, 393, 2019; S. Abend et al., Phys. Rev. Lett. 117, 203003, 2016.
(4) D. Schlippert et al., arXiv:1909.08524; J. Hartwig et al., New J. Phys. 17, 035011, 2015.
(5) A. Trimeche et al., Class. Quantum Grav. 36, 215004, 2019; K. Douch et al., Adv. Space. Res. 61, 1301, 2018.

How to cite: Schubert, C., Herr, W., Abend, S., Gaaloul, N., Schlippert, D., Ertmer, W., and Rasel, E. M.: Quantum sensors for gravimetry and gravity gradiometry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19873, https://doi.org/10.5194/egusphere-egu2020-19873, 2020.

EGU2020-19987 | Displays | SM5.1

Assembly of optical fiber sensors for rotational seismology - data coherence and comparability issues in field application

Anna Kurzych, Leszek R. Jaroszewicz, Michał Dudek, Zbigniew Krajewski, Jerzy K. Kowalski, Sławomir Niespodziany, Felix Bernauer, Joachim Wassermann, and Heiner Igel

Nowadays rotational seismology has become rapidly developing field of study which can deliver completely new  perspectives for earthquakes analysis or torsional effects in engineering structures. Rotational seismology as a scientific field has been clarified in 2009 as a field for researching all aspects of rotational ground movements generated by earthquakes, explosions, and ambient vibrations. Nevertheless, technical requirements for sensors in this field are very strict and rigorous, especially taking into account measuring range from 10-7 rad/s event up to few rad/s. In order to fulfill all technical requirements for sensors which can be applied in rotational seismology measurements we designed and constructed device based on an optical fiber gyroscope (FOG). Fibre-Optic System for Rotational Events&phenomena Monitoring (FOSREM) is a an interferometric optical fiber sensor designed to continuously observe rotational effects. It uses closed-loop configuration which is based on the compensatory phase measurement method as well as specific electronic system. It should be noticed, that the coupling of the FOSREM’s optical part which detects critical low value of signal with specialized electronic system which requires precise analog to digital conversion as well as data transfer with different sampling rate is the source of differences between constructed devices even in the same technology. In this paper we present laboratory investigation of FOSREMs including Allan variance analysis indicating that Angle Random Walk is equal to 10-7 rad/s. Expect laboratory verification of proper FOSREMs’ operation we carried out field tests taking into account that validity and reliability of the research instruments are crucial during field application. The quality of data utilised in any research determines the outcome of the research and its importance for further research work and relevance to scientific community and knowledge. The data reliability can be determined by comparison between records from several sensors. We present first data from international field research which was focused on efforts to achieve uniformity in collecting and data processing. This experiment involved more than 40 rotational and strain sensors and took place in Geophysical Observatory Fürstenfeldbruck, LMU Munich, Germany. Authors applied four FOSREMs in this experiment and the presented analysis was focused on their data comparability as well as consistency.

How to cite: Kurzych, A., Jaroszewicz, L. R., Dudek, M., Krajewski, Z., Kowalski, J. K., Niespodziany, S., Bernauer, F., Wassermann, J., and Igel, H.: Assembly of optical fiber sensors for rotational seismology - data coherence and comparability issues in field application , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19987, https://doi.org/10.5194/egusphere-egu2020-19987, 2020.

EGU2020-11887 | Displays | SM5.1

Extending the Pegasus Portable Technology Platform to Apply to More Geophysical Monitoring Use

Bruce Townsend, Andrew Moores, and Sylvain Pigeon

The new Nanometrics Pegasus now available to the scientific community provides a compelling comprehensive solution for easily and quickly deploying portable seismic stations. Use cases include RAMP, local and regional hazard monitoring, passive seismic imaging and local or regional seismicity assessments. Because the technology platform on which Pegasus Portable is based is extensible and versatile, it has the potential to address additional use cases. The benefits of the Pegasus technology would apply to new use cases: optimal SWaP (Size, Weight and Power), Modularity (the versatility permits wide choice of sensors and power to serve various situations), Ease-of-Use (workflows designed for planning-to-publishing efficiency), Complete Ready-to-Use Datasets (including automatically generated station response), and Quick (such as ultra-fast boot, rapid data download). We explore the potential extensions to Pegasus that can enable additional use cases for autonomous geophysical monitoring.

An example is large-N mixed-mode nodal deployments in which hundreds of stations are quickly deployed that can include a mix of sensor types such as broadband seismometers, geophones, microbarometers, and weather stations. A key focus for large-N campaigns is to scale efficiently. One proposed element for consideration is a cloud-based campaign planning and post-deployment auditing service in which a master plan can be readily distributed to many field operators to facilitate automatic station configuration and later reconciliation of on-the-ground actions with the master plan. 

Another compelling use case for Pegasus is ocean bottom seismometry, where technology enablers would include OBS-specific actions and workflows (managing the datalogger and its power sources without having to open marine pressure vessels, synchronizing timing to GNSS, applying time corrections to retrieved data and the like). These and other use cases and related technology extensions are discussed.

How to cite: Townsend, B., Moores, A., and Pigeon, S.: Extending the Pegasus Portable Technology Platform to Apply to More Geophysical Monitoring Use, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11887, https://doi.org/10.5194/egusphere-egu2020-11887, 2020.

A standard ocean bottom seismometer (OBS) package of the U.S. OBS Instrument Pool (OBSIP) carries a seismometer and a pressure sensor. For broadband applications, the seismometer typically is a wide-band or broad-band three-components seismometer, and the pressure sensor is a differential pressure gauge (DPG). The purpose of the pressure sensor is manifold and includes the capture of pressure signals not picked up by a ground motion sensor (e.g. the passage of tsunami), but also for purposes of correcting the seismograms for unwanted signals generated in the water column (e.g. p-wave reverberations).
Unfortunately, the instrument response of the widely used Cox-Webb DPG remains somewhat poorly known, and can vary by individual sensor, and even by deployment of the same sensor.

Efforts have been under way to construct and test DPG responses in the laboratory. But the sensitivity and long‐period response are difficult to calibrate as they  vary with temperature and pressure, and perhaps by hardware of the same mechanical specifications.  Here, we present a way to test the response for each individual sensor and deployment in situ in the ocean. This test requires a relatively minimal and inexpensive modification to the OBS instrument frame and a release mechanism that allows a drop of the DPG by 3 inches after the OBS package settled and the DPG equilibrated on the seafloor. The seismic signal generated by this drop is then analyzed in the laboratory upon retrieval of the data. 

The results compare favorably with calibrations estimated independently through post‐deployment data analyses of other signals such as Earth tides and the signals from large teleseismic earthquakes. Our study demonstrates that observed response functions can deviate from the nominal response by a factor of two or greater with regards to both the sensitivity and the time constant. Given the fact that sensor calibrations of DPGs in the lab require very specific and stable environments and are time consuming, the use of in-situ DPG calibration frames pose a reliable and inexpensive alternative. 

How to cite: Laske, G. and Doran, A.: In-Situ Calibration of Differential Pressure Gauges on OBSIP Ocean Bottom Seismometers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6145, https://doi.org/10.5194/egusphere-egu2020-6145, 2020.

EGU2020-4256 | Displays | SM5.1

Rotation and Strain Instrument Performance Tests with Active Seismic Sources

Felix Bernauer, Joachim Wassermann, Katrin Behnen, Heiner Igel, Stefanie Donner, Pascal Edme, David Sollberger, Patrick Paitz, Jonas Igel, Gizem Izgi, Eva P.S. Eibl, Stefan Buske, Christian Veress, Frederic Guattari, Olivier Sebe, Basil Brunner, Anna T. Kurzych, Piotr Bonkovsky, Piotr Bobra, and Johana Brokesova and the Fürstenfeldbruck Experiment Team

Interest in measuring seismic rotation and strain is growing in many areas of geophysical research. This results in a great need for reliable and field deployable instruments measuring ground rotation and strain. To further establish a high quality standard for rotation and strain measurements in seismology, researchers from the Ludwig-Maximilians University of Munich (LMU), the German Federal Institute for Geosciences and Natural Resources, the University of Potsdam and the ETH Zürich organized a comparative sensor test experiment which took place in November 2019 at the Geophysical Observatory of the LMU in Fürstenfeldbruck, Germany. More than 40 different sensors such as ring-laser and fiber optic gyroscopes, a Distributed Acoustic Sensing (DAS) cable and interrogator, liquid-based as well as mechanical rotation sensors were involved in addition to 12 classical broadband
seismometers and a 80 channel, 4Hz geophone chain. The experiment consisted of two parts: during the first part, the sensors were co-located in a huddle test recording self noise and signals from small, nearby explosions. In a second part, the sensors were distributed into the field in various array configurations recording active seismic signals generated by small amounts of explosive and a vibro-seis truck. This contribution presents details on the setup of the experiment and first results on sensor performance characteristics and signal similarities.

How to cite: Bernauer, F., Wassermann, J., Behnen, K., Igel, H., Donner, S., Edme, P., Sollberger, D., Paitz, P., Igel, J., Izgi, G., Eibl, E. P. S., Buske, S., Veress, C., Guattari, F., Sebe, O., Brunner, B., Kurzych, A. T., Bonkovsky, P., Bobra, P., and Brokesova, J. and the Fürstenfeldbruck Experiment Team: Rotation and Strain Instrument Performance Tests with Active Seismic Sources, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4256, https://doi.org/10.5194/egusphere-egu2020-4256, 2020.

EGU2020-15759 | Displays | SM5.1

Assessing a 6C Kalman filter using experimental datasets from an industrial robot

Yara Rossi, Eleni Chatzi, Markus Rothacher, John Clinton, and Cédric Schmelzbach

Current best practice in monitoring earthquake strong motion are dense networks comprising strong motion accelerometers that measure acceleration over a broad frequency and amplitude range. These instruments are capable of measuring translational motions of large earthquakes, but lack sensitivity to very low frequencies or permanent displacements. However, it is widely accepted that during earthquakes the rotational component of the ground motion, both static and dynamic, is large enough to contaminate the derived displacements from these sensors. Modern rotational sensors, are also very broadband, have a large dynamic range, and are not sensitive to translational motions. We explore the value of complementing accelerometers with these rotational sensors at seismic strong motion monitoring stations.

The assessment of the errors introduced into accelerometer records from rotational ground motions is only possible with co-located rotational instruments sensitive enough to record the small rotation rates accompanying the translational motion. Operating accelerometers alongside gyros and additionally GNSS instrumentation should allow us to record the full 6 components (6C) of near-field earthquake motions, with increasing fidelity across a very broad frequency band for the strongest motions.

We aim to demonstrate how, using a combination of the three sensor types, we can recover the full 6C ground motion, and hence also more reliable displacement records, using a versatile industrial six-axis robot that can produce controlled and repeatable 6C motion across a broad frequency band. Through the precise feedback loop used by the robot to stabilize its precise trajectory, we get a 6C recording of the driven motion represented by Euler rotations and displacements, which we use as ground truth. By simulating a combination of translational and rotational motions on the robot, we show that the 6C Kalman filter can accurately reproduce the clean simulated translational motion. By using a Kalman filter, we attempt to combine the different data sets using prediction and weighting of the observation data for an optimal solution. Our methodology tries to take into account the strengths and weaknesses of the individual instruments that are providing partly redundant and partly complementary ground motion information.

How to cite: Rossi, Y., Chatzi, E., Rothacher, M., Clinton, J., and Schmelzbach, C.: Assessing a 6C Kalman filter using experimental datasets from an industrial robot, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15759, https://doi.org/10.5194/egusphere-egu2020-15759, 2020.

EGU2020-7759 | Displays | SM5.1

Deployment recommendation for Distributed Acoustic Sensing at the surface

Pascal Edme, Patrick Paitz, Ana Nap, Francois Martin, Valentin Metraux, Luca Guglielmetti, Cedric Schmelzbach, Vincent Perron, Daniel Bowden, David Dupuy, Andrea Moscariello, Andreas Fichtner, and Johan O. A. Robertsson

Distributed Acoustic Sensing (DAS) is an optical interferometry based ground motion sensing technology which has the potential to revolutionize the field of seismological data acquisition. It offers the possibility to replace very large numbers of cost-intensive conventional point sensors (seismometers or geophones) by interrogating a single low-cost optic-fibre cable. Being unaffected by spatial aliasing, DAS is emerging as a potential next-generation broad-band geo-hazard (e.g. earthquakes, landslides) and reservoir (e.g. geothermal, oil and gas) seismic monitoring tool.

For borehole applications, with the cable appropriately coupled with the casing, the reliability and benefit of DAS-based VSP acquisition is now widely recognized. At the surface however, for reflection seismic for example, the adequate deployment procedure is less well documented, and experiments are performed with cables sometimes directly deployed on the surface, or sometimes buried quite deep (e.g. one meter) in the ground. Especially for non-permanent monitoring, the trenching effort can be substantial or unaffordable due to logistic or permitting issues. One may wonder if such an effort with its associated cost is actually beneficial.

We present here the results of a surface-based active seismic experiment conducted in Switzerland in the context of a geothermal reservoir characterization project with “co-located” stretches of cable deployed at different depths. The repeatability of the DAS measurements is quantified and compared to a dense array of conventional multi-component geophones. The study shows that deeply (50 cm) deployed cables offers only marginal data quality improvements compared to very shallow (2 cm) cables. In contrast, the parts of the cable directly laid down at the surface exhibit much larger noise levels and very poor repeatability (approximately one order of magnitude larger NRMS). Our study suggests that only a minor amount of elastic material covering the cable is enough to provide a good coupling and that a modest machine to conveniently perform such a shallow deployment would greatly benefit the growing DAS user community.

How to cite: Edme, P., Paitz, P., Nap, A., Martin, F., Metraux, V., Guglielmetti, L., Schmelzbach, C., Perron, V., Bowden, D., Dupuy, D., Moscariello, A., Fichtner, A., and Robertsson, J. O. A.: Deployment recommendation for Distributed Acoustic Sensing at the surface, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7759, https://doi.org/10.5194/egusphere-egu2020-7759, 2020.

EGU2020-5602 | Displays | SM5.1

Multiple 6C-station Huddle Test in Fürstenfeldbruck, Germany

Gizem Izgi, Stefanie Donner, Felix Bernauer, Daniel Vollmer, Klaus Stammler, Mathias Hoffmann, and Eva P.S. Eibl

Rotational motions play a key role in measuring seismic wavefield properties. To fully understand and describe the behavior of seismic waves, both translational and rotational components should be properly investigated. Portable blueSeis-3A (iXblue) sensors allow to measure 3 components of rotational motions with high sensitivity in a frequency range from 0.001 Hz to 50 Hz.

A huddle test was performed in Fürstenfeldbruck, Germany by the University of Potsdam in collaboration with the Ludwig-Maximilians University of Munich (LMU) and Federal Institute for Geosciences and Natural Resources (BGR) between 26 of August and 02 of September 2019, in order to further investigate the performance of multiple rotational instruments in combination with seismometers. Within the scope of this test, 5 rotational and 3 translational sensors were deployed on the basement of the observatory on decoupled plinth. Our preliminary results show good correlation between all components and rotational sensors. To investigate the coherent noise between sensors, we applied a 50 Hz low-pass filter and 100 Hz sampling rate. To better illustrate, probabilistic power spectral densities and spectrograms have been created. In general, we will discuss the reliability of the data recorded by rotational sensors for further investigations.

 

How to cite: Izgi, G., Donner, S., Bernauer, F., Vollmer, D., Stammler, K., Hoffmann, M., and Eibl, E. P. S.: Multiple 6C-station Huddle Test in Fürstenfeldbruck, Germany , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5602, https://doi.org/10.5194/egusphere-egu2020-5602, 2020.

SM5.6 – Pan-European Seismic Waveform Data, Services and Products

EGU2020-8389 | Displays | SM5.6

ORFEUS Services for Coordinated High-Quality Seismic Waveform Data Access in Pan-Europe

Carlo Cauzzi, Jarek Bieńkowski, Susana Custódio, Christos Evangelidis, Philippe Guéguen, Christian Haberland, Florian Haslinger, Giovanni Lanzano, Thomas Meier, Alberto Michelini, Lars Ottemöller, Helle Pedersen, Javier Quinteros, Reinoud Sleeman, Angelo Strollo, and Luca Trani

ORFEUS (Observatories and Research Facilities for European Seismology) is a non-profit foundation that promotes seismology in the Euro-Mediterranean area through the collection, archival and distribution of seismic waveform data, metadata and closely related products. The data and services are collected or developed at national level by more than 60 contributing Institutions in Pan-Europe and further developed, integrated, standardized, homogenized and promoted through ORFEUS. Among the goals of ORFEUS are: (a) the development and coordination of waveform data products; (b) the coordination of a European data distribution system, and the support for seismic networks in archiving and exchanging digital seismic waveform data; (c) the encouragement of the adoption of best practices for seismic network operation, data quality control and data management; (d) the promotion of open access to seismic waveform data, products and services for the broader Earth science community.  These goals are achieved through the development and maintenance of services targeted to a broad community of seismological data users, ranging from earth scientists to earthquake engineering practitioners. Two Service Management Committees (SMCs) are consolidated within ORFEUS devoted to managing, operating and developing (with the support of one or more Infrastructure Development Groups): (i) the European Integrated waveform Data Archive (EIDA; https://www.orfeus-eu.org/data/eida/); and (ii) the European Strong-Motion databases (SM; https://www.orfeus-eu.org/data/strong/). A new SMC is being formed to represent the community of European mobile pools. Products and services for computational seismologists are also considered for integration in the ORFEUS domain. ORFEUS services currently provide access to the waveforms acquired by ~ 10,000 stations in Pan-Europe, including dense temporary experiments, with strong emphasis on open, high-quality data. Contributing to ORFEUS data archives means long-term archival, state-of-the-art quality control, improved access and increased  usage. Access to data and products is ensured through state-of-the-art information and communications technologies, with strong emphasis on federated web services that considerably improve seamless user access to data gathered and/or distributed by ORFEUS institutions. The web services also facilitate the automation of downstream products. Particular attention is paid to adopting clear policies and licences, and acknowledging the crucial role played by data providers / owners, who are part of the ORFEUS community. There are significant efforts by ORFEUS participating Institutions to enhance the existing services to tackle the challenges posed by the Big Data Era, with emphasis on data quality, improved user experience, and implementation of strategies for scalability, high-volume data access and archival. ORFEUS data and services are assessed and improved through the technical and scientific feedback of a User Advisory Group (UAG), comprised of European Earth scientists with expertise encompassing a broad range of disciplines. All ORFEUS services are developed in coordination with EPOS and are largely integrated in the EPOS Data Access Portal. ORFEUS is one of the founding Parties and fundamental pillars of EPOS Seismology. This contribution presents the current products and services of ORFEUS and introduces the planned key future activities. We aim at stimulating Community feedback about the current and planned ORFEUS strategies.

How to cite: Cauzzi, C., Bieńkowski, J., Custódio, S., Evangelidis, C., Guéguen, P., Haberland, C., Haslinger, F., Lanzano, G., Meier, T., Michelini, A., Ottemöller, L., Pedersen, H., Quinteros, J., Sleeman, R., Strollo, A., and Trani, L.: ORFEUS Services for Coordinated High-Quality Seismic Waveform Data Access in Pan-Europe, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8389, https://doi.org/10.5194/egusphere-egu2020-8389, 2020.

EGU2020-19125 | Displays | SM5.6

Yasmine : A new tool for stationXML

Jean-Marie Saurel, Sidney Hellman, Robert Casey, Mike Hagerty, Stefan Lisowski, Constanza Pardo, Helle Pedersen, Catherine Péquegnat, Tim Ronan, Jonathan Schaeffer, Oleksandr Sukhotskyi, Mary Templeton, Chad Trabant, and David Wolyniec

In 1987 The International Federation of Digital Seismograph Networks (FDSN) was formed and the SEED (Standard for the Exchange of Earthquake Data) format was adopted as its standard for digital seismic data exchange.  In addition, since at least 1991, it has been common practice to generate dataless SEED volumes, containing only station metadata, for distribution.  The success of the SEED format as a global standard can be judged from the high level of data exchange in the  seismological community.

A few years ago FDSN working group II was tasked with updating the representation of seismic metadata.  The result, stationXML, is defined in a modern XML schema and extends the SEED representation of metadata.  Today, most of the worldwide seismic datacenters, including the entire EIDA framework, are already distributing metadata in stationXML format, or will do so soon.

While some client-side software (e.g., ObsPy) exists for reading stationXML, there are relatively few standalone and dedicated solutions available for metadata producers to generate and edit stationXML.  Here we describe a tool for the creation and management of stationXML, initially developed by IRIS and ISTI. Currently, RESIF has undertaken, along with ISTI, to continue the development of an improved version of the tool which has been named “yasmine” (Yet Another Station Metadata INformation Editor).

This software, with a web-based GUI, offers the user the ability to create and edit native stationXML metadata complying with the latest FDSN approved standard (currently v 1.1). It offers the ability to create channel responses from scratch using templates in both the IRIS Nominal Response Library (NRL) and a new Atomic Response Objects Library (AROL). The NRL/AROL wizard in yasmine allows the user to browse these generic libraries and select the sensor and datalogger at the site and returns the full (combined) response.  The tool uses ObsPy Inventory python objects (Station, Channel, Response, etc) in the backend, and maintains collections of these for editing and assembly in a persistent, user-defined database.  Existing stationXML may be imported, saved into network, station, channel and response templates and stored in user-defined libraries for future use.  Channel responses may be readily plotted in the tool for confirmation.

While the web-based GUI permits both local standalone and server deployments, a full set of command line options will allow users to create their own batch scripts to drive yasmine’s stationXML editing capabilities including stationXML file splitting/merging, batch modification of objects, insertion of objects at various levels, and more.

The software will be released under the GNU GPL v3 licence and the code will be made available from IRIS github repositories.

How to cite: Saurel, J.-M., Hellman, S., Casey, R., Hagerty, M., Lisowski, S., Pardo, C., Pedersen, H., Péquegnat, C., Ronan, T., Schaeffer, J., Sukhotskyi, O., Templeton, M., Trabant, C., and Wolyniec, D.: Yasmine : A new tool for stationXML, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19125, https://doi.org/10.5194/egusphere-egu2020-19125, 2020.

EGU2020-22285 | Displays | SM5.6

Fling-step recovering from near-source waveforms and ground displacement attenuation models

Maria D'Amico, Erika Schiappapietra, Giovanni Lanzano, Sara Sgobba, and Francesca Pacor

We present a processing scheme (eBASCO, extended BASeline COrrection) to remove the baseline of strong-motion records by means of a piece-wise linear de-trending of the velocity time history. Differently from standard processing schemes, eBASCO does not apply any filtering to remove the low-frequency content of the signal. This approach preserves both the long-period near-source ground-motion, featured by one-side pulse in the velocity trace, and the offset at the end of the displacement trace (fling-step). Hence, the software is suitable for the identification of fling-containing strong-motion waveforms. Here, we apply eBASCO to reconstruct the ground displacement of more than 400 three-component near-source waveforms recorded worldwide (NESS1, http://ness.mi.ingv.it/; Pacor et al., 2019) with the aim of: 1) extensively testing the eBasco capability to capture the long-period content of near-source records; 2) calibrating attenuation models for peak ground displacement (PGD), 5% damped displacement response spectra (DS), permanent displacement amplitude (PD) and period (Tp). The results could provide a more accurate estimate of ground motions, to be adopted for different engineering purposes such as performance-based seismic design of structures.

Pacor F., Felicetta C., Lanzano G., Sgobba S., Puglia R., D’Amico M., Russo E., Baltzopoulos G., Iervolino I. (2018). NESS v1.0: A worldwide collection of strong-motion data to investigate near source effects. Seismological Research Letters. https://doi.org/10.1785/0220180149

How to cite: D'Amico, M., Schiappapietra, E., Lanzano, G., Sgobba, S., and Pacor, F.: Fling-step recovering from near-source waveforms and ground displacement attenuation models , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22285, https://doi.org/10.5194/egusphere-egu2020-22285, 2020.

EGU2020-17790 | Displays | SM5.6

Revamping the GFZ Energy Magnitude computation procedure to establish a new service

Angelo Strollo, Domenico Di Giacomo, Dino Bindi, and Riccardo Zaccarelli

Location and magnitude are the primary information released by any seismological observatory to characterize an earthquake. Nowadays, the size of large enough earthquakes are routinely measured in terms of released seismic moment (moment magnitude, Mw). Whereas events with Mw above about 5.5 populate seismological archives connected to global monitoring networks, the moment magnitude of smaller events require the analysis of regional and local dense networks, or the establishment of empirical relationships to convert other magnitude scales into Mw (e.g., local magnitude to moment magnitude conversions). Since Mw is constructed over a physical parameter, it does not saturate. Moreover, being the seismic moment connected to tectonic features such as fault area and the average slip, Mw became the reference magnitude for seismic hazard studies. Although Mw accomplishes perfectly the task of characterizing the earthquake size, it does not provide the most complete view about the earthquake strength since Mw is insensitive to changes in the rupture dynamics. An assessment of the amount of the seismic energy released by an event (energy magnitude Me) is allowing to complement Mw with a measure of the earthquake size more suitable to evaluate the earthquake shaking potential.

Aiming at introducing soon a new real-time service providing Me for major earthquakes we are presenting in this study the results of benchmark tests against the procedure proposed by Di Giacomo et al., in 2008 [1] as well as the analysis performed on a larger data set including all major events available in the GEOFON catalogue with a published moment magnitude since 2011. The initial procedure has been translated to a python code within the Stream2segment package [2] and leveraging on EIDA and IRIS data services, more than 2000 station for ~5000 events since 2011 have been downloaded and processed. The large data set used and the real-time application pose new challenges, among them the teleseismic distance, the strongly unbalanced network and the real-time data flow making the data set used dynamic. We present and discuss here the effects of these complications and how we are tackling them towards the implementation of new service at GFZ computing Me in real-time.

 

[1] Di Giacomo, D., Grosser, H., Parolai, S., Bormann, P., and Wang, R. (2008), Rapid determination of Me for strong to great shallow earthquakes, Geophys. Res. Lett., 35, L10308, doi:10.1029/2008GL033505.

[2] Riccardo Zaccarelli, Dino Bindi, Angelo Strollo, Javier Quinteros, Fabrice Cotton; Stream2segment: An Open‐Source Tool for Downloading, Processing, and Visualizing Massive Event‐Based Seismic Waveform Datasets. Seismological Research Letters ; 90 (5): 2028–2038. doi: https://doi.org/10.1785/0220180314

How to cite: Strollo, A., Di Giacomo, D., Bindi, D., and Zaccarelli, R.: Revamping the GFZ Energy Magnitude computation procedure to establish a new service, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17790, https://doi.org/10.5194/egusphere-egu2020-17790, 2020.

EGU2020-7929 | Displays | SM5.6

Towards an Open Access European Database for Deep Seismic Sounding data

Ramon Carbonell, Irene DeFelipe, Juan Alcalde, Monika Ivandic, and Roland Roberts

Controlled source seismic data acquisition experiments have produced a vast amount of Deep Seismic Sounding (DSS) data since its development in the late 50’s. These datasets provide critical information on the structure and nature of the crust and the lithosphere, which constitutes a fundamental research tool within Solid Earth Sciences. The DSS datasets are unique and constitute the output of an expensive (in time, effort and cost) scientific process, which evidences the need for their preservation, both the recently acquired and the legacy data. Furthermore, the new developments in processing and imaging techniques generate new possibilities for re-use of the vintage datasets. The availability and accessibility of these datasets, therefore, is of foremost importance for scientists, decision-makers and the general public.

The research community, aware of the value of these data, has pushed forward Open Data policies based on the FAIR principles of data management (Findable, Accessible, Interoperable and Reusable). In this respect, a long-term plan has been launched by the European Plate Observation System (EPOS, https://www.epos-ip.org/) e-infrastructure. The focus is to streamline the integrated use of scientific data, data products and services. In close link with EPOS, the Seismology and Earthquake Engineering Research Infrastructure Alliance for Europe (SERA, http://www.sera-eu.org/home, a Horizon 2020 project) includes a working package to set up a network on DSS data and products management. This initiative ensures the traceability of the data allowing that third parties can freely access, exploit and disseminate the data by means of permanent, international identifiers: a Digital Object Identifier (DOI) and a Uniform Resource Identifier (URI) or handle. Furthermore, the current aim is to go beyond the FAIR principles by linking the data with its related peer-reviewed publications, other scientific contributions and technical reports, facilitating its re-use.

A prototype DSS data exchange system has been developed jointly between the DIGITAL.CSIC (the Spanish National Research Council) services and the Institute of Earth Sciences Jaume Almera-CSIC (https://digital.csic.es/handle/10261/101879, last access January 2020). Within the platform, each dataset includes the acquired raw data and a metadata file. The metadata provides information of the nature of the data itself, list of authors, the context of the data (time and location of the experiments), funding agencies and other relevant legal aspects. The technical information includes the acquisition parameters, data processing and format of the data (SEGY standard in this case - www.seg.org-, broadly used in the geophysics community). In the developed storage protocol, a permanent identifier is assigned once it has been checked that the data meets all the described requirements. This permanent identifier ensures that any visit or download is accounted for. This information is entered into a statistics referencing database and can also be used as a measure of the impact of the data and/or data product.

This work is funded by the European Commission (Grant Agreement no: 676564-EPOS IP, Call H2020-INFRADEV-2014-2015/H2020-INFRADEV-1-2015-1, SERA 730900).

How to cite: Carbonell, R., DeFelipe, I., Alcalde, J., Ivandic, M., and Roberts, R.: Towards an Open Access European Database for Deep Seismic Sounding data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7929, https://doi.org/10.5194/egusphere-egu2020-7929, 2020.

EGU2020-7692 | Displays | SM5.6

The Spanish node of the multidisciplinary integrated e-infrastructure EPOS

Irene DeFelipe, Juan Alcalde, Jose Luis Fernandez-Turiel, Jordi Diaz, Adelina Geyer, Cecilia Molina, Isabel Bernal, Jose Fernandez, and Ramon Carbonell

The European Plate Observation System (EPOS, https://www.epos-ip.org/) is an e-infrastructure of ESFRI, the European Strategy Forum on Research Infrastructures (https://www.esfri.eu/), aimed at facilitating and promoting the integrated use of data, data products, services and facilities from internationally distributed research infrastructures for Solid Earth Science. This e-infrastructure is greatly committed to tackle viable solutions for Solid Earth challenges. It is a long-term plan that integrates research infrastructures of different European countries into a single inter-operable platform through different thematic core services (e.g., Seismology, Satellite data, Volcano Observations, Multi-Scale Laboratories). The Spanish EPOS node is coordinated by CSIC (the Spanish National Research Council) that hosts its own institutional repository, the DIGITAL.CSIC.

CSIC has adopted the European open data mandate and supports that data archives follow the FAIR principles of data management: Findable, Accessible, Interoperable, and Reusable. Therefore, data are broadly accessible to reuse for other researchers, industry, teaching, training and for the general public. Following these principles, the Institute of Earth Sciences Jaume Almera is updating and enlarging its database (https://digital.csic.es/handle/10261/101879, last access January 2020). These datasets include, among other, geophysical data acquired in the Iberian Peninsula since the 90’s. They comprise seismic studies of the structure of the crust in different geological settings, both on and offshore, and ranging from continental to exploration scale. These projects have been funded by public calls as well as from industry-funded research projects. As an example, these datasets contain data addressing the characterization of the shallow subsurface for the development of CO2, radioactive waste geologic storage sites, and to assess geologic hazards in the nearby of active faults. These datasets provide access to data that are relevant to assess sustainable and secure exploration and exploitation of the subsurface, a key societal challenge.

This work is a contribution of Project EPOS Sustainability Phase (EPOS SP), funded by the European Commission (Grant Agreement no: 871121 - EPOS SP-H2020-INFRADEV-2018-2020/H2020-INFRADEV-2019-2).

How to cite: DeFelipe, I., Alcalde, J., Fernandez-Turiel, J. L., Diaz, J., Geyer, A., Molina, C., Bernal, I., Fernandez, J., and Carbonell, R.: The Spanish node of the multidisciplinary integrated e-infrastructure EPOS, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7692, https://doi.org/10.5194/egusphere-egu2020-7692, 2020.

EGU2020-8273 | Displays | SM5.6

Updated determination of earthquake magnitudes at the Swiss Seismological Service

Roman Racine, Carlo Cauzzi, John Clinton, Donat Fäh, Benjamin Edwards, Tobias Diehl, Stefan Heimers, Nicholas Deichmann, Philipp Kästli, Florian Haslinger, and Stefan Wiemer

The Swiss Seismological Service (SED; http://www.seismo.ethz.ch) at ETH Zürich is the federal agency in charge of monitoring earthquakes in Switzerland and neighboring areas, and for the assessment of seismic hazard and risk for the region. The SED seismic network largely relies on software and databases integrated in the SeisComP3 monitoring suite for waveform acquisition, automatic and manual event processing, event alerting, web infrastructure, data archiving and dissemination. Data from all digital seismic stations acquired by the SED over the last 30 years - broadband (presently ~230), strong-motion (~185), short-period (~65), permanent and temporary - are homogeneously integrated in the seismic network processing tools and products. Waveform data from the Swiss National Seismic Networks are openly available through the SED website and ORFEUS EIDA / Strong-Motion (http://orfeus-eu.org/data/) data gateways. The SED earthquake catalogue is publicly available through FDSN Event web services at  the SED (http://arclink.ethz.ch/fdsnws/event/1/). The Swiss seismic hazard maps are integrated in the EFEHR portal (http://www.efehr.org). The SED is updating its strategy for magnitude determination to make it fully consistent with the state-of-the-art in engineering seismology and seismic hazard studies in Switzerland, and to optimise the use of its dense seismic monitoring infrastructure. Among the planned changes are the: (a) adoption of a new ML relationship applicable in the near-source region at epicentral distances smaller than 15-20 km; (b) inclusion of ML station corrections based on empirically observed (de)amplification with respect to the Swiss reference rock velocity model and associated predictions; (c)  seamless computation of Mw based on spectral fitting of recorded FAS using a Swiss specific model. In this contribution we present and discuss the updated magnitude computations for a playback dataset of thousands of recorded earthquakes, and compare them with the current official estimates. We discuss the expected impacts of the new magnitude determination strategy on the SED event processing chain in SeisComP3, the SED catalogues and other seismological products. We welcome community feedback on our planned transition strategy.

How to cite: Racine, R., Cauzzi, C., Clinton, J., Fäh, D., Edwards, B., Diehl, T., Heimers, S., Deichmann, N., Kästli, P., Haslinger, F., and Wiemer, S.: Updated determination of earthquake magnitudes at the Swiss Seismological Service, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8273, https://doi.org/10.5194/egusphere-egu2020-8273, 2020.

EGU2020-21693 | Displays | SM5.6

Estimation of seismic network detection thresholds for Austria

Maria-Theresia Apoloner, Helmut Hausmann, and Nikolaus Horn and the AlpArray Working Group

Seismic networks are expanding and changing continuously: station instrumentation breaks and improves, new stations are set up permanently and temporarily for projects, or get available online from seismological services. For routine processing, it is important to know if and where adding an existing station to processing or building or improving a station will add the most value to the detection an location capabilities.

Therefore, in this study we calculate seismic network detectionthresholds for Austria using data available to us from different sources: From the Seismic Network of Austria (OE), which consists of unevenly distributed high quality low noise broadband and strong-motion stations, with station spacing up to 100 km. Cross-border from neighboring countries, where each of them operates at least one seismic network with very different station quality and coverage. As well as from temporary regional scientific projects (i.a. AlpArray (Z3), the SWATH (ZS)) and local infrastructure monitoring (GeoTief EXPLORE 3D).

Additionally to comparing different methods (SN-CAST by Möllhoff et al. 2019, Net-Sim by Niko Horn, GT5-criterium) with each other, we also analyze how strong-motion stations, recently added due to the interregio project ARMONIA, improve the detection capabilities.

How to cite: Apoloner, M.-T., Hausmann, H., and Horn, N. and the AlpArray Working Group: Estimation of seismic network detection thresholds for Austria, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21693, https://doi.org/10.5194/egusphere-egu2020-21693, 2020.

EGU2020-13619 | Displays | SM5.6

The Swedish National Seismic Network

Michael Roth and Björn Lund

The Swedish National Seismic Network (SNSN) is operating 69 broadband stations in a latitude range from about N55.5 to N68.5 deg. The southern and northern parts of Sweden are covered more or less evenly with stations having about 100km interstation distances. In the center, between latitudes N61 - N65 deg the stations are situated in a band of about 100 km width following the coast of the Bothnian Sea. The maintenance of this large and distributed network - parts of it in Arctic environment - is challenging. All stations are recording at 100 samples per second and are sending continuous data in near real-time to the SNSN centre at Uppsala University. Seismic data are shared via seedlink directly with seismological institutes in the neighbouring countries, and a subset of the network is made available at ORFEUS. The density, spatial distribution and data avalability of the network allow the production of  a reviewed seismic bulletin with a magnitude completeness down to 0.5. We are currently running several independent automatic processing systems at SNSN: Seiscomp3, Earthworm, SIL/MSIL and an in-house developed waveform-backpropagation algorithm. The SIL system was put in operation 1990 and was originally designed to work decentralized (i.e. phase detection processing at each station computer) and to work with segmented data, suitable for a network with narrow communication bandwidth. SIL was further developed into a version called MSIL, which now performs all steps (detection, associaton and localization) centrally. This not only facilitates station and software maintenance, but also reduces the number of potential points of failure, thereby increasing the data acquisition and processing performance. All the automatic systems are set up for regional and local monitoring. Solutions obtained by the Seiscomp3 and Earthworm system are consistent in location and magnitude for more than 90% of the detected events. The SIL/MSIL and the backpropagation system are targeted to weaker events and they provide additional seismic event locations, but also more spurious events. The current setup of several automatic systems provides operational redundancy and it increases the confidence in the automatic solutions (when detected by more than one system). Eventually we are going to merge the automatic solutions of all systems into one automatic bulletin in order to decrease the workload for analyst review.

How to cite: Roth, M. and Lund, B.: The Swedish National Seismic Network, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13619, https://doi.org/10.5194/egusphere-egu2020-13619, 2020.

SM6.1 – Tectonic and volcanic earthquake swarms: From a multi-disciplinary imaging and tracking of crustal fluids to characterization of transient forcing.

EGU2020-21279 | Displays | SM6.1

Three-dimensional Magnetotelluric Crustal Model of High Agri Valley seismic area to identify and to quantify the resistivity variation in depth

Anna Eliana Pastoressa, Marianna Balasco, Juanjo Ledo, Pilar Queralt, Gerardo Romano, Agata Siniscalchi, and Simona Tripaldi

The High Agri Valley (HAV) is an axial zone of the Southern Apennines thrust belt chain with a strong seismogenic potential as shown by different stress indicators and space geodesy data that suggest an NE-SW extensional stress regime still active. Moreover, the HAV hosts the Europe’s largest onshore oil and gas field, which give it further strategic importance.

