Content:

SM – Seismology

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

EGU21-1581 | vPICO presentations | SM1.1 | Highlight

The major (Mw=7.0) earthquake of 30th October 2020 north Samos Island, Greece: Analysis of seismological and geodetic data 

Alexandra Moshou, Antonios Konstantaras, and Panagiotis Argyrakis

On 30th October 2020, at 11.51 (UTC), a very strong earthquake of magnitude Mw = 7.0 struck north of the Greek island of Samos in the Aegean coast of Turkey, south of Izmir. The epicentre was determined 17km north of Samos, in the Gulf of Ephesus and was felt in many parts of Greece and western Turkey. The geographical coordinates as calculated of the manual analysis of the National Observatory of Athens (http://bbnet.gein.noa.gr/Events/2020/10/noa2020vipzs_info.html) was determined as  φ= 37.9001⁰N, λ=26.8167⁰E at a focal depth at 11.8km. The earthquake triggered a tsunami that flooded the coastal district of Seferihisar (Turkey), Cesme, Izmir and the port of Samos (Greece). In the next 8 minutes after the detection of the earthquake, tsunami bulletins were issued to national focal points by the Tsunami Service Providers accredited by UNESCO’s IOC Intergovernmental Coordination Group for the Tsunami Early Warning and Mitigation System in the North-eastern Atlantic, the Mediterranean and connected seas (ICG/NEAMTWS). Greece and Turkey were put on Tsunami Watch (highest level of alert). In Seferishar the tsunami swept away many boats in the marina and the water level reached 1.5 meters causing damage to shops.

Three hours later, 15:14 (UTC) a second strong event (Mw = 5.3) occurred in the same region some kilometres south of the main earthquake (φ=37.8223⁰N,λ=26.8652⁰E, http://bbnet.gein.noa.gr/Events/2020/10/noa2020viwsi_info.html). By the end of the same day that the earthquake took place, there were 65 aftershocks while a total of 576 aftershocks up to 31/12 with magnitude greater than 1.0. For the aftershocks with 3.7<ML<7.0 we applied the moment tensor inversion to determine the focal mechanism, 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 analysed. 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. Finally, an extensive kinematic analysis from data provided by two private sector companies networks was done.

References:

Athanassios Ganas, Penelope Kourkouli, Pierre Briole, Alexandra Moshou, Panagiotis Elias and Isaak Parcharidis. Coseismic Displacements from Moderate-Size Earthquakes Mapped by Sentinel-1 Differential Interferometry: The Case of February 2017 Gulpinar Earthquake Sequence (Biga Peninsula, Turkey), Remote Sensing, 2018, pp. 237 – 248

Athanassios Ganas, Zafeiria Roumelioti, Vassilios Karastathis, Konstantinos Chousianitis, Alexandra Moshou, Evangelos Mouzakiotis. The Lemnos 8 January 2013 (Mw=5.7) earthquake: fault slip, aftershock properties and static stress transfer modeling in the north Aegean Sea J Seismol (2014) 18:433–455 DOI 10.1007/s10950-014-9418-3

Konstantaras A. Deep Learning and Parallel Processing Spatio-Temporal Clustering Unveil New Ionian Distinct Seismic Zone. Informatics, 7(4), 39, 2020

KONSTANTARAS, A. Expert knowledge-based algorithm for the dynamic discrimination of interactive natural clusters. Earth Science Informatics 9, (2016), 95-100

How to cite: Moshou, A., Konstantaras, A., and Argyrakis, P.: The major (Mw=7.0) earthquake of 30th October 2020 north Samos Island, Greece: Analysis of seismological and geodetic data , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1581, https://doi.org/10.5194/egusphere-egu21-1581, 2021.

EGU21-9320 | vPICO presentations | SM1.1

Aftershock signature of the M7.5 Palu 2018 supershear rupture from a rapidly deployed nodal array 

Karen Lythgoe, Muzi Muzli, Win Oo, Hongyu Zeng, Rahmat Triyono, Phyo Maung Maung, Dwikorita Karnawati, and Shengji Wei

Supershear earthquakes have significant implications for seismic hazard, in terms of  ground shaking and aftershock pattern. It has been suggested that supershear ruptures are associated with fewer aftershocks on the supershear rupture segment, however this needs to be tested using high resolution event locations. Current aftershock catalogues for the M7.5 Palu 2018 supershear rupture are not of sufficient resolution to identify any characteristic aftershock pattern. Additionally it is unclear whether the supershear rupture speed occurred from the time of earthquake initiation, or at a later time on a certain segment of the fault.

We deployed a nodal array to record aftershocks following the main event. The array comprised of twenty short-period nodes, which can be deployed rapidly, making them ideal for post-rupture investigations in areas of sparse coverage. We expand the earthquake catalogue by applying template matching to the nodal array data. We then relocate seismicity recorded by the array using a double difference method. We also relocate seismicity that occurred before the array was active, using a relative relocation method. To do this, we calibrate the more distant permanent stations using events well-located by the nodal array. We further derive moment tensors for the largest events by waveform modelling using short-period and broadband records.

Our results show that the aftershocks cluster at the northern and southern extents of rupture. There is a relative dearth of aftershocks in the middle part of the rupture, particularly in the Palu valley, where rupture terminated to the surface. The fault here is a long and straight distinctive geomorphic feature. Many secondary faults were triggered, particularly in the southern Sapu valley fault system. An earthquake swarm was triggered 1 month after the main event on a strike-slip fault 200km away.

How to cite: Lythgoe, K., Muzli, M., Oo, W., Zeng, H., Triyono, R., Maung Maung, P., Karnawati, D., and Wei, S.: Aftershock signature of the M7.5 Palu 2018 supershear rupture from a rapidly deployed nodal array , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9320, https://doi.org/10.5194/egusphere-egu21-9320, 2021.

EGU21-10516 | vPICO presentations | SM1.1

Insights into seismic activity of Central Adriatic offshore (Italy) evidenced by the 2013-2014, Conero seismic sequence

Guido Maria Adinolfi, Elvira Battimelli, Ortensia Amoroso, and Paolo Capuano

The Adriatic region has always attracted the interests of researchers involved in the study of the tectonic processes that controlled the evolution of the Alpine-Mediterranean area. It has been considered as an undeformed area, an aseismic, rigid block located between two active orogenic belts, the Apennines and External Dinarides thrust belts. Nevertheless, new scientific evidences reveal a complex structural framework in which active faults are capable to produce seismic activity not only along the borders of Adriatic Sea, but also in the offshore areas. In fact, the outer thrusts of Apennines and Dinarides orogenic belts propagated from the coasts to the offshore areas originating active, NW-SE trending anticlines and thrust faults that affects the Plio-Quaternary sequences.

Defining the seismotectonics of Adriatic domain and studying the active tectonics of the area with its seismogenic potential represent a challenge because the sea prevents direct observation of main geological and structural lineaments and the deployment of standard seismic networks for a more accurate analysis of seismicity. Despite the existence of new evidences, derived from seismic profiles and borehole data, by hydrocarbon exploration, correct seismic hazard estimates of Adriatic Sea require original and accurate data on the