There is a certain ambiguity concern the causative fault of the large event (M=7.0) occurred in 1857 in Agri Valley, although two well-documented fault systems are recognised as potentially seismogenic: the Monti della Maddalena Fault System (MMFS) and the Eastern Agri Fault System (EAFS).

With the aim to bring new information on identification and characterization of the principal structures, on the fluids distribution and their possible relationship with the developed of kinematics in upper fragile crust, several multiscale and multidisciplinary surveys are currently running in the HAV. Here we present the first results of a 3D Magnetotelluric (MT) investigation composed of 58 MT soundings in the period range [10-2 Hz, 103 Hz] which cover an area of approximately of 15 km x 30 km. All the 3D results were obtained by using the 3D inversion conde ModEM: Modular EM Inversion Software.

The work carried out so far has been mainly focused on the definition of the best mesh to adopt, both in terms of cell size and orientation, and on the correct choice of the inversion parameters: resistivity of the starting model, smoothing model parameter, minimum error floor attributed to the data, regularization parameter (trade-off).

The 3D MT preliminary model obtained shows a good agreement with 2D models previously realized using a part of the same dataset and defines the main geo-structural features of the HAV.

The resistivity variations in HAV subsurface will be jointly interpreted with accurate seismic data collected by seismic broadband network INSIEME (composed by 8 stations distributed throughout the Agri Valley) and other available geophysical and geological data.

The interconnection between the conductivity and seismicity information will be useful to implement the knowledge on the role that fluids play in fault zones and in earthquake processes. 

How to cite: Pastoressa, A. E., Balasco, M., Ledo, J., Queralt, P., Romano, G., Siniscalchi, A., and Tripaldi, S.: Three-dimensional Magnetotelluric Crustal Model of High Agri Valley seismic area to identify and to quantify the resistivity variation in depth , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21279, https://doi.org/10.5194/egusphere-egu2020-21279, 2020.

EGU2020-7061 | Displays | SM6.1

Evidences of high diffusivity near a waste water injection well in the Val d’Agri oil field (Italy) from noise-based monitoring

Andrea Berbellini, Lucia Zaccarelli, Andrea Morelli, Licia Faenza, Alexander Garcia-Aristizabal, Luigi Improta, and Pasquale De Gori

We monitor the relative variations of crustal velocity during a stop of water injection at the Val d’Agri oilfield (Italy) in January-February 2015 from the analysis of the ambient seismic noise cross-correlations. This technique allows the continuous estimations of the relative velocity variations occurred in the superficial layers of the Earth crust independently from the earthquake occurrence. Our results show a relative decrease in seismic velocity of about 0.08%, detected seven days after the injection restart of fluids injection and can be compatible with an increase of fluids in the medium. We estimate the medium diffusivity from this delay time obtaining a value of about 2.0 m2/s. Independently, we compute diffusivity from the observed delay time of small-magnitude (ML ≤ 1.8) seismicity induced by the first injection tests in June 2006, finding a similar value. The high diffusivity values found from the two independent analysis are compatible with the hydraulic properties of the hydrocarbon reservoir. Finally, we estimate the spatial distribution of the observed variations finding that the largest changes are located in the North-West direction, where the oilfield is elongated. Our results show that fluids propagate efficiently from the wellbore in the reservoir direction through a strongly fractured medium following efficient hydraulic pathways, and that the noise-based monitoring technique adequately map in time and space this perturbation.

How to cite: Berbellini, A., Zaccarelli, L., Morelli, A., Faenza, L., Garcia-Aristizabal, A., Improta, L., and De Gori, P.: Evidences of high diffusivity near a waste water injection well in the Val d’Agri oil field (Italy) from noise-based monitoring, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7061, https://doi.org/10.5194/egusphere-egu2020-7061, 2020.

EGU2020-15845 | Displays | SM6.1

Anomalous Vp/Vs in highly pressurized rocks: Evidence for anisotropy or mafic composition?

Lucas Pimienta, Alexandre Schubnel, Jerome Fortin, Yves Guéguen, Helene Lyon-Caen, and Marie Violay

         Anomalously high seismic P- to S-wave velocity ratios (Vp/Vs) have been observed in subduction zones, in locations where varieties of earthquakes and slips are expected to occur. From qualitative laboratory knowledge of rocks Poisson’s ratio, these results were interpreted as evidence of near-lithostatic pore fluid pressure. Because most laboratory data did not document such high Vp/Vs values, these were further linked to additional constrains of anisotropy or the dominance of minerals of very high intrinsic Vp/Vs, e.g. mafic rocks.However, does high Vp/Vs necessarily imply anisotropy and/or mafic composition?

         Recently, the measuring frequency (f) was shown to play a major role on rocks’ resulting Poisson’s ratio, so that usual laboratory results (at f = 1 MHz) might not directly transfer to field ones (at f = 1 Hz). From this consideration, we investigate Vp/Vs of a variety of crustal rocks in the elastic regime relevant at the field scale, the undrained elastic regime.Accounting for rocks dispersive properties, this work aims to show that:

  • In the laboratory, in isotropic rocks, one might attain Vp/Vs values as high as the anomalous ones observed in subduction zones.
  • No mineralogical control is needed for such high Vp/Vs values, which could be consistent with the inherent mineral variability in different settings across the globe.
  • High pore fluid pressure is a major parameter, but not alone: such high values cannot be achieved without very high degree of micro-fracturing of the rock, opened by high fluid pressures, an information of potential importance to understand those seismogenic zones.

How to cite: Pimienta, L., Schubnel, A., Fortin, J., Guéguen, Y., Lyon-Caen, H., and Violay, M.: Anomalous Vp/Vs in highly pressurized rocks: Evidence for anisotropy or mafic composition?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15845, https://doi.org/10.5194/egusphere-egu2020-15845, 2020.

EGU2020-19708 | Displays | SM6.1

Induced Fault Reactivation and Hydraulic Diffusivity Enhancement : Insights from Pressure Diffusion Inversion in Laboratory Injection Tests

Michelle Almakari, Hervé Chauris, François Passelègue, and Pierre Dublanchet

Understanding how the permeability of a fault evolves during injection induced fault reactivation process is of great interest. The interactions between fluids and faults can be complex, as the confining pressure, effective stress and shear slip can affect the hydro-mechanical properties of the fault. The relationship between induced slip (reactivation) front and fluid front requires a better understanding of what controls hydraulic diffusivity as well.
In this study, we investigate shear induced fluid flow and permeability enhancement during fracture shearing. We used a series of laboratory injection reactivation tests on saw cut Andesite rock sample, under triaxial conditions, at different confining pressures (30, 60 and 95 MPa). The sample was connected to two pressure sensors, at two opposite locations of the fault, and equipped by strain gauges along strike.
We thus propose a numerical method, in the context of deterministic and probabilistic inversion approaches, that allows to estimate the temporal evolution of the effective hydraulic diffusivity (and its associated uncertainties) of an experimental fault throughout an injection test, using the pressure history at two points on the fault.
The numerical method was able to reproduce the experimental data for a wide time range of the different experiments. The hydraulic diffusivity was found to largely depend on the confining pressure and to increase (by one order of magnitude) throughout the injection experiment with the reduction of the mean effective stress acting along the fault plane. As well, the shear slip was observed to have an effect on the hydraulic diffusivity evolution. Instantaneous short term diffusivity enhancement accompanied slip events with large slip velocities, while long term increases accompanied slow slip events.

How to cite: Almakari, M., Chauris, H., Passelègue, F., and Dublanchet, P.: Induced Fault Reactivation and Hydraulic Diffusivity Enhancement : Insights from Pressure Diffusion Inversion in Laboratory Injection Tests, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19708, https://doi.org/10.5194/egusphere-egu2020-19708, 2020.

EGU2020-3446 | Displays | SM6.1

Assessing fault criticality using seismic monitoring and fluid pressure analysis

Léa Perrochet, Giona Preisig, and Benoît Valley

The stability of a critically stressed fault depends on the surrounding stresses acting on it. Fluids, by reducing the effective normal stress, play a major role. It has been observed that in karstic regions, an increase in groundwater pressure following significant recharge (precipitations and/or seasonal snowmelt) can result in a fault re-activation, inducing microseismicity. This study combines the natural microseismicity and the groundwater level fluctuations observations to estimate the fault criticality. The research is carried out on two major strike-slip faults in the folded Jura in Switzerland – La Lance Fault and La Ferrière Fault – most likely critically stressed according to their position in the global stress-regime. Data acquisition mainly consists in hydrogeologic and seismic monitoring. The objectives are to have continuous discharge rates of the major karstic springs and to produce a seismic catalog for the area of interest. Combining both data sets will allow to determine relations between increasing spring discharge rates and low magnitude earthquakes and eventually to acquire a quantitative knowledge on what pressure change is affecting the fault’s stability. This knowledge will be used to develop a straighforward methodology to assess fault criticality.  In addition, the study of a possible time lag between aquifer response and fault activation, as well as back-analysis of seismic events can provide, respectively, important information about the deep-seated fluid circulation and the local stress-regime.

How to cite: Perrochet, L., Preisig, G., and Valley, B.: Assessing fault criticality using seismic monitoring and fluid pressure analysis , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3446, https://doi.org/10.5194/egusphere-egu2020-3446, 2020.

In northeastern Japan, low-frequency earthquakes (LFEQs) occur preferentially at depths from the lower crust to the uppermost mantle near the active volcanoes. Many researchers have suggested the contribution of geofluid to the occurrence of these unusually deep LFEQs. Recent observations show that relatively low-frequency earthquakes occur even in the upper crust as well. Investigation of the generation mechanism of shallow LFEQs is quite important because it is directly related to the mechanism of closely located high-frequency earthquakes in the brittle upper crust. One of the areas of enhanced shallow LFEQ seismicity is the aftershock zone of the 2008 Iwate-Miyagi Nairiku Earthquake (Mw 6.8) located to the west of the 2011 great Tohoku earthquake. We detected LFEQs by using the frequency index (FI) defined by the logarithm of a ratio of high- and low-frequency spectral amplitudes. We used 2–4 Hz and 10–20 Hz bands for low- and high-frequency ranges. We analyzed more than 4000 events observed by a dense temporary seismic network deployed just after the occurrence of the mainshock. Our detection revealed that there are five LFEQs dominant clusters in the aftershock zone trending NNE-SSW with a length of about 40 km: the northern and the southern edge of the aftershock zone, to the north of the mainshock epicenter, the eastern and western edge of the central aftershock zone. In the area near the mainshock epicenter, hypocenter distribution shows two planes: mainshock fault dipping to the west and a conjugate fault dipping to the east. The previous study has shown that the events with N-S trending largest principle stress axis are distributed along the conjugate plane. In contrast, the events along the mainshock fault have E-W trending largest principle axis that is consistent with the relative motion of the subducting Pacific plate beneath the Tohoku region. The former anomalous groups are interpreted to be caused by local stress change by the mainshock applied to a neutral stress field with high pore pressure suggested by high Vp/Vs ratio. An interesting feature is the preferential distribution of LFEQs along the conjugate plane. Also, the hypocenter of LFEQs migrated with time from deeper to the shallower part of the plane. These observations strongly suggest that the existence and movement of geofluid are responsible for both the unusual stress field and the occurrence and migration of LFEQs. The location of LFEQs at the northern and eastern edge of the aftershock zone is close to the areas of postseismic slip detected by GNSS observation, which is suggestive of the increased pore pressure in the area. The LFEQs at the southern and western edge of the aftershock zone occur in calderas, suggesting that these LFEQs occur in hotter and/or fluid-rich areas where the ductile deformation occurs. Thus, though the interpretation of the cause of LFEQs is not unique, the distribution of LFEQs plays a crucial role in understanding the contribution of geofluids not only to the seismogenic processes of aftershocks but to the faulting mechanism in the upper crust.

How to cite: Kosuga, M.: Spatial distribution of low-frequency earthquakes suggestive of geofluid among the aftershocks of the 2008 Iwate-Miyagi Nairiku Earthquake in northeastern Japan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13654, https://doi.org/10.5194/egusphere-egu2020-13654, 2020.

EGU2020-9809 | Displays | SM6.1

New insights on temporal and spatial evolution of Yellowstone earthquake swarms: a multidisciplinary geological-seismological approach

Luca Carbone, Elena Russo, Rita de Nardis, Giuseppina Lavecchia, Alessandro Tibaldi, and Fabio Bonali

The Yellowstone volcanic field, in western United States, is well known for intense seismic activity, abundant geothermal features and a violent volcanic history that includes a caldera-forming eruption 640 ka ago. Even though the recentmost eruption dates back to 70 ka ago, a very high seismicity, quasi-continuous surficial deformation through uplift and subsidence stages (at rates of up to 70 mm/yr) and intense hydrothermal activity are clear evidences of a still very active volcanic field. Thanks to a recently improved seismic network, here we analyze the rate of occurrence of 19’538 relocated earthquakes belonging to the temporal window between 1988 and 2016. Starting from this dataset, we identify and characterize the seismic swarm activity occurring in the study area after 2007. We also evaluate the analogies and differences of their seismic behavior through the analysis of frequency-magnitude distribution of seismic events. We investigate the identified seismic swarms clustered in space and time, their relation with active volcanic and tectonic processes and stress field variations caused by the migration of magmatic and hydrothermal fluids. Calculated b-values associated with the recentmost seismic swarms have been related to past swarms that occurred in the area, thus revealing the temporal and spatial evolution of such phenomena. Our study gives new crucial insights to understand the relation between seismic and magmatic activity in the Yellowstone volcanic plateau, with important implications for a better comprehension of the local seismic and volcanic hazards.

How to cite: Carbone, L., Russo, E., de Nardis, R., Lavecchia, G., Tibaldi, A., and Bonali, F.: New insights on temporal and spatial evolution of Yellowstone earthquake swarms: a multidisciplinary geological-seismological approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9809, https://doi.org/10.5194/egusphere-egu2020-9809, 2020.

EGU2020-10012 | Displays | SM6.1

Fault geometry and rupture patterns of the 2018 Lombok earthquakes – complex thrust faulting in a volcanic retro-arc setting

Shengji Wei, Karen Lythogoe, Muzli Muzli, Andri Dian Hugraha, Kyle Bradley, and Zulfakriza zulhan

The 2018 Lombok earthquake sequence, which took place ~10km to the north of the Rinjani volcano on the Flores thrust fault, are distributed beneath the northern coast of the island, composing of two Mw6.4 and two Mw6.9 earthquakes and numerous aftershocks. The first Mw6.4 earthquake was followed by the first Mw6.9 event in a week, which was located only a few kilometers to the west of the Mw6.4 event, characterized with strong westwards rupture directivity and multiple asperities (rougher source time function). Two weeks later, the second Mw6.4 event took place a few km to the east of the first Mw6.4 event and triggered the second Mw6.9 event 12 hours later. In contrast, the second Mw6.9 ruptured towards east with a single major asperity, with a centroid depth of ~18km, ~5km shallower than the first Mw6.9 event. The seismicity was well captured by 7 broadband stations and 6 short period nodes deployed just before the first Mw6.9 event, mostly concentrated within a depth range of 5km. Relocated seismicity shows shallower depth to the west and deeper to the east, in consistent with the coseismic rupture of the largest events. Aftershocks are shallowest below the volcano due to an elevated Brittle-Ductile-Transition (BDT) zone depth controlled by the thermal structure. A few anomalous earthquakes were identified between the Mw6.9 events below the BDT zone that could be related to the basaltic conduit of the volcano. Several sets of repeating earthquakes were identified and are mostly located in the rupture area of the first Mw6.9 event, indicating a highly heterogeneous friction on the fault that is probably caused by to the stronger thermal gradient compared with the second Mw6.9 event. The earthquake sequence highlights the strong interaction between the volcanic system and the tectonic faulting process. 

How to cite: Wei, S., Lythogoe, K., Muzli, M., Hugraha, A. D., Bradley, K., and zulhan, Z.: Fault geometry and rupture patterns of the 2018 Lombok earthquakes – complex thrust faulting in a volcanic retro-arc setting, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10012, https://doi.org/10.5194/egusphere-egu2020-10012, 2020.

EGU2020-13584 | Displays | SM6.1

The Global Importance of Continental Seismic Sequences

Richard Walters, Tim Craig, Laura Gregory, and Russell Azad Khan

Large continental earthquakes necessarily involve cascading rupture of multiple faults or segments (e.g. El Mayor-Cucapah 2010). But these same critically-stressed systems sometimes rupture in drawn-out sequences of smaller earthquakes over days or years (e.g. Central Italy 2016), instead of in a single large event. Due to the similarity in the initial conditions of both scenarios, seismic sequences may be considered as ‘failed’ multi-segment earthquakes, whereby cascading rupture is prematurely halted before all available slip deficit is released.

These two modes of strain-release have vastly different implications for seismic hazard. Recent work on the 2016 Central Italy earthquake sequence, which is the first seismic sequence to be studied with modern high-quality geodetic and seismological datasets, showed that complexity in fault structure appeared to exercise a dual control on both the timing and sizes of events throughout this sequence. However, it is unclear if this structural control is common for all continental seismic sequences, how important seismic sequences are for the global seismic moment budget, and how this contribution to moment budget may vary between different tectonic regions.

Here we select shallow crustal continental earthquakes from the Global Centroid Moment Tensor catalog, and identify seismic sequences as agglomerates of clustered pairs of earthquakes where the summed moment (M0) of all aftershocks is greater than 50% of the M0 of the first event in the sequence. We analyse the relative number of seismic sequences compared to other earthquakes for normal, reverse, and strike-slip faulting regions, and also calculate the relative M0 release of seismic sequences and other earthquakes in these three regimes.

We find that although seismic sequences are equally common by number in all continental tectonic regimes, seismic sequences account for a much higher proportion of M0 release for normal faults (~20%) than for reverse faults (~10%), with strike-slip faults intermediate between these two end-members. We also find that the proportion of M0 release in seismic sequences is higher for events that occur in regions characterised by a diversity of different earthquake types (e.g. both reverse and strike-slip faulting) than for events that occur in regions characterised by a single earthquake type (e.g. strike-slip faulting only). Together these findings imply that complexity of fault network is an important factor in controlling the occurrence of large-M0 seismic sequences, and that ‘failed’ multi-segment earthquakes and therefore large-M0 seismic sequences are more likely to occur in regions with complex fault networks.

How to cite: Walters, R., Craig, T., Gregory, L., and Azad Khan, R.: The Global Importance of Continental Seismic Sequences, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13584, https://doi.org/10.5194/egusphere-egu2020-13584, 2020.

Swarms of microearthquakes on a network of conjugate strike-slip faults in the rift zone in Central Iceland have been detected and located using a dense local seismic network operational since 2007. These swarms have been recorded since the 1970s, but the cause of their clear swarm-like nature remains enigmatic.

We use the QuakeMigrate earthquake detection and location software – which is able to detect earthquakes separated by very small inter-event times – to produce a highly complete catalogue. Automatic hypocentre locations have been refined using waveform cross-correlation and double-difference relocation, and focal mechanisms and manual earthquake locations have been produced for a subset of events by manual picking. Analysis of the resulting high-resolution earthquake catalogue reveals systematic migration of hypocentres at velocities of ~ 1 km/day along sharply defined fault planes ranging from 1 – 10 km in length. In the majority of swarms we also observe clusters of identical repeating events, providing evidence for re-loading of the brittle asperities that produce earthquakes.

For a selection of swarms, our high resolution seismic observations are complemented by GPS and InSAR measurements, allowing us to constrain the amount of fault slip. Comparing this, and the area of the fault plane activated in the swarm, to the seismic moment release reveals a significant contribution of aseismic slip, or very low effective stress drop. Analysis of swarms triggered on these faults by the static coulomb stress increase induced by the 2014 Bárðarbunga-Holuhraun dike intrusion provides a further estimate of the amplitude of the stress cycle.

We combine our observations with comparisons to numerical & laboratory modelling studies, observed swarm scaling properties and knowledge of the geological and permeability structure of the Icelandic crust to determine the nature of the transient forcing driving these exceptionally well-recorded tectonic earthquake swarms.

How to cite: Winder, T. and White, R. S.: Slowly migrating tectonic microearthquake swarms in the Icelandic Rift Zone: driven by pore-pressure or aseismic slip transients?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19827, https://doi.org/10.5194/egusphere-egu2020-19827, 2020.

EGU2020-19267 | Displays | SM6.1

Recent seismic swarms in the Tjornes fracture zone, N-Iceland

Kristín Jónsdóttir, Gunnar B. Guðmundsson, Luigi Passarelli, Sigurjón Jónsson, Yesim Cubuncu, Thomas Lecocq, Corentin Caudron, and Felix Rodriguez Cardozo

The Tjörnes fracture zone (TFZ) in N-Iceland is a seismically active zone with on average 4000 earthquakes detected annually since 1993 by the regional seismic network operated by the Icelandic Meteorological Office (IMO). Most of the earthquakes occur offshore and with only one seismic station on the Grímsey island north of Iceland, the seismic network detects earthquakes down to magnitude M-0.5. The fracture zone, essentially a transform between the northern volcanic zone of Iceland and the Mid-Atlantic Ridge north of Iceland, has three major segments; the Grímsey Oblique Rift (GOR) farthest to the North which accounts for 60% of the seismicity of the TFZ, the Húsavík-Flatey Fault (HFF) in the middle, where 38% of the TFZ earthquakes occur and the least active Dalvík Lineament (DL) farthest to the south (only 2% of TFZ seismicity). The IMO’s seismic catalogue clearly draws up the most active segments of the TFZ, where each extends laterally roughly 100 km. The largest earthquakes occur on the HFF where the accumulated seismic moment release is an order of magnitude higher than the GOR and three orders of magnitude higher than the DL.

There are other interesting differences between the segments. There are several known central volcanoes aligned along the GOR and the oblique rifting is likely to cause both tectonic and volcanic seismicity which shows up as a catalogue of many but similarly sized earthquakes, in other words a catalogue with a higher b-value than the neighbouring HFF. Despite these differences, seismic swarms, without a clear mainshock or aftershock sequences, counting thousands of earthquakes with a duration of a few days upto weeks, are recorded every 2-3 years both in GOR and HFF. In late March 2019, one of this seismic swarms took place on GOR, mostly on a single NNE-SSW striking fault near Kópasker. Relative earthquake locations draw the fault up nicely and in addition a few shorter faults with similar strike of 15°deg. The temporal evolution of the swarm shows an upwards migration and how the seismicity starts at the middle of the fault, jumps a little to the north and migrates in two days to the southern end of the fault over 7 km. When that point is reached, the largest earthquake in the swarm takes place, M4.2, however in the very northern end of the fault. The focal mechanism of this largest event shows a left-lateral strike-slip as do the smaller earthquakes. A b-value plot of the 2300 earthquakes that were recorded during the swarm reveal a value of 1.2, which is typical for volcanic seismicity. The size of active fault is considerable larger than expected from a M4.2 earthquake and the question rises if part of the motion is taken up as aseismic slip.

We will present examples of recent swarms in the TFZ along with new results of a cross-correlation study of the waveforms recorded during the swarm activity.

How to cite: Jónsdóttir, K., Guðmundsson, G. B., Passarelli, L., Jónsson, S., Cubuncu, Y., Lecocq, T., Caudron, C., and Rodriguez Cardozo, F.: Recent seismic swarms in the Tjornes fracture zone, N-Iceland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19267, https://doi.org/10.5194/egusphere-egu2020-19267, 2020.

The Central-Southern Apennines of Italy are a region with high seismic risk zone and experienced destructive earthquakes both in historical and in instrumental time. Geological data and historical seismicity indicate that the fault structures in this area are able to produce earthquakes with magnitude greater than 6.5. In fact the sector, stretching between the Irpinia 1980 (Mw 6.9) and the Accumuli-Visso-Norcia 2016 (Mw 6.5) seismic sequences, was struck by eleven events (MW ≥ 6.5) among the largest historical and early-instrumental earthquakes  since 1349. On the contrary, if we exclude the Barrea seismic sequence occurred in 1984 (Mw 5.9), the instrumental catalogue shows that this area is predominantly characterized by a low background level of seismicity and by earthquake clustering characterized by low release of strain energy.

We analyzed the seismicity occurring in this area from 1985 to 2018 (0.0 ≤ ML ≤ 5.0)  and  by a declustering algorithm  we indentified a set of 45 spatio-temporal clusters where the earthquake number stem out significantly from the background seismicity rate. The background seismicity (6196 events, 0.0≤ML≤4.1) is characterized by a b value of 0.96 ± 0.4, a magnitude of completeness of 1.4 and it is strictly controlled by known fault patterns. The earthquake clusters accounts for a non-negligible (45%) part of the total seismicity. A close inspection to the individual clusters allowed us to identify 4 seismic sequences characterized by isolated mainshock-aftershocks behaviour and 41 tectonic earthquake swarms (TESs). TESs have a duration ranging 2-12 days, 2.5-3.0 characteristic magnitude and 1.2 km/d migration rate. They are constituted by mono and/or polyphase episodes and they do not show a spatial complementary along the system of activated fault rather they are often spatially overlaid occupying the same fault segment. The latter behavior seems to indicate TES occurrences be driven by an underlying transients loading of the fault faster than the few mm/yr long-term extension active along the Apennines chain. The best candidates to explain these transients are likely presence of pressurized fluid abundant and/or  possible small scale creeping. The focal mechanisms and the depth of foci well correlate with the mapped normal fault systems and TESs illuminate regions of these faults adjoining ruptures of past large earthquakes. The spatio-temporal distribution of TESs suggests that the system of faults in the southern and central Apennines is characterized by heterogeneous rheology where small fault patches systematically release strain through TESs and other parts are to date locked. These findings are of great importance to better improve models for the assessment of seismic risk in the area.

How to cite: de Nardis, R., Carbone, L., Pandolfi, C., Passarelli, L., and Lavecchia, G.: Quantitative analyses of background seismicity and earthquake clustering in extensional seismogenic settings - case studies from the Central-Southern Apennines of Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9480, https://doi.org/10.5194/egusphere-egu2020-9480, 2020.

EGU2020-10865 | Displays | SM6.1

Migration patterns of earthquake clusters

Tomáš Fischer and Sebastian Hainzl

Earthquake hypocenter migration is the most characteristic pattern which indicates aseismic processes triggering the observed seismicity. These processes can involve creep, fluid migration or similar. While interactions among earthquakes can also lead to some expansion of seismic clouds, these expansions are rather small and not comparable to migration patterns related to pore-pressure diffusion, slow slip events, or growing hydraulic fractures. Thus, identification and modeling of migration patterns, which has not been studied in detail, is important for the characterization of fault dynamics.

Advance of the triggering front is usually analyzed using distance-time plots that show the time dependence of the distance of individual events from the origin. If event order is used instead of time as the argument on the horizontal axis, event migration is analyzed in dependence on the seismic activity itself, which brings a new view to the running seismicity. We applied this approach to the relocated earthquake swarm catalogs from West Bohemia, California and Iceland and found a striking linear growth of the triggering front. This indicates that the advance of the front is likely to be driven by the rupture of individual earthquakes rather than by the running time. It also turned out that the growth velocity measured in meters per event increases with the magnitude of the data set.

Using the basic concepts of earthquake physics, we propose the relation of the growth velocity on earthquake magnitudes and compare it with measurements on the analyzed swarm catalogues. We show that the spreading velocity of the triggering front is closely related to source parameters, which gives hints to the understanding of the background mechanism.

How to cite: Fischer, T. and Hainzl, S.: Migration patterns of earthquake clusters, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10865, https://doi.org/10.5194/egusphere-egu2020-10865, 2020.

EGU2020-10497 | Displays | SM6.1

Earthquake-earthquake triggering in natural swarms and fluid-induced seismicity

Kamran Karimi and Joern Davidsen

Aftershock cascades and aftershock zones play an important role in forecasting seismic activity in both natural and human-made situations. While their behavior including the spatial aftershock zone scaling has been the focus of many studies in tectonic settings finding, for example, long-range earthquake-earthquake triggering in the near-field, this is not the case in situations where the seismic activity is primarily driven by fluids and the diffusion of excessive pore pressure. Here, we probe three different seismic settings that are believed to be influenced by fluid diffusion. The natural swarm in i) the Long Valley Caldera and the suspected swarms in ii) the Yuha Desert, both located in California, and associated earthquake-earthquake triggering behavior are compared against induced seismicity related to large scale wastewater disposal in iii) Oklahoma and southern Kansas. All settings exhibit a significant amount of event-event triggering highlighting the importance of secondary processes for the overall seismicity. We find an almost identical temporal event-event triggering behavior including the Omori-Utsu relation and the associated productivity relation. In terms of the spatial triggering density, both cases i) and iii) show a rapid decay beyond their rupture length. This proves that narrow spatial “aftershock” zones are not specific to induced seismicity but also occur in natural settings. Typical of most tectonic settings, a relatively long-range behavior is observed in case ii) suggesting that fluid migration might not be the dominant driving mechanism of the seismic activity and/or that the underlying structure of the fault network may control the secondary earthquake-earthquake triggering and its spatial evolution.

How to cite: Karimi, K. and Davidsen, J.: Earthquake-earthquake triggering in natural swarms and fluid-induced seismicity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10497, https://doi.org/10.5194/egusphere-egu2020-10497, 2020.

EGU2020-20892 | Displays | SM6.1

Seismic Precursors in Central Italy using Autocorrelation Measurements

Estelle Delouche, Laurent Stehly, and Christophe Voisin

The aim of this study is to identify precursors related to fluid pressure changes at depth to large earthquakes that occured in central Italy such as the 2009 Mw6.3 L'Aquila and the 2016 Mw6.2 Amatrice earthquake. To that end, we monitor the temporal evolution of the crust using a new method called Coherence of Correlated Waveforms [CCW] that uses seismic noise autocorrelation measurements. This allow us to look for changes in the medium with a high temporal resolution of 5 days. Our measurements of the CCW show that the L'Aquila Earthquake (2009-M6.3) is preceded by a 150-day oscillation whose amplitude and frequency progressively increases until the rupture. The high Vp/Vs ratios measured on the foreshocks of L’Aquila earthquake correspond to the CCW drop periods, suggesting the sensitivity of the measurement to crusty fluids.

Analysing 17 years of data, we found that this signal occurred only before the L'Aquila and the Amatrice earthquakes. This suggests the existence of a unique nucleation process.

Finally, for the 2016 Amatrice Earthquake, using an array of 25 seismic stations we are able to map the geographical extension of this precursory signal. This pattern, evolving over time, suggests diffusion phenomena in the upper crust.

 

How to cite: Delouche, E., Stehly, L., and Voisin, C.: Seismic Precursors in Central Italy using Autocorrelation Measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20892, https://doi.org/10.5194/egusphere-egu2020-20892, 2020.

In 2014 t In the framework of the MEDSUV project, the Solfatara area, part of the Campi Flegrei caldera, was chosen as test site for the RICEN (Repeated InduCed Earthquakes and Noise) experiment mainly oriented to the use of seismic waves (both in passive and active mode) as a diagnostic tool to investigate the changes in the properties of the medium at small scales. Besides the study of seismic waves (both in passive and active mode), part of the RICEN activities was focused on the detection and characterization of the Seismo-Electromagnetic signals (SES) associated with their propagation.

Considering the abundance of fluids that characterize the shallow hydrothermal system of Solfatara area, SES arewere expected to be detectable and informative of the subsoil structure. On May 2014, Hence for their detection during RICEN experimthree magnetotelluric (MT) stations were installed outside the seismic grid and close to the main volcanic fumaroles in the Solfatara area. Thusing electrical and magnetic components concurrently with seismic and geochemical measurements were recorded. As a result, SES related to Vibrosesis seismic source energization in 100 sites, distributed on an almost regular grid on an area of about 115m x 90m, were analyzed. The m.

Although Unfortunately the electrical part of the SES could not be extracted by the recorded time series due to the severe effects of the Solfatara volcanic environment on the unmpolarizable electrodes. ,Converselyagnetic components, instead of the electrical ones, were generally better appreciable, in terms of amplitude, with respect to the natural electromagnetic fluctuations(s?). This circumstance allowed to verify the strict causality relationship between of the SES with and seismic signals for interstation distances (Seismic source -MT stations) ranging between 100 m and 200 m..

Focusing on the magnetic part, a comparative Wavelet analysis on SES and on seismic source permitted to evaluate that in the time domain that SES signals are mainly associated with Rayleigh wave, due to relatively large average distance between shot and MT sites, ranging between 60 m and 200 m.

Once defined SES characteristics, in terms of frequency content and amplitude, possible information on the subsurface status and new inferences on fluids characterizing the subsoil of the studied area were obtained. This was possible by investigating the spatial distribution of SES amplitude and by comparing it with 3D model of Vp, Vs and resistivity as well as with temperature and CO2 flux maps.

How to cite: Siniscalchi, A., Balasco, M., Romano, G., and Tripaldi, S.: Study of Seismo-Electromagnetic signals in an area characterized by an intense hydrothermal activity: a case study from the Solfatara area (Campi Flegrei, Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19048, https://doi.org/10.5194/egusphere-egu2020-19048, 2020.

After the occurrence of the 1999 magnitude 7.3 Chi-Chi earthquake, a cluster of NE-SW trending earthquakes,
almost along the surface trace of the Lishan fault, has been detected in the northern portion of the Central Range
in northern Taiwan. From the spatiotemporal distribution of hypocenters based on cluster analysis, the Lishan
fault cluster (LFC) can quantify the evolution of seismicity as aftershocks of the 1999 Chi-Chi earthquake. The
results of seismic tomographic inversion indicate that the LFC extends down to about 10 km depth and seems
to be distributed in high Vp areas rather than in low Vp areas. This temporal expansion is attributed to fluid
diffusion. Seismic activity in the upper crust tends to be high above broad zone with low Vp in the lower crust. Our
tomographic images demonstrate a series of relatively high Vp/Vs anomalies dipping to the east which seems to
form a fluid upwelling conduit beneath the Central Range. We thus suggest that the Lishan Fault might play a role
of an active fluid conduit, fluid or fluid fluxed a partial melt of the Philippines Sea plate would be released along
the east-dipping conduit and rise gravitationally to the upper crust.

How to cite: Cheng, W.-B.: Tomographic imaging of a seismic cluster in northern Taiwan and its implications for crustal fluid migration, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2006, https://doi.org/10.5194/egusphere-egu2020-2006, 2020.

We investigated temporal changes in the waveform of wave packets in S-coda associated with a swarm-like earthquake sequence, and estimated the original location of the wave packets via an array analysis. The earthquakes are located around the Moriyoshi-zan volcano in northeastern Japan, and were triggered by the 2011 off the Pacific coast of Tohoku earthquake, forming the largest cluster to the north of the volcano. A notable feature of seismograms from the triggered earthquakes is the appearance of the distinct scattered wave-packets (DSW) that are S-to-S scattered waves from the localized strong heterogeneity in the mid-crust. The DSW appear about 2–3 s after the onset of S-wave with a dominant frequency of 8–24 Hz and with a duration of around 1 s. Furthermore, the DSW show the variation in their shapes even in the roughly near events. 
To investigated the variation of DSW in detail, we first grouped events in the largest cluster with short inter-event distances and high cross-correlation coefficients (CC) in the time window of direct waves. Then we focused on the DSW part. Even in the same group, DSW showed temporal changes in their amplitudes and shapes. The change occurred gradually in some cases, but temporal variation were much more complicated in many cases. For example, the shapes of DSW changed from unclear peak to clear double peaks and suddenly back to the unclear. We also found that the shape of DSW changed in a very short time interval, for example, within ~ 12 h. 
Next, we estimated the location of DSW origin by applying the semblance analysis to the data of the temporary small-aperture array deployed to the north of the largest cluster. The DSW origin is located between the largest cluster within which hypocentral migration had occurred and the low-velocity zone depicted by a previous tomographic study. These observations imply the existence of crustal fluid and the DSW origin was composed of crustal fluid accumulated midway in the upward fluid pathway from the low-velocity zone to the earthquake cluster. 
Though we could not entirely exclude the possibility of the effect of the event location and focal mechanisms, the remarkable temporal changes in DSW waveforms possibly reflect the temporal changes in and/or near the origin. The short term change in DSW implies that fast movement of crustal fluid can occur at the depth of the mid-crust. 

 

How to cite: Amezawa, Y., Kosuga, M., and Maeda, T.: Temporal changes in the distinct scattered wave packets and their origin associated with triggered earthquake swarm beneath the Moriyoshi-zan volcano, northeastern Japan , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2243, https://doi.org/10.5194/egusphere-egu2020-2243, 2020.

EGU2020-15110 | Displays | SM6.1

Groundwater changes affect crustal deformation, elastic properties and seismicity rates in the Southern Alps (Italy)

Francesco Pintori, Enrico Serpelloni, Laurent Longuevergne, Carlos Almagro-Vidal, Lucia Zaccarelli, Alexander Garcia-Aristizabal, Licia Faenza, Maria Elina Belardinelli, and Pier Luigi Bragato

We show the results of a multidisciplinary study on hydrologically-induced deformation in the Southern Alps (Italy) developed integrating geodetic, seismological and hydrological observations. The study region, located across the Belluno Valley and the Montello Hill, is part of the Adria-Eurasia boundary, where ~1 mm/yr of N-S shortening is accommodated across a S-verging fold-and-thrust belt. GNSS time-series show the occurrence of non-seasonal horizontal transient displacements, characterized by a sequence of extensional and contractional deformation episodes oriented along the direction of the tectonic shortening. This signal is temporally correlated with water storage changes that are estimated using a lumped hydrological model based on precipitation, temperature, potential evapotranspiration and Piave river flow measurements. Geodetic and hydrological information are integrated in a 2D mechanical model with the goal of defining possible geological structures responsible for the measured subcentimetric geodetic displacements. Our interpretation implies that precipitation water rapidly penetrates the epikarst developed at the hinge of the anticline associated with the Bassano-Valdobbiadene thrust, converging toward a sub-vertical, deeply rooted hydrologically-active fracture (associated with its back-thrust), which tend to focus groundwater fluxes and pressure changes, generating ground displacements. Accordingly, seismic velocity changes computed from the analysis of ambient seismic noise cross-correlation show a temporal (anti) correlation with the evolution of water storage changes, suggesting that fluid increase in the aquifer perturb the Earth crust at depth by decreasing the seismic velocity (and vice-versa, during water storage decrease phases). Finally, by analyzing the seismicity recorded between 2012 and 2017 by a local network using a covariate model, we found that seismicity rates from a cluster of background seismicity correlate with changes in water storage. Although a spatial correlation between these seismic events and Coulomb stress changes associated with transient deformation episodes is not clear, it is worth noting that our model suggests stress perturbations of the order of 5-10 KPa down to 5-10 km of depth.

How to cite: Pintori, F., Serpelloni, E., Longuevergne, L., Almagro-Vidal, C., Zaccarelli, L., Garcia-Aristizabal, A., Faenza, L., Belardinelli, M. E., and Bragato, P. L.: Groundwater changes affect crustal deformation, elastic properties and seismicity rates in the Southern Alps (Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15110, https://doi.org/10.5194/egusphere-egu2020-15110, 2020.

EGU2020-3505 | Displays | SM6.1

Microseismic and Induced Seismicity Monitoring and Tomography of the Changning-Zhaotong shale gas play in China using dense nodal arrays

Wen Yang, Junlun Li, Yuyang Tan, Yaxing Li, Jiawei Qian, and Haijiang Zhang

With the development of shale gas in the Changning-Zhaotong play in the southern Sichuan basin of China, which is the largest shale gas prospect in China, the frequency and magnitude of earthquakes in this region have increased significantly in recent years. For example, a M5.7 earthquake occurred on December 16, 2018, and a M5.3 earthquake on January 6, 2019 in addition to many M4.0+ earthquakes in this area. Some studies argue the large magnitude earthquakes are triggered by hydraulic fracturing in for the local shale gas development, which commenced in 2011. The frequency of the earthquake occurrence has been on steady increase in the past few years that local residents often reported felt quakes. To further understand the correlation between the shale gas development and local seismic activity, we conducted a two-phase dense array seismic monitoring with about 200 Zland 3C and SmartSolo 3C 5 Hz seismic nodes, from late February to early May, 2019 for a period of 70 days. The survey consists of roughly 340 deployments at 240 sites, with an average interstation distance of 1.5 km, covering 500 km2 in total. We have processed seismic records from late February to early April, 2019 (phase I), and picked some 600,000 P- and S-wave arrival times from 4385 detected local earthquakes. The earthquake hypocenters and the subsurface velocity structure of the Changning-Zhaotong area are inverted for using the double-difference tomography method. The relocation results show that the majority of hypocenters were located at depths ranging from 1.0km to 4.0km, in the proximity of the horizontal hydraulic fracturing wells. The tomographic results (< 3 km) correlate well with the known surface geological units, and most earthquakes occurred along the velocity discontinuities, likely characterizing a large hidden fault which, interestingly, is where the January 2019 M5.3 occurred. Our study is very important for understanding the seismic potentials in this area, and should provide useful information for the shale gas development in this region and other areas in China with similar geological, tectonic and stress conditions.

How to cite: Yang, W., Li, J., Tan, Y., Li, Y., Qian, J., and Zhang, H.: Microseismic and Induced Seismicity Monitoring and Tomography of the Changning-Zhaotong shale gas play in China using dense nodal arrays, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3505, https://doi.org/10.5194/egusphere-egu2020-3505, 2020.

This study focuses on the the active fault system that caused the 1980 M_S 6.9 Irpinia earthquake (Irpinia fault zone (IFZ)) that is presently interested by a continuous and frequent micro-earthquake activity occurring within the volume in the volume enclosed by two antithetic faults. It is therefore important to improve the knowledge of the IFZ dynamics, with reference to potential future occurrence of moderate to large earthquakes, especially in terms of earthquake triggering mechanisms. Several previous works evaluated the spatial distribution of elastic/anelastic fault-embedded medium properties and related rock physical micro-parameters in connection with the seismicity rate. These studies showed a spatial correlation between high Vp/Vs, low seismic attenuation in rock volumes where most of seismicity occurs, suggesting that fluid-driven pore-pressure changes may plays a key role in controlling the seismicity production at the IFZ.
Here we reconstruct accurate 4D seismic velocity images of the volume embedding IFZ which allows to detect and track space-time changes of medium elastic properties possibly induced by fluid pore pressure migration and investigate the related seismicity production.
We analyzed the arrival time phase catalogue of about ten years (2005-2016) of Mw < 3.1 events recorded by the ISNet (Irpinia Seismic Network) and INGV network. We divided the catalog in 5 not-overlapping epochs by selecting in each of them , approximately the same number of events and an uniform volume coverage, in order to ensure that the 3D P and S velocity models could be equally well resolved for each epoch. By comparing the Vp, Vs and Vp/Vs images at each epoch in the equally resolved volume, we are able to detect medium velocity changes. Some regions, in the first 6 km of depth of NE part, do not show velocity changes with time, which is interpreted as the main effect of unperturbed lithology mainly controlling the average seismic velocity. In other regions, in the central part of the model at about 8-10 km depth, we clearly detect velocity changes causing an up to 10% Vp/Vs variation between different epochs. Based on the rock physical modelling, we associate the time-varying Vp/Vs and the observed amplitude of variation to fluid-driven changes in rock physical properties related to their spatial migration or pore-pressure induced changes. The regions where large Vp/Vs changes occur appear correlated with the largest seismicity production volumes, suggesting a direct link between the physical processes associated with fluid mobility and/or pore pressure migration and earthquake generation at the IFZ.

How to cite: De Landro, G., Esposito, R., Ortensia, A., and Zollo, A.: Time-lapse tomographic images of the Irpinia Fault System (Southern Italy) reveal Vp/Vs ratio changes that correlate with micro-seismicity production and evolution, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14615, https://doi.org/10.5194/egusphere-egu2020-14615, 2020.

EGU2020-8620 | Displays | SM6.1

Seismic imaging of St Gallen (Switzerland) deep geothermal field medium properties

Paolo Capuano, Vincenzo Convertito, Raffaella De Matteis, Ortensia Amoroso, Umberto Napoli, Bruno Massa, Grazia De Landro, and Gian Paolo Donnarumma

Sub-surface operations for energy production may originate various environmental risks among which, of great relevance is the seismic risk due to the induced seismicity associated with field operations.

In the framework of the H2020 Science4CleanEnergy project, S4CE, a multi-disciplinary project aimed at understanding the underlying physical mechanisms underpinning sub-surface geo-energy operations and to measure, control and mitigate their environmental risks, we have investigated the role of fluids in the generation of the seismicity induced during the deep geothermal drilling project close to the city of St.Gallen, Switzerland. To this aim we applied the Focal Mechanism Tomography (FMT) technique and the velocity and attenuation tomography using data collected by the Swiss Seismological Service in 2013 while realizing well control measures after drilling and acidizing the GT-1 well. The dataset consists of 347 earthquakes with magnitude (MLcorr) between -1.2 and 3.5. P and S phases were initially hand-picked on three-component ground velocity recordings. As an additional enhancement, a refined re-picking algorithm based on the waveforms cross-correlation was applied providing accurate travel-times data set. The revised picks and P polarities were used both to re-locate the events, using probabilistic approach considering both the absolute both the differential arrival times, and to estimate fault mechanisms using the FPFIT code. Only those events having at least 6 clear P-wave polarities have been analysed. To better constrain the focal mechanisms, for the larger magnitude events the BISTROP code (Bayesian Inversion of Spectral-Level Ratios and P-Wave Polarities) has been also applied.

Using the FMT technique we estimated the 3D excess pore fluid pressure field at the events hypocentre. Basically, the technique assumes that fault strength is controlled by Coulomb failure criterion and, under the hypothesis of uniform stress field, it ascribes the focal mechanism variations to pore fluid pressure acting on faults.

The velocity model and the attenuation model have been estimated by using an iterative tomographic inversion of P and S arrival times and t* quantities, which are defined as the ratio of the travel time and quality factor (Q). The t* measures for both P and S wave have been obtained from the analysis of the displacement spectra. We found that fault mechanisms do not fit a uniform stress-field. Based on the events depth, at least two different stress-fields are required. FMT results indicate that fluids contributed to the generation of the induced events. Taking into account for the uncertainties, the inferred excess pore fluid pressure is consistent with the wellhead pressure. Moreover, a correlation exists between the high excess pore fluid pressure and the high Vp/Vs values.

 

This work has been supported by S4CE ("Science for Clean Energy") project, funded from the European Union’s Horizon 2020 - R&I Framework Programme, under grant agreement No 764810 and by MATISSE project funded by Italian Ministry of Education and Research.

How to cite: Capuano, P., Convertito, V., De Matteis, R., Amoroso, O., Napoli, U., Massa, B., De Landro, G., and Donnarumma, G. P.: Seismic imaging of St Gallen (Switzerland) deep geothermal field medium properties, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8620, https://doi.org/10.5194/egusphere-egu2020-8620, 2020.

EGU2020-13536 * | Displays | SM6.1 | Highlight

The project FLUIDS: Detection and tracking of crustal fluids by multi-parametric methodologies and technologies

Aldo Zollo, Grazia De Landro, Antonio Caracausi, Raffaele Castaldo, Nicola D'Agostino, Michele Paternoster, Agata Siniscalchi, Tony Alfredo Stabile, and Andrea Tallarico

Fluids permeate and diffuse within the shallow crust being as originated by internal or external natural sources or by industrial activities for modern energy exploitation and production.

Fluid-induced stress changes can reactivate faults and cause earthquakes. In volcanic environments fluids play a key role in controlling the evolution of magmatic processes and eruption. The reliable imaging of fluid storages and accurate tracking of their movements is therefore critical in evaluating the nature and likelihood of future natural/induced earthquake or volcanic activity and their relative hazard monitoring and assessment.

The project FLUID has been recently approved by the Italiam Ministry for Research and has the ambitious goal to build up and experiment the next generation of deep ( crust and mantle-derived) fluid monitoring systems aimed at their timely detection and space-time tracking. This objective is achieved by developing and applying an integrated, multi-parametric and multi-disciplinary approach for mapping and tracking fluid movements in volcanic, tectonic and industrial exploitation, sub-surface, geological environments. Innovative methodologies and technologies will be developed to reconstruct the 4D (space and time) variations of rock properties in the fluid-filled porous medium and to detect and characterize fluid-triggered natural effects as well as the induced micro-seismicity, electric crustal properties changes, earth surface ground deformation and geochemical signatures of fluid presence and diffusion.

The project will develop activities and scientific products in the following research directions:

  • Multi-parametric data acquisition and management aimed at data acquisition, integration and sharing/publishing for scientific and public information purposes;
  • 4D multi-parametric crustal imaging aimed at setting up and testing different geophysical/geological methodologies to image the underground in space and time, and at comparing the obtained images for an effective and reliable tracking of fluids;
  • Induced phenomena and/or triggered effects by fluid diffusion aimed at investigating fluid properties and movements, developing new methods & technologies for their detection and tracking through their triggered effects and finding their correlation with geophysical observables;
  • Characterization and modeling of fluids migration at test-sites from regional to the local scale: through the application of the developed multi-parametric and multi-disciplinary approaches to different test-sites in volcanic, tectonic and industrial exploitation geological environments in Italy.

Results of this project are expected to have a broad scientific-technological impact through the development and application of new, integrated multi-parametric methods & technologies for fluid detection and space-time tracking. As for its socio-economic impact, the project will deliver “best practice” recommendations for managing fluid-induced seismicity for a sustainable and safe exploitation of all the energy resources that involve injection/withdrawal of fluid into/from the subsoil. Forecasts of induced seismicity using multi-parametric observed systems of induced seismicity represent the planning and decision-making tools for mitigating the associated risk for population living nearby industrial sites but also active hazardous tectonic and volcanic regions.

How to cite: Zollo, A., De Landro, G., Caracausi, A., Castaldo, R., D'Agostino, N., Paternoster, M., Siniscalchi, A., Stabile, T. A., and Tallarico, A.: The project FLUIDS: Detection and tracking of crustal fluids by multi-parametric methodologies and technologies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13536, https://doi.org/10.5194/egusphere-egu2020-13536, 2020.

EGU2020-12453 | Displays | SM6.1

Characterization of Tectonic - Magmatic Seismic Source at Chiles - Cerro Negro Volcanic Complex (CCNVC)

Andrea Córdova, Pedro Espin, and Daniel Pacheco

The Chiles - Cerro Negro Volcanic Complex (CVCCN) is located in the Western cordillera at the Ecuador – Colombia border. This volcanic complex has showed an anomalous seismic activity since late 2013, with high activity peaks in 2014, specially in October and November with up to 6000 earthquakes per day mostly volcanic-tectonics events. The most important earthquake in this sequence occurred on October 20, 2014 with a 5.7 Mw. In order to obtain a better characterization of the seismic source in the CVCCN area, a new 1D velocity model was computed using 300 earthquakes with magnitudes larger than 3.0 MLv, and high quality of P and S pickings.  This model has 8 layers over a semi-space and starts with a Vp = 2.96 Km/s and  Vs = 1.69 Km/s highlighting strong variations at 7km with Vp = 5.87 Km/s and  Vs = 3.52 Km/s and at 24 km Vp = 6.58 Km/s and  Vs = 3.79 Km/s . A value of 1.73 of Vp/Vs was determined, which is a normal for the continental crust. Computed hypocenters with the new velocity model highlighted two sources: one is defined by a concentration of shallow earthquakes on the southern flank of Chiles Volcano, and the second one contains events deeper than 7 km and follows a N-S tectonic structure that crosses the CVCCN and matches the Cauca-Patía fault. This structure obtained with this new model is confirmed by interferograms from Sentinel images after the earthquake MLv 4.2 of September 27, 2019 where a mostly dextral movement is defined. Focal mechanisms were computed for earthquakes larger than MLV 4.0 using waveform inversion (SeisComp3). Most events show ~N-S planes and dextral with inverse component. Focal mechanisms exhibit a Non-Double Couple component (CLVD), which in most of these events is more than 40 percent including the CLVD = 71% calculated for the earthquake of Mw 5.7 on October 20, 2014. This value suggests the presence of a volumetric component that could be induced by magma or fluid movements. This is corroborated by the presence of LP and VLP events inside of the CVCCN system.

How to cite: Córdova, A., Espin, P., and Pacheco, D.: Characterization of Tectonic - Magmatic Seismic Source at Chiles - Cerro Negro Volcanic Complex (CCNVC), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12453, https://doi.org/10.5194/egusphere-egu2020-12453, 2020.

EGU2020-654 | Displays | SM6.1

Hybrid long-period volcanic events observed in off Nicobar region, the Andaman Sea from a passive OBS experiment

Karanam Kattil Aswini, Pawan Dewangan, Kattoju Achuta Kamesh Raju, Yatheesh Vadakkeyakath, Pabitra Singha, Ramakrushana Reddy, and Lalit Arya

The off Nicobar region in the Andaman Sea is witnessing frequent earthquake swarms after December 2004 Tsunamigenic earthquake in January 2005, March and October 2014, November 2015 and April 2019. In this study, we present the geophysical evidence of active volcanism in the Off Nicobar back-arc region on 21st and 22nd March 2014 based on a passive Ocean Bottom Seismometer (OBS) experiment. We detected a series of hybrid earthquake events characterized by the onset of high–frequency signal (1-10 Hz) which is followed by a long period waveform of up to 600s having a range of 0.1-1 Hz. The waveforms appear to be emergent and lack the onset of a distinct S-phase. We also observed a very high frequency (10-40 Hz) hydro-acoustic phase in the coda of long-period events.  These hybrid events are considered to be volcano-tectonic (VT) events that may trigger magmatic activities in the Off Nicobar region. We have identified and located 141 high-frequency events on 21st and 22nd March 2014 using hypocent v.3.2 program and they are distributed along NW-SE direction aligning with the submarine volcanoes defining the volcanic arc as observed in the high-resolution bathymetry data. The fault plane solution of the major high-frequency events suggests strike-slip faulting with the strike, dip and rake values of 334°, 89° and 171°, respectively along the direction of the prevalent sliver strike-slip faulting in the Andaman back-arc region. We propose that the upward movement of magma is a plausible mechanism which can explain the frequent occurrence of earthquake swarms in the off Nicobar region. The stress generated from magma movement may initially trigger shallow VT events such as faulting or dike intrusions and later generate long period ringing associated with the resonance of the magma chamber. The shallow nature of the events also generates a hydroacoustic wave which is detected in the OBS experiment as the source region is in the SOFAR channel.

How to cite: Aswini, K. K., Dewangan, P., Kamesh Raju, K. A., Vadakkeyakath, Y., Singha, P., Reddy, R., and Arya, L.: Hybrid long-period volcanic events observed in off Nicobar region, the Andaman Sea from a passive OBS experiment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-654, https://doi.org/10.5194/egusphere-egu2020-654, 2020.

EGU2020-8498 | Displays | SM6.1

Volcano-tectonic earthquake statistics for Campi Flegrei monitoring

Anna Tramelli, Cataldo Godano, Flora Giudicepietro, Patrizia Ricciolino, and Stefano Caliro

The knowledge of the dynamic of the Campi Flegrei calderic system is essential to mitigate the volcanic risk in one of the most densely populated volcanic areas in the world. From 1950 to 1985 three bradyseismic crises occurred with a total uplift of almost 3 m (Del Gaudio et al., 2010). After more than 20 years of subsidence, at the end of 2005 the uplift started again accompanied by a low increment in the seismicity rate. In 2012 a further increment in the seismicity rate was observed and a variation in the gas composition of the fumaroles of Solfatara (central area of the caldera) revealed the injection of magmatic fluids into the hydrothermal system (Chiodini et al., 2017). This suggests that the investigation of the seismicity can represent a very useful tool for the risk mitigation. Here we analyze the seismic catalogue of Campi Flegrei (collected by INGV - Osservatorio Vesuviano) to check for any variation of the observed seismicity. This can be eventually associated with geochemical monitored parameters. In addition, we analyzed the most energetic swarms recorded in this period by comparing their locations, waveforms and source mechanisms. We find that occurrence rate, location and b-value change in time. The seismicity occurs in swarms, which, in the last years, tends to became closer but with a smaller number of events. The observed variations are correlated also with the geochemical monitoring parameters suggesting that the uplift process has probably modified the elastic and permeability properties of the shallow part of the crust. 

How to cite: Tramelli, A., Godano, C., Giudicepietro, F., Ricciolino, P., and Caliro, S.: Volcano-tectonic earthquake statistics for Campi Flegrei monitoring, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8498, https://doi.org/10.5194/egusphere-egu2020-8498, 2020.

EGU2020-20361 | Displays | SM6.1

Seismic activity along a Cretaceous magmatic intrusion in Monchique, SW Iberia

Analdyne Soares, Susana Custódio, Marta Neres, Dina Vales, and Luís Matias

Iberia, located at the southwestern end of Europe, displays a complex pattern of seismic activity, with most known active faults slipping at low rates (< 1 mm/yr). However, the seismic activity is remarkable, with numerous earthquakes in the historical record proving destructive. The earthquake cluster in mainland Portugal that has a highest rate of seismic activity is very localized (small spatial extent), extends vertically from 5 to 20 km depth and lays on the Monchique late Cretaceous magmatic intrusion, in SW Portugal. This magmatic intrusion forms strong rheological contrast between the intruded magmatic rocks and surrounding Paleozoic rocks. Furthermore, it is the locus of abundant natural water springs. Several pertinent questions remain to be answered concerning earthquakes in Monchique: Are earthquakes in Monchique simply a response to tectonic stresses (given the proximity of Monchique to the EU-AF plate boundary), with the localization of brittle failure in the region facilitated by the rheological contrast between the Cretaceous intrusion and surrounding Paleozoic rocks? Do fluids play a role in facilitating slip in existing fractures? Or, conversely, is the circulation of fluids facilitated by the faulting that results from the rheological contrasts? Are there hazardous faults in Monchique? In this presentation, we re-analyze in detail the seismic data recorded by the regional permanent seismic network, in order to better understand the relationship between seismic activity and igneous intrusion. In particular, we re-locate earthquakes using NonLinLoc and PRISM3D, a 3D velocity model for the region. At a subsequent step, we re-locate earthquakes using HypoDD. We also perform a clustering analysis based on waveform similarity and compute focal mechanisms for the region. The results show that earthquakes align along two main directions, E-W and NNE-SSW, coinciding with surface features of the magmatic intrusion. Focal mechanisms indicate dominantly strike-slip faulting, with the possible fault planes coinciding with the favored directions of earthquake lineations. We investigate the spatio-temporal evolution of seismicity and address possible forcing mechanisms, including tidal forcing.

 

 

The author would like to acknowledge the financial support  FCT through project UIDB/50019/2020 – IDL and PTDC/GEO-FIQ/2590/2014 - SPIDER.

How to cite: Soares, A., Custódio, S., Neres, M., Vales, D., and Matias, L.: Seismic activity along a Cretaceous magmatic intrusion in Monchique, SW Iberia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20361, https://doi.org/10.5194/egusphere-egu2020-20361, 2020.

EGU2020-2626 | Displays | SM6.1

The source scaling of swarm-genic slow slip events

Luigi Passarelli, Eleonora Rivalta, Paul Antony Selvadurai, and Sigurjón Jónsson

Slow slip events (SSEs) are slow fault ruptures that do not excite detectable seismic waves although they are often accompanied by some forms of seismic strain release, e.g., clusters of low- and very-low frequency earthquakes, and/or episodic or continuous non-volcanic tremor (i.e. tremor-genic SSEs) and earthquake swarms (swarm-genic SSEs). At subduction zones, increasing evidence indicates that aseismic slip and seismic strain release in the form of non-volcanic tremor represent the evolution of slow fracturing. In addition, aseismic slip rate modulates the release of seismic slip during tremor-genic SSEs. No general agreement has been reached, however, on whether source duration-moment scaling of SSEs is linear or follows that of ordinary earthquakes (cubic). To date, investigations on the source scaling has been based on global compilations of tremor-genic SSEs while no studies have looked into the source scaling of swarm-genic SSEs.

We present the first compilation of source parameters of swarm-genic slow slip events occurring in subduction zones as well as in extensional, transform and volcanic environments. We find for swarm-genic SSEs a power-law scaling of aseismic to seismic moment release during episodes of slow slip that is independent of the tectonic setting. The earthquake productivity, i.e., the ratio of seismic to aseismic moment released, of shallow SSEs is on average higher than that of deeper ones and scales inversely with rupture velocity. The inferred source scaling indicates a strong interplay between the evolution of aseismic slip and the associated seismic response of the host medium and that swarm-genic SSEs and tremor-genic SSEs arise from similar fracturing mechanisms. Depth dependent rheological conditions modulated by fluid pore pressure, temperature and density of asperities appear to be the main controls on the scaling. Large SSEs have systematically high earthquake productivity suggesting static stress transfer as an additional factor in triggering swarms of ordinary earthquakes. Our data suggest that during the slow slip evolution the proportion of seismic strain release is always smaller than the aseismic part although transient changes in stress and fault rheology imparted by swarm-genic SSEs can lead to delayed triggering of major and devastating earthquakes like in the Tohoku, Iquique and L’Aquila cases. The evidence of source scaling reported here will help constraining theoretical models of SSEs rupture propagation and seismic hazard assessments that should take into account the new scaling between aseismic and seismic moment release. 

How to cite: Passarelli, L., Rivalta, E., Selvadurai, P. A., and Jónsson, S.: The source scaling of swarm-genic slow slip events, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2626, https://doi.org/10.5194/egusphere-egu2020-2626, 2020.

EGU2020-10252 | Displays | SM6.1

Detection and characterization of fluid-driven earthquake clusters

Sebastian Hainzl and Tomas Fischer

Natural earthquake clusters are often related to a mainshock, which triggers the sequence by its induced stress changes. These clusters are called mainshock-aftershock sequences and statistically well explained by earthquake-earthquake interactions according to the Epidemic Type Aftershock Sequence (ETAS) model. Additionally, aseismic processes such as slow slip, dike propagation or fluid flow might also play a role in the initiation and driving of the earthquake sequence. Earthquake swarms, which lacks a dominant earthquake, are often believed to indicate such transient aseismic forcing signals. However, swarm-type clusters can also occur by chance in ETAS-simulations and thus not necessarily related to aseismic drivers. Thus, more sophisticated quantification of the space-time-magnitude characteristics of earthquake sequences are required for discrimination. Migration patterns are one of those properties which can be indicative for aseismic triggering. We suggest simple measures to identify and quantify migration patterns and test those for synthetic data, data from fluid injection experiments, and natural swarm activity related to fluid flow in NW Bohemia and Long Valley caldera. We analyze their potential to discriminate from ETAS-type clusters and compare it with those of time-magnitude characteristics of the activity such as seismic moment ratios and skewness. Our results are finally used to discriminate earthquake clusters in California and elsewhere.

How to cite: Hainzl, S. and Fischer, T.: Detection and characterization of fluid-driven earthquake clusters, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10252, https://doi.org/10.5194/egusphere-egu2020-10252, 2020.

EGU2020-5153 | Displays | SM6.1

On seismic clusters, swarms and repeating earthquakes in Central Chile.

Carla Valenzuela Malebrán, Simone Cesca, Sergio Ruiz, Luigi Passarelli, Felipe Leyton, and Torsten Dahm

Along the Chilean subduction segment, the seismicity tends to display characteristics of mainshock-aftershocks sequences. However, besides large and destructive earthquakes, central Chile has been also characterized by the occurrence of localized seismicity clusters with weak to moderate magnitudes, appearing either in form of repeated short-duration swarms or in form of sustained long-lasting activity. Seismic swarms were observed prior to large earthquakes and were hypothesized as possible precursors, although they did not always develop into major earthquakes. The origin and driving processes of this localized seismic activity have not yet been identified. Here, we characterize the seismicity at two seismic clusters in Central Chile, by analyzing hypocentral locations, spatio-temporal migration, magnitude, and inter-event time distributions and moment tensors. Both clusters are characterized by weak to moderate seismicity and manifest as clear seismicity rate and Benioff strain anomalies. We discuss these seismic clusters over a period of 18 years (2000-2017) and investigate their interactions with the Maule earthquake. We find repeating thrust earthquakes on the slab interface at one cluster beneath Vichuquén slipping at a rate comparable to the tectonically accumulated one. At the offshore Navidad cluster, the seismicity occurs in forms of swarms, with the largest episodes in 2001, 2002, 2004, 2012, 2014, 2016 and 2017 showing some rough temporal recurrence. Moment tensor indicates the occurrence of similar thrust mechanisms along a west-dipping structure across the subducting plate. Clusters persist before and after the Maule earthquake. However, at the Vichuquén cluster, the increased seismicity rate following the Maule earthquake remains to date higher than the background rate and the system is still far from recovery.

How to cite: Valenzuela Malebrán, C., Cesca, S., Ruiz, S., Passarelli, L., Leyton, F., and Dahm, T.: On seismic clusters, swarms and repeating earthquakes in Central Chile., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5153, https://doi.org/10.5194/egusphere-egu2020-5153, 2020.

EGU2020-5767 | Displays | SM6.1

The October 2019 earthquake swarm in the Mineral Mountains, Utah and its relation to the geothermal system

Maria Mesimeri, Kristine Pankow, Ben Baker, and Mark Hale

In October 2019 an earthquake swarm initiated in the Mineral Mountains, Utah near the Roosevelt Hot Springs. The area has been characterized as swarm-genic after the recording of an energetic swarm (1044 microearthquakes, M less than 1.5) during the summer of 1981. This study primarily aims to investigate the spatio-temporal properties of the newly detected earthquake swarm and compare its occurrence to prior seismic activity. The October, 2019 earthquake swarm lasted four days and consists of forty-three shallow earthquakes that were cataloged by the University of Utah Seismograph Stations (UUSS) with magnitudes -0.7 to 1.31. All the events were recorded by a dense local broadband seismic network located around the Frontier Observatory for Research in Geothermal Energy (FORGE) in southcentral Utah, ~10 km west of the activated area. The close proximity of the seismic network along with the density of the seismicity allows us to apply techniques for improving the detection level and earthquake location. To achieve this, we use the earthquakes detected by the UUSS as templates and scan the continuous data for new events by applying a matched filter technique. To perform a detailed spatial analysis of the earthquake swarm and look for migration patterns, we create a high-resolution earthquake catalog using a double difference technique and differential times from both catalog and cross correlation data. To gain insight into the stress regime, we compute fault plane solutions from first motions for individual events and composite focal mechanisms for families of similar events. We further attempt to explore the underlying mechanism by examining the presence of repeating earthquakes comprising the earthquake swarm and their relation to aseismic slip. Such observations may shed insights into the role of fluids and the influence of the high heat flow, due to the geothermal system, on earthquake triggering and migration.

 

How to cite: Mesimeri, M., Pankow, K., Baker, B., and Hale, M.: The October 2019 earthquake swarm in the Mineral Mountains, Utah and its relation to the geothermal system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5767, https://doi.org/10.5194/egusphere-egu2020-5767, 2020.

EGU2020-7904 | Displays | SM6.1

Earthquake swarms in West Bohemia-Vogtland and South-West Iceland: are they of similar nature?

Josef Horálek, Hana Jakoubková, Jana Doubravová, and Martin Bachura

Earthquake swarms occurred worldwide in diverse geological units, however, their origin is still unclear. West Bohemia-Vogtland represents one of the most active intraplate earthquake-swarm areas in Europe, South-West Iceland is characterized by intense interplate earthquake swarms. Both these areas exhibit high activity of crustal fluids.

We investigated earthquake swarms from W-Bohemia and from different areas in SW-Iceland: the Hengill volcanic complex, Ölfus transition zone (the edge of the SISZ), and Reykjanes Peninsula, from the perspective of their magnitude-time development, seismic moment release with time, the magnitude-frequency distribution and distribution of the inter-event times, and the space and time distribution of the foci. The aim was to determine the swarm characteristics that are dependent or vice-versa independent on the tectonic environment, and also the characteristics which should help us to distinguish more precisely earthquake swarms from mainshock-aftershock sequences.

We found that the frequency-magnitude (b-values) and inter-event-time distributions are similar for both areas, while total seismic moment release and its rate are much larger for the SW Icelandic activities compared to the W-Bohemia ones. One dominant short-term swarm phase with one or a few dominant events in which significant part of M0tot released, is typical of the SW Icelandic swarms, whereas the W-Bohemia swarms are characterised by stepwise seismic moment release, which is manifested by several swarm phases. MFDs of the SW-Iceland swarms indicate significantly lower a-value (number of ML > 0 evens), particularly of those on the Reykjanes Peninsula, compared to W-Bohemia swarms; it is due to the fact that considerable amount of M0tot released in quasi-mainshocks and the rest in aftershocks; lower a-value was also found for the W-Bohemian mainshock-aftershock sequence in 2014. The W-Bohemian swarms took place in a bounded focal zone consisting of several fault segments but the SW-Icelandic swarms correspond well to tectonic structures along the Mid Atlantic Ridge. We conclude that most of the W-Bohemia earthquake swarms were series of subswarms with one or more embedded mainshock-aftershock sequences, while the SW-Icelandic swarms (particularly those on the Reykjanes Peninsula appear to be a transition between earthquake swarm and mainshock-aftershock sequence. The W-Bohemia and SW-Iceland focal zones are characterized by complex system of short, differently oriented faults/fault segments; interestingly, the W-Bohemia and some SW-Icelandic focal zones exhibit coexistence of faults susceptible to earthquake swarms and differently oriented faults predisposed to common earthquakes (mainshock-aftershocks).

How to cite: Horálek, J., Jakoubková, H., Doubravová, J., and Bachura, M.: Earthquake swarms in West Bohemia-Vogtland and South-West Iceland: are they of similar nature?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7904, https://doi.org/10.5194/egusphere-egu2020-7904, 2020.

EGU2020-8391 | Displays | SM6.1

Advances in earthquake-swarms monitoring networks WEBNET and REYKJANET

Jakub Klicpera, Jana Doubravová, and Josef Horálek

The IG CAS in cooperation with IRSM CAS operates two local seismic networks deployed to monitor the seismic swarms in West Bohemia/Vogtland, Czechia and Reykjanes Peninsula, Iceland. 

WEBNET monitors the region of West Bohemia since 1991 developing from 4 short period stations to 24 broadband stations today. The seismoactive region West Bohemia/Vogtland lies in the border area between Czechia and Germany in the western part of Bohemian Massif. It is an intra-continental area with persistent swarm-like seismicity but rarely also main-shock after-shock sequences may occur. 

REYKJANET local seismic network is situated in Reykjanes Peninsula on Southwest Iceland. The area is an onshore part of the mid-Atlantic plate boundary between the North America and Eurasia Plates. The seismic activity of Reykjanes peninsula is represented by typical main-shock after-shock sequences as well as earthquake swarms. The REYKJANET network was built in 2013 and it consists of 15 stations placed around the epicentral area.

Both networks have been substantially upgraded during the last years. In case of REYKJANET the replacement of old sensors and digitizers with new ones made the operation easier and ready for near future plan to stream the waveform files in real time. WEBNET network which was long years divided into two subnets – on-line permanent stations and off-line autonomous stations, was recently homogenized by eco-powering and 4G LTE data connecting of the off-line stations. Additonally, the micro network HORNET was deployed within the WEBNET epicentral zone to monitor Horka water dam.

Data from both above mentioned networks are automatically searched for seismic events by the neural-network-based detector designed by Doubravová et al. (2016, 2019) providing event list with completeness magnitude Mc=0 for REYKJANET and Mc=-0.5 for WEBNET. The main difference of sensitivity is given by different noise levels of the two networks.

How to cite: Klicpera, J., Doubravová, J., and Horálek, J.: Advances in earthquake-swarms monitoring networks WEBNET and REYKJANET, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8391, https://doi.org/10.5194/egusphere-egu2020-8391, 2020.

EGU2020-16840 | Displays | SM6.1

Detection of repeating earthquakes in the West Bohemia swarm region

Ali Salama and Tomas Fischer

 

 

 

Repeating earthquakes, sequences of microseismic events with highly similar seismograms and magnitudes, suggest quasi-periodic rupturing of the same asperity. They are observed on creeping fault segments surrounded by aseismic slip area and also in earthquake swarms. However, so far, they have not been documented in the West Bohemia/Vogtland seismic swarm area. These local swarms consist of thousands of ML < 4 events occurring along a small area of fault zone with repeated activation of some patches during the swarms and weak background activity in the intermediate periods. Detecting and analyzing the repeating earthquakes would help revealing the continuing background activity and identifying fault areas that are active permanently. This could point to the possible sources of fluids or aseismic creep that are supposed to play significant role in swarm generation. Repeating earthquakes are identified by waveform cross-correlation analysis comparing waveforms of repeaters with continuous seismic data set. We developed efficient detection algorithm to identify repeating earthquakes using selected event templates to reveal continuing seismic activity along the main Nový Kostel fault zone, namely in the areas with only episodic activity. The results provide a robust basis for routine application to the long-term seismic dataset that will allow also for further applications including analysis of the source parameters of the repeaters and/or detecting possible seismic velocity variations in the focal zone.

 

How to cite: Salama, A. and Fischer, T.: Detection of repeating earthquakes in the West Bohemia swarm region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16840, https://doi.org/10.5194/egusphere-egu2020-16840, 2020.

In the Hualien area, two Mw6.4 and Mw6.2 earthquakes, 20 km apart, occurred in February 2018 and April 2019 respectively. The former to the northeast, located offshore to ​​the Liwu river, triggered several earthquake clusters along the Milun fault and southward to the Longitudinal Valley, the suture of the Eurasian and the Philippine Sea plates; the latter to the southwest, located in the Central Range, also triggered several seismic swarms in the Central Range,  along the Liwu river to the northeast and at Ji'an to the southeast. Except for the Milun fault, neither GPS nor InSAR observations detects significant surface deformation after the occurrence of these two main shocks, indicating that the earthquake ruptures mainly developed within the crust. Therefore, seismic observation becomes an efficient tool for revealing the seismotectonics of the two earthquake sequences. For monitoring the aftershock sequences, two days after the main shocks, we deployed two geophone arrays, 70 Z-component RefTek 125A TEXANs for two weeks in 2018 and 47 three-component Fairfield Nodal Z-Lands for one month in 2019, with 1-5 km station spacing around the Hualien City. These earthquake swarms were well recorded and analyzed through the dense seismic networks. The numbers of aftershock sequences manually identified are two-fold more than that issued by the Central Weather Bureau, Taiwan. The seismicity of the 2018 aftershock sequence, to depths of between 5-15 km, was significantly reduced within 10 days after the main shock. however, the seismicity of the 2019 aftershock sequence, to depths of between 2-50 km, was still above background seismicity rate 30 days after the main shock. The spatial distribution of the 2018 aftershock sequence could be related to a fault zone of the plate boundary, but that of the 2019 and the relocated 1986 aftershock sequences show a conjugate thrust fault pair beneath the eastern Central Range. Our results clearly depict several local tectonic structures that have not been observed at the northern tip of the Longitudinal Valley, not only a suture but also a transitional area from collision to subduction.

How to cite: Sun, W.-F., Kuo-Chen, H., Guan, Z.-K., and Chang, W.-Y.: Seismotectonics of the northern Longitudinal Valley, Taiwan, inferred from aftershock sequences of 2018 Mw6.4 and 2019 Mw6.2 Hualien earthquakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21398, https://doi.org/10.5194/egusphere-egu2020-21398, 2020.

SM6.3 – Fault mechanics and earthquakes from near fault observations

EGU2020-5996 | Displays | SM6.3 | Highlight

The role of fluids in the seismicity of the Western Gulf of Corinth (Greece)

Georgios Michas, Vasilis Kapetanidis, George Kaviris, and Filippos Vallianatos

The present study focuses on the Western Gulf of Corinth (WGoC), which is one of the most seismically active sites in Europe and also the region where the Corinth Rift Laboratory (CRL) Near Fault Observatory (NFO) has been installed. The WGoC exhibits high extension rates and intense microseismicity, with frequent occurrence of clustered seismicity, as in the cases of the 2001 Agios Ioannis swarm, the seismic sequences of 2003-2004 and 2006-2007 in the central part of the Gulf and the 2013 Helike swarm. These outbreaks of seismicity, lasting a few days to months, are characterized by a high frequency and density of earthquakes, with magnitudes generally not exceeding 4.5, with the strongest ones usually occurring in the middle or towards the end of the sequence. These short-lived seismic crises often exhibit patterns of spatio-temporal migration in their hypocentral distribution, which has been associated with the effects of diffusion and circulation of fluids to the seismogenic crust of the WGoC. Fluids appear to play an important role in both triggering and evolving seismic sequences. In the framework of the present study, earthquake hypocenters, relocated with high resolution by employing waveform cross-correlation and the double difference-method, are used to perform an upper crust Shear-Wave Splitting (SWS) study at the WGoC area. The temporal variation of the SWS is investigated, in relation with the temporal evolution of seismicity, to possibly identify patterns related to changes of the stress-field due to fluid migration, or before the occurrence of moderate to strong earthquakes. In addition, the relocated catalogue is analyzed using nonlinear statistical physics for the definition of the spatio-temporal scaling properties of clustered seismicity, as well as for the quantification and modeling of seismic diffusion phenomena associated with fluid circulation at the upper crust of the WGoC.

Acknowledgements

We would like to thank the personnel of the Hellenic Unified Seismological Network and the Corinth Rift Laboratory Network (https://doi.org/10.15778/RESIF.CL) for the installation and operation of the stations used in the current article. The present work was co-funded by the European Union (ESF) and Greek national funds through the Operational Program "Human Resources Development, Education and Lifelong Learning", project title “Τhe effect of fluids on the seismicity of the Western Gulf of Corinth” (project code MIS: 5048127).

How to cite: Michas, G., Kapetanidis, V., Kaviris, G., and Vallianatos, F.: The role of fluids in the seismicity of the Western Gulf of Corinth (Greece), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5996, https://doi.org/10.5194/egusphere-egu2020-5996, 2020.

EGU2020-20869 | Displays | SM6.3

Seismotectonic analysis of the 2014 seismic swarm at the Western Corinth Gulf (Greece)

Anna Serpetsidaki, Efthimios Sokos, Sophie Lambotte, Pascal Bernard, and Helene Lyon-Caen

The Corinth Rift (Greece) is one of the most seismically active regions in Europe and has been studied extensively during the past decades. It is characterized by normal faulting and extension rates between 6 and 15 mm yr−1 in an approximately N10E° direction. The seismicity of the area is continuously monitored by the stations of the Corinth Rift Laboratory Network (CRL Net). The availability of a dense permanent seismological network allows the extensive analysis of the seismic swarms which occur frequently. In this study, the September 2014 swarm located at the western part of the Corinth Gulf is analyzed. Initially, more than 4000 automatically located events, of a two month period, were relocated using the HYPODD algorithm, incorporating both catalogue and cross-correlation differential traveltimes. Consequently, the initial seismic cloud was separated into several smaller, densely concentrated clusters. Double difference relocation was also applied to 707 manually located events in order to investigate the Vp/Vs ratio variation, due to its sensitivity in pore fluids. The swarm’s parameters such as seismicity distribution and moment tensors were combined with the seismotectonic data of the area. The results indicate an initial activation of the Psathopyrgos normal fault; afterwards the seismicity extended both towards East and West, while most events occurred at the western part of the study area. The seismicity distribution revealed a main activation of the North – dipping faults. The seismicity migration with respect to pore pressure changes due to fluid movements was investigated through diffusivity calculations. The diffusivity value was found to be 4.5m2s-1, which is consistent with results of previous studies in the area. The results of the investigation of the fault- zone hydraulic behavior provide evidence for the fluid – triggered earthquake swarms and the related rock physical properties.

How to cite: Serpetsidaki, A., Sokos, E., Lambotte, S., Bernard, P., and Lyon-Caen, H.: Seismotectonic analysis of the 2014 seismic swarm at the Western Corinth Gulf (Greece), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20869, https://doi.org/10.5194/egusphere-egu2020-20869, 2020.

EGU2020-19156 | Displays | SM6.3

Monitoring natural CO2 flow in the mofettes of the West Bohemia seismoactive region

Josef Vlček, Tomáš Fischer, and Martin Lanzendörfer

Monitoring of CO2 degassing in seismoactive areas allows the study of correlations of gas
release and seismic activity. Reliable continuous monitoring of the gas flow rate in rough field
conditions requires robust methods capable of measuring gas flow at different types of gas
outlets such as wet mofettes, mineral springs and boreholes. In this paper we focus on the
methods and results of the long-term monitoring of CO2 degassing in the West
Bohemia/Vogtland region in Central Europe, which is typified by the occurrence of
earthquake swarms and emanations of carbon dioxide of magmatic origin. Besides direct
flow measurement using flowmeters, we introduce a novel indirect technique based on
quantifying the gas bubble contents in a water column, which is capable of functioning in
severe environmental conditions. The method calculates the mean bubble fraction in a water-
gas mixture from the pressure difference along a fixed depth interval in a water column.
Laboratory tests indicate the nonlinear dependence of the bubble fraction on the flow rate,
which is confirmed by empirical models found in the chemical and nuclear engineering
literature. Application of the method in a pilot borehole shows a high correlation between the
bubble fraction and measured gas flow rate. This was specifically the case of two coseismic
anomalies in 2008 and 2014, when the flow rate rose during a seismic swarm to a multitude
of the pre-seismic level for several months and was followed by a long-term flow rate decline.
However, three more seismic swarms occurring in the same fault zone were not associated
with any significant CO2 flow anomaly. We surmise that this could be related to the slightly
farther distance of the hypocenters of these swarms than the two ones which caused the
coseismic CO2 flow rise. Further long-term CO2-flow monitoring is required to verify the
mutual influence of CO2 degassing and seismic activity in the area.

How to cite: Vlček, J., Fischer, T., and Lanzendörfer, M.: Monitoring natural CO2 flow in the mofettes of the West Bohemia seismoactive region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19156, https://doi.org/10.5194/egusphere-egu2020-19156, 2020.

EGU2020-5735 | Displays | SM6.3

Variations of gas compositions during a drilling process: A key study on the Hartoušov Mofette, Czech Republic

Kyriaki Daskalopoulou, Heiko Woith, Martin Zimmer, Samuel Niedermann, Cemile D. Bağ, Ralf Bauz, Jakub Trubač, and Tomáš Fischer

The Eger Rift (Czech Republic) is an intraplate region without active volcanism but with emanations of magma-derived gases and the recurrence of mid-crustal earthquake swarms with small to intermediate magnitudes (ML < 5) in the Cheb Basin. To understand the anomalous earthquake activity and CO2 degassing, an interdisciplinary well-based observatory is built up for continuous fluid and earthquake monitoring at depth.

The fluid observatory is located at the Hartoušov Mofette (Cheb Basin), an area characterized by intense mantle degassing with a subcontinental lithospheric mantle (SCLM) contribution of He that increased from 38% in 1993 to 89% in 2016. Two drillings with depths of 30 and 108 m (F1 and F2, respectively) are being monitored since August 2019 for the composition of ascending fluids. Additionally, the environmental air composition is monitored. Gas concentrations were determined in-situ at 1-min intervals, while direct sampling campaigns took place periodically and samples were analyzed for their chemical and isotope composition. Samples of gases emerging in the mofette were also collected. During this period, a third borehole (F3) with a depth of 238 m was drilled.

At Hartoušov, carbon dioxide is the prevailing gas component (concentrations above 99.5%), with helium presenting a mantle origin (up to 90% considering a SCLM-type source). The atmospheric contribution is negligible, even though during drilling of F3 enrichments in atmospheric components such as Ar and N2 have been observed. An increase in both CH4 and He has been noticed in F2 (108 m borehole) at 40 m depth, whilst a decrease in He has been observed at 193 m depth in both F1 and the natural mofette. Enrichments in less soluble gases (eg. He and N2) at various depths accompanied by a minor CO2 decrease have also been noticed. Such variations may have been caused by the different solubilities of gases in aquatic environments. Moreover, a decrease in CO2 followed by a subsequent enrichment of CH4 and CxHy during the first days after the initial drilling could promote the hypothesis of the generation of microbialy derived CH4. Diurnal variations were observed for the majority of the gas components during the last phase of the F3 drilling, when the well reached a depth >200 m.

This research is a part of the MoRe - “Mofette Research” project, which is included in the ICDP project “Drilling the Eger Rift: Magmatic fluids driving the earthquake swarms and the deep biosphere”). This work was supported by the DFG grant# WO 855/4-1 and BA 2207/19-1.

How to cite: Daskalopoulou, K., Woith, H., Zimmer, M., Niedermann, S., Bağ, C. D., Bauz, R., Trubač, J., and Fischer, T.: Variations of gas compositions during a drilling process: A key study on the Hartoušov Mofette, Czech Republic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5735, https://doi.org/10.5194/egusphere-egu2020-5735, 2020.

EGU2020-13025 | Displays | SM6.3 | Highlight

Complex system for earthquakes forecast using gas emission observations

Victorin - Emilian Toader, Iren-Adelina Moldovan, Constantin Ionescu, Alexandru Marmureanu, and Andrei Mihai

Romanian National Institute of Earth Physics (NIEP) develops a gas emission monitoring network as part of a multidisciplinary activity. The goal is to help organizations specialized in emergency situations with short-term earthquakes forecast and information related to pollution and effects of climate change. In Romania, the important seismic area is Vrancea where there are seismic and multidisciplinary monitoring stations. The methods and monitoring solutions are general and they could be applied in any place. The main part of our system is related to CO2, CO, radon, air ionization in correlation with earth radiation, air ionization, telluric currents, ULF radio waves disturbance, magnetic field, temperature in borehole, infrasound, acoustic waves and meteorological data. The monitoring stations are located on the faults in the curvature of the Carpathian Mountains. The first step is to determine the daily, seasonal and annual evolutions of gas emissions and ionization of the air for at least one year. We are looking for time intervals during which the seismic activity was reduced to determine the normal evolutions of the measured parameters. Then we can determine the effects of active seismic periods on gas emissions. We will apply several methods of analysis and will correlate the particularities of the geological structure in which the monitoring stations are located and the position of the epicenters of earthquakes. The present results are favorable to the analysis by integrating the values measured on variable time windows according to the case. Instantaneous values also include local effects that are not related to tectonic stress. Current measurements indicate the presence of CO at certain times of the day and at certain stations. This is not possible due to tectonic stress, but may be the result of pollution in short-distance cities and air currents that spread it.

Key words: gas emission, multidisciplinary analysis, radon concentration, air ionization, multi-parametric monitoring, earthquake forecast, earthquake precursors

How to cite: Toader, V.-E., Moldovan, I.-A., Ionescu, C., Marmureanu, A., and Mihai, A.: Complex system for earthquakes forecast using gas emission observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13025, https://doi.org/10.5194/egusphere-egu2020-13025, 2020.

EGU2020-11381 | Displays | SM6.3

Detection and characterization of microseismicity and its relation with fluids at the Irpinia Near Fault Observatory

Gaetano Festa, Matteo Picozzi, Guido Maria Adinolfi, Alessandro Caruso, Simona Colombelli, Grazia De Landro, Luca Elia, Rosario Riccio, Antonio Scala, Mariano Supino, and Aldo Zollo

The Irpinia Near Fault Observatory (NFO) is an EPOS infrastructure located in the Campania-Lucania Region (Southern Italy). Its goal is to characterize the microseismicity and to understand the underlying chemical and physical processes occurring along the fault systems that can potentially generate large earthquakes in the area. The Irpinia NFO is currently composed of ISNet – Irpinia Seismic Network, with associated products and services. ISNet is a local network of 32 accelerometric, short-period and broad band stations, that cover the seismogenic areas related to the main earthquakes that occurred in the region in the last centuries, including the 23 November 1980 , Ms = 6.9 event.  Also, ISNet provides real-time analysis and it represents the prototype network for the testing of early warning systems in Italy.

Here we present tools and techniques to accurately locate and characterize the seismicity within the Observatory. Accurate event detection of weak signals from small magnitude events is based on the coherence of arrival times and migration techniques. This method is then coupled with advanced picking and double differences techniques to accurately locate events with a sub-kilometric scale resolution. Source parameters are thus computed inverting the displacement amplitude spectrum, with a probabilistic approach based on the conjunction of states of information in the data and model spaces. This technique is able to automatically rule out unconstrained solutions, while accounting for correlation among parameters. Source location and characterization is here used to investigate the role of the fluids in the region embedding the fault systems; space and time changes in the medium properties or in the source parameters can be used to detect a deviation from the present mechanical state of the faults owing to changes in fluid pore pressure and migration.

Finally, local dense arrays are used here to improve our capability to detect events within the noise level and to move from a point-source to an extended source description of small events in the area.

How to cite: Festa, G., Picozzi, M., Adinolfi, G. M., Caruso, A., Colombelli, S., De Landro, G., Elia, L., Riccio, R., Scala, A., Supino, M., and Zollo, A.: Detection and characterization of microseismicity and its relation with fluids at the Irpinia Near Fault Observatory, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11381, https://doi.org/10.5194/egusphere-egu2020-11381, 2020.

EGU2020-19792 | Displays | SM6.3

A Strainmeter Array Along the Alto Tiberina Fault System, Central Italy

Lauro Chiaraluce, Rick Bennett, David Mencin, Massimiliano Barchi, and Marco Bohnhoff

The Alto Tiberina fault (ATF) in the Northern Apennines (Central Italy) is a low-angle normal fault (mean dip 20°) that is the target of TABOO (The Alto Tiberina Near Fault Observatory), a state-of-art research and monitoring infrastructure based on multidisciplinary sensors. With the STAR’s project, we intend to deploy a strain- and seismo-meter array in six shallow boreholes to complement and enhance TABOO. This will happen with the active contribution of US National Science Foundation and International Continental Scientific Drilling Program (ICDP Project ID: ICDP-2018/05).

Existing seismic data from TABOO reveal microseismicity, at a consistently high rate on the ATF fault plane, including repeating earthquakes (RE). REs together with a steep gradient in crustal velocities measured by GPS and transient surface motion lasting for few months and coinciding with seismic swarms, support the hypothesis that portions of the ATF are creeping aseismically.

Recent studies document that any given patch of a fault can creep, nucleate slow earthquakes, and also host large earthquakes. Thus, these observations are forcing a revolution in our way of thinking about how faults accommodate slip. However, the interaction between creep, slow, and regular earthquakes is still poorly documented by observation. The ATF fault is perhaps the best place in the world to understand at local scale the mechanisms and implications of stress transfer process between seismic and aseismic fault segments. With STAR we will collect the Open Access data to illuminate the physics that allows for both seismic and aseismic slip on a single fault patch, with potentially transformational implications for seismic hazard and risk assessment globally.

STAR will consist of six 80-160m deep vertical boreholes covering the portion of the ATF that exhibits REs at shallow depth (~4 km), identified with waveforms analysis. The observatory will provide the international community an opportunity to study creep at local scale and over periods of minutes to months poorly constrained by other geophysical instruments. We will also deploy downhole seismometers and pressure transducers co-located with the strainmeters, and each station will be equipped with surface GPS and a meteorological instrument. The suite of instruments will enable the collection and calibration of strain records with exquisitely high precision, allowing for a quantitative characterization of ATF creep (~1mm over <1km2), enhanced monitoring of microseismicity (below Mc 0.5), and allowing correlation between degassing (CO2, Rn) measurements and subsurface strain.

How to cite: Chiaraluce, L., Bennett, R., Mencin, D., Barchi, M., and Bohnhoff, M.: A Strainmeter Array Along the Alto Tiberina Fault System, Central Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19792, https://doi.org/10.5194/egusphere-egu2020-19792, 2020.

EGU2020-22582 | Displays | SM6.3

Seismic arrays and frequency-wavenumber spectrum analysis to detect and monitor natural/induced microseismicity

Guido Maria Adinolfi, Matteo Picozzi, Enrico Priolo, Marco Romanelli, Rosario Riccio, Stefano Parolai, and Aldo Zollo

Seismic arrays are instruments capable to lower the magnitude threshold of earthquake detection by improving the signal-to-noise ratio of recordings. Seismic arrays have been used since 1960s, to investigate global earthquakes or small-scale structure of the Earth’s interior. We designed a cost-effective seismic monitoring array-system made by autonomous stations specifically designed for studying local micro-seismicity. The data collected by this system allow to develop and apply a new method for earthquake detection and location of micro-seismicity based on a frequency-wavenumber (f-k) domain analysis of continuous data recorded by small seismic arrays, in order to separate the coherent signal of low magnitude events from the surrounding noise.
Field surveys have been carried out in two Italian regions, which are seismically active and already monitored by high quality, standard seismic networks, so to allow us to test the performance of the proposed array configuration and processing algorithm.
The first survey was carried out in the Irpinia region (Southern Italy), near the main fault segment activated during the Ms 6.9 Irpinia earthquake occurred in 1980. The natural seismicity of the area features occasional small seismic sequences, with magnitude (ML) less than 3, that are recorded by the local seismic network that monitors the Irpinia fault-system (ISNet - Irpinia Seismic Network). Three small aperture seismic arrays (few hundred meters wide) were deployed at distance of few tens of kilometers each other for three months. Each array was made up of seven 3-component stations, arranged in irregular geometry.
The second experiment was carried out in the Montello-Collalto area (Veneto region, North-East Italy), where an underground gas storage concession, known as “Collalto Stoccaggio”, exists. The gas storage activity is monitored by a local seismic network named “Collalto Seismic Network” (Rete Sismica di Collalto, or RSC). This dense network was designed and is managed by the National Institute of Oceanography and Applied Geophysics (OGS) on behalf of Edison Stoccaggio S.p.A., the storage concession holder. A seismic array composed of eight 3-component stations with 2 km of maximum aperture was deployed for a couple of months to monitor the micro-seismicity occurring nearby.
Our preliminary results suggest that the f-k earthquake detection algorithm of micro-seismicity using seismic arrays can become a valid tool to complement standard seismic networks in monitoring and studying natural and induced seismicity. In the Irpinia region, for instance, we have detected and located about six times the earthquakes recorded by the local network lowering the magnitude down to 0.1, smaller than the catalogue minimum magnitude that is equal to 1.2.

How to cite: Adinolfi, G. M., Picozzi, M., Priolo, E., Romanelli, M., Riccio, R., Parolai, S., and Zollo, A.: Seismic arrays and frequency-wavenumber spectrum analysis to detect and monitor natural/induced microseismicity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22582, https://doi.org/10.5194/egusphere-egu2020-22582, 2020.

EGU2020-4657 | Displays | SM6.3

Postojna Cave as Near Fault Observatory site in SW Slovenia

Stanka Šebela, Giovanni Costa, Janja Vaupotič, Mladen Živčić, Nataša Viršek Ravbar, and Magdalena Năpăruș-Aljančič

Within EPOS IP project (H2020, 676564) four organizations (ZRC SAZU as EPOS IP partner, University of Trieste as member of EPOS Italy, and Jožef Stefan Institute and Slovenian Environment Agency as members of consortium EPOS-SI) started with development of Postojna Cave as possible Near Fault Observatory (NFO). Intensive geological, meteorological, hydrogeological, seismological and karstological studies are taking place in Postojna Cave. Being a show cave, it has good infrastructure (electricity, cave train, optical cable etc.) what is necessary for on-line scientific measurements inside the cave and transfer of data.

Postojna Cave is situated in SW part of Slovenia in External Dinarides with tectonically active Alpine thrusts and Dinaric (NW-SE) and cross-Dinaric (NE-SW) faults. It belongs to NE part of Adria microplate. Postojna Cave is situated between regionally important Dinaric-oriented Idrija and Predjama Faults. Idrija Fault is supposed to be responsible for 1511 earthquake (M=6.8), which is the strongest earthquake in the territory of Slovenia. In Postojna Cave there are numerous broken speleothems, some of them can be due to tectonic activity others due to karst processes.

Postojna Cave NFO includes regular micro-climatic monitoring as cave air temperature, water temperature, rock temperature, CO2, humidity, air pressure, wind speed and direction. At several locations such measurements are going on since 2009 to assess impact of tourism on cave environment and to study natural cave meteorological conditions.

Radon (222Rn) monitoring in cave atmosphere started in 1995. In the first period seasonal measurements of radon activity concentration, equilibrium factor, radon progeny activity concentrations in attached and unattached form (EQF-3020-2, Sarad) have been carried out to establish the reliable methodology for dose estimates of cave workers. Contemporary measurements of radon progeny activity concentrations (EQF-3020-2, Sarad) and number concentrations and size distribution of general (non-radioactive) aerosol particles (SMPS, Grimm) started in 2010 and were carried out periodically. In the period 2011-16 continuous radon monitoring (once an hour) was conducted (Radon Scout, Sarad), using radon as a tracer for cave ventilation.

3D micro-displacement monitoring on two Dinaric oriented fault zones in the cave is performed with four TM 71 extensometers. First two instruments were installed in 2004 and the second ones in 2016. Small micro-displacements of up to 0.08 mm in one month are registered.

Seismic station in Postojna Cave is operating since 2010, with periods of inoperability due to power supply problems and hardware malfunctions. The station in the Tartarus tunnel (TTPJ) recorded more than hundred earthquakes of the sequence near Ilirska Bistrica that started on 15 September 2010, with two MLV=3.5 earthquakes and lasted till the end of the year 2010, without accurate timing at that time. A fibre optic cable was installed later on and a Quanterra Q330 data logger with accurate timing and real-time telemetry was installed with an Episensor accelerometer and a Streckeisen STS2 seismometer.

In the future we are planning to enlarge the underground seismic station with equipment provided from RI-SI-EPOS project (EU Cohesion Funds). Micro-deformation monitoring sites will be additionally studied with methane and radon measurements (H2020 871121 EPOS SP).

How to cite: Šebela, S., Costa, G., Vaupotič, J., Živčić, M., Viršek Ravbar, N., and Năpăruș-Aljančič, M.: Postojna Cave as Near Fault Observatory site in SW Slovenia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4657, https://doi.org/10.5194/egusphere-egu2020-4657, 2020.

This study presents observations and analysis from a high-sampling-rate micro-seismic network, located at the north of the Sea of Galilee, Israel. Stations’ locations were chosen following the seismic swarm at the North of the Sea of Galilee, in October 2013, aiming to perceive a better understanding of the seismicity and structure of this area, in light of that anomaly seismic swarm, and of the seismic activity along the Dead Sea Fault. The micro-seismic network was active between May 2016 to August 2018, with six stations altogether, in distances of 3-5km around the northern Sea of Galilee.  Each of the micro-seismic stations had two collocated sensors: 1) GS-1 Geospace, 1 Hz vertical seismometers, sampled at 500 samples per second, and 2) 3-channel Episensor embedded in a Rock+ Kinemetrics datalogger, sampled at 200 samples per second. Towards the dismantling of the network, another swarm, stronger in magnitude, and longer in duration, has occurred in July-August 2018, roughly at the same location. Meanwhile, a significant upgrade of the Israel Seismic Network (ISN) was taking place, also densifying the number of stations around the Sea of Galilee.

The seismic processing presented here has many steps of verification, at all levels: detection, association, and location.  Processing begins with the local high-sampling-rate micro-seismic stations, tuning the most appropriate micro-seismic detectors, and association, location and magnitude parameters. Then this new generated micro-seismic catalogue is used to reveal lower magnitude events within the ISN stations, followed by relocation and re-magnitude estimations, done to those events that have additional information from the ISN stations. Running this process for increasing time-windows, it is demonstrated how the use of micro-seismic instrumentation can increase the seismic catalogue by an order of magnitude, providing higher resolution of the seismicity, both in space and time.

These efforts, of increasing the seismic catalogue, and improving their locations, are utilised for two main goals: a) obtaining a clearer picture of the seismicity and structure in the area before and during the seismic swarm of July-August 2018, b) Zooming into the interesting micro-seismic activity just before the initiation of the swarm.

How to cite: Kurzon, I.: Micro-seismicity of the Northern Sea of Galilee - Before and During the July-August 2018 Seismic Swarm, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20684, https://doi.org/10.5194/egusphere-egu2020-20684, 2020.

EGU2020-13079 | Displays | SM6.3

Tracking the development of seismic fracture network by considering the fault rupture method

Taghi Shirzad, Stanisław Lasocki, and Beata Orlecka‐Sikora

In Enhanced Geothermal Systems pressurized injections play a role in developing fracture networks and enhancing the water transmissivity. However, the fractures may also coalesce into undesired pathways for fluid migration to enable the fluids to reach pre-existing faults. The properties of observed seismicity can shed some light on the fracture network development and from the standpoint of the possibility to form such undesired pathways. However, to reach this goal the seismic events should be well parameterized. In particular, the information on fault plane mechanisms is essential, which is often not readily accessible. In this study, we use the rupturing process with an accurate P-wave velocity model, which is obtained by the first arrival P-wave tomography approach, to compensate for an eventual lack of source mechanisms of micro-events. For this purpose, four characteristics of the sources (final/average displacement on the fault, the dimension of fault, rupture velocity and particle velocity) can be considered. A 3D model is defined around the hypocenter of each event, so that the size of this model directly depends on the event magnitude. After calculating the arrival time of the selected phase (e.g., P, S, p or s) for each station, all waveforms are then aligned, and stacked by different stacking (e.g., phase weight, Nth-root) methods. By considering the maximum amplitude of the stacked waveform which is stimulated by each grid, the rupturing plane and the average velocity of rupturing can be obtained. This information of source can be replaced by the double-couple mechanism to investigate the fractures linking and tracking.

This work was supported under the S4CE: "Science for Clean Energy" project, which has received funding from the European Union’s Horizon 2020 research and innovation program, under grant agreement No 764810.

How to cite: Shirzad, T., Lasocki, S., and Orlecka‐Sikora, B.: Tracking the development of seismic fracture network by considering the fault rupture method, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13079, https://doi.org/10.5194/egusphere-egu2020-13079, 2020.

EGU2020-3288 | Displays | SM6.3

Stress state in the upper margin of the aftershock zone of the 2014 Orkney earthquake (M5.5), South Africa, estimated from analyses of drill cores and borehole breakouts of ICDP-DSeis drillings

Yasuo Yabe, Makoto Kanematu, Mitsuya Higashi, Ryogo Tadokoro, Shunsuke Yoshida, Kosuke Sugimura, Hiroshi Ogasawara, Takatoshi Ito, Akio Funato, Martine Ziegler, Bennie Liebenberg, Bryan Watson, Siyadnda Mngadi, Musa Manzi, and Raymond Durrheim

The 2014 Orkney earthquake (M5.5) occurred below the Moab Khotsong gold mine in South Africa. The shallowest aftershocks were located only several hundred meters below the deepest level of the mine. Two boreholes (Holes A (817 m) and B (700 m)) were drilled toward the upper margin of the aftershock zone from a specially excavated chamber at 2.9 km depth by the ICDP-DSeis project. Hole A deflected from the aftershock zone, while Hole B intersected it. Hole C was branched from Hole B to recover more samples from the aftershock zone. Except for the intersection in Hole B, the drill core recovery was ~100%. In-hole geophysical logging, including the surveys of the borehole wall geometry were carried out along the entire length of Hole A, while they could be done only as far as the intersection with the aftershock zone in Hole B due to hole closure. Hole C was not logged.

The focal mechanism solutions of mining induced earthquakes shallower than 3 km are usually of the normal faulting type, while those of the Orkney earthquake and its aftershocks deeper than 3.5 km have a strike-slip signature. In this study, we applied the Deformation Rate Analysis (DRA) and the Diametrical Core Deformation Analysis (DCDA) techniques to rock cores recovered from Holes A, B and C to explore the depth variation in the stress state that would cause the depth variation in the faulting regime. In the DRA, a cyclic loading is applied to a sub-sample cut from a drill core to determine the normal stress in the loading direction from hystereses of the stress-strain curve. We determined the normal stresses in 9 directions at each depth to recover the principal stress state redundantly. However, because it takes much time for sub-sample preparations and loading, we applied this technique only at 3 depths in Hole A. With the DCDA, the differential stress in the plane normal to a borehole is evaluated from the ellipsoidal cross-sectional shape of the rock cores. Though only the differential stress can be measured by the DCDA, it takes only several minutes for measurement at each depth. We evaluated the differential stress as densely as every several meters along Holes A, B and C.

Rock cores of Hole A were oriented by comparing joints and veins identified on the borehole wall optical-televiewer images and in the cores. Thus, the stress orientations in the plane normal to Hole A can be determined as the orientation of the maximum and the minimum core diameter. The stress orientation is obtained also from the breakout of borehole wall identified by the acoustic televiewer. Further, by combining the differential stress magnitude evaluated by the DCDA and the width of the breakout, magnitudes of the maximum and the minimum compression are estimated. We introduce the depth variations in the stress state along Holes A, B and C, as well as those of in-hole logging data to discuss spatial heterogeneity of stresses in the source region of the Orkney earthquake.

How to cite: Yabe, Y., Kanematu, M., Higashi, M., Tadokoro, R., Yoshida, S., Sugimura, K., Ogasawara, H., Ito, T., Funato, A., Ziegler, M., Liebenberg, B., Watson, B., Mngadi, S., Manzi, M., and Durrheim, R.: Stress state in the upper margin of the aftershock zone of the 2014 Orkney earthquake (M5.5), South Africa, estimated from analyses of drill cores and borehole breakouts of ICDP-DSeis drillings, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3288, https://doi.org/10.5194/egusphere-egu2020-3288, 2020.

EGU2020-4155 | Displays | SM6.3

Application of on-site EEW technology in South Korea.

Ho Jun Lee, Jeong Beom Seo, Jin Koo Lee, and Inchan Jeon

The potential seismic hazard in South Korea is known to be that mid-sized earthquakes could occur nationwide. Because the damages of mid-sized earthquake are concentrated only in the vicinity of the epicenter, on-site EEW technology is known to be effective as a means to reduce absence of alarm near the epicenter and to ensure safety from earthquake threat. This study aims to simulate on-site EEW suitable for South Korea's seismic observation environment and verify its reliability. Seismic observations of 267 events occurred in South Korea, have been collected over the past five years for the simulation . Filter Picker was utilized to detect P-wave features from more than 37,000 data sets using a time window suitable for mid-sized earthquakes. The ground noises are removed from the detected P-waves, and a linear empirical relationship between the maximum P-wave amplitudes in vertical direction and observed PGVs on the base rock are derived. Convert the forecasted and observed PGVs to MMI, respectively. Assuming a successful prediction within the MMI±1 margin of error by comparing the two values, the results of this study showed an 80% success rate in the range above MMI 4. Through this study, feasibility and performance of on-site EEWS using domestic earthquake records were verified in South Korea. It is expected that this will contribute to the reduction of earthquake damage near the epicenter through an on-site warning in Korea.

How to cite: Lee, H. J., Seo, J. B., Lee, J. K., and Jeon, I.: Application of on-site EEW technology in South Korea. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4155, https://doi.org/10.5194/egusphere-egu2020-4155, 2020.

SM6.5 – Volcano Seismology: Observations and Modelling

We have developed a 3D model of the Mount St Helens (MSH) magmatic plumbing system extending from the upper magma storage zone (> 3.5 km bsl) to Moho depths (40-45 km) by combining results from 2D and 3D active source seismic tomography and reflection imaging, and autocorrelation reflectivity imaging. The data are from the ~6000 high frequency seismographs used in the 2014 iMUSH active seismic experiment.

We developed a 3D Vp tomography model of melt distribution in the upper-middle crust (Kiser et al, 2018). The model suggests the plumbing system is a complex sill structure consisting of several interconnected bodies that lie beneath MSH at 3.5-14 km depth and that extend ~25 km laterally. Bright reflections in 3D autocorrelation reflectivity depth migrations are strongly correlated with the melt model, illuminating its interior as well as a system of more geographically extensive thin sills that are invisible to the tomography. High amplitude reflectivity occurs near the top of the sill complex, suggesting the system grows by successive emplacement at the top of the complex. Inversion of the autocorrelation reflection volume for melt content suggests melt concentrations exceed 30% locally in the sill complex.  The highly reflective center of the sill complex is likely the magma storage zone that feeds dacitic composition MSH eruptions. We speculate that some of the more geographically widespread dikes feed the Indian Heaven basalt fields.

Deeper reflectivity trends to the northeast of MSH and intersects the Lower Crustal Conductor in Bedrosian et al’s (2018) MT interpretation. They interpret high conductivity values as indicative of 3-10% interconnected melt in the crust at depths > 20 km, which is consistent with our reflectivity images. We also observe asymmetric crustal thickening toward and thinning away from MSH along the strike of the Cascades. Moho reflectivity is weak directly beneath MSH, agreeing with previous studies (Kiser et al, 2016; Hansen et al, 2016). Zones of strong autocorrelation and wide-angle reflectivity cross the refraction Moho and extend some distance into the upper mantle. 

How to cite: Levander, A. and Kiser, E.: iMUSH Autocorrelation Reflectivity and Active Seismic Imaging of the Magma Plumbing System under Mount St Helens, Washington, USA, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11763, https://doi.org/10.5194/egusphere-egu2020-11763, 2020.

EGU2020-18971 | Displays | SM6.5

Relation of Time-Varying Vp/Vs ratio to Inflation and Deflation Episodes near Hengill Volcano, Iceland

Ari Tryggvason, Alex Hobé, Olafur Gudmundsson, and Halldor Geirsson and the SIL Seismological Group

The Hengill area experienced an intensive and long-lived series of earthquakes in the 1990s. This coincided with a period of inflation near the Hengill volcano, which was interpreted as new influx of magma at ~7 km depth. Feigl et al. (2000) postulated that the observed seismicity was triggered by the strain accumulation associated with the magma-influx. In a similar area ~3 km to the NW, subsidence has been occurring since 2006. The timing of this subsidence coincides with the onset of geothermal production at Hellisheidi in the west and enlargement of the Nesjavellir powerplant in the North. The source of the subsidence near Hengill volcano is however estimated between 5.6 and 7 km depth and at significant lateral distances from these production sites (Juncu et al. 2016). In this study we apply newly developed methods in time-dependent seismic tomography (Hobé et al. 2020) in the Hengill area, to study if significant velocity changes can be attributed to these inflation/deflation episodes. The dataset employed for the tomography covers the inflation period, the subsidence period, and the time in-between, with varying station coverage and geometry. In this study, the artificial velocity variations due to variations in source and receiver geometries are first separated from “true” velocity variations. In the approximate source region of the 2006-onwards deflation the preliminary results show a low Vp/Vs ratio anomaly between ~4-7 km depth, with an EW extent of ~8-10 km and an NS extent of ~4 km. This anomaly coincides with a significant amount of seismicity. This may indicate an increase in the amount of compressible fluids, accompanied with hydro-fracturing. The seismicity terminates below this low Vp/Vs anomaly, underneath which there is an area of increased Vp/Vs ratios (associated with melt) in the approximate center of the inflation episode in the 1990s. Thus, this investigation provides new information about the nature of the deformation sources, and the surrounding hydrothermal system. We will further investigate the apparent connection between the current subsidence and geothermal production.

 

References:

Feigl et al. (2000): Crustal deformation near Hengill volcano, Iceland 1993-1998: Coupling between magmatic activity and faulting inferred from elastic modeling of satellite radar interferograms, J. Geophys. Res.

Hobé et al. (2020): Imaging the 2010-2011 inflationary source at Krysuvik, SW Iceland, using time-dependent Vp/Vs tomography, WGC 2020, forthcoming

Juncu et al. (2016): Anthropogenic and natural ground deformation in the Hengill geothermal area, Iceland, J. Geophys. Res.

How to cite: Tryggvason, A., Hobé, A., Gudmundsson, O., and Geirsson, H. and the SIL Seismological Group: Relation of Time-Varying Vp/Vs ratio to Inflation and Deflation Episodes near Hengill Volcano, Iceland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18971, https://doi.org/10.5194/egusphere-egu2020-18971, 2020.

EGU2020-14124 | Displays | SM6.5

Hidden earthquakes unveil the dynamic evolution of a large-scale explosive eruption

Ricardo Garza-Giron, Emily Brodsky, Zack Spica, and Matt Haney

Volcanic eruptions progress by co-evolving the fluid and solid systems. The fluid mechanics can be observed through the evolution of plumes and ejecta. How does the solid evolve? When does the conduit open? When does it close? Seismology can potentially tell us about these processes by measuring the failure of the solid rock. However, such inferences require detection of earthquakes during an explosive eruption. Standard earthquake detection methods often fail during this time as the eruption itself produces seismic noise that obscures the earthquakes. In this work, we address this problem by applying both a supervised and unsupervised search techniques to the existing catalog of the 2008 Okmok Caldera eruption to find brittle failure signals during the continuous eruptive sequence. We were able to detect >4500 new earthquakes using the 419 events previously located by the Alaska Volcano Observatory (AVO). A spatiotemporal analysis of the occurrence of earthquakes during the eruption reveal interesting observations: Seismic bursts during the eruption are not synchronized with the exhalation of large ash and steam plumes, suggesting that the dynamics of the eruption are controlled by a clog-and-crack mechanism; most of the Caldera co-eruptive seismicity that is not located at the focus of the eruption occurs under the intra-Caldera cones, showing the activation of their hydrological system due to a system-wide pressurization; the end of the eruption is marked by a large burst of small, deep earthquakes trending SW-NE, possibly related to a propagating lateral dike similar to those observed in other basaltic calderas; the magnitude distribution of seismicity through time shows that the largest earthquakes in the bursts do not happen at the beginning of the sequence like in typical mainshock-aftershock sequences. Furthermore, high precision earthquake relocations highlight a ring-fault structure inside of Okmok Caldera which is thought to be acting as the pathway for fluids to the surface.

How to cite: Garza-Giron, R., Brodsky, E., Spica, Z., and Haney, M.: Hidden earthquakes unveil the dynamic evolution of a large-scale explosive eruption, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14124, https://doi.org/10.5194/egusphere-egu2020-14124, 2020.

EGU2020-18862 | Displays | SM6.5

Rotational sensor on a volcano: New insights from Etna, Italy

Eva P. S. Eibl, Gilda Currenti, Joachim Wassermann, Philippe Jousset, Daniel Vollmer, Graziano Larocca, Daniele Pellegrino, Mario Pulviventi, Danilo Contrafatto, and Shihao Yuan

Rotational seismology is an emerging field of seismology with rotational sensors such as blueSeis-3A as portable devices. We deployed one of these rotational sensors on Etna volcano from August to September 2019 in the middle of a 26 stations broadband seismic array and a fibre-optic cable deployed for Distributed Acoustic Sensing (DAS). We, therefore, recorded continuously the full seismic wavefield using a 6C station (rotational sensor co-located with a broadband seismometer) for 30 days.

We will present an overview of our work on the rotational data in combination with a broadband seismometer. We will (i) compare the translational with rotational data and show how they complement each other, (ii) calculate back azimuths using only a 6C station or using merely the horizontal components of the rotational sensor, (iii) determine Love and Rayleigh wave velocities from the rotation rate and (iv) perform a simple inversion for the shallow velocity structure below the station, and finally (v) discuss the usefulness of such a sensor in a volcanic environment and (vi) highlight what new it would bring to volcano-related research.

How to cite: Eibl, E. P. S., Currenti, G., Wassermann, J., Jousset, P., Vollmer, D., Larocca, G., Pellegrino, D., Pulviventi, M., Contrafatto, D., and Yuan, S.: Rotational sensor on a volcano: New insights from Etna, Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18862, https://doi.org/10.5194/egusphere-egu2020-18862, 2020.

EGU2020-511 | Displays | SM6.5

Geomorphological controls on seismic recordings in volcanic areas

Simona Gabrielli, Luca De Siena, and Matteo Spagnolo

In volcanoes, topography, shallow heterogeneity, and even shallow morphology can substantially modify seismic coda signals. Coda waves are an essential tool to monitor eruption dynamics and model volcanic structures jointly and independently from velocity anomalies: it is thus fundamental to test their spatial sensitivity to seismic path effects. Here, we apply the Multiple Lapse Time Window Analysis (MLTWA) to measure the relative importance of scattering attenuation vs absorption at Mount St. Helens volcano (MSH) before its 2004 eruption. The results show the typical dominance of scattering attenuation in volcanoes at lower frequencies (3 - 6 Hz), while absorption is the primary attenuation mechanism at 12 Hz and 18 Hz. Still, the seismic albedo (measuring the ratio between seismic energy emitted and received from the area) is anomalously-high (0.95) at 3 Hz.

A radiative-transfer forward model of far- and near-field envelopes confirms this is due to strong near-receiver scattering enhancing anomalous phases in the intermediate and late coda across the 1980 debris avalanche and central crater. Only above this frequency and in the far-field, diffusion onsets at late lapse times.  We also implemented a layered model with a shallower layer with increased scattering properties to model late coda envelopes. While the broadening of late coda phases improves, this model cannot explain the phases of the intermediate coda with higher amplitude than the direct waves.

The scattering and absorption parameters derived from MLTWA are used as inputs to construct 2D frequency-dependent bulk sensitivity kernels for the S-wave coda in the multiple-scattering (using the Energy Transport Equations - ETE) and diffusive (AD, independent of MLTWA results) regimes. At 12 Hz, high coda-attenuation anomalies characterise the eastern side of the volcano using both kernels, in spatial correlation with low-velocity anomalies from literature. At 3 Hz, the anomalous albedo, the forward modelling, and the results of the tomographic imaging confirm that shallow heterogeneity beneath the extended 1980 debris-avalanche and crater enhance anomalous intermediate and late coda phases, mapping shallow geological contrasts.

The geomorphological map of MSH highlights extremely rough landforms (hummocky structures) of the already complex morphology of the debris avalanche. The comparison with the attenuation tomography reveals several matches, not only with the debris avalanche itself but also with other areas in the south flank of MSH, like the volcanoclastic plane, affected by intense eruptions in the past (e.g. Cougar stage, 28-18 ka).

We remark the effect those may have on coda-dependent source inversion and tomography, currently used across the world to image and monitor volcanoes.

How to cite: Gabrielli, S., De Siena, L., and Spagnolo, M.: Geomorphological controls on seismic recordings in volcanic areas, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-511, https://doi.org/10.5194/egusphere-egu2020-511, 2020.

EGU2020-8251 | Displays | SM6.5

Bubble nucleation and growth in basaltic magmas as a possible source of Deep Long Period Volcanic earthquakes

Nikolai M. Shapiro, Oleg Melnik, Vladimir Lyakhovsky, Natalia Galina, and Olga Bergal-Kuvikas

Deep Long Period (DLP) earthquakes have been observed in many volcanic regions and are often considered as one of the important precursors to volcanic eruptions. At the same time, the physics of the source of these earthquakes remains unclear. We focus our study on Klyuchevskoy group of volcanoes in Kamchatka, Russia, one of the World’s most active volcanic system. The DLP earthquakes in this region occur at the limit between the lower crust and the upper mantle at depths of 30-35 km where ductile flow is expected to dominate rock deformation. Their occurrence also appears to correlate with the eruptive activity. Therefore, this is natural to consider that their generating mechanism is not related to brittle mechanism but rather to pressure fluctuations in the magmatic system as often suggest for the LP seismicity in general. We suggest a possible generating mechanism related to the rapid pressure changes caused by nucleation and growth of gas bubbles in response to the slow decompression of over-saturated magma. The pressure variation is simulated using the mathematical model of bubble nucleation and growth accounting for multiple dissolved volatiles (H2O-CO2) and diffusive gas transfer from magma into growing bubbles. Results of simulations show that fast pressure increase followed by its relaxation almost to its initial level is not very sensitive to the assumptions on the values of governing parameters. Typical pressure changes of a few tens of MPa in a volume of 3500 m3 occurring on time scales of fractions of a second to a second following bubble nucleation and growth can generate seismic waves with amplitudes similar to those recorded by seismographs in the vicinity of the Klyuchevskoy volcano.

How to cite: Shapiro, N. M., Melnik, O., Lyakhovsky, V., Galina, N., and Bergal-Kuvikas, O.: Bubble nucleation and growth in basaltic magmas as a possible source of Deep Long Period Volcanic earthquakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8251, https://doi.org/10.5194/egusphere-egu2020-8251, 2020.

EGU2020-6813 | Displays | SM6.5

The seismic sound of deep volcanic processes

Simone Cesca, Torsten Dahm, Sebastian Heimann, Martin Hensch, Jean Letort, Hoby N. T. Razafindrakoto, Marius Isken, and Eleonora Rivalta

Deep volcanic processes and magma intrusion episodes through the crust are typically accompanied by a variety of seismic signals, including volcano-tectonic (VT) seismicity, very long period (VLP) signals and deep low-frequency (DLF) events. These signals can reveal the migration of magma batches and the resonance of magma reservoirs and dikes. The recent 2018-2019 unrest offshore the island of Mayotte, Comoros archipelago, represents the first case of a geophysically monitored magmatic intrusion from a deep sub-Moho reservoir through the whole crust reaching the surface. At Mayotte, a huge magma movement and the following drainage of a deep reservoir were accompanied by a complex seismic sequence, including a massive VT swarm and energetic long-duration very long period (VLP) signals recorded globally. The identification and characterization of ~7000 VTs and ~400 VLPs by applying waveforms-based seismological methods allowed us to reconst the unrest phases: early VTs, migrating upward, were driven by the ascent of a magmatic dike, and tracked its propagating from Moho depth to the seafloor, while later VTs marked the progressive failure of the reservoir’s roof, triggering its resonance and the generation of long-duration VLPs. At the Eifel, Germany, weak DLFs earthquakes have been recorded over the last decades and located along a deep channel-like structure, extending from sub-Moho depth (~40-45 km) to the upper crust (~5-10 km). While not showing any clear migration, they reveal a different way of fluid transfer from depth towards the surface, possibly marking intermediate small reservoirs along a feeding channel. Here, brittle failure occurring in the vicinity of the reservoirs may cause their resonance. The Mayotte and Eifel observations are example of end member models for deep fluid transfer processes through the crust. These examples show that, by listening to seismic signals at different distances and by analysing them with modern waveform based methods, we can provide a detailed picture of deep magmatic processes and enable future eruption early warning.

How to cite: Cesca, S., Dahm, T., Heimann, S., Hensch, M., Letort, J., Razafindrakoto, H. N. T., Isken, M., and Rivalta, E.: The seismic sound of deep volcanic processes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6813, https://doi.org/10.5194/egusphere-egu2020-6813, 2020.

EGU2020-19068 | Displays | SM6.5

Towards real-time monitoring with a seismic antenna at Merapi volcano

Jean-Philippe Metaxian, Agus Budi Santoso, François Beauducel, Nabil Dahamna, Vadim Monteiller, and Ali Fahmi

Seismic antennas are often used on volcanoes to analyse emergent signals as LP events or tremor.  In fact, they can be used for any kind of seismicity whether the signal is impulsive or emergent. In this work we are using a seismic antenna as an instrument for monitoring the continuous seismic signal, with the objective of a real-time application.

A seismic antenna composed of 5 broadband stations equipped with Guralp CMG-6TD stations was installed in November 2013 close to the summit of Merapi, on the site called Pasar Bubar. Sensors have a flat response characteristic from 30 s to the Nyquist frequency (50 Hz). This network has an aperture of 280 m. The shortest distance between sensors is 100 m.

 

In the perspective of a real-time application, the main analysis, which consists of estimating the slowness vector, requires a shorter computation time than the data acquisition time. We thus focused on a signal processing technique based on the calculation of time delays on the vertical component only and in a single frequency band. Given a set of time delays and associated errors calculated between each couple of sensors in the frequency domain, the corresponding slowness vectors can be recovered by inversion. Slowness vectors are estimated for successive time-windows in the frequency band 0.5-3 Hz. Temporal series of back-azimuth and apparent slowness are deduced with respect to time.

The analysis strategy for monitoring is then the following: A weight function expressed as a function of the derivatives of the time delays is calculated for successive moving time-windows. This function was designed in order to identify areas of stability of the back-azimuth values as function of time. A PDF of the back-azimuth and apparent slowness is then estimated for time series of 1 hour. This gives information on the dominant activity by time unit.

We will show the results obtained with the analysis of several months of continuous signal which are including different types of events generated by the on-going eruptive activity of Merapi: 1) volcano-tectonic events, 2) Multi-Phase (MP) events related with magma ascent in the conduit, 3) low-frequency events, 4) Rock-falls and 5) Pyroclastic density currents.

How to cite: Metaxian, J.-P., Santoso, A. B., Beauducel, F., Dahamna, N., Monteiller, V., and Fahmi, A.: Towards real-time monitoring with a seismic antenna at Merapi volcano, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19068, https://doi.org/10.5194/egusphere-egu2020-19068, 2020.

EGU2020-10234 | Displays | SM6.5

Automatic classification of seismo-volcanic signals at La Soufrière of Guadeloupe

Alexis Falcin, Jean-Philippe Metaxian, Jérôme Mars, Eléonore Stutzmann, Roberto Moretti, and Jean-Christophe Komorowski

Seismic activity at La Soufrière volcano of Guadeloupe is composed of various transient signals, which are classified manually by the Observatoire Volcanologique et Sismologique de Guadeloupe (OVSG-IPGP) considering waveforms recorded at several stations. Although five main types of signals are recognized in the data analysis by the observatory (Moretti et al., 2020), only three main classes readily distinguishable on seismic traces during the daily analytical protocol have been catalogued: Volcano-Tectonic events, Long-Period events and Nested events, each related to a distinct physical process.

Automatic classification of seismo-volcanic signals of La Soufrière was performed by using an architecture based on supervised learning, available at github.com/malfante/AAA. Seismic waveforms are transformed into a large set of features (34 features for each representation domain) computed from three representation domain of the signal (time, frequency, quefrency). The resulting vectors of features are then used for the modeling. We are using the Random Forest Classifier algorithm from the scikit-learn library.

At first, we trained the model with the dataset given by the OVSG consisting of 845 available labeled events (542 VT, 217 nested and 86 LP) recorded in the period 2013-2018. We obtained an average classification rate of 72 %. We determined that the VT class includes a variety of signals covering the LP, Nested and VT classes. Reviewing in details the waveforms and the spectral characteristics of the signals belonging to the 3 classes we then introduced Hybrid events and also defined a monochromatic class (so-called Tornillo) of LP signals, thus matching the full description of signals provided in Moretti et al. (2020).

Then, using the new information, a new model was trained with 5 classes and tested. We obtained a much better classification average rate of 84 %. The classification is excellent for Nested events (93 % of accuracy and precision) and Tornillo events (93% of accuracy and precision). The classification of VT events (90% accuracy, 89% precision) and LP events (86% accuracy, 82% precision) were also very good. The most difficult class to recognize is the Hybrid class (64 % accuracy, 69 % precision). Hybrid events are often mixed with VT and LP events. This may be explained by the nature of this class and the physical process that includes both a fracturing and a resonating component with different modal frequencies.

Machine learning is a powerful tool to handle large datasets. From a dataset built manually, the processing we applied allowed to obtain a reliable automatic classification by refining class definitions. This has important implications for observatory data processing during unrest and eruptive activity.

How to cite: Falcin, A., Metaxian, J.-P., Mars, J., Stutzmann, E., Moretti, R., and Komorowski, J.-C.: Automatic classification of seismo-volcanic signals at La Soufrière of Guadeloupe, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10234, https://doi.org/10.5194/egusphere-egu2020-10234, 2020.

EGU2020-11749 | Displays | SM6.5

Discriminating icequakes from volcanic seismicity at Cotopaxi volcano (Ecuador)

Jean Battaglia, Silvana Hidalgo, Agnes Helmstetter, Cristian Espín, Luis Velez, Marco Cordova, and Antonio Proaño

Cotopaxi volcano (5,897 m) is located in Central Ecuador, 50 km south of Quito. It has a long eruptive history including more than 70 eruptions with an estimated VEI between 2 and 4 since 1534. Its last low magnitude eruption occurred in 2015. The summit of the volcano is covered by a glacier down to about 5000 m elevation. The volcano is monitored by the Instituto Geofísico (IG) whose monitoring network includes permanent seismic stations. The closest station to the summit (BREF) is located 1 km below the summit (2.2 km distance), about 400 m from the base of the glacier. It is used as a reference station by the IG to characterize the seismicity. The station records transient events related to volcanic activity such as Long Period (LP) and Volcano Tectonic (VT) events, as well as icequakes (IQ) issued from the neighboring glacier. IQs may have various origins including fracture propagation or opening, collapse of ice blocks, basal friction or forced water flow within the glacier. These signals may be difficult to distinguish from VTs or LPs.

We examined data from station BREF recorded between January 2013 and October 2018, with the aim of identifying families of characteristic similar events. We applied a 3-step procedure including: (1) an automatic detection of transient events, (2) a classification of the detected events into families of similar events and (3) a re-composition of the temporal evolution of the largest families using matched-filtering. This procedure outlines the presence of numerous families and points out 4 characteristic temporal evolutions with respect of the 2015 eruption. These evolutions allow to distinguish precursory LP events from background seismicity and outline the presence of long lasting families which may persist for years. We use amplitude ratios calculated between BREF and a station more distant from the summit to distinguish shallow families from deeper ones. We also locate sources of long-lasting families with a seismic antenna installed at the foot of the glacier from April to September 2018. Locations indicate shallow sources below the glacier corresponding to IQs. These results confirm that background seismicity close to the summit of Cotopaxi is dominated by IQs. Temporal evolutions of these families also suggest that the large (Mw=7.8) subduction earthquake which occurred near Pedernales on April 16, 2016, 250 km from the volcano, had a stronger influence on the glacier or its shallow substratum than the 2015 eruption.

How to cite: Battaglia, J., Hidalgo, S., Helmstetter, A., Espín, C., Velez, L., Cordova, M., and Proaño, A.: Discriminating icequakes from volcanic seismicity at Cotopaxi volcano (Ecuador), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11749, https://doi.org/10.5194/egusphere-egu2020-11749, 2020.

EGU2020-9986 | Displays | SM6.5

Space-weighted seismic attenuation multi-frequency tomography at Deception Island volcano (Antartica)

Roberto Guardo, Luca De Siena, Alberto Caselli, Janire Prudencio, and Guido Ventura

Deception Island is the most active and documented volcano in the South Shetland Islands (Antarctica). Since its last eruption (1970) several experiments have targeted the reconstruction of its magmatic systems. Geophysical imaging has provided new insight into Deception's interior, particularly when using space-weighted seismic attenuation tomography for coda waves. Here, sensitivity kernels have been used to invert coda wave attenuation (Qc−1). We obtain a multifrequency-dependent model of the magmatic systems at Deception Island using active data, paying particularly attention to data selection and model optimisation. The results have been framed in the extensive knowledge of the tectonics and the geomorphology of the volcano with a GIS, underlining a spatial correlation between high-attenuation anomalies and high thermal activity regions. This inter- and multi-disciplinary analysis improves the interpretation of the dynamics of Deception Island and its related hazards.

How to cite: Guardo, R., De Siena, L., Caselli, A., Prudencio, J., and Ventura, G.: Space-weighted seismic attenuation multi-frequency tomography at Deception Island volcano (Antartica), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9986, https://doi.org/10.5194/egusphere-egu2020-9986, 2020.

EGU2020-18954 | Displays | SM6.5

Recent microseismicity observed at Hekla volcano and first velocity inversion results

Martin Möllhoff, Meysam Rezaeifar, Christopher J. Bean, Kristin S. Vogfjörd, Bergur H. Bergsson, and Heiko Buxel

Hekla is one of the most active and dangerous volcanoes in Iceland presenting a high hazard to air travel and a growing tourist population. Until now the pre-eruption warning time at Hekla is only around one hour.  In 2018 we installed the real-time seismic network HERSK directly on Hekla's edifice. If microseismicity on Hekla increases prior to the next eruption the network could possibly provide a means to improve early warning. In addition it is hoped that HERSK will better our understanding of the processes driving the evolution of pre-eruptive seismicity. The configuration and tuning of a dedicated real-time detection and location system requires the determination of a suitable velocity model and station corrections. We present a catalogue of recently detected local events that we use to invert for a 1-D velocity model. We observe significant variations in station corrections and conclude that it is important to account for these in the real-time detection and location system which we are developing based on the SeisComp3 software.

How to cite: Möllhoff, M., Rezaeifar, M., Bean, C. J., Vogfjörd, K. S., Bergsson, B. H., and Buxel, H.: Recent microseismicity observed at Hekla volcano and first velocity inversion results, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18954, https://doi.org/10.5194/egusphere-egu2020-18954, 2020.

EGU2020-8521 | Displays | SM6.5

Interseismic stress field variations in Hjalli-Ölfus, SW Iceland

Ingi Th. Bjarnason, Revathy M. Parameswaran, and Bergthóra S. Thorbjarnardóttir

Western South Iceland Seismic Zone (SISZ) plate boundary lies adjacent to the Hengill central volcano. The sinistral SISZ connects the two arms of the divergent Mid-Atlantic Ridge (MAR) plate boundaries (Western and Eastern Volcanic Zones; WVZ, EVZ), while Hengill is a part of the WVZ. Seismicity in western SISZ, also known as the Hjalli-Ölfus region, closely interacts with the seismicity and magmatism in Hengill. For instance, the  4 June 1998 Mw 5.4 Hengill earthquake witnessed aftershocks that extended south to meet the Hjalli-Ölfus segment. This segment then hosted the Mw 5.1 Hjalli-Ölfus earthquake that occurred on 13 November 1998; elucidating the Hengill-Ölfus interaction. Relative relocations of earthquakes from July 1991 to December 1999 in Hjalli-Ölfus indicate that the seismogenic zone is predominant at 4-8 km depth, with 80% of the events occuring along an ~ENE-WSW trending seismic zone with lateral extension of ~12 km. The remaining occur along N-S faults, much like the observed norm of dextral faulting along the rest of the SISZ (e.g., 17 June 2000, 29 May 2008 earthquakes; Árnadottir et al., 2001; Brandsdottir et al., 2010). These relocated earthquake sequences were used to perform stress inversions within specified spatio-temporal grids. The results show that from 1994 to 1997, the western part of the Hjalli-Ölfus region exhibits an oblique normal stress regime, while the eastern part remains consistently strike-slip in nature. From mid-1997 to June 1998 western Hjalli-Ölfus shifts from an oblique normal to a strike-slip stress regime, while the eastern part maintains the strike-slip character of the SISZ. However, two months after the 4 June 1998 Hengill earthquake, the western part shifts back to an oblique normal regime, which loses a part of its normal-faulting tendency after the 13 November 1998 Hjalli-Ölfus earthquake. This variation in stress fields between two significant events on conjugately oriented prodominantly strike-slip faults is a clear example of these features influencing one another between seismic episodes. 

How to cite: Bjarnason, I. Th., Parameswaran, R. M., and Thorbjarnardóttir, B. S.: Interseismic stress field variations in Hjalli-Ölfus, SW Iceland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8521, https://doi.org/10.5194/egusphere-egu2020-8521, 2020.

EGU2020-16535 | Displays | SM6.5

Seismic Eruption Catalog of Strokkur Geyser, Iceland

Eva P. S. Eibl, Sebastian Hainzl, Nele. I. K. Vesely, Thomas R. Walter, Philippe Jousset, Gylfi P. Hersir, and Torsten Dahm

Geysers are hot springs whose frequency of water eruptions remain poorly understood. We setup a local broadband seismic network for one year at Strokkur geyser, Iceland, and developed an unprecedented catalog of 73,466 eruptions. We detected 50,135 single eruptions, but find that the geyser is also characterized by sets of up to six eruptions in quick succession. The number of single to sextuple eruptions exponentially decreased, while the mean waiting time after an eruption linearly increased (3.7 to 16.4 min). While secondary eruptions within double to sextuple eruptions have smaller mean seismic amplitudes, the amplitude of the first eruption is comparable for all eruption types. We statistically assess and model the eruption frequency assuming discharges proportional to the eruption multiplicity and a constant probability for subsequent events within a multi‐tuple eruption. We conclude that the waiting time after an eruption is predictable, but not the type or amplitude of the next one.

How to cite: Eibl, E. P. S., Hainzl, S., Vesely, N. I. K., Walter, T. R., Jousset, P., Hersir, G. P., and Dahm, T.: Seismic Eruption Catalog of Strokkur Geyser, Iceland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16535, https://doi.org/10.5194/egusphere-egu2020-16535, 2020.

EGU2020-16388 | Displays | SM6.5

The Great Geysir and tectonic interactions in South Iceland

Bergthóra S. Thorbjarnardóttir, Ingi Th. Bjarnason, and Revathy M. Parameswaran

The Great Geysir is within a tectonically active region bordering the eastern flank of the Western Volcanic Zone (WVZ), south of Langjökull glacier. The geothermal area has been active at least throughout the Holocene (Torfason, 1985). It is a high-temperature system, which is not common in Iceland for geothermal areas located outside the neovolcanic zones. Its longevity suggests continuously active tectonics in the region. Indeed, half a century of seismic monitoring shows relatively high activity of minor earthquakes (magnitude<4.0). The general pattern of seismicity is rather constant through time, but comes in bursts of activity. We attempt to elucidate the driving forces in this unusual and poorly tectonically understood area, by analyzing the most modern seismic data collected in the years 1995-2016 within a study area ~25x25 km2 enclosing the Geysir area. It is, for instance, observed how the large (Mw~6.5) earthquakes in June 2000, located ~45 km south and southwest of Geysir in the South Iceland Seismic Zone (SISZ), induced seismicity kilometers away within hours after their occurrence. The heightened level of activity, an order of magnitude in terms of number of earthquakes, lasted half a year after the 2000 events in large parts of the study area and finally tapered down in 2001. Within the first two weeks of the 2000 events, the main activated faults are within 5 km of the Great Geysir. The activation is mostly at shallow depth (< 4 km). However, none pass directly through the Geysir geothermal area. That may explain the only minor change observed in the dormant state of the Great Geysir, which has now lasted approximately a century. There are historic accounts on how several large South Iceland earthquakes in the SISZ activated the Great Geysir, lasting for years or decades. The last such activation was in 1896. In its full might, it erupts up to a height of 70-80 m (Torfason, 1985).  Its currently active neighbor, Strokkur geysir, usually erupts to heights of 15-20 m.  Cross-sections of the seismicity near Geysir suggest several near vertical  right-lateral ~NNE trending faults.  Focal mechanisms indicate strike-slip movements, but also oblique-normal and thrust events in between. This may suggest fault jogs and high horizontal stresses. Approximately 6 km north of Geysir, in the Sandfell and Sandvatn area, there is a persistent ~ENE trending ~5 km long seismic pattern with main activity between 4-8 km depth. This seismicity has occurred, on and off, through the history of seismic observations. Here the faulting is also complicated (strike-slip and thrust), but focal mechanisms suggest the main component to be normal to oblique normal. Cross-sections, although unclear, suggest possible dip to the ~SSE. We intend to calculate stress inversions in the study area prior to the conference.

How to cite: Thorbjarnardóttir, B. S., Bjarnason, I. Th., and Parameswaran, R. M.: The Great Geysir and tectonic interactions in South Iceland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16388, https://doi.org/10.5194/egusphere-egu2020-16388, 2020.

EGU2020-4714 | Displays | SM6.5

Characterizing the long-period seismicity of Teide volcano in Tenerife (Canary Islands, Spain)

Jean Soubestre, Luca D'Auria, José Barrancos, Germán D. Padilla, Léonard Seydoux, and Nemesio M. Peréz

The volcanic long-period seismicity, composed of long-period events and volcanic tremors, constitutes an important attribute of volcanic unrest. Its detection and characterization is therefore a key aspect of volcano monitoring. In the present work, a method based on the seismic network covariance matrix, the equivalent in the frequency domain of the cross-correlation matrix, is used to automatically detect and locate long-period events of the Teide volcano on the island of Tenerife (Canary Islands, Spain). The method is based on the analysis of eigenvalues and eigenvectors of the network covariance matrix.

Long-period events are detected through the time evolution of the width of the network covariance matrix eigenvalues distribution, which is a proxy of the number of sources acting in the wavefield. Each detected long-period event is then located using the moveout information of the corresponding first eigenvector. Three years of seismic data (from 2017 to 2019) continuously recorded by the Red Sísmica Canaria (C7), a permanent monitoring network composed of 17 broadband stations operated by the Instituto Volcanológico de Canarias (INVOLCAN), are analysed. The obtained locations are compared with potential locations from INVOLCAN’s catalog, obtained by a standard approach based on manual phases picking.

How to cite: Soubestre, J., D'Auria, L., Barrancos, J., Padilla, G. D., Seydoux, L., and Peréz, N. M.: Characterizing the long-period seismicity of Teide volcano in Tenerife (Canary Islands, Spain), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4714, https://doi.org/10.5194/egusphere-egu2020-4714, 2020.

EGU2020-9767 | Displays | SM6.5

The 11 November 2018 Mayotte event was observed at the Iranian Broadband seismic stations

Hossein Sadeghi and Sadaomi Suzuki

The 11 November 2018 Mayotte event was first introduced in the media by Maya Wei-Haas (2018) on National Geographic Magazine as a strange earthquake of which seismic waves were recorded by instruments around the world, but unusually nobody felt them. The Mayotte event in the absence of body waves caused long-duration, long-period surface waves traveling around the globe. Cesca et al. (2020) by analyzing regional and global seismic and deformation data suggested drainage of a deep magma reservoir. Tono Research Institute of Earthquake Science recorded the data with the broadband seismometer (STS-1) and gravimeter (gPhone) installed in Mizunami, Japan (Murakami et al., 2019). The records by Iranian broadband stations clearly showed the long-period seismic signals around 10 (UTC) on November 11, 2018. We studied records by 26 stations distributed throughout the country. The stations are operated by National Center of Broadband Seismic Network of Iran, International Institute of Earthquake Engineering and Seismology (IIEES). Since the frequency content of Fourier amplitude spectra appeared the signal of the surface waves as a peak around 0.06 Hz, we applied a bandpass filter of 0.05-0.07 Hz to the waveform data. To separate Rayleigh from love in surface waves, the filtered horizontal components were rotated to the radial and transverse components based on an assumed epicenter location at the latitude of 12.7S and longitude of 45.4E degrees. The stations considered as an array and the investigation was carried out in two ways. First, the position of each station was taken as the reference point of the array coordinate, and arrival delay times at the other stations relative to the reference were calculated. The phase velocity and the back-azimuth of each station were estimated through the least-square regression method. The estimated back azimuths were within 13 degrees from the back azimuths from the assumed epicenter. The average phase velocity for Rayleigh and Love phases are calculated as 2.97 and 3.31 km/sec, respectively. Second, we applied semblance analysis to six stations with the shortest spacing distances. However, the distance between the adjacent stations relative to the signal wavelength was not enough short to prevent spatial aliasing. Nevertheless, the interesting was that the semblance results were different for radial and transverse components. We calculated surface-wave magnitude (Ms) for the event and a number of recorded earthquakes occurring in the Mayotte area from May 13 to June 1, 2018. Linear regression was used to define relationships between the calculated Ms and the USGS body-wave magnitude (mb) and the local magnitude by BRGM catalog (Bertil et al. , 2019), and the moment magnitude (Mw) from the CMT solutions of HRVD and USGS.

How to cite: Sadeghi, H. and Suzuki, S.: The 11 November 2018 Mayotte event was observed at the Iranian Broadband seismic stations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9767, https://doi.org/10.5194/egusphere-egu2020-9767, 2020.

Deep Low-Frequency earthquakes (DLFs) beneath volcanoes are possible evidence for deep-seated magmatic activities in the crust and uppermost mantle. After the 2011 Tohoku Earthquake (Mw 9.0), the number of DLFs beneath Zao volcano in the northeast Japan started increasing. The hypocenters of these DLFs form two clusters at shallow (20~28 km) and deep (28~38 km) depths. The shallow and deep clusters are located central and lower part of a high Vp/Vs zone, respectively (e.g., Okada et al., 2015), and the fact suggests different fluid involvement and source processes of DLFs at two clusters. In addition, after the activation of DLFs in 2012, increase in shallow ( < 2 km depth) seismicity has been observed since 2013, which implies the interaction of shallow and deep volcanic fluids. However, the small magnitude of DLFs makes it rather difficult to discuss detailed spatio-temporal characteristics of DLFs and focal mechanism of individual DLF. Therefore, in this study, we first detected DLFs using waveform correlation and determine their hypocenter, and then classified DLFs using waveform correlation to reveal the spatio-temporal characteristics and focal mechanism of each event type.

To detect DLFs, we applied the matched filter method to the continuous three-components waveform data recorded at stations operated by Tohoku Univ., NIED, and JMA. 146 DLFs listed in the JMA unified earthquake catalog between Jan. 2012 and Sept. 2016 were selected as templates. For each newly detected DLF, we estimated the differential arrival times using cross-correlation between the detected DLF and the template having maximum correlation, and determined the relative hypocenter using the master event method. As a result, we determined hypocenters of 1202 DLFs between Jan. 2012 to May 2018, which is about 4 times the number of DLFs listed in the JMA catalog.

We then classified newly detected DLFs using the hierarchical clustering method based on the waveform correlation, and classified 939 events into seven types (Type A: 241 events, B: 222, C: 295, D: 79, E: 42, F: 37, G: 23). The characteristics of individual waveform types are summarized as follows: Type C shows high frequency components (4-8 Hz) superimposed on the P wave, while the other types only have low frequency components (1-4 Hz); S-wave/P-wave spectral ratio of type C observed at each station shows larger azimuthal variation than that of the other types, and shows maximum peaks in northeast and southwest direction; Type C occurs mainly in the deep cluster while the other types occur in the shallow cluster; The activity of type C started in 2012 and showed rapid increase in 2015, while the other types show similar temporal changes in 2013 and 2016.

These results of this study suggest fluid transportation in the crust and different dynamic processes at each depth beneath the volcano.

How to cite: Ikegaya, T. and Yamamoto, M.: Spatio-temporal characteristics and focal mechanisms of deep low-frequency earthquakes beneath Zao volcano, Japan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12533, https://doi.org/10.5194/egusphere-egu2020-12533, 2020.

Sierra Negra is a basaltic shield volcano in the Galapagos Archipelago (Ecuador) and is the largest of the Galapagos volcanoes. The 2018 eruption was a complex event that included eruptive fissures opening on the northern rim and north-western flank. In this study, we report observations of seismic signals recorded on a temporary dense local network consisting of 14 seismometers and nearby permanent seismic stations, and analyze this data set to retrieve the source mechanisms of moderate pre- and co-eruptive seismic events (body-wave magnitude range of M3.5-5.3). Because of the shallow depths of the seismic events (<2 km) and short source-receiver distances (~1.5-10 km), that are comparable to or shorter than the wavelengths of radiated waves, the effect of near- and intermediate-field terms on dynamic displacements can be significant and hence the far-field assumption may not be well-suited for determining fault-plane solutions. Therefore, we pay special attention on the polarization properties of seismic waves excited at the near-field and intermediate-field ranges, and model and analyze complete displacement wave-fields to determine seismic sources. The source mechanism solutions are also interpreted in light of the volcanic unrest leading to the 2018 eruption, GPS observations, and reported regional centroid moment tensors.

How to cite: Nooshiri, N., Lokmer, I., Bean, C., Bell, A., Möllhoff, M., and Ruiz, M.: Source mechanisms of seismic events during the 2018 eruption of Sierra Negra Volcano (Galapagos) determined by using polarization properties of complete (near-field and far-field) body waves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11297, https://doi.org/10.5194/egusphere-egu2020-11297, 2020.

EGU2020-18213 | Displays | SM6.5

Multiplet Based Time Lapse Velocity Changes Prior to the 2018 Eruption of Sierra Negra Volcano, Galapagos Island Observed with Coda Wave Interferometry

Mariantonietta Longobardi, James Grannel, Christopher Bean, Andrew Bell, and Mario Ruiz

Changes in external stress state and fluid content alter the mechanical properties of an geological media. Variations in seismic wave velocity can be used as proxies for changes in stress the onset of mechanical demage and/or possible fluid ingression. Temporal variations in seismic wave velocity have previously been monitored and observed prior to volcanic eruptions. In the absence of additional constraints related to stress or fluid changes on the volcano, these pre-eruptive changes are difficult to interpret and hence the causes of them are often not well understood. In this study, Coda Wave Interferometry (CWI) is used to measure time-lapse changes in seismic velocity on seismic multiplets (repeating similar earthquakes). In particular, we focus our analysis on using this technique to calculate the velocity changes on the data recorded prior to the 2018 eruption of Sierra Negra volcano, Galapagos Island. On 26th June 2018 at 09:15 UTC, a magnitude 5.3 earthquake occurred near the south-west caldera rim and an intense seismic swarm started around 17:15 UTC. Seismic tremor dominated at about 19:45 UTC, which marked the onset of the eruption. A very large seismicity sequence preceded the eruption. The pricise relationship between the magnitude 5.3 event and the eruption is not fully constraind. Here we search for multiplets in order to achieve high time resolution velocity change information in the hours between the large earthquake and the eruption. Our aim is to understand whether changes in seismic velocity measured with CWI on multiplets method provide new insight into the physical processes related to the eruption.



How to cite: Longobardi, M., Grannel, J., Bean, C., Bell, A., and Ruiz, M.: Multiplet Based Time Lapse Velocity Changes Prior to the 2018 Eruption of Sierra Negra Volcano, Galapagos Island Observed with Coda Wave Interferometry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18213, https://doi.org/10.5194/egusphere-egu2020-18213, 2020.

EGU2020-18975 | Displays | SM6.5

Different mechanisms of the pre- and co-eruptive tremor during the 2018 eruption at Sierra Negra volcano, Galapagos

Ka Lok Li, Meysam Rezaeifar, Christopher J. Bean, James Grannell, Andrew Bell, Mario Ruiz, Stephen Hernandez, and Martin Möllhoff

Volcanic tremor are persistent seismic signals observed near active volcanoes. They are often associated with eruptions, although the exact relationships are not well constrained. To gain a better insight into the generation mechanisms of volcanic tremor, we study tremor that occurred during the 2018 eruption at Sierra Negra volcano, Galapagos. Located 1000 km west of continental Ecuador, Sierra Negra is a shield volcano with a large summit caldera and is one of the most active volcanoes in the Galapagos archipelago. The 2018 eruption started at about 19:55 UTC on 26th June and lasted about two months. Two tremor phases with very different frequency characteristics are identified before and after the eruption onset. The pre-eruptive phase is characterized by a narrow frequency band (2.5 – 4 Hz) and the co-eruptive phase has a broad frequency band (1 – 15 Hz). Location of the two phases by a seismic amplitude ratio method suggests that they are likely to be generated by different physical processes. The pre-eruptive phase is likely generated by dike opening while the co-eruptive phase is associated with lava flow. This interpretation is consistent with a time-lapse P-wave velocity structure of the volcano imaged by local-earthquake travel-time tomography.

How to cite: Li, K. L., Rezaeifar, M., Bean, C. J., Grannell, J., Bell, A., Ruiz, M., Hernandez, S., and Möllhoff, M.: Different mechanisms of the pre- and co-eruptive tremor during the 2018 eruption at Sierra Negra volcano, Galapagos, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18975, https://doi.org/10.5194/egusphere-egu2020-18975, 2020.

EGU2020-11365 | Displays | SM6.5

An Experimental Study of Volcanic Tremor Driven by Magma Wagging

vahid dehghanniri and A. Mark Jellinek

 


Volcanic tremor is a feature of most explosive eruptions. Pre-eruptive tremors can be characterized by monotonic increases in the maximum frequency, frequency bandwidth and amplitude that are correlated with increases in gas flux from a volcanic vent. An enigmatic feature of this behavior is that is observed at volcanoes with widely ranging conduit geometries and structures. Accordingly, the ``magma wagging'' model introduced by [1] and extended by [2] hypothesizes an underlying mechanism that is only weakly-sensitive to volcano architecture: Within active volcanic conduits, the flow of gas through a permeable foamy annulus of gas bubbles excites and maintains an oscillation of a central magma column through a well-known Bernoulli effect. Furthermore, this oscillation has spectral properties that evolve depending on annulus thickness and permeability and the total flow of gas. 

In this thesis, we carry out a critical experimental test of the underlying mechanism for excitation. We explore the response of columns with prescribed elastic and linear damping properties to forced air annular airflows. From high-speed video measurements of linear and orbital displacements and time series of accelerometer measurements we characterize and understand the excitation, evolution, and steady-state oscillating behaviors of analog magma columns over a broad range of conditions. Where the time scale for damping is much longer than the natural period of free oscillation, column oscillation is continuously excited by relatively short period Bernoulli modes through a reverse energy cascade. We also identify three distinct classes of wagging: i. rotational modes that confirm predictions for whirling modes by [3]; as well as ii. mixed-mode; and iii. chaotic modes that are extensions to previous studies[1,2]. Our results show that rotational modes are favored for symmetric, and high-intensity forcing. Mixed-mode responses are favored for a symmetric and intermediate intensity forcing. Chaotic modes occur in asymmetric or low intensity forcing. To confirm and better understand our laboratory results and also extend them to conditions beyond what is possible in the laboratory we carry out two-dimensional numerical simulations of our analog experiments.

Taken together, results from our experimental and numerical studies can be extended to a natural system to make qualitative predictions testable in future studies of pre- and syn-eruptive volcano seismicity. Far before an eruption, the total gas flux is low and magma wags in a chaotic mode no matter what is the spatial distribution of the gas flux. At a pre-eruptive state, as gas flux increases, if the distribution of gas flux is approximately symmetric, we expect a transition to mixed and possibly rotational modes. During an eruption, fragmentation and explosions within the foamy annulus can cause spatial heterogeneity in permeability resulting in non-uniform gas flux that favors chaotic wagging behavior. 

[1] A. M. Jellinek and D. Bercovici. Seismic tremors and magma wagging during explosive volcanism. Nature, 470(7335):522-525, 2011

[2] D. Bercovici, A. M.  Jellinek, C. Michaut, and D. C. Roman. Volcanic tremors and magma wagging: gas flux interactions and forcing mechanism. Geophys. J.Int., 195(2):1001-1022, 2013

[3] Y. Liao and D. Bercovici. Magma wagging and whirling: excitation by gas flux. Geophys. J.Int., 215(1):713-735, 2018

How to cite: dehghanniri, V. and Jellinek, A. M.: An Experimental Study of Volcanic Tremor Driven by Magma Wagging, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11365, https://doi.org/10.5194/egusphere-egu2020-11365, 2020.

EGU2020-851 | Displays | SM6.5

Identifying icequakes at ice-covered volcanoes in Southern Chile

Oliver Lamb, Jonathan Lees, Luis Franco Marin, Jonathan Lazo, Andres Rivera, Michael Shore, and Stephen Lee

Volcanoes and glaciers are both productive sources of seismic activity which may be easily confused for each other, leading to potential missed warnings or false alarms. This presents a challenge for organizations monitoring active volcanoes with glaciers on or near the edifice. Cryogenic earthquakes (i.e. icequakes) have been studied at only a few volcanoes around the world and there is a ready need to develop robust methods for efficiently differentiating them from volcanic events. Here we present results from an ongoing study of icequakes at active ice-covered volcanoes in the Southern Chilean Volcanic Zone. The primary focus of the project so far has been on seismo-acoustic data collected at Llaima volcano, one of the largest and most active volcanoes in the region. The data, recorded in 2015 and 2019, was analysed using a combination of automatic multi-station event detection and waveform cross-correlation to find candidate repeating icequakes. We identified 11 persistent families of repeating events in 2015, and over 50 families in 2019; the largest family containing over 1000 events from January to April 2019. The persistent, repetitive nature of these events combined with their waveform characteristics and source locations suggest they originated from multiple sub-glacial sources on the upper flanks of the volcano. Low levels of volcanic activity at Llaima volcano since 2009 have allowed this clear discrimination of icequake events. We are also targeting Villarrica volcano in early 2020 with a network of seismo-acoustic sensors and to record icequake activity in concurrence with the ongoing eruptive activity at the summit. Altogether, the results from this project so far suggest icequakes may be more common than previously thought and has strong implications for how seismic data at ice-covered volcanoes may be interpreted.

How to cite: Lamb, O., Lees, J., Franco Marin, L., Lazo, J., Rivera, A., Shore, M., and Lee, S.: Identifying icequakes at ice-covered volcanoes in Southern Chile, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-851, https://doi.org/10.5194/egusphere-egu2020-851, 2020.

SM7.1 – Advances in Modelling, Inversion and Interpretation of Geophysical data

EGU2020-3060 | Displays | SM7.1

Joint inversion and collaborative interpretations in complex geodynamical context

Christel Tiberi, Adeline Clutier, Matthieu Plasman, Stéphanie Gautier, Fleurice Parat, and Marie Lopez

Active regions concentrate different geodynamical processes sometimes with complex interactions and retroactions. In order to understand the associated lithospheric deformation and evolution, scientists deduce crustal and mantle structures from sparse, inaccurate and indirect observations. In particular, geophysics aims at retrieving physical properties of crustal or lithospheric media from gravity, electric or seismic measurements. Those indirect tools have been used for decades now to image the Earth Interior at many different scales, from the surface down to the Core.

Besides, density, resistivity or seismic velocity retrieved from geophysical inversions are sensitive to many different factors (temperature, pressure, melt, composition…), each of them impacting the parameters variously. Finally, each of these methods presents its own depth investigation and accuracy, which depends on time lap, network configuration, data wavelength, etc.

In order to distinguish the role of each factor in the lithospheric structure heterogeneity, and to counteract the different method limits, geophysicists have combined their observations in combined schemes for decades now. We will present here how jointly inverting seismic tomography and gravity may help to better understand complex zones implying melt, faults, crustal modification and plate interaction. When mathematical link between the parameters doesn’t exist, we will present a combination of petrophysics and geophysics, that brings new information on past and present dynamical evolution in a magmatic area (East African Rift, Tanzania). Finally, we will address the question of the real benefit of a joint inversion, and whether we can combine all kind of data.

How to cite: Tiberi, C., Clutier, A., Plasman, M., Gautier, S., Parat, F., and Lopez, M.: Joint inversion and collaborative interpretations in complex geodynamical context, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3060, https://doi.org/10.5194/egusphere-egu2020-3060, 2020.

A lens having a diameter of about 40 km and a thickness of 10 km was determined at depths 14-22 km in the junction of the Kyrgyz Range and the Chu Basin Depression Trough of the Northern Tien Shan area by 3D seismic tomography carried out earlier. The following questions are still unanswered: 

- what are its petrophysical characteristics?

- what is the nature of the geophysical anomalies?

- what is the mechanism of its formation?

- how long does it exist within their present boundaries?

In order to address these key issues, it is insufficient to analyze the depth behavior of the P-waves velocities as it was done before. To this end we have built additionally the electrical resistivity, density, lithotypes, temperature, porosity, and fluid saturation models along the N-S collocated seismic and magnetotelluric profile intersecting the study area.

Their integrated analysis enabled to propose a conceptual model of a lens in the Earth’s crust which answers the questions enumerated above.  In particular, it was determined that the lens is characterized by low VP and VS velocities and their ratio VP / VS;  low resistivity (3–30 Ω.m); low density (at most 2.45 g/cm3); high porosity (above 1.2%) and fluid saturation (above 0.1%); pressure range of 4–6 Kbar; temperature range from TSCF  = 350-400°C at the lens’ top to TBDT  = 600–650°C at the bottom, characteristic for the emergence of supercritical fluids and for the solidus of granite, respectively; presence of a cap (a relatively dense, poorly permeable zone) that shields the forming fluid reservoir from above.

Joint analysis of these models made it possible to rule out the molten rocks as a responsible factor for high electrical conductivity and, with a high degree of confidence, assume supercritical fluid nature of the observed petrophysical anomalies. It was supposed that the lens is most likely to be a giant reservoir of supercritical fluids located at the depths between isotherms  TSCF  and  TBDT corresponding to the PT-conditions of existence of supercritical fluids, on the one hand, and granite solidus (brittle / ductile transition), on the other hand.

The mechanism of its formation could be explained by dehydration of amphibolites accompanied by dissolution of chlorides which, in turn, leads to the emergence of films with sufficiently high electrical conductivity typical of supercritical highly mineralized solutions. Although this formation scenario fairly well explains the observed anomalies, it does not exclude another mechanism associated with the partially melted material risen from the large depths.

The lens lifetime was determined from properties of the cap. Assuming that for the Cenozoic folding regions, the rock permeability is around 10-21 m2 we could roughly estimate the rate of fluid migration through it. Accordingly the lens lifetime is around 33 million years which is consistent with the age of the Cenozoic activation zones.

How to cite: Spichak, V. and Goidina, A.: Conceptual model of a lens in the upper crust determined from joint analysis of petrophysical models (Northern Tien Shan case study), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-110, https://doi.org/10.5194/egusphere-egu2020-110, 2020.

EGU2020-7242 | Displays | SM7.1

Shallow structural setting of an active normal fault zone in the 30 October 2016 Mw 6.5 central Italy earthquake imaged through a multidisciplinary geophysical approach.

Fabio Villani, Stefano Maraio, Pier Paolo Bruno, Lisa Serri, Vincenzo Sapia, and Luigi Improta

We investigate the shallow structure of an active normal fault-zone that ruptured the surface during the 30 October 2016 Mw 6.5 Norcia earthquake (central Italy) using a multidisciplinary geophysical approach. The survey site is located in the Castelluccio basin, an intramontane Quaternary depression in the hangingwall of the SW-dipping Vettore-Bove fault system. The Norcia earthquake caused widespread surface faulting affecting also the Castelluccio basin, where the rupture trace follows the 2 km-long Valle delle Fonti fault (VF), displaying a ~3 m-high fault scarp due to cumulative surface slip of Holocene paleo-earthquakes. We explored the subsurface of the VF fault along a 2-D transect orthogonal to the coseismic rupture on recent alluvial fan deposits, combining very high-resolution seismic refraction tomography, multichannel analysis of surface waves (MASW), reflection seismology and electrical resistivity tomography (ERT).

We acquired the ERT profile using an array of 64 steel electrodes, 2 m-spaced. Apparent resistivity data were then modeled via a linearized inversion algorithm with smoothness constraints to recover the subsurface resistivity distribution. The seismic data were recorded by  a190 m-long single array centered on the surface rupture, using 96 vertical geophones 2 m-spaced and a 5 kg hammer source.

Input data for refraction tomography are ~9000 handpicked first arrival travel-times, inverted through a fully non-linear multi-scale algorithm based on a finite-difference Eikonal solver. The data for MASW were extracted from common receiver configurations with 24 geophones; the dispersion curves were inverted to generate several S-wave 1-D profiles, subsequently interpolated to generate a pseudo-2D Vs section. For reflection data, after a pre-processing flow, the picking of the maximum of semblance on CMP super-gathers was used to define a velocity model (VNMO) for CMP ensemble stack; the final stack velocity macro-model (VNMO) from the CMP processing was smoothed and used for post-stack depth conversion. We further processed Vp, Vs and resistivity models through the K-means algorithm, which performs a cluster analysis for the bivariate data set to individuate relationships between the two sets of variables. The result is an integrated model with a finite number of homogeneous clusters.

In the depth converted reflection section, the subsurface of the VF fault displays abrupt reflection truncations in the 5-60 m depth range suggesting a cumulative fault throw of ~30 m. Furthermore, another normal fault appears in the in the footwall. The reflection image points out alternating high-amplitude reflections that we interpret as a stack of alluvial sandy-gravels layers that thickens in the hangingwall of the VF fault. Resistivity, Vp and Vs models provide hints on the physical properties of the active fault zone, appearing as a moderately conductive (< 150 Ωm) elongated body with relatively high-Vp (~1500 m/s) and low-Vs (< 500 m/s). The Vp/Vs ratio > 3 and the Poisson’s coefficient > 0.4 in the fault zone suggest this is a granular nearly-saturated medium, probably related to the increase of permeability due to fracturing and shearing. The results from the K-means cluster analysis also identify a homogeneous cluster in correspondence of the saturated fault zone.

How to cite: Villani, F., Maraio, S., Bruno, P. P., Serri, L., Sapia, V., and Improta, L.: Shallow structural setting of an active normal fault zone in the 30 October 2016 Mw 6.5 central Italy earthquake imaged through a multidisciplinary geophysical approach., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7242, https://doi.org/10.5194/egusphere-egu2020-7242, 2020.

EGU2020-15067 | Displays | SM7.1

Utilisation of stochastic MT inversion results to constrain potential field inversion

Jérémie Giraud, Hoël Seillé, Gerhard Visser, Mark Lindsay, and Mark Jessell

We introduce a methodology for the integration of results from 1D stochastic magnetotelluric (MT) data inversion into deterministic least-square inversions of gravity measurements. The goal of this study is to provide a technique capable of exploiting complementary information between 1D magnetotelluric data and gravity data to reduce the effect of non-uniqueness existing in both methodologies. Complementarity exists in terms of resolution, the 1D MT being mostly sensitive to vertical changes and gravity data sensitive to lateral property variations, but also in terms of the related petrophysics, where the sensitivity to different physical parameters (electrical conductivity and density) allows to distinguish between different contrasts in lithologies.  To this end, we perform a three-step workflow. Stochastic 1D MT inversions are performed first. The results are then fused to create 2D model ensembles. Thirdly, these ensembles are utilised as a source of prior information for gravity inversion. This is achieved by extracting geological information from the ensemble of resistivity model realisations honouring MT data (typically, ensemble comprising several thousands of models) to constrain gravity data inversion.

In our investigations, we generate synthetic data using the 3D geological structural framework of the Mansfield area  (Victoria, Australia) and subsequently perform stochastic MT inversions using a 1D trans-dimensional Markov chain Monte Carlo sampler. These inversions are designed to account for the uncertainty introduced by the presence of non-1D structures.  Following this, the 1D probabilistic ensembles for each site are fused into an ensemble of 2D models which can then be used for further modelling. The fusion method incorporates prior knowledge in terms of spatial lateral continuity and lithological sequencing, to create an image that reflects different scenarios from the ensemble of models from 1D MT inversion. It identifies several domains across the considered area where it is plausible for the different lithologies to occur. This information is then used to constrain gravity inversion using a clustering algorithm by varying the weights assigned to the different lithologies spatially accordingly with the domains defined from MT inversions.

Our results reveal that gravity inversion constrained by MT modelling results in this fashion provide models that present a lower model misfit and are geologically closer to the causative model than without MT-derived prior information. This is particularly true in areas poorly constrained by gravity data such as the basement. Importantly, in this example, the basement is better imaged by the combination of both gravity and MT data than by the separate techniques. The same applies, to a lesser extent, to dipping geological structures closer to surface. In the case of the Mansfield area, the synthetic modelling investigation we performed shows the potential of the workflow introduced here and that it can be confidently applied to real world data.

How to cite: Giraud, J., Seillé, H., Visser, G., Lindsay, M., and Jessell, M.: Utilisation of stochastic MT inversion results to constrain potential field inversion , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15067, https://doi.org/10.5194/egusphere-egu2020-15067, 2020.

The EarthScope USArray project provides high quality magnetotelluric and seismic observations, which have been used to identify tectonic boundaries of the USA. Combining these data sets together with satellite gravity observations, we investigate how the different data sets can complement each other in order to find a consistent model of the subsurface. Using a cross-gradient constraint, we first invert the magnetotelluric and gravity data sets in order to demonstrate the feasibility of our approach and to identify any difficulties. Once a joint conductivity and density model is found, we perform a full joint inversion of all three data sets. By comparison with models derived from separate inversions of the individual observables we can show how the different data sets interact. Examining the magnitude of the cross-gradient lets us distinguish parts of the model where a good agreement of the recovered structures has been achieved from those where differing patterns are necessary in order to achieve an acceptable data fit. In this presentation we will give an overview of our approach, highlight our strategy and show results from individual and joint inversions.

How to cite: Weise, B., Moorkamp, M., and Fishwick, S.: Coupling USArray and satellite gravity data – an integrated conductivity, density and seismic velocity model of the western USA, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7436, https://doi.org/10.5194/egusphere-egu2020-7436, 2020.

EGU2020-497 | Displays | SM7.1

Integration of multi parameters geophysical models using PCA : application to geothermal exploration

jean-michel ars, Pascal Tarits, Sophie Hautot, Mathieu Bellanger, and Olivier Coutant

Geophysical exploration of natural resources is challenging because of complex and/or narrow geological structures to image. Geophysical models should provide an image at a scale large enough to understand the complex geology but with the adequate resolution to resolve features like faults. One solution to overcome this difficulty is to integrate large multiphysics datasets to provide complementary insight of the geology. New approaches involve joint inversion of all datasets in a common process where models are coupled together. Geometrical or quantitative interpretation of the joint models image several physical properties shaping the same pattern of the target resources. In reality, models resulting from joint inversion are still challenging to interprete. Most of the joint inversion techniques are based on parameters relationship or geometrical constraint which imply common interfaces between models. This assumption may be wrong since geophysical methods have different sensitivity to the same geological object.

Geophysical integration cover a wide range of approach from the visual interpretation of model presented side by side to sophistical statistical analyses such as automatic clustering. We present here a geophysical models integration based on principal component analysis (PCA). PCA allow to gain insight on a multi-variable system with high level of interaction. PCA aims to reorganize the system by finding a new set of variables distributed along new orthogonal axis and keeping most of the variance from the data. Thus geophysical interaction are highlighted along components that can be interpreted in terms of patterns. We applied this integration method to gravity, ambient noise tomography and resistivity models obtained from joint inversion in the framework of unconventional geothermal exploration in Massif Central, France. PCA of the log-resistivity, the density contrast and the Vs velocity model has 3 independent components. The first one (PC1) representing 69% of the total variance of the system is highly influenced by the parameter coupling enforced in the joint inversion process. PC1 allows to point to geophysical structures that may be related to the geothermal system. The second component (PC2) represents 22% of the total variance and is strongly correlated to the resistivity distribution The correlation with the surface geology suggests that it may be a fault marker. The third component (PC1: 9% of the total variance) is still above the nul hypothesis and seems to describe the 3D geometry of the geological units. This statistical approach may help the geophysical interpretation into a possible geothermal conceptual model

 

How to cite: ars, J., Tarits, P., Hautot, S., Bellanger, M., and Coutant, O.: Integration of multi parameters geophysical models using PCA : application to geothermal exploration, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-497, https://doi.org/10.5194/egusphere-egu2020-497, 2020.

Reflections of seismic waves are strongly distorted by the presence of complex geological structures (e.g. salt bodies) and their vertical resolution is usually of the order of a few tens of meters, imposing limitations in the construction of subsurface models. One way to improve the reliability of such models is to integrate reflection seismic data with other types of geophysical data, such as gravimetric data, since the latter provide an additional link to map geological structures that exhibit density contrasts with respect to their surroundings. In a previous study, we developed a cooperative inversion method of 2D post-stack and migrated reflection seismic data, and gravimetric data. Using that inversion method, we minimize two problems: (1) the problem of the distortion of reflection seismic data due to the presence of complex geological bodies and (2) the problem of the greater ambiguity and the commonly lower resolution of the models obtained only from gravimetric anomalies. The method incorporates a technique to decrease the number of variables and is solved by optimization of the gravity inverse problem, thus reducing computing time. The objective function of cooperative inversion was minimized using three different methods of optimization: (1) simplex, (2) simulated annealing, and (3) genetic algorithm. However, these optimization methods have internal parameters which affect the convergence rate and objective function values. These parameters are usually chosen accordingly to previous references. Although the usage of these standard values is widely accepted, the best values to assure effectiveness and stability of convergence are case-dependent. In the present study, we propose a sensitivity analysis on the internal parameters of the optimization methods for the previously presented cooperative inversion. First, we developed the standard case, which is an inversion performed using all parameters at their standard values. Then, the sensitivity analysis is performed by running multiple inversions, each one with a set of parameters. Each set is obtained by modifying the value of a single parameter either for a lower or for a higher value, keeping all other values at their standard values. The results obtained by each setting are compared to the results of the standard case. The compared results are both the number of evaluations and the final value of the objective function. We then classify parameters accordingly to their relative influence on the optimization processes. The sensitivity analysis provides insight into the best practices to deal with object-based cooperative inversion schemes. The technique was tested using a synthetic model calculated from the Benchmark BP 2004, representing an offshore sedimentary basin containing salt bodies and small hydrocarbons reservoirs.

How to cite: Gonçalves, M. and Leite, E.: Sensitivity analysis of optimization parameters on cooperative inversion of seismic reflection and gravity data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-881, https://doi.org/10.5194/egusphere-egu2020-881, 2020.

EGU2020-6016 | Displays | SM7.1

Variational Probabilistic Tomography

Andrew Curtis and Xin Zhang

Seismic Tomography is a method to image the interior of solid media, and is often used to map properties in the subsurface of the Earth. In order to better interpret the resulting images it is important to assess imaging uncertainties. Since tomography is significantly nonlinear, Monte Carlo sampling methods are often used for this purpose but the ‘curse of dimensionality’ generally makes them computationally intractable for large data sets and high-dimensional parameter spaces. To extend uncertainty analysis to larger systems we introduce variational inference methods to conduct seismic tomography. In contrast to Monte Carlo stochastic sampling, variational methods solve the Bayesian inference problem as an optimization problem, yet still provide probabilistic results.

We apply variational inference to solve two types of tomographic problems using synthetic and real data: travel time tomography and full waveform inversion. We test two different variational methods: automatic differential variational inference (ADVI) and Stein variational gradient descent (SVGD). In each case we compare the results to solutions given by a variety of Monte Carlo methods.

In the travel time tomography example we show that ADVI provides a robust mean velocity model but biased uncertainties due to an implicit Gaussian approximation, and that it cannot be used to find multi-modal Bayesian posterior probability distributions. SVGD produces an accurate match to the fully probabilistic results of Markov chain Monte Carlo analysis, but at significantly reduced computational cost – provided that gradients of model parameters with respect to data can be calculated efficiently.

In our waveform inversion example, the SVGD method produces results of similar quality to published results from an efficient Hamiltonian Monte Carlo analysis, at around the same cost. However, that particular Monte Carlo method has significant ‘hidden’ costs: these are necessarily incurred by running a substantial number of pre-run tests to determine suitable settings of run-time parameters, and are not generally included in quoted cost estimates. By contrast, SVGD has relatively low pre-run costs. In addition, SVGD is significantly easier to parallelize, and for very large problems can be run in minibatch mode; this is impossible for Monte Carlo methods without incurring probabilistic errors as so-called ‘detailed balance’ can not be maintained in minibatch Hamiltonian Monte Carlo. We therefore contend that the variational method may have greater potential to extend probabilistic analysis to higher dimensional tomographic systems than current Monte Carlo methods.

 

How to cite: Curtis, A. and Zhang, X.: Variational Probabilistic Tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6016, https://doi.org/10.5194/egusphere-egu2020-6016, 2020.

A Bayesian model for prediction and uncertainty quantification of subsurface lithology/fluid classes, petrophysical properties and elastic material properties conditional on seismic amplitude-versus-offset measurements is defined. We demonstrate the proposed methodology  on a real Norwegian Sea gas discovery in 3D in a seismic inversion framework.

The likelihood model is assumed to be Gaussian, and it is constructed in two steps. First, the reflectivity coefficients of the elastic material properties are computed based on the linear Aki Richards approximation valid for weak contrasts. The reflectivity coefficients are then convolved in depth with a wavelet.  We assume a Markov random field prior model for the lithology/fluid classes with a first order neighborhood system to ensure spatial coupling. Conditional on the lithology/fluid classes we define a Gauss-linear petrophysical and rock physics model. The marginal prior spatial model for the petrophysical properties and elastic attributes is then a multivariate Gaussian mixture random field.

The convolution kernel in the likelihood model restricts analytic assessment of the posterior model since the neighborhood system of the lithology/fluid classes is no longer a simple first order neighborhood. We propose a recursive algorithm that translates the Gibbs formulation into a set of vertical Markov chains. The vertical posterior model is approximated by a higher order Markov chain, which is computationally tractable. Finally, the approximate posterior model is used as a proposal model in a Markov chain Monte Carlo algorithm. It can be verified that the Gaussian mixture model is a conjugate prior with respect to the Gauss-linear likelihood model; thus, the posterior density for petrophysical properties and elastic attributes is also a Gaussian mixture random field.

We compare the proposed spatially coupled 3D model to a set of independent vertical 1D inversions. We obtain an increase of the average acceptance rate of 13.6 percentage points in the Markov chain Monte Carlo algorithm compared to a simpler model without lateral spatial coupling. At a blind well location we obtain a reduction of at most 60 % in mean absolute error and root mean square error for the proposed spatially coupled 3D model.

How to cite: Fjeldstad, T. and Omre, H.: Joint Bayesian spatial inversion of lithology/fluid classes, petrophysical properties and elastic attributes – A Norwegian Sea gas discovery, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6859, https://doi.org/10.5194/egusphere-egu2020-6859, 2020.

EGU2020-8164 | Displays | SM7.1

Kite - bridging InSAR displacement analysis and earthquake modelling: the 2019 Ridgecrest earthquakes

Marius Paul Isken, Henriette Sudhaus, Sebastian Heimann, Hannes Vasyura-Bathke, Andreas Steinberg, and Torsten Dahm

We present a modular open-source software framework - Kite (http://pyrocko.org) - for rapid post-processing of spaceborne InSAR-derived surface displacement maps. The software enables swift parametrisation, post-processing and sub-sampling of the displacement measurements that are compatible with common InSAR processors (e.g. SNAP, GAMMA, ISCE, etc.) and online processing centers delivering unrwapped InSAR data products, such as NASA ARIA or LiCSAR. The post-processing capabilities include removal of first-order atmospheric phase delays through elevation correlation estimations and regional atmospheric phase screen (APS) estimations based on atmospheric models (GACOS), masking of displacement data, adaptive data sub-sampling using quadtree decomposition and data error covariance estimation.

Kite datasets integrate into forward modelling and optimisation frameworks Grond (Heiman et al., 2019) and BEAT (Vasyura-Bathke et al., 2019), both software packages aim to ease and streamline the joint optimisation of earthquake parameters from InSAR and GPS data together with seismological waveforms. These data combinations will improve the estimation of earthquake rupture parameters. Establishing this data processing software framework we want to bridge the gap between InSAR processing software and seismological modelling frameworks, to contribute to a timely and better understanding of earthquake kinematics. This approach paves the way to automated inversion of earthquake models incorporating space-borne InSAR data.

Under development is the processing of InSAR displacement time series data to link simultaneous modelling of co- and post-seismic transient deformation processes from InSAR observations to physical earthquake cycle models.

We demonstrate the framework’s capabilities with an analysis of the 2019 Ridgecrest earthquakes from InSAR surface displacements (provided by NASA ARIA) combined with GNSS displacements using the Bayesian bootstrapping strategy from the Grond inverse modelling tool.

How to cite: Isken, M. P., Sudhaus, H., Heimann, S., Vasyura-Bathke, H., Steinberg, A., and Dahm, T.: Kite - bridging InSAR displacement analysis and earthquake modelling: the 2019 Ridgecrest earthquakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8164, https://doi.org/10.5194/egusphere-egu2020-8164, 2020.

EGU2020-1365 | Displays | SM7.1

Inhomogeneous waves in isotropic anelastic media: explicit expressions for Q

Xu Liu, Stewart Greenhalgh, Bing Zhou, and Huijian Li

Seismic waves propagating in attenuative materials are generally inhomogenous waves which, unlike homogeneous waves, have different directions of propagation and attenuation. The degree of wave inhomogeneity can be represented by the inhomogeneity parameter D which varies from 0 to infinity (Cerveny & Psencik, 2005). The dissipation (1/Q)  factors of   inhomogeneous waves vary according to the different definitions. Based on the complex energy balance equations (Carcione, 2001) and the mixed specification of the slowness vector (Cerveny & Psencik, 2005), explicit formulas for the dissipation factors of P- and SV-waves are developed under the two different definitions, (1) 1/QV, the ratio of the time-averaged dissipated energy density to the time-averaged strain-energy density, and (2) 1/QT, the time-averaged dissipated energy density to the time-averaged energy density. By setting the degree of wave inhomogeneity D as zero, these dissipation factor expressions are reduced to their special case versions as homogeneous waves, i.e., 1/QVH = -Im(M)/Re(M) and 1/QTH = 2αv/ω , where, M is the wave modulus, α the attenuation coefficient, v the phase velocity and ω the frequency. An example viscoelastic material is chosen to represent the dissipative features of a reservoir for which P-waves are normally more dissipative than S-waves. The calculated dissipation factors of P-waves under the two definitions (i.e. 1/QPV and 1/QPT) decrease with increasing degree of wave inhomogeneity. For the counterpart S waves, 1/QSV is independent of the degree of wave inhomogeneity and 1/QST shows the trend of increasing with increasing degree of wave inhomogeneity. These findings can be explained by the limiting dissipation factors (defined at the infinite degree of inhomogeneity) which all depend only on the shear modulus.  To ensure the correctness of our results, we repeated each step of the investigation  in a parallel way based on Buchen’s (1971) classic real value energy balance equation, including derivation of explicit formulas for 1/QPV and 1/QPT , with inhomogeneity angle γ  ( -π/2 < γ < π/2) representing the degree of inhomogeneity of the plane wave. We also obtain the inhomogeneity-independent formula for 1/QSV, and  exactly the same phase velocity and dissipation factor dispersion results for the example material.

Acknowledgements

We are grateful to the College of Petroleum Engineering & Geosciences, King Fahd University of Petroleum and Minerals, Kingdom of Saudi Arabia for supporting this research.

References

Buchen, P.W., 1971. Plane waves in linear viscoelastic media, Geophysical Journal of the Royal Astronomical Society, 23, 531-542.

Carcione, J. M., 2001. Wave fields in real media:Wave propagation in anisotropic, anelastic and porous media: Pergamon Press, Inc.

Cerveny, V. & Psencik, I., 2005. Plane waves in viscoelastic anisotropic media—I. Theory, Geophysical Journal International, 161, 197–212.

How to cite: Liu, X., Greenhalgh, S., Zhou, B., and Li, H.: Inhomogeneous waves in isotropic anelastic media: explicit expressions for Q, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1365, https://doi.org/10.5194/egusphere-egu2020-1365, 2020.

We present a novel numerical method called the Distributional Finite Difference method (DFD) for modeling elastodynamic wave propagation in complex heterogeneous media. Efficient wave propagation modeling is crucial for solving the inverse problem where thousands of simulations are needed to infer the Earth's internal structure. The proposed method elegantly combines advantages of the finite difference method (FD) and of the spectral element method (SEM). In the past decades, the Spectral Element Method has become a popular alternative to the Finite Difference method for modeling wave propagation through the Earth. Though this can be debated, SEM is often considered to be more accurate and flexible than FD. This is because SEM has exponential convergence, it allows to accurately model material discontinuities, and complex structures can be meshed using multiple elements. In the mean time, FD is often thought to be simpler and more computationally efficient, in particular because it relies on structured meshed that are well adapted to computational architectures.  The DFD method divides the computational domain in multiple blocks or elements that can be arbitrary large. Within each block, the computational operations needed to model wave propagation are very similar that of FD which leads to high efficiency. When using smaller elements, the DFD approach allows to mesh certain regions of space having complex geometry, thus ensuring high flexibility. The DFD method permits simple specification of boundary conditions and accurately account for free surfaces and solid/fluid interfaces. Further, depending on the chosen basis functions, the DFD method can achieve "spectral like accuracy",  only a few (say 3) points per wavelength are need to model wave propagation accurately, i.e., with acceptable numerical dispersion. This results in significant reduction in memory usage. We present numerical examples demonstrating the advantages of the DFD method in various situations. We show that the DFD method si well adapted for modeling 3D wave propagation through the Earth. 

How to cite: masson, Y. and Faucher, F.: The distributional finite difference method: an efficient method for modeling seismic wave propagation through 3D heterogeneous geological media., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20071, https://doi.org/10.5194/egusphere-egu2020-20071, 2020.

EGU2020-9650 | Displays | SM7.1

Source-structure trade-offs in noise correlation functions in 3-D

Korbinian Sager, Christian Boehm, and Victor Tsai

Noise correlation functions are shaped by both noise sources and Earth structure. The extraction of information is thus inevitably affected by source-structure trade-offs. Resorting to the principle of Green’s function retrieval deceptively renders the distribution of ambient noise sources unimportant and existing trade-offs are typically ignored. In our approach, we consider correlation functions as self-consistent observables. We account for arbitrary noise source distributions in both space and frequency, and for the complete seismic wave propagation physics in 3-D heterogeneous and attenuating media. We are therefore not only able to minimize the detrimental effect of a wrong (homogeneous) source distribution on 3D Earth structure by including it as an inversion parameter, but also to quantify underlying trade-offs.

The forward problem of modeling correlation functions and the computation of sensitivity kernels for noise sources and Earth structure are implemented based on the spectral-element solver Salvus. We extend the framework with the evaluation of second derivatives in terms of Hessian-vector products. In the context of probabilistic inverse problems, the inverse Hessian matrix in the vicinity of an optimal model with vanishing first derivatives and under the assumption of Gaussian statistics can be interpreted as an approximation of the posterior covariance matrix. The Hessian matrix therefore contains all the information on resolution and trade-offs that we are trying to retrieve. We investigate the geometry of trade-offs and the effect of the measurement type. In addition, since we only invert for sources at the surface of the Earth, we study how potential scatterers at depth are mapped into the inferred source distribution.

A profound understanding of the physics behind correlation functions and the quantification of trade-offs is essential for full waveform ambient noise inversion that aims to exploit waveform details for the benefit of improved resolution compared to traditional ambient noise tomography.

How to cite: Sager, K., Boehm, C., and Tsai, V.: Source-structure trade-offs in noise correlation functions in 3-D, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9650, https://doi.org/10.5194/egusphere-egu2020-9650, 2020.

EGU2020-22320 | Displays | SM7.1

Intrinsic non-uniqueness of the acoustic full waveform inverse problem

Yann Capdeville, Chao Lyu, David Al-Attar, and Liang Zhao

In the context of seismic imaging, the full waveform inversion (FWI) is more and more popular. Because of its lower numerical cost, the acoustic approximation is often used, especially at the exploration geophysics scale, both for tests and for real data. Moreover, some research domains such as helioseismology face true acoustic medium for which FWI can be useful. In this work, we show that the general acoustic inverse problem based on limited frequency band data is intrinsically non-unique, making any general acoustic FWI impossible. Our work is based on two tools: particle relabelling and homogenization. On the one hand, the particle relabelling method shows it is possible to deform a true medium based on a smooth mapping into a new one without changing the signal recorded at seismic stations. This is a potentially strong source of non-uniqueness for an inverse problem based a seismic data. Nevertheless, in the elastic case, the deformed medium loses the elastic tensor minor symmetries and, in the acoustic case, it implies density anisotropy. It is therefore not a source of non-uniqueness for elastic or isotropic acoustic inverse problems, but it is for the anisotropic acoustic case. On the other hand, the homogenization method shows that any fine-scale medium can be up-scaled into an effective medium without changing the waveforms in a limited frequency band. The effective media are in general anisotropic, both in the elastic and acoustic cases, even if the true media are isotropic at a fine scale. It implies that anisotropy is in general present and needs to be inverted. Therefore, acoustic anisotropy can not be avoided in general. We conclude, based on a particle relabelling and homogenization arguments, that the acoustic FWI solution is in general non-unique. We show, in 2-D numerical FWI examples based on the Gauss-Newton iterative scheme, the effects of this non-uniqueness in the local optimization context. We numerically confirm that the acoustic FWI is in general non-unique and that finding a physical solution is not possible.

How to cite: Capdeville, Y., Lyu, C., Al-Attar, D., and Zhao, L.: Intrinsic non-uniqueness of the acoustic full waveform inverse problem, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22320, https://doi.org/10.5194/egusphere-egu2020-22320, 2020.

EGU2020-8163 | Displays | SM7.1 | Highlight

Quantitative CO2 monitoring at the CaMI Field Research Station (CaMI.FRS), Canada, using a hybrid structural-petrophysical joint inversion

Dennis Rippe, Michael Jordan, Marie Macquet, Don Lawton, Anouar Romdhane, and Peder Eliasson

A key requirement by the European CCS directive for the safe operation of geological CO2 storage is the operator's responsibility to demonstrate containment of the injected CO2 and conformance between its actual and modelled behavior. Understanding the subsurface behavior and long-term fate of the injected CO2 requires the quantification of key reservoir parameters (e.g. pore pressure, CO2 saturation and strain in the overburden). Reliable quantification of these parameters and distinction between them pose a challenge for conventional monitoring techniques, which could be overcome by combining advanced multi-disciplinary and multi-method monitoring techniques in a joint inversion.

Within the aCQurate project, we aim to develop a new technology for accurate CO2 monitoring using Quantitative joint inversion for large-scale on-shore and off-shore storage applications. In previous applications of joint inversion to CO2 monitoring, we successfully combined the strengths and advantages of different geophysical monitoring techniques (i.e. seismics with its high spatial resolution and geoelectrics with its high sensitivity to changes in CO2 saturation), using a cross-gradient approach to achieve structural similarity between the different models. While this structural joint inversion provides a robust link between models of different geophysical monitoring techniques, it lacks a quantitative calibration of the model parameters based on valid rock-physics models. This limitation is addressed by extending the previously developed structural joint inversion method into a hybrid structural-petrophysical joint inversion, which allows integration of cross-property relations, e.g. derived from well logs.

The hybrid structural-petrophysical joint inversion integrates relevant geophysical monitoring techniques in a modular way, including seismic, electric and potential field methods (FWI, CSEM, ERT, MMR and gravity). It is implemented using a Bayes formulation, which allows proper weighting of the different models and data sets, as well as the relevant structural and petrophysical joint inversion constraints during the joint inversion.

The hybrid joint inversion is designed for on-shore and off-shore CO2 storage applications and will be demonstrated using synthetic data from the CaMI Field Research Station (CaMI.FRS) in Canada. CaMI.FRS is operated by the Containment and Monitoring Institute (CaMI) of CMC Research Institutes, Inc., and provides an ideal platform for the development and deployment of advanced CO2 monitoring technologies. CO2 injection occurs at 300 m depth into the Basal Belly River sandstone formation, which is monitored using a large variety of geophysical and geochemical monitoring techniques. In preparation for the application to real monitoring data, we present the application of the joint inversion to synthetic full waveform inversion (FWI) and electrical resistivity tomography (ERT) data, derived for a geostatic model with dynamic fluid flow simulations.

In addition to obtaining a better understanding of the subsurface behavior of the injected CO2 at CaMI.FRS, our goal is to mature the joint inversion technology further towards large-scale CO2 storage applications, e.g. on the Norwegian Continental Shelf.

Acknowledgements

Funding is provided by the Norwegian CLIMIT program (project number 616067), Equinor ASA, CMC Research Institutes, Inc., University of Calgary, Lawrence Berkeley National Laboratory (LBNL), Institut national de la recherche scientifique (INRS), Quad Geometrics Norway AS and GFZ German Research Centre For Geosciences (GFZ).

How to cite: Rippe, D., Jordan, M., Macquet, M., Lawton, D., Romdhane, A., and Eliasson, P.: Quantitative CO2 monitoring at the CaMI Field Research Station (CaMI.FRS), Canada, using a hybrid structural-petrophysical joint inversion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8163, https://doi.org/10.5194/egusphere-egu2020-8163, 2020.

EGU2020-11834 | Displays | SM7.1

Muon Tomography applied in the Lousal Mine (Portugal)

Pedro Teixeira, Lorenzo Cazon, Bento Caldeira, Alberto Blanco, José Borges, Sofia Adringa, Pedro Assis, Bernardo Tomé, Ricardo Luz, José Nogueira, Luís Lopes, Mourad Bezzeghoud, Miguel Ferreira, Pedro Nogueira, Catarina Espírito Santo, Daniel Galaviz, Fernando Barão, and Mário Pimenta

Muon Tomography is an imaging technique that uses muons, a natural background radiation, as a means of observing the the earth’s subsurface. Muons are elementary particles like electrons but with a much greater mass that gives them a high penetrative power across matter. With suitable detectors it is possible to create muographs (muon radiographs) to obtain the column density distribution of the surveyed region. This project is a collaboration between University of Évora and the Laboratory of Instrumentation and Experimental Particle Physics (LIP). Both are Portuguese institutions that intend to apply the muon tomography in the geophysics field. The chosen location was the Lousal Mine, an abandoned and well mapped mine in Portugal with all the support infrastructures necessary that make it an ideal location to test the muon telescope developed by us. The detection will take place inside a mine gallery about 18 m below the surface. The telescope will do a geological reconnaissance of the ground above the gallery with the intention of mapping structures and ore masses already known and of improving the existing information with new data. This will serve to test the performance and sensitivity of the muon telescope, made of particle detectors called RPCs. A working prototype was put in place to gather preliminary information and establish the requirements of the equipment. After that, a muon telescope equipped with four RPC detectors, with an area of 1 m2 each, was assembled and has been collecting muons inside the Lousal Mine for the last few months. The tomographic aspect of the work is born from placing the telescope in different locations inside the mine and by orienting it to observe in different directions. Simulations of the muons detection have been made using GEANT4 software. The simulations allow to study the expected result of muographs produced by the muon flux passing through a simulated ground with different characteristics. The aim of this work is to combine the muography information with gravimetry data, from a gravimetric survey that will be carried on site, through a joint inversion of both data sets in order to obtain 3D density profiles of the observed region. Other geophysical methods are being applied above the mine to survey the surface, using photogrammetry, and the ground, using GPR and seismic refraction. These methods give knowledge about the arrangement of the ground, can be compared with previous acquired information and will help to perfect the 3D density profiles.

How to cite: Teixeira, P., Cazon, L., Caldeira, B., Blanco, A., Borges, J., Adringa, S., Assis, P., Tomé, B., Luz, R., Nogueira, J., Lopes, L., Bezzeghoud, M., Ferreira, M., Nogueira, P., Espírito Santo, C., Galaviz, D., Barão, F., and Pimenta, M.: Muon Tomography applied in the Lousal Mine (Portugal), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11834, https://doi.org/10.5194/egusphere-egu2020-11834, 2020.

There are many inversion algorithms that have been developed in the literature to obtain the resistivity distribution of the subsurface. Recovered resistivity values are usually lower/higher than the actual resistivity as a consequence of the inversion algorithms. As a consequence, Identification of geologic units based on resistivity distribution can be done on a relative scale. In general, identification of different geologic units is a post step inversion process based on resistivity distribution in the study region.  I have presented a technique to enhance the resistivity image using cooperative inversion (named as fuzzy cooperative resistivity tomography) where two additional input parameters are added as the number of geologic units in the model (i.e. number of cluster) and the cluster center values of the geologic units (mean resistivity value of each geologic unit). Fuzzy cooperative resistivity tomography fulfills three needs: (1) to obtain a resistivity model which will satisfy the fitting between measured and modeled data, (2) the recovered resistivity model will be guided by additional a priori parametric information, and (3) resistivity distribution and geologic separation will be accomplished simultaneously (i.e. no post inversion step will be needed). Fuzzy cooperative resistivity tomography is based on fuzzy c-means clustering technique which is an unsupervised machine learning algorithm. The highest membership value which is a direct outcome from the FCRT corresponds to a geology separation result. To obtain a geology separation result, I adopted the defuzzification method to assign a single geologic unit for each model cell based on the membership values. Various synthetic and field example data show that FCRT is an effective approach to differentiate between various geologic units. However, I have also noted that this approach is only effective when measured data sets are sensitive to particular geologic units. This is the limitation of the presented FCRT.

How to cite: Singh, A.: Unstructured Grid based Fuzzy Cooperative Resistivity Tomography for Electrical and Electromagnetic data , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-379, https://doi.org/10.5194/egusphere-egu2020-379, 2020.

This work consists on the interpretation of multichannel seismic profiles complemented and supported by gravimetric and magnetic forward modeling, on the region surrounding the underwater volcano Fontanelas (Estremadura Spur, west of Lisbon).

The Fontanelas seamount (FSM) is a volcanic cone about 3000 m high from its top to its submerged base that coincides with a strong magnetic anomaly (~350 nT). From dredged samples it is known that it is consists of altered pillow-lavas of ultrabasic and basic alkaline composition (foidites and alkaline basalts) (Miranda et al., 2010). It has been associated with onshore Upper Cretaceous alkaline magmatic events due to its enrichment in incompatible elements and similar isotopic elementary signatures (Miranda et al., 2009 and 2010). The FSM is located halfway between the onshore Sintra intrusive complex and the Tore seamount, between which a 300 km long tectono-magmatic lineament of intrusive/extrusive alkaline bodies of Upper Cretaceous age has been proposed, based on the existence of several other magnetic anomalies (Neres et al., 2014).

Magnetic and gravimetric modeling allowed to constrain the location, depth, extension and geometry of the magmatic bodies in the seismic reflection profiles that were used to map and dating the magmatic bodies and tectonic events.

The joint modeling of these three geophysical methods (seismic, magnetic and gravimetric) allowed for the production of an integrated tectono-magmatic-sedimentary model of the Estremadura Spur. The existence of a complex volcanic and subvolcanic system in the Estremadura Spur was confirmed, including several intrusive bodies, besides the Fontanelas volcano: sills, secondary volcanic cones, large laccolith-type intrusions in the Upper Jurassic. Some extensional rift faults were used as magma conduits for sills plugs and volcanoes.  Magmatic bodies localized compressive strain during the tectonic inversion of the Lusitanian basin during the Alpine compression.

The age of the magmatic bodies is constrained by seismic stratigraphy as prior to the Campanian (83.9 Ma), which allows to associate them with the onshore Upper Cretaceous alkaline magmatic event (Sintra, Sines, Monchique, Lisbon Volcanic Complex, minor intrusive bodies), also correlative of the alkaline magmatism existing offshore along the Madeira-Tore Rise (Merle et al., 2018).

This work will be the basis of future studies regarding the heat dissipation from the intrusion of the magmatic bodies over time in order to estimate the temperatures that surrounding rocks have reached.

Support by Landmark Graphics Corporation, Oasis Montaj (Geosoft), FCT (project UID/GEO/50019/2019- Instituto Dom Luiz) and DGEG is acknowledged.      

 

Merle, R., et al. (2018). Australian Journal of Earth Sciences, 65(5), 591-605. https://doi.org/10.1080/08120099.2018.1471005

Miranda R., et al. (2009). Cretaceous Research, 30, Elsevier, 575-586. https://doi.org/10.1016/j.cretres.2008.11.002.

Miranda, R., et al. (2010). In X Congresso de Geoquímica dos Países de Língua Portuguesa e XVI Semana de Geoquímica, 28 de Março a 1 de Abril de 2010. http://hdl.handle.net/10400.9/1246

Neres, M., et al. (2014). Geophysical Journal International, 199(1), 78-101. https://doi.org/10.1093/gji/ggu250

Pereira, R., et al. (2016). Journal of the Geological Society, 174(3), 522-540. https://doi.org/10.1144/jgs2016-050

How to cite: Simões, P., Neres, M., and Terrinha, P.: Joint modeling of seismic, magnetic and gravimetric data unravels the extent of the Late Cretaceous Magmatic Province on the Estremadura Spur offshore West Iberia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-571, https://doi.org/10.5194/egusphere-egu2020-571, 2020.

EGU2020-20720 | Displays | SM7.1

Comparison of coupling methods in joint inversion along the Namibian continental margin

Gesa Franz, Max Moorkamp, Marion Jegen, Christian Berndt, and Wolfgang Rabbel

Understanding the driving mechanisms of continental breakup is still a key question in global geodynamics. The Namibian continental margin and Walvis Ridge offer an ideal area for related studies, because it accumulates on- and offshore magmatic features, associated with crustal stretching, a potential plume impact, and continental breakup.

 

While previous studies in the area all agree on the general occurrence of these features, they have shown some contradictory results for their extent and depth. Therefore, we jointly invert different geophysical data sets to gain a deeper, three-dimensional insight into the continent-ocean-transition zone. In this study, we test three different cross-gradient coupling approaches for Magnetotelluric, Gravity and Seismic data sets or models. First, a fixed 3D density model is used as a structural constraint to MT data inversion. It’s impact is limited, due to large model areas with constant density values, and thus zero density gradients. Second, satellite gravity and MT data are jointly inverted. Both data sets reach a satisfactory misfit and the gravity data constraint slightly modifies the interpreted earth model. Third, a fixed 2D velocity model is used as a structural constraint for a 3D MT data inversion. Some assumptions had to be made to account for the dimensionality difference, but a sufficiently good data fit was achieved, and inversion benefits from a gradient structural model for the cross-gradient coupling. Earth model modifications through this velocity model constraint resemble the results from the joint Gravity-MT data inversion. The analysis of the three approaches, yields new insights into the cross-gradient coupling concept for joint inversion.

 

Interpreting these three earth models, we believe, that continental break-up in the South Atlantic is neither driven solely by a large plume, nor by pure tectonic forces. High resistivites, velocities and densities in the lower crust point to an accumulation of plume material. However, the size of these features is not big enough to explain the Gondawana break-up as a result of a mega-plume arrival. Indications for a tectonically driven break-up initiation include evidence for extensive crustal stretching, and often an abrupt change to oceanic regime, with the upwelling asthenosphere in juxtaposition to the stretched continental lithosphere. As our models indicate a broader transitional zone, we exclude a pure tectonically driven continental break-up. Our favoured explanation incorporates aspects of both hypothesis, where an accumulation of so-called secondary plumes initiate rifting and break-up. These would be smaller plumes, rising from mid-mantle depths, which might have a common source in the deep mantle.

How to cite: Franz, G., Moorkamp, M., Jegen, M., Berndt, C., and Rabbel, W.: Comparison of coupling methods in joint inversion along the Namibian continental margin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20720, https://doi.org/10.5194/egusphere-egu2020-20720, 2020.

EGU2020-7145 | Displays | SM7.1

The role of a laterally varying density contrast for gravity inversion of the Moho depth

Peter Haas, Joerg Ebbing, Wolfgang Szwillus, and Philipp Tabelow

We present a new inverse approach to invert satellite gravity gradients for the Moho depth under consideration of a laterally varying density contrast between crust and mantle. The inverse problem is linearized and solved with the classical Gauss-Newton algorithm in a spherical geometry. To ensure stable solutions, the Jacobian is smoothed with second-order Tikhonov regularization. During the inversion, the Moho depth is discretized into tesseroids by reference Moho depth and density contrast, from which the gravitational effect can be calculated. As a computational benefit, the Jacobian is calculated only once and afterwards weighted with the laterally varying density contrast. We look for a Moho depth model that simultaneously explains the gravity gradient field and a least misfit to existing seismic Moho depth determinations. We perform the inversion both on regional and global scale.

The laterally varying density contrast is based on different tectonic units, which are defined by independent global geological and geophysical data, such as regionalization of dispersion curves. This is beneficial in remote areas, where seismic investigations are very sparse and the crustal structure is to a large extent unknown. Applying the inversion to the Amazonian Craton and its surroundings shows a lower density contrast at the Moho depth for the continental interior compared to oceanic domains. This is in accordance with the tectono-thermal architecture of the lithosphere. The inverted values of the density vary between 300-450 kg/m3. The inverted Moho depth shows a clear separation between the Sao Francisco Craton and shallower Amazonian Craton.

Gravity inversion with a laterally varying density contrast requires a uniform reference Moho depth. On a global scale, we utilize our inversion to estimate a reference Moho depth that is in accordance with crustal buoyancy. The inverted density contrasts show a similar trend like the regional study area. The inverted Moho depth shows expected tectonic features. Our method of computing the Jacobian once and weighting with lateral variable density contrasts is a valuable optimization of standard gravity inversion.

How to cite: Haas, P., Ebbing, J., Szwillus, W., and Tabelow, P.: The role of a laterally varying density contrast for gravity inversion of the Moho depth, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7145, https://doi.org/10.5194/egusphere-egu2020-7145, 2020.

EGU2020-12461 | Displays | SM7.1

Cooperative inversion of gravity and seismic data with different spatial coverage

mahtab Rashidifard, Jérémie Giraud, Vitaliy Ogarko, Mark Lindsay, and Mark Jessell

Combining two or more geophysical datasets with different resolutions and characteristics is now a common practice to recover one or more physical properties. Building 3D geological models for mineral exploration targeting is often an expensive task even for inversion of a single dataset, because of extremely complicated structures with small scale targets. In this context, seismic methods, among all other traditional techniques in mineral exploration, are receiving increasing attention due to their higher resolution in depth. With more limited spatial coverage and higher resolution, they are usually used to refine the interpretation of potential field data.

As each seismic survey is designed for a particular intention with specific targets and may not be available in all regions of interests, we develop an iterative cooperative inversion algorithm for inverting gravity and seismic travel-time data. This enables the utilization of localized high-resolution seismic data in a larger full 3D volume which is covered by gravity data. Geological information in the form of probabilistic geological modelling is used to extend information away from the high-resolution data and constrain the inversion result. We use these data as the prior model and to derive constraints incorporated into the objective function of gravity inversion. This allows us to obtain information about the probability of the presence of lithologies associated with the formation of mineral systems. To ensure structural consistency between density and velocity we develop a geologically constrained structure-based coupling technique following the same principle as the cross-gradient technique but with a higher degree of freedom in spatial directions. We apply local structure-based constraints conditioned by a geological probability distribution, which is considering direction and magnitude and provide a higher degree of freedom for model variations. An investigation of the proposed methodology and a proof-of-concept using realistic synthetic data are presented. Our results reveal that the methodology has the potential to constrain the gravity inversion results using the limited seismic data.

How to cite: Rashidifard, M., Giraud, J., Ogarko, V., Lindsay, M., and Jessell, M.: Cooperative inversion of gravity and seismic data with different spatial coverage, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12461, https://doi.org/10.5194/egusphere-egu2020-12461, 2020.

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Decoupling strategy for large-scale multiphysics joint inversion

Dmitry Molodtsov, Duygu Kiyan, and Christopher Bean

We present a generalized 3-D multiphysics joint inversion scheme with a focus on large-scale regional problems. One of the key features of this scheme is the formulation of the structure coupling as a sparsity-promoting joint regularization. This approach makes it possible to simplify the structure of the objective function and to keep the number of hyperparameters relatively low, so that the inversion framework complexity scales well with respect to the number of geophysical methods and possible reference models used. To further simplify adding geophysical solvers to the framework and to optimize the discretization, we propose an alternating minimization scheme that decouples the inversion and the joint regularization steps. Decoupling is achieved by introducing an auxiliary multi-parameter model. This allows the individual subproblems to make use of problem-tailored grids and specialized optimization algorithms. As we will see, this is in particular important for the regularization subproblem. In contrast to straightforward 'cooperative inversion' formulation, decoupled inversion steps appear to be regularized by a standard quadratic model-norm penalty, and as a result existing separate inversion codes can be used with minimal, if any, modifications. The developed scheme is applied to magnetotelluric, seismic and gravity data and tested on synthetic model examples.

How to cite: Molodtsov, D., Kiyan, D., and Bean, C.: Decoupling strategy for large-scale multiphysics joint inversion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11958, https://doi.org/10.5194/egusphere-egu2020-11958, 2020.

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Geophysical basin characterization using seismic noise H/V spectral ratio and gravity data (Vallès Basin NE-Spain).

Anna Gabàs, Albert Macau, Fabián Bellmunt, Beatriz Benjumea, Jose Sedano, and Sara Figueras

Vallès Basin (NE-Spain) is a neogene basin with mainly granitic bedrock and delimited at NW by one normal fault (Vallès fault). This basin presents significant geothermal anomalies reflected with surficial hot thermal waters. Previous studies carried out in the 70s, to define its energy resource potential using one single geophysical technique, were not enough to clearly interpret the subsoil structure and many uncertainties remain still unsolved.

The aim of this work is to combine two different geophysical techniques for collaborative interpretation of the Vallès Basin structure in order to reduce the uncertainties: 2D gravity profiles and seismic noise H/V spectral ratio measurements distributed in the whole basin area. 2D gravity profiles provide subsurface structural information and basement depth from density models obtained after modelling and inversion processes; whereas the seismic noise H/V spectral ratio technique determines the soil fundamental frequency, which helps to locate the boundary between soft sediments and hard rock materials using empirical equations. Therefore, bedrock geometry and infill sediments structure can be estimated, which is crucial to understand ongoing processes related to the surface geothermal evidences.

The work methodology consists of combining both geophysical methods comparing the density models obtained from Bouguer anomaly in the 2D gravity profiles with the soft soil-hard rock contact surface obtained from the seismic noise H/V spectral measurement. The co-validation between them is carried out overlapping these two individual geophysical results and complementing models between them to obtain the best fit. Despite using different geophysical techniques to reduce ambiguities, a final discussion about lithology of sediments, geometry of basement and location of main faults is always needed. In this case, two equally probable models are proposed to interpret the Vallès Basin structure. One of them presents a shallow basin with granitic basement below. The other one presents a deeper basin, the granitic bedrock is located at 2000 m depth, with conglomerate deposits near the main fault. In both cases, the obtained models detect the Vallès fault as a sub-vertical fault which slightly diminishes its slope from 1200-1400 m in depth.

These new results in the Vallès Basin provide valuable information for geothermal purposes, but should be completed with more geophysical data to assure the geological model. As a future work, the gravity data will be extended at the whole basin in order to create a 3D geological model. To accomplish this objective, it will be fundamental to construct a very dense mesh of gravity points (good resolution) which affords a plausible hypothesis about the basin geological structure.

How to cite: Gabàs, A., Macau, A., Bellmunt, F., Benjumea, B., Sedano, J., and Figueras, S.: Geophysical basin characterization using seismic noise H/V spectral ratio and gravity data (Vallès Basin NE-Spain)., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19542, https://doi.org/10.5194/egusphere-egu2020-19542, 2020.

Local and teleseismic body wave inversions are two approaches commonly used to obtain 3D Earth velocity models for shallow and mantle scale, respectively. However, each method used separately is poorly resolved at the mantle/crust boundary while imaging that interface is important to understand the geodynamic processes (e.g. magmatic underplating, mantle delamination, crustal thinning or thickening) occurring at this depth. In order to develop a high-resolved final velocity model, the two approaches were combined. First, an irregular grid was settled, with a higher density of nodes at crustal scale (from 0 to 40 km) and an increasing node step when approaching the limits of the model. Then, an a priori 3D crustal velocity model (from an independent local tomography) was inserted within the 1D IASP91 lithospheric one. Finally, the teleseismic tomographic inversion was carried out at crust-to-upper mantle scale using this new mixed initial model and teleseismic data. We applied the method on a real case that includes both tectonic and magmatic processes, the North Tanzanian Divergence (NTD). Synthetic tests showed that we had no resolution between 0 and 35 km. However, a fine crustal grid with the 3D local model helps to better constrain ray paths, limiting the artefacts and smearing from the mantle to the crust, enhancing details, sharpening the velocity anomalies and modifying the geometry of anomalies at depth (> 150 km). Following these tests, we propose then a final scheme in which we include the a priori crustal 3D velocity model in the finer crustal grid, and we prevent the inversion from modifying it. This insertion of strong crustal constraints in teleseismic inversion provides sharper spatial resolution at both crustal and mantle scales, including areas with poor ray coverage, beneath the NTD region. Our strategy allows to counteract the degradation of the results in areas with low velocity zones (such as rift and hotspot), where the seismic rays go around these anomalies.

How to cite: Clutier, A., Gautier, S., and Tiberi, C.: High-resolution teleseismic body-wave tomography with a 3D initial crustal model for crust-to-upper mantle images in highly heterogeneous media., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3564, https://doi.org/10.5194/egusphere-egu2020-3564, 2020.

EGU2020-18459 | Displays | SM7.1

Whole Earth Full-Waveform Inversion With Wavefield Adapted Meshes

Solvi Thrastarson, Dirk-Philip van Herwaarden, Lion Krischer, Christian Boehm, Martin van Driel, Michael Afanasiev, and Andreas Fichtner

With the steadily increasing availability and density of seismic data, full-waveform inversion (FWI) can reveal the Earth's subsurface with unprecedented resolution. FWI, however, carries a significant computational burden. Even with the ever-increasing power of high-performance computing resources, these massive compute requirements inhibit substantial progress, and require algorithmic and technological innovations for global and continental scale inversions.
In this contribution, we present an approach to FWI where we achieve significant computational savings through wavefield adapted meshing [1] combined with a stochastic optimization scheme [2]. This twofold strategy allows us (a) to solve the wave equation at lower costs, and (b) to reduce the number of required simulations. In laterally smooth media, we can construct meshes which are adapted to the expected complexity of the wavefield. By optimally designing a unique mesh for each source, we can reduce the computational cost of the forward and adjoint simulations by an order of magnitude. The stochastic optimization scheme is based on a dynamic mini-batch L-BFGS approach, which adaptively subsamples the event catalogue and requires significantly fewer wavefield simulations to converge to a model than conventional FWI. An additional benefit of the dynamic mini-batches is that they seamlessly allow for the inclusion of more sources in an inversion without a considerable additional computational cost.
We demonstrate a prototype FWI for this approach towards a global scale inversion with real data.

[1] Thrastarson, S., van Driel, M., Krischer, L., Afanasiev, M., Boehm, C., van Herwaarden, DP., Fichtner, A., 2019. Accelerating numerical wave propagation by wavefield adapted meshes, Part II: Full-waveform inversion. Submitted to Geophysical Journal International
[2] van Herwaarden, DP., Boehm, C., Afanasiev, M., Krischer, L., van Driel, M., Thrastarson, S., Trampert, J., Fichtner, A. 2019. Accelerated full-waveform inversion using dynamic mini-batches. Submitted to Geophysical Journal International

How to cite: Thrastarson, S., van Herwaarden, D.-P., Krischer, L., Boehm, C., van Driel, M., Afanasiev, M., and Fichtner, A.: Whole Earth Full-Waveform Inversion With Wavefield Adapted Meshes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18459, https://doi.org/10.5194/egusphere-egu2020-18459, 2020.

EGU2020-17921 | Displays | SM7.1

Evolutionary full-waveform inversion with dynamic mini-batches

Dirk-Philip van Herwaarden, Christian Boehm, Michael Afanasiev, Solvi Thrastarson, Lion Krischer, Jeannot Trampert, and Andreas Fichtner

We present an evolutionary full-waveform inversion based on dynamic mini-batch optimization, which naturally exploits redundancies in observed data from different sources and allows the model to evolve along with the amount of available information in the data.

Quasi-random subsets (mini-batches) of sources are used to approximate the misfit and the gradient of the complete dataset. The size of the mini-batch is dynamically controlled by the desired quality of the approximation of the full gradient. Within each mini-batch, redundancy is minimized by selecting sources with the largest angular differences between their respective gradients, and spatial coverage is maximized by selecting candidate events with Mitchell’s best-candidate algorithm. Information from sources included in a previous mini-batch is incorporated into each gradient calculation through a quasi-Newton approximation of the Hessian, and a consistent misfit measure is achieved through the inclusion of a control group of sources.

By design, the dynamic mini-batch approach has several main advantages: (1) The use of mini-batches with adaptive sizes minimizes the number of redundant simulations per iteration, thus potentially leading to significant computational savings. (2) Curvature information is accumulated and used during the inversion, using a stochastic quasi-Newton method. (3) Data from new events or different time windows can seamlessly be incorporated during the iterations, thereby enabling an evolutionary mode of full-waveform inversion.

To illustrate our method, we start an inversion for upper mantle structure beneath the African plate. Starting from a smooth 1-D background model for a dataset recorded in the years 1990 to 1995, we then sequentially add more and more recent data into the inversion and show how the model can evolve as a function of data coverage. The mini-batch sampling approach allows us to incorporate data from several hundred earthquakes without increasing the computational burden, thereby going significantly beyond previous regional-scale full-waveform inversions.

How to cite: van Herwaarden, D.-P., Boehm, C., Afanasiev, M., Thrastarson, S., Krischer, L., Trampert, J., and Fichtner, A.: Evolutionary full-waveform inversion with dynamic mini-batches, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17921, https://doi.org/10.5194/egusphere-egu2020-17921, 2020.

EGU2020-12246 | Displays | SM7.1

Earthquake-based Full-Waveform Inversion at the Exploration Scale from Dense Broadband Array Data

Qiancheng Liu, Frederik Simons, Fuchun Gao, Paul Williamson, and Jeroen Tromp

In challenging environments with natural seismicity and where active source acquisition is expensive and dangerous, the question arises whether naturally occurring earthquakes offer useful information for hydrocarbon exploration. Here, we report on an experiment that installed 252/247 receivers to acquire data for two periods of 7 months, in the presence of significant rugged topography (elevations from 500 m to 3500 m). The station density is about 1 per 25 km^2 (compared to, e.g., USArray, where the average station spacing was 70 km). Data were recorded in a frequency band from 0.2 Hz to 50 Hz. Several thousand seismic events originating within the array bounds were identified in these data. A compressional-speed tomographic velocity model was derived using first-arriving phases. Centroid moment tensor (CMT) solutions have been obtained for about 4% of the identified events using Green’s function-based multicomponent waveform inversion, assuming a layered velocity model. We are now working to improve that model by performing elastic full-waveform inversion for three-dimensional compressional and shear-wave speed perturbations, honoring the topography, after a prior full-wavefield-based reassessment of the earthquake source mechanisms. We are also aiming to increase the number of events considered in the inversion while weighting the data based on estimates of data quality. This is assessed with a flexible automated procedure that considers a variety of data attributes over a range of frequencies. We run simulations using the spectral-element package SPECFEM3D on a cluster that employs 4 GPU cards per simulation. We identify the promising areas of good initial fit from the highest-quality seismic traces and gradually bring the predictions in line with the observations via LBFGS model optimization. We review the results of our work so far, discuss how to continue to bring best practices from global seismology down to the regional scale, and consider the implications for using such passive experiments to complement or replace active exploration in such challenging zones.

How to cite: Liu, Q., Simons, F., Gao, F., Williamson, P., and Tromp, J.: Earthquake-based Full-Waveform Inversion at the Exploration Scale from Dense Broadband Array Data , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12246, https://doi.org/10.5194/egusphere-egu2020-12246, 2020.

EGU2020-21047 | Displays | SM7.1

Complex topography and media implementation using hp-adaptive DG-FEM for seismic wave modeling

Yang Xu, Xiaofei Chen, Dechao Han, and Wei Zhang

Numerical simulation of seismic wavefield is helpful to understand the propagation law of seismic wave in complex media. In addition, accurate simulation of seismic wave propagation is of great importance for seismic inversion. The discontinuous Galerkin finite element method(DG-FEM) combines the advantages of finite element method(FEM) and finite volume method(FVM) to effectively simulate the propagation characteristics of seismic waves in complex medium.

In this study, we use the hp-adaptive DG -FEM to perform accurate simulation of seismic wave propagation in complex topography and medium, and compare the results with the analytical solution of the Generalized Reflection/Transmission(GRT) coefficient method. Furthermore, ADE CFS-PML is modified and applied to DG-FEM, which greatly reduces the impact of artificial boundaries.

How to cite: Xu, Y., Chen, X., Han, D., and Zhang, W.: Complex topography and media implementation using hp-adaptive DG-FEM for seismic wave modeling , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21047, https://doi.org/10.5194/egusphere-egu2020-21047, 2020.

In-seam seismic methods have been widely used in underground coal mine exploitation since early 80’s. They are helpful for identification of stress concentration zones or to locate geological disturbances within the coal seam. Usually, such surveys are optimized to perform seismic tomography. Therefore, sources and receivers are located on the opposite sides of the longwall. Results are produced in form of velocity maps of body-waves for rock-coal-rock medium or maps of group velocity and frequency of Airy-phase of dispersive waves trapped inside the coal seam, so-called channel waves. However, with the above geometry, the high-resolution imaging of the rock mass close to the roadway, including excavation-damaged zone (EDZ), is hampered by the available ray coverage.  In order to overcome this limitation, sources and receivers should be mounted in the same roadway. There is also a fundamental problem contributing to the lack of a robust method to image such area, which is the complexity of the seismic wavefield in the vicinity of the EDZ in a coal seam, where both surface tunnel waves and Rayleigh and Love-type channel waves overlap. We address this problem using numerical simulations. We use finite-difference method and viscoelastic model with petrophysical parameters for coal and host rock layers representative for the Upper Silesia mining district. First, we analyze seismic waves propagation within simple rock-coal-rock model, particularly channel waves dispersion properties. Then, we add a roadway with 3-meter thick EDZ to the model. Velocity and density within the EDZ linearly decrease up to 70% close to the free surface of excavation. By analyzing particle motion close to the free-surface, we observe that for very short wavelengths, the main energy is traveling as a fundamental mode of Rayleigh surface tunnel wave (for horizontal components). However, for longer wavelengths, the main energy is focused around frequency of Airy-phase of fundamental mode of Love-type channel wave. Eventually, we insert 10% Gaussian-shape velocity anomaly with 20 m width in the middle of the roadway to the model and investigate changes in frequency and group velocity of Airy-phase of Love-type channel waves for different offsets. We notice that the group velocity and frequency of maximum energy correspond to the velocity anomaly. For longer offsets, these parameters are approaching theoretical values for undisturbed medium. We conclude that because the group velocity of the Airy-phase is close to the coal S-wave velocity, it can be possible to image the velocity of such wave in the vicinity of the roadway, especially when the thickness of the coal seam is known.  

 

This research is supported by Polish National Science Centre grant no UMO-2018/30/Q/ST10/00680.

How to cite: Czarny, R. and Malinowski, M.: Modeling of seismic wave propagation around coal mine roadway with presence of excavation-damaged zone , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4797, https://doi.org/10.5194/egusphere-egu2020-4797, 2020.

EGU2020-3504 | Displays | SM7.1

Waveform Energy Focusing Tomography for Passive Sources

Yueqiao Hu, Junlun Li, and Haijiang Zhang

Full waveform inversion (FWI) is one of the most attractive geophysical inversion methods that reconstruct models with higher quality by exploiting the information of full wave-field. Despite its high resolution and successful practical applications, there still exist several obstacles to the successful application of FWI for passive earthquake sources, such as the high non-linearity for model convergence and demand for accurate source information, such as the moment tensor, the source time function, etc. To alleviate the requirement for a priori source information in waveform inversion, we propose a new method called Waveform Energy Focusing Tomography (WEFT), which backpropagates the observed wavefield from the receivers, not the data residuals like in conventional FWI, and tries to maximize the back-propagated wavefield energy around the source location over a short period around the origin time. Therefore, there is no need to provide the focal mechanism and source time function in advance. To better reconstruct the passive sources, the least-squares moment tensor migration approach is used, and the Hessian matrix is approximated using either analytic expression or raytracing. Since waveform fitting is superseded by simpler energy maximization, the nonlinearity of WEFT is weaker than that of FWI, and even less-accurate initial velocity model can be used. These advantages of WEFT make it more practical  for challenging earthquake data, especially for local small magnitude earthquakes where both velocity model and earthquake source information are unknown.

How to cite: Hu, Y., Li, J., and Zhang, H.: Waveform Energy Focusing Tomography for Passive Sources, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3504, https://doi.org/10.5194/egusphere-egu2020-3504, 2020.

EGU2020-9394 | Displays | SM7.1

HMCtomo: A framework for Hamiltonian Monte Carlo sampling of Bayesian geophysical inverse problems

Lars Gebraad, Andrea Zunino, Andreas Fichtner, and Klaus Mosegaard
We present a framework to solve geophysical inverse problems using the Hamiltonian Monte Carlo (HMC) method, with a focus on Bayesian tomography. Recent work in the geophysical community has shown the potential for gradient-based Monte Carlo sampling for a wide range of inverse problems across several fields.
 
Many high-dimensional (non-linear) problems in geophysics have readily accessible gradient information which is unused in classical probabilistic inversions. Using HMC is a way to help improve traditional Monte Carlo sampling while increasing the scalability of inference problems, allowing access to uncertainty quantification for problems with many free parameters (>10'000). The result of HMC sampling is a collection of models representing the posterior probability density function, from which not only "best" models can be inferred, but also uncertainties and potentially different plausible scenarios, all compatible with the observed data. However, the amount of tuning parameters required by HMC, as well as the complexity of existing statistical modeling software, has limited the geophysical community in widely adopting a specific tool for performing efficient large-scale Bayesian inference.
 
This work attempts to make a step towards filling that gap by providing an HMC sampler tailored for geophysical inverse problems (by e.g. supplying relevant priors and visualizations) combined with a set of different forward models, ranging from elastic and acoustic wave propagation to magnetic anomaly modeling, traveltimes, etc.. The framework is coded in the didactic but performant languages Julia and Python, with the possibility for the user to combine their own forward models, which are linked to the sampler routines by proper interfaces. In this way, we hope to illustrate the usefulness and potential of HMC in Bayesian inference. Tutorials featuring an array of physical experiments are written with the aim of both showcasing Bayesian inference and successful HMC usage. It additionally includes examples on how to speed up HMC e.g. with automated tuning techniques and GPU computations.

How to cite: Gebraad, L., Zunino, A., Fichtner, A., and Mosegaard, K.: HMCtomo: A framework for Hamiltonian Monte Carlo sampling of Bayesian geophysical inverse problems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9394, https://doi.org/10.5194/egusphere-egu2020-9394, 2020.

In this study, we develop a new adjoint- and full-waveform inversion approach for low-amplitude seismic phases that are typically below noise in individual recordings. The methodology aims at enhancing weak signals from body wave phases, which can be used in full-waveform inversion for inferring structural and boundary parameters in the earth. The new approach is based on the formulation of misfit functionals and corresponding adjoint sources for stacks of suitably time-shifted recordings. 

To tackle this problem, we compute synthetic waveforms using spectral-elements for models with and without topographic variations along mantle discontinuities. We focus on global underside reflections which are reportedly almost always undetectable in real seismograms due to their low amplitudes and are considerably affected by topography. We enforce phase alignment on a chosen reference seismogram recorded at an average distance among the selected stations. A time shift towards the reference is applied to all seismograms according to their epicentral distance calculated by 1-D ray tracing. A set of time shifts is calculated by cross-correlation in time windows around predicted traveltimes of the desired phase. Using this set of time shifts, we sum the waveforms creating the main stack for each model.

We use the two linear stacks as observed and synthetic (with and without topography, respectively) and develop a least-squares misfit measurement which gives rise to an adjoint source determined by the time shift between stacks. The expectation is that computing the traveltime Fréchet kernel with respect to volumetric and boundary model parameters will show the exact sensitivity of the enhanced signal and save time from computing each station kernel separately. Upon achieving signal enhancement of the desired phases, we can ensure that these can be used for better informing updates of the initial model given the higher quality measurement of the observable.

This method once fully developed will allow us to leverage information of many recordings by reducing incoherent signal and enhancing weak seismic phases. The computation of sensitivity kernels in our study has a twofold importance. Firstly, it helps us realise whether the stacking technique indeed enhances the desired signal and whether it is ideal for precursor waves. Secondly, the exact sensitivity kernels show us the way of incorporating finite-frequency effects of weak but informative phases and introducing non-linear inversion for improving imaging while reducing some computational cost. 

 

How to cite: Koroni, M. and Fichtner, A.: Full-waveform inversion for signal enhancement of weak amplitude phases using beamforming and adjoint methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9096, https://doi.org/10.5194/egusphere-egu2020-9096, 2020.

EGU2020-4739 | Displays | SM7.1

Influence of subduction zone complexities on teleseismic records

Hector Perez Alemany, Anthony Sladen, Vadim Monteiller, and Bertrand Delouis

 

 

How to cite: Perez Alemany, H., Sladen, A., Monteiller, V., and Delouis, B.: Influence of subduction zone complexities on teleseismic records, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4739, https://doi.org/10.5194/egusphere-egu2020-4739, 2020.

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Pyrocko - A Versatile Software Framework for Seismology

Sebastian Heimann, Marius Kriegerowski, Marius Isken, Hannes Vasyura-Bathke, Simone Cesca, Nima Nooshiri, Gesa Petersen, Malte Metz, Andreas Steinberg, Henriette Sudhaus, and Torsten Dahm

Pyrocko is an open source seismology toolbox and library, written in the Python programming language. It can be utilized flexibly for a variety of geophysical tasks, like seismological data processing and analysis, modelling of waveforms, InSAR or GPS displacement data, or for seismic source characterization. At its core, Pyrocko is a  library  and  framework  providing  building  blocks  for researchers  and  students  wishing  to  develop  their  own applications. Pyrocko contains a few standalone applications for everyday seismological practice. These include the Snuffler program, an extensible seismogram browser and workbench, the Cake tool, providing travel-time and ray-path computations for 1D layered earthmodels, Fomosto, a tool to manage pre-calculated Green’s function stores, Jackseis, a command-line tool for common waveform archive data manipulations, Colosseo, a tool to create synthetic earthquake scenarios, serving waveforms and static displacements, and new, Sparrow, a 3D geophysical data visualization tool. This poster gives a glimpse of Pyrocko’s features, for more examples and tutorials visit https://pyrocko.org/.

How to cite: Heimann, S., Kriegerowski, M., Isken, M., Vasyura-Bathke, H., Cesca, S., Nooshiri, N., Petersen, G., Metz, M., Steinberg, A., Sudhaus, H., and Dahm, T.: Pyrocko - A Versatile Software Framework for Seismology, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18734, https://doi.org/10.5194/egusphere-egu2020-18734, 2020.

SM7.3 – Physics-based earthquake modeling and engineering

EGU2020-5587 | Displays | SM7.3 | Highlight

A new instrument in earthquake early warning system by detection and modeling of prompt gravity signals

Kevin Juhel, Jean-Paul Montagner, Jean-Paul Ampuero, Matteo Barsuglia, Pascal Bernard, Giovanni Losurdo, and Martin Vallée

The recent finding of prompt elastogravity signals (PEGS) before the arrival of P-waves, associated with the M9.1 2011 Tohoku earthquake (Montagner et al., Nat. Comm., 2016; Vallée et al., Science, 2017) and a few earthquakes of magnitude larger than 8.5  (Vallée and Juhel, JGR, 2019) opens the new field of speed-of-light seismology.  The systematic detection of PEGS on real-time might help saving a few seconds before the arrival of destructive seismic waves and to obtain an accurate determination of the magnitude of the earthquake at the end of rupture. So the potential application to earthquake early warning is obvious.

However, the use of classical broadband seismometers for detecting PEGS has severe limitations for detecting earthquakes of magnitude smaller than 8.5: first of all, the background seismic noise and second of all, the partial cancellation of the gravitational perturbation by the inertial induced acceleration recorded by seismometers (Heaton, Nature Comm., 2017). Two different approaches can be explored for detecting for earthquakes of magnitude smaller than 8.5. Either, by using a dense array of broadband seismometers  (more than 100 receivers) or by designing completely new instruments such as gravity strainmeters. These new detectors must be able to measure the difference in gravity acceleration between two masses, making this instrument isolated from the seismic noise. A sensitivity of 10-15 Hz-1/2 at 0.1 Hz is required for detecting earthquakes of M>7 (Juhel et al., JGR, 2019) and the technology developed by the gravitational wave physicists can be used for reaching such a sensitivity. The simulation of the expected gravity strain signals based on analytical model of gravity perturbations associated with a network-based matched filter approach show that a network of 3 gravity strainmeters might make it possible to reach such a challenging goal. Gravity strainmeters could therefore open new ways to investigate the first seconds of the earthquake rupture, speed up the estimate of earthquake magnitude, enhance tsunami warning systems and  complement other EEWS in the future.

 

 

How to cite: Juhel, K., Montagner, J.-P., Ampuero, J.-P., Barsuglia, M., Bernard, P., Losurdo, G., and Vallée, M.: A new instrument in earthquake early warning system by detection and modeling of prompt gravity signals, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5587, https://doi.org/10.5194/egusphere-egu2020-5587, 2020.

EGU2020-12092 | Displays | SM7.3 | Highlight

Near-field directionality of earthquake strong ground motions measured by displaced geological objects

Tamarah King, Mark Quigley, and Dan Clark

Coseismically displaced rock fragments (chips) in the near-field (less than 5 km) of the 2016 moment magnitude (MW) 6.1 Petermann earthquake (Australia) preserve directionality of strong ground motions. Displacement data from 1437 chips collected over an area of 100 km2 along and across the Petermann surface rupture is interpreted to record combinations of co-seismic directed permanent ground displacements associated with elastic rebound (fling) and transient  ground shaking, with intensities of motion increasing with proximity to the surface rupture. The observations provide a proxy test for available models for directionality of near-field reverse fault strong ground motions in the absence of instrumental data. This study provides a dense proxy record of strong ground motions at less than 5 km distance from a surface rupturing reverse earthquake, and may help test models of near-field dynamic and static pulse-like strong ground motion for dip-slip earthquakes.

How to cite: King, T., Quigley, M., and Clark, D.: Near-field directionality of earthquake strong ground motions measured by displaced geological objects, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12092, https://doi.org/10.5194/egusphere-egu2020-12092, 2020.

The objective of our study is the improvement of shallow rupture characterization in kinematic rupture models used in strong ground motion simulations. Based on geological investigations, earthquake stress drop, depth-variation of seismicity, as well as recorded near-fault ground motion, there is clear evidence for depth variation of frictional properties of crustal materials. The material ductility in the weak zone (upper 3-5 km of the crust) and the transition from ductile state to brittle state in the upper seismogenic zone, determine how the fracture energy is consumed by the earthquake rupture, and  how generated seismic energy is distributed in space and time.

 

Using plausible stress models for crustal ruptures, we performed dynamic rupture simulations on vertical strike slip faults that break the free surface. We used a 3D staggered-grid finite-difference method (Pitarka and Dalguer, 2009) and regional 1D velocity model. The stress drop as a function of slip was modeled using a linear slip weakening frictional law that reflects the depth and lateral variations of frictional properties of crustal materials. Through dynamic rupture modeling we were able to extract kinematic rupture characteristics, such as changes in the shape of slip rate functions, rupture velocity, and peak slip  rate across the weak zone, and in the slip asperity areas. These results were then used to refine our existing rupture generating model (Graves and Pitarka, 2016) for crustal  earthquakes. The modifications to the rupture generator code include changes to the shape of slip-rate function at shallow depths, rise time variation with depth and stronger correlation with slip at shallow depths.

 

The effects of the new characterization of shallow rupture kinematics on simulated ground motion was thoroughly investigated in broad-band (0-10Hz) simulations of the M7.1 2019 Ridgecrest California earthquake. The ground motion time histories were computed using the hybrid method of Graves and Pitarka (2010. In our simulations we considered several slip distributions, including two that were obtained by inverting recorded velocity and displacement ground motion, respectively.  Finally, through comparisons with recorded data, we analyzed the sensitivity of computed near-fault broad-band ground motion characteristics, including amplitude of ground motion velocity pulse, peak acceleration, and response spectra, to shallow slip characterization and location of strong motion generation areas for each rupture model. The proposed modifications to kinematic rupture models of crustal earthquakes provide improved simulation of broadband strong ground motion and seismic hazard assessment.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344

How to cite: Pitarka, A. and Graves, R.: Characterization of Shallow Rupture Kinematics in Strong Ground Motion Simulations of Strike-Slip Earthquakes Constrained by Dynamic Rupture Modeling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4180, https://doi.org/10.5194/egusphere-egu2020-4180, 2020.

EGU2020-6782 | Displays | SM7.3

Stress transfer process in doublet events studied by numerical TREMOL simulations: Study case Ometepec 1982 Doublet.

Josep de la Puente, Marisol Monterrubio-Velasco, Quetzalcoátl Rodríguez-Pérez, Francisco Ramón Zúñiga, and Otilio Rojas

How to cite: de la Puente, J., Monterrubio-Velasco, M., Rodríguez-Pérez, Q., Zúñiga, F. R., and Rojas, O.: Stress transfer process in doublet events studied by numerical TREMOL simulations: Study case Ometepec 1982 Doublet., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6782, https://doi.org/10.5194/egusphere-egu2020-6782, 2020.

EGU2020-8976 | Displays | SM7.3

A small-scale numerical study of fault slip mechanisms using DEM

Nathalie Casas, Guilhem Mollon, and Ali Daouadji

How do earthquakes start? What are the parameters influencing fault evolutions? What are the local parameters controlling the seismic or aseismic character of slip?

To predict the dynamic behaviour of faults, it is important to understand slip mechanisms and their source. Lab or in-situ experiments can be very helpful, but tribological experience has shown that it is complicated to install local sensors inside a mechanical contact, and that they could disturb the behaviour of the sheared medium. Even with technical improvements on lab tools, some interesting data regarding gouge kinematics and rheology remains very difficult or impossible to obtain. Numerical modelling seems to be another way of understanding physics of earthquakes.

Fault zone usually present a granular gouge, coming from the wear material of previous slips. That is why, in this study, we present a numerical model to observe the evolution and behaviours of fault gouges. We chose to focus on physics of contacts inside a granular gouge at a millimetre-scale, studying contact interactions and friction coefficient between the different bodies. In order to get access to this kind of information, we implement a 2D granular fault gouge with Discrete Element Modelling in the software MELODY (Mollon, 2016). The gouge model involves two rough surfaces representing the rock walls separated by the granular gouge.

One of the interests of this code is its ability to represent realistic non-circular grain shapes with a Fourier-Voronoï method (Mollon et al., 2012). As most of the simulations reported in the literature use circular (2D)/spherical (3D) grains, we wanted to analyse numerically the contribution of angular grains. We confirm that they lead to higher friction coefficients and different global behaviours (Mair et al., 2002), (Guo et al., 2004).

In a first model, we investigate dry contacts to spotlight the influence of inter-particular cohesion and small particles on slip behaviour and static friction. A second model is carried out to observe aseismic and seismic slips occurring within the gouge. As stability depends on the interplay between the peak of static friction and the stiffness of the surrounding medium, the model includes the stiffness of the loading apparatus on the rock walls.

The work presented here focuses on millimetre-scale phenomena, but the employed model cannot be extended to the scale of the entire fault network, for computational cost reasons. It is expected, however, that it will lead to a better understanding of local behaviours that may be injected as simplified interface laws in larger-scale simulations.

How to cite: Casas, N., Mollon, G., and Daouadji, A.: A small-scale numerical study of fault slip mechanisms using DEM, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8976, https://doi.org/10.5194/egusphere-egu2020-8976, 2020.

EGU2020-10243 | Displays | SM7.3 | Highlight

Signature of coseismic off-fault damage in intermediate- and far-field radiation

Kurama Okubo, Harsha S. Bhat, Esteban Rougier, and Marine A. Denolle

Off-fault damage is observed around fault cores in a wide range of length scales, which is identified as an aggregation of localized fractures via geological and geodetic observations, or as low-velocity zone via seismological tomography. However, its seismological observables in earthquake traces, e.g. change in source spectra and/or radiation pattern, remains to be investigated. 

Okubo et al. (2019) proposed an approach framework of physics-based dynamic earthquake rupture modeling with coseismic off-fault damage using the combined finite-discrete element method (FDEM). It shows a non-negligible contribution of coseismic damage to rupture dynamics, high-frequency radiation and overall energy budget, whereas the model domain is limited in the near-field region. This study efficiently computes intermediate- and far-field radiation propagating from earthquake sources with coseismic off-fault damage, and to identify its signature in the seismic traces.

We first conduct the dynamic earthquake rupture with coseismic damage and compute synthetic near-field radiation using FDEM-based software tool, HOSSedu, developed by Los Alamos National Laboratory. We then couple the output of HOSSedu to SPECFEM2D in order to compute intermediate- and far-field radiation. The HOSS-SPECFEM2D coupling can resolve complexities over wide range of length scales associated with earthquake sources with coseismic damage and wave propagation.

We conduct 2D dynamic earthquake rupture modeling with a finite planar fault as canonical simplest model. The comparison between the cases with and without allowing for coseismic off-fault damage shows differences in intermediate- and far-field radiation. 1) High-frequency components in ground motion are enhanced all around the fault. 2) The rupture arresting phase, which clearly appears at the stations located orthogonal to the fault for the case without off-fault damage, is damped due to the smoothed rupture arrest by coseismic damage around fault edges. 3) Radiated energy is enhanced in the direction parallel to the fault due to the substantial damage around fault edges.

These fundamental observables will help identify the existence of coseismic off-fault damage in real earthquakes. It would also contribute to resolve the mechanisms of earthquake sources and the potential distribution of aftershock locations. We also attempt to replace the planar fault to the real fault geometry, e.g. the fault system associated with the 2019 Ridgecrest earthquake sequence, and will investigate the signature of off-fault damage in the seismic traces recorded in intermediate- and far-field range.

How to cite: Okubo, K., S. Bhat, H., Rougier, E., and A. Denolle, M.: Signature of coseismic off-fault damage in intermediate- and far-field radiation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10243, https://doi.org/10.5194/egusphere-egu2020-10243, 2020.

EGU2020-4044 | Displays | SM7.3

Dynamic rupture and seismic radiation in a damage-breakage rheology model

Vladimir Lyakhovsky, Ittai Kurzon, and Yehuda Ben-Zion

We present simulations of dynamic ruptures in a continuum damage-breakage rheological model and waves radiated by the ruptures observed in the far field. The model combines aspects of a continuum viscoelastic damage framework for brittle solids with a continuum breakage mechanics for granular flow. The brittle instability is associated with a phase transition between a damaged solid with distributed cracks and a granular medium within the generated rupture zone. The formulation significantly extends the ability to model brittle processes in structures with complex volumetric geometries and evolving elastic properties, compared to the traditional models of pre-existing frictional surface(s) in a solid with fixed properties. A set of numerical simulations examines the sensitivity of dynamic ruptures, seismic source properties and radiated waves to material properties controlling the coupled damage-breakage evolution, the thickness and geometry of the damage zone, and fluidity of the granular material. The simulations are performed in two stages. First, details of the rupture process are simulated using adaptive fine grid model. The results of these simulations include source parameters such as rupture velocity, potency, stress and strain drop, heat generation, and others. In the second stage, the obtained velocity source function is used for simulating radiated seismic waves and synthetic seismograms sampled by stations around the rupture zone and in the far field.

Detailed comparisons between the simulated source properties and those obtained by analyzing the synthetic seismograms demonstrate the relations between different source processes and inferred seismic parameters (potency, strain drop, directivity, rupture velocity, corner frequency, and others). One main effect shown in these simulations emphasizes the important role of rock damage and granulation process generating dynamic expansion-compaction around the process-zone. This expansion-compaction process leads to isotropic source term, while shear motion that accumulates behind the propagating front produces deviatoric deformation and shear heating behind the rupture front. Changing through our simulations, source geometries, and fault zone properties, we demonstrate that the process-zone dissipation due to the damage-breakage mechanism, and the isotropic source component, significantly affect the radiation pattern, rupture directivity, S/P energy partitioning, seismic potency and moment, and more. The results are significant for understanding better the proper usage and limitations of methods applied within the observational framework of earthquake seismology.

How to cite: Lyakhovsky, V., Kurzon, I., and Ben-Zion, Y.: Dynamic rupture and seismic radiation in a damage-breakage rheology model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4044, https://doi.org/10.5194/egusphere-egu2020-4044, 2020.

EGU2020-1800 | Displays | SM7.3

Strong ground motion simulation of the 2013 MS 7.0 Lushan, China, earthquake

Wenbo Zhang and Xiangwei Yu

The near source strong ground motions of the 2013 MS 7.0 Lushan, China, earthquake were simulated using empirical Green's function (EFG) method. At first, we estimated the amount and location of strong motion generation areas (SMGA) based on the character of both slip distributions from far-field seismic inversion and the envelopes of recorded acceleration from main shock, and determined the amount of subfaults on SMGAs referring to the scaling law introduced by Somerville et al.. Then, we implemented the genetic algorithm searching for the optimized source parameters. Based on the source models, we synthetized the waveforms for the 30 stations selected near the source region. Our results show that the comparison between the synthetic waveforms and the observed records agree very well with each other, especially for the part of high-frequency larger than 1.0 Hz. We found that there are two obvious SMGAs on the fault, which take the position that the asperities from far-field seismic inversion take. The combined strong motion generation areas we obtained were smaller than those values predicted by extension of the scaling law by Somerville et al..

How to cite: Zhang, W. and Yu, X.: Strong ground motion simulation of the 2013 MS 7.0 Lushan, China, earthquake, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1800, https://doi.org/10.5194/egusphere-egu2020-1800, 2020.

Rate- and state-dependent friction laws (RSF laws) are empirical laws derived from laboratory experiments related to rock friction. They have been used to quantitatively describe complex fault friction processes. With a combination of the RSF laws and the McKenzie-Brune frictional heat generation model, we have studied the effects of frictional heating processs on the fault strength variation and temporal evolution of temperature based on the spring-slider-fault system subjected to Ruina and Chester-Higgs RSF laws. The system equations are solved efficiently by Dormand-Prince method with adaptive steps. First, with a comparison to the Ruina- model in which the temperature effect due to frictional heating on frictional strength is neglected, the numerical results show that the fault will be unstable slightly earlier for the Chester-Higgs- model in which the temperature effect due to frictional heating on frictional strength is taken into consideration, which indicates that the rise of temperature caused by frictional heating can lead to a slight time advance of fault instability. Second, by contrast with Ruina- model, the frictional strength will keep a little bit higher for the Chester-Higgs- model when the fault sliding at high speed, indicating that frictional heat can strengthen faults to a certain extent. Third, the simulation results also suggest that, at the same rupture velocity, the temperature change for the Chester-Higgs- model is much smaller than that given by the Ruina- model, indicating that frictional heat can also restrain the sharp rise of temperature on fault surface. In addition, under the same parameters and initial conditions, the seismic occurrence time giving by the Chester-Higgs- model is obviously shorter than that by the Ruina- model, indicating that a significant effect of friction heating generated on entire fault temporal evolution could greatly reduce the seismic recurrence time. Correspondingly, both static stress drop and total slip resulted from the Chester-Higgs- model is also smaller than that from the Ruina- model, respectively.

How to cite: Gao, Y. and Shi, B.: Numerical investigation of frictional heating effect on the earthquake faulting based on the Ruina- and Chester-Higgs- models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3226, https://doi.org/10.5194/egusphere-egu2020-3226, 2020.

EGU2020-3607 | Displays | SM7.3 | Highlight

Characterizing seismic scattering in 3D heterogeneous Earth by a single parameter

Jagdish Chandra Vyas, Martin Galis, and Paul Martin Mai

The rise of observations from Distributed Acoustic Sensing (e.g., Zhan 2020) and high-rate GNSS networks (e.g., Madariaga et al., 2019) highlight the potential of dense ground motion observations in the near-field of large earthquakes. Here, spectral analysis of >100,000 synthetic near-field strong motion waveforms (up to 2 Hz) is presented in terms of directivity, corner frequency, fall-off rate, moment estimates and static displacements.

The waveforms are generated in 3‐D large-scale dynamic rupture simulations which incorporate the interplay of complex fault geometry, topography, 3‐D rheology and viscoelastic attenuation (Wollherr et al., 2019). A preferred scenario accounts for off-fault deformation and reproduces a broad range of observations, including final slip distribution, shallow slip deficits, and spontaneous rupture termination and transfers between fault segments. We examine the effects of variations in modeling parameterization within a suite of scenarios including purely elastic setups and models neglecting viscoelastic attenuation. 

First, near-field corner frequency mapping implementing a novel spectral seismological misfit criterion reveals rays of elevated corner frequencies radiating from each slipping fault at 45 degree to rupture forward direction. The azimuthal spectral variations are specifically dominant in the vertical components indicating we map rays of direct P-waves prevailing (Hanks, 1980). The spatial variation in corner frequencies carries information on co-seismic fault segmentation, slip distribution, focal mechanisms and stress drop. Second, spectral fall-off rates are variably inferred during picking the associated corner frequencies to identify the crossover from near-field to far-field spectral behaviour in dependence on distance and azimuth. Third, we determine static displacements with the help of near-field seismic spectra.

Our findings highlight the future potential of spectral analysis of spatially dense (low frequency) ground motion observations for inferring earthquake kinematics and understanding earthquake physics directly from near-field data; while synthetic studies are crucial to identify "what to look for" in the vast amount of data generated.

References:

Hanks, T.C., 1980. The corner frequency shift, earthquake source models and Q.

Madariaga, R., Ruiz, S., Rivera, E., Leyton, F. and Baez, J.C., 2019. Near-field spectra of large earthquakes. Pure and Applied Geophysics, 176(3), pp.983-1001.

Wollherr, S., Gabriel, A.-A. and Mai, P.M., 2019.  Landers 1992 “reloaded”: Integrative dynamic earthquake rupture modeling. Journal of Geophysical Research: Solid Earth, 124(7), pp.6666-6702.

Zhan, Z., 2020. Distributed Acoustic Sensing Turns Fiber‐Optic Cables into Sensitive Seismic Antennas. Seismological Research Letters, 91(1), pp.1-15.

How to cite: Schliwa, N. and Gabriel, A.-A.: Near-field spectral analysis of data-integrative dynamic rupture earthquake simulations of the 1992 Landers earthquake , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22673, https://doi.org/10.5194/egusphere-egu2020-22673, 2020.

EGU2020-12490 | Displays | SM7.3 | Highlight

Spontaneous dynamic rupture modeling of 2017 M 5.4 Pohang, South Korea, earthquake, using the slip-weakening friction law

Seok Goo Song, Chang Soo Cho, and Geoffrey Ely

An M 5.4 earthquake occurred in the southeastern part of the Korean Peninsula in 2017. It is an oblique thrust event that occurred at a relatively shallow depth (~ 5 km) although it did not create coseismic surface rupture. A coseismic slip model was successfully obtained by inverting the ground displacement field extracted by the InSAR data (Song and Lee, 2019). In this study, we performed spontaneous dynamic rupture modeling using the slip weakening friction law. The static stress drop distribution obtained by the coseismic slip model was used as an input stress field. We adopted high performance computing (HPC) using the parallelized dynamic rupture modeling code (SORD, Support Operator Rupture Dynamics). Although our target event is moderate-sized one, we can successfully produce a spontaneous dynamic rupture model using a relatively small initial nucleation patch (radius ~ 1 km) with a relatively small slip weakening distance (~ 5 cm). Our preliminary results show that the rupture creates an asperity near the initial nucleation zone with approximately 4 MPa stress drop, then propagates obliquely upward both in the northeast and southwest directions. Although we assumed a single planar fault plane in our current rupture modeling, it seems worthwhile to dynamically model the rupture process, including complex fault geometry in following studies. Dynamic rupture modeling for a natural earthquake provides an opportunity to understand the dynamic rupture characteristics of the earthquake, including both stress drop and fracture energy.

How to cite: Song, S. G., Cho, C. S., and Ely, G.: Spontaneous dynamic rupture modeling of 2017 M 5.4 Pohang, South Korea, earthquake, using the slip-weakening friction law, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12490, https://doi.org/10.5194/egusphere-egu2020-12490, 2020.

Soil layer shear wave velocity is a key parameter in numerical simulation models of ground motion of various sites. For three-dimensional models, there is a high cost to measure the shear wave velocity. It is a common method to estimate the shear wave velocity by a empirical relationship of depth and velocity depends on several drilling data. This paper studies the depth-shear wave velocity empirical relationships of various soil layers in Yuxi, Qingdao, and Fuzhou. It is found that the correlation degree between depth and shear wave velocity is higher in the soil layers with obvious grain characteristics, such as breccia layer, round gravel layer, gravel layer and fine sand layer, and the error of the empirical relationship is lower. Conversely, the correlation degree is lower and the error of the empirical relationship is high in clay layers. The possible reason for this phenomenon is: the layer description in the drilled histogram cannot represent the clay layers with different properties effectively.

For soil layers with obvious particle characteristics, the shear wave velocity has a significant positive correlation with the particle size. The size of the sediment particles is related to the carrying capacity of the surface water. A larger the water flow and faster flow velocity lead to a larger sediment particles. Therefore, this paper considers that the shear wave velocity of the soil layer in the study area is related to the hydrodynamic deposition environment. Smaller sediments carry longer distances in the water stream, resulting in lower sedimentary layer wave velocity; larger sediment particles carry shorter distances in the water stream, resulting in higher sedimentary layer wave velocity. Further analysis shows that the shear wave velocity of the clay layer has a certain relationship with the particle characteristics of the other soil layers in the same drill. In the environment where the sedimentary soil layer with larger particles is formed, the shear wave velocity of the clay layer is also higher. This article discusses this phenomenon and further analyzes the influence of the porosity ratio of the clay layer on its depth-shear wave velocity empirical relationship in the Yuxi area. It is found that the void ratio of the clay layer has a negative correlation with its shear wave velocity. The depth-shear wave velocity empirical relationship of the clay layer in Yuxi area was modified to improve accuracy.

The study of the relationship between the sedimentary characteristics, particle characteristics of the soil layer and the shear wave velocity, a key factor in the site conditions, is an attempt to improve the accuracy of geophysical model parameters using geological data. In the research of numerical simulation of site ground motion, it is possible to use abundant geological data to supplement models using few geophysical exploration data, or areas where it is difficult to carry out geophysical exploration, and it has certain application value.

How to cite: Li, T., Chen, X., and Li, Z.: Study on the Relationship Between Void Ratio and Empirical Relationship of Depth and S-WAVE Velocity in Clay, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12157, https://doi.org/10.5194/egusphere-egu2020-12157, 2020.

EGU2020-16610 | Displays | SM7.3

Rupture parameters of dynamic source models compatible with NGA-West2 GMPEs

Frantisek Gallovic and Lubica Valentova

Dynamic source inversions of individual earthquakes provide constraints on stress and frictional parameters, which are inherent to the studied event. However, general characteristics of both kinematic and dynamic rupture parameters are not well known, especially in terms of their variability. Here we constrain them by creating and analyzing a synthetic event database of dynamic rupture models that generate waveforms compatible with strong ground motions in a statistical sense.

We employ a framework that is similar to the Bayesian dynamic source inversion by Gallovič et al. (2019). Instead of waveforms of a single event, the data are represented by Ground Motion Prediction Equations (GMPEs), namely NGA-West2  (Boore et al., 2014). The Markov chain Monte Carlo technique produces samples of the dynamic source parameters with heterogeneous distribution on a fault. For all simulations, we assume a vertical 36x20km strike-slip fault, which limits our maximum magnitude to Mw<7. For dynamic rupture calculations, we employ upgraded finite-difference code FD3D_TSN (Premus et al., 2020) with linear slip-weakening friction law. Seismograms are calculated on a regular grid of phantom stations assuming a 1D velocity model using precalculated full wavefield Green's functions. The procedure results in a database with those dynamic rupture models that generate ground motions compatible with the GMPEs (acceleration response spectra in period band 0.5-5s) in terms of both median and variability.

The events exhibit various magnitudes and degrees of complexity (e.g. one or more asperities). We inspect seismologically determinable parameters, such as duration, moment rate spectrum, stress drop, size of the ruptured area, and energy budget, including their variabilities.  Comparison with empirically derived values and scaling relations suggests that the events are compatible with real earthquakes (Brune, 1970, Kanamori and Brodsky, 2004). Moreover, we investigate the stress and frictional parameters in terms of their scaling, power spectral densities, and possible correlations. The inferred statistical properties of the dynamic source parameters can be used for physics-based strong-motion modeling in seismic hazard assessment.

How to cite: Gallovic, F. and Valentova, L.: Rupture parameters of dynamic source models compatible with NGA-West2 GMPEs, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16610, https://doi.org/10.5194/egusphere-egu2020-16610, 2020.

EGU2020-19007 | Displays | SM7.3

Physics-based constraints for probabilistic seismic hazard assessment in Húsavík–Flatey fault zone, Northern Iceland

Bo Li, Alice-Agnes Gabriel, Sara A. Wirp, Thomas Chartier, Thomas Ulrich, and Benedikt Halldórsson

Probabilistic seismic hazard assessment (PSHA) is widely used to generate national seismic hazard maps, design building codes for earthquake resilient structures, determine earthquake insurance rates, and in general for the management of seismic risk. However, standard PSHA is generally based on empirical, time-independent assumptions that are simplified and not based on earthquake physics. Physics-based numerical models such as dynamic rupture simulations account for the non-linear coupling of source, path and site effects, which can be significant in their respective contributions depending on the generally complex geological environment (e.g., Wollherr et al., 2019), and could potentially complement standard PSHA. In this study we demonstrate the benefits of such an approach by modeling various rupture scenarios in the complex Húsavík–Flatey fault zone (HFFZ), Northern Iceland. The HFFZ consists of multiple right-lateral strike slip segments distributed across ~100 km. The moment accumulated on the HFF since the last major earthquake in 1872 can result in an earthquake of magnitude 6.8 to 7 (Metzger and Jonsson, 2014) posing a high risk to Húsavík’s community, flourishing tourism and heavy industry.

We perform high-resolution 3D dynamic rupture simulations using the open-source software SeisSol (www.seissol.org), which can efficiently model spontaneous earthquake rupture across complex fault networks and seismic wave propagation with high order accuracy in space and time. Our models incorporate regional topography, bathymetry, 3D subsurface structure and varying models of the complex fault network while accounting for off-fault damage.

Synthetic ground motions suggest highly heterogenous radiation patterns and intense localization of shaking in the vicinity of geometric complexities, such as fault bends or rupture transition between segments. In our models, the hypocenter location does not affect the plausible moment magnitude of large events. However, changes in rupture directivity affect the spatial distribution of ground motion significantly.  We run hundreds of dynamic rupture scenarios to generate a physics-based dynamic earthquake catalog of mechanically plausible events. Based on this, we identify a possible maximum magnitude earthquake and generate model-based ground motion prediction equations to complement standard empirical ground motion models. In addition, we use the open-source python code SHERIFs (Chartier et al., 2019) to estimate the likelihood of each rupture event, which is mainly constrained by the fault slip rate estimated and fault-to-fault (f2f) rupture scenarios that are determined by the dynamic simulations. Finally, combining the fault seismic rates and the f2f probabilities with dynamic rupture scenarios and the OpenQuake framework allows us to perform physics-based PSHA for the HFFZ, the largest strike-slip fault in Iceland.

How to cite: Li, B., Gabriel, A.-A., Wirp, S. A., Chartier, T., Ulrich, T., and Halldórsson, B.: Physics-based constraints for probabilistic seismic hazard assessment in Húsavík–Flatey fault zone, Northern Iceland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19007, https://doi.org/10.5194/egusphere-egu2020-19007, 2020.

EGU2020-20600 | Displays | SM7.3

Modeling earthquake rupture dynamics across diffuse deforming fault zones

Jorge Nicolas Hayek Valencia, Duo Li, Dave A. May, and Alice-Agnes Gabriel

Earthquakes are a multi-scale, multi-physics problem. For the last decades, earthquakes have been modeled as a sudden displacement discontinuity across a simplified (potentially heterogeneous) surface of infinitesimal thickness in the framework of linear elastodynamics. Thus, earthquake models are commonly forced to distinguish artificially between on-fault frictional failure and the off-fault response of rock. 

While complex volumetric failure patterns of fault networks are observed from well-recorded large earthquakes (e.g., the 2016 Mw7.8 Kaikōura event, Klinger et al. 2018) and small earthquakes (e.g., events in the San Jacinto Fault Zone, Cheng et al. 2018) as well as in laboratory experiments (e.g., in high-velocity friction experiments, Passelègue et al., 2016) inelastic deformation within a larger volume around the fault is generally neglected when studying kinematics, dynamics and the energy budget of earthquakes. Fault behaviour is then dominantly controlled by lab-derived friction on a surface. Recent 2D collapsing of material properties, stresses, geometry, and strength conditions from seismo-thermo-mechanical models to elastodynamic frictional interfaces illustrated resulting earthquake complexity and modeling challenges (van Zelst et al., 2019).

To understand the mechanics of slip in extended fault zones the ERC project TEAR (https://www.tear-erc.eu) aims to solve the governing equations of earthquake sources based on the conservation of mass, momentum and energy and rheological models for generalized visco-elasto-plastic materials. We here present (i) 2D numerical experiments of rupture dynamics and displacement decoupling under loading for varying fault zone properties resembling observations from the San Jacinto Fault Zone in a weak discontinuity approach  sing a diffuse fault representation (adapted stress-glut approach, Madariaga et al., 1998) within a PETSc spectral element discretisation of the seismic wave equation; (ii) Verification of modeling rupture dynamics using a novel diffuse interface approach using ExaHyPE (www.exahype.eu, Reinarz et al. 2019) that allows spontaneous, finite crack formation (Tavelli et al., in prep.) and adaptive mesh refinement (AMR) zooming into the process zone at the rupture tip.

By this means, we start exploring scalable software for modelling shear rupture across extended, spontaneously developing fault systems for testing the hypothesis, that earthquake dynamics in fault zones can be jointly captured based on the theory of generalized visco-elasto-plastic materials.

References:

  • Cheng, Y. et al. Diverse volumetric faulting patterns in the San Jacinto fault zone. JGR: Solid Earth, 123.6, 5068-5081 (2018). https://doi.org/10.1029/2017JB015408
  • Klinger, Y. et al. Earthquake damage patterns resolve complex rupture processes. GRL, 45, 10,279– 10,287 (2018). https://doi.org/10.1029/2018GL078842
  • Madariaga, R. et al. Modeling dynamic rupture in a 3D earthquake fault model. BSSA, 88.5 (1998): 1182-1197.
  • Passelègue, F. X. et al. Frictional evolution, acoustic emissions activity, and off‐fault damage in simulated faults sheared at seismic slip rates. JGR: Solid Earth, 121(10), 7490-7513 (2016). doi:10.1002/2016JB012988
  • Reinarz, A. et al. ExaHyPE: An Engine for Parallel Dynamically Adaptive Simulations of Wave Problems. arXiv preprint (2019), arXiv:1905.07987.
  • Tavelli, M. et al. Space-time adaptive ADER discontinuous Galerkin schemes for nonlinear hyperelasticity with material failure, in prep.
  • Van Zelst, I. et al. Modeling Megathrust Earthquakes Across Scales: One-way Coupling From Geodynamics and Seismic Cycles to Dynamic Rupture. JGR: Solid Earth, 124, 1141411446 (2019). https://doi.org/10.1029/2019JB017539

How to cite: Hayek Valencia, J. N., Li, D., May, D. A., and Gabriel, A.-A.: Modeling earthquake rupture dynamics across diffuse deforming fault zones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20600, https://doi.org/10.5194/egusphere-egu2020-20600, 2020.

EGU2020-11757 | Displays | SM7.3

Towards coupling fluid flow and rate-and-state friction in compacting visco-poro-elasto-plastic reservoirs

Mohsen Goudarzi, Ylona van Dinther, Meng Li, René de Borst, Casper Pranger, Taras Gerya, Claudio Petrini, and Femke Vossepoel

Induced seismicity as a result of natural gas production is a major challenge from both an industrial and a societal perspective. The compaction caused by gas production leads to changes of the effective pressure fields in the reservoir and stress redistributions occur particularly in the surrounding faults. In addition, the strong coupling between fluid flow and solid rock deformations and the role of fluid flow regarding the frictional properties of the faults necessitate a coupled and comprehensive modeling framework. A general and fully coupled thermo-hydro-mechanical finite difference formulation is developed herein and the results are verified against numerical benchmarks. A visco-elasto-plastic rheological behavior is assumed for the bulk material and a return-mapping algorithm is implemented for accurate simulation of the stress evolution. The geometrical features of the faults are incorporated into a regularized continuum framework, while the response of the fault zone is governed by a rate-and-state-dependent friction model. Numerical simulations are provided for large-scale problems and their efficiency is assured through the evaluation of the consistently linearized systems of equations along with the use of advanced numerical solvers and parallel computing. Although the proposed framework is a step towards the modeling of earthquake sequences for induced seismicity applications, the features of the numerical model are highlighted for other applications, including seismic events in subduction settings where the role of fluid flow inside the faults is considerable. Another application of the present, fully coupled hydro-thermo-mechanical formulation is the prediction of the fluid pressurization phenomena, where the frictional heating increases the magnitude of the pore fluid pressure inside the faults, and the resultant degradation of dynamic frictional strength is naturally captured. 

How to cite: Goudarzi, M., van Dinther, Y., Li, M., de Borst, R., Pranger, C., Gerya, T., Petrini, C., and Vossepoel, F.: Towards coupling fluid flow and rate-and-state friction in compacting visco-poro-elasto-plastic reservoirs, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11757, https://doi.org/10.5194/egusphere-egu2020-11757, 2020.

In 1999, Chi-Chi earthquake hit Taiwan and caused severe damage to the infrastructures along the Chelungpu fault because of overburden deformation. Previous study excavated several trenches near the Chelungpu fault to study the fault characteristics and the fault deformation zone. The most important trench, Chushan site, records the Chi-Chi earthquake with 1.7m vertical offset and other four large paleoseismic events. This fault trench was now retained in the Chelungpu Fault Preservation Park, Taiwan that greatly contributes to observing the deformation pattern of overburden layer induced by repeated thrust faulting. For the north wall of the Chushan trench, the east-dipping basal thrust with a dip angle of 24° splits into two branches and the sedimentary layer, which consists of silt layer and gravel layer, is deformed into an asymmetric anticline fold. This observation indicates that the overburden layer in natural is the composite strata and the presence of gravel layer in the composite strata could be an indicator for the coseismic deformation.

In this study, three-dimensional DEM simulations are conducted to identify the deformation pattern of composite strata under repeated thrust faulting. The numerical model was constructed based on the Chushan trench. Silt layers are made by balls and the gravel layer is compose of balls and ellipsoid particles. Results show that a fault-propagation fold forms during the initial stage of the deformation, and an asymmetric anticline fold with one limb slightly overturned forms in the Chi-Chi earthquake. The rotation of ellipsoid particles in the numerical model indicates the evolution of folding, which conduces to understand the deformation progress in the full faulting process.

How to cite: Hung, C.-H., Lin, C.-H., and Lin, M.-L.: Discrete Element Modeling on Deformation Pattern of Composite Strata Induced by Repeated Thrust Faulting: Case Study of Chushan Site, Central Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6377, https://doi.org/10.5194/egusphere-egu2020-6377, 2020.

EGU2020-9691 | Displays | SM7.3 | Highlight

Fault opening related to free surface interaction on reverse faults: insights from numerical modeling

Lucile Bruhat, Esteban Rougier, Kurama Okubo, and Harsha S. Bhat

Thrust faults are commonly known to produce significant amounts of slip, damage and ground acceleration, especially close to the free surface. The effect of the free surface on faulting has always been a standing issue in theoretical mechanics. While static solutions exist, they still cannot explain the large amounts of slip, damage and ground acceleration observed on low dipping faults. Dynamics effects raised by the presence of a free surface were first evaluated by Brune [1996] using analog experiments, which hinted at a torque mechanism induced in the hanging wall leading to a natural reduction in elastic compressive normal stress as the rupture approaches the surface. This solution was recently supported by preliminary work from Gabuchian et al. [2017], which, combining numerical and experimental simulations, also showed that the earthquake rupture, propagating up dip, induces rotation of the hanging wall, and might promote fault opening.


In this work, we take advantage of new numerical algorithms for dynamic modeling of earthquake rupture to confirm and document this opening effect. We use enhanced numerical solutions for earthquake rupture, based on the Combined Finite-Discrete Element Methodology (FDEM), which were recently developed by the Los Alamos National Laboratory. Through a systematic analysis of case studies, we investigate the effect of fault geometry, friction parameters and rupture behavior on the deformation pattern. Fault opening is observed in all simulations, growing dramatically as the rupture reaches the surface. Evolution of slip, fault-normal displacement and velocities, and of the predicted surface displacements and velocities are documented for each simulation. These predictions will serve as synthetic data when comparing with recorded surface deformation from real-case earthquakes.

How to cite: Bruhat, L., Rougier, E., Okubo, K., and Bhat, H. S.: Fault opening related to free surface interaction on reverse faults: insights from numerical modeling , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9691, https://doi.org/10.5194/egusphere-egu2020-9691, 2020.

EGU2020-15268 | Displays | SM7.3 | Highlight

Untangling the dynamics of the 2019 Ridgecrest sequence by integrated dynamic rupture and Coulomb stress modeling across an immature 3D conjugate fault network

Alice-Agnes Gabriel, Taufiqurrahman Taufiqurrahman, Sara Carena, Alessandro Verdecchia, Bo Li, Duo Li, Thomas Ulrich, Frantisek Gallovic, and Sara Aniko Wirp

We present combined 3D dynamic rupture scenarios of the 2019 Mw6.4 Searles Valley and Mw7.1 Ridgecrest earthquakes closely constrained by observations, incorporating complex subsurface material properties, high-resolution topography and off-fault plastic deformation empowered by supercomputing. A detailed 3D non-vertical fault model of the active quasi-orthogonal intersecting fault network is built by integrating relocated aftershocks and surface ruptures constrained by space geodesy and field observations. All faults are exposed to a 3D SCEC community stress model as well as long- and short-term static and dynamic stress transfers, which impact rupture dynamics, particularly in the vicinity of complexities in fault geometry.

By assuming apparently weak faults due to the effect of rapid velocity-weakening friction and elevated fluid pressure, we determine initial stresses and fault strength. Multi-fault rupture directivity and velocity of both events are constrained by aftershock calibrated back-projection. In the presented scenario two conjugate faults simultaneously rupture in the Mw6.4 event, while only the SW-segment breaks the surface. The Mw7.1 event experiences the full final state of stress (dynamic plus static effects) of the Searles Valley scenario, leading to complex rupture including re-activation of the conjugate Mw6.4 segment, mixed crack and pulse-like propagation, tunneling beneath the fault intersection and choosing one Southern branch only. Both events exhibit a high dynamic stress drop reflecting the immature fault system. The foreshock induces a considerable Coulomb stress change in the Mw7.1 hypocentral region; however, not enough to trigger rupture across the stress-shadowed main fault. Both scenarios match key observations including magnitude, rupture speed, directivity, off-fault damage, slip distribution from kinematic inversion, teleseismic waveforms, GPS, and InSAR ground deformation; while shedding light on geometric, strength and stress factors governing the complex rupture evolution and interaction of the Ridgecrest sequence.

How to cite: Gabriel, A.-A., Taufiqurrahman, T., Carena, S., Verdecchia, A., Li, B., Li, D., Ulrich, T., Gallovic, F., and Wirp, S. A.: Untangling the dynamics of the 2019 Ridgecrest sequence by integrated dynamic rupture and Coulomb stress modeling across an immature 3D conjugate fault network, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15268, https://doi.org/10.5194/egusphere-egu2020-15268, 2020.

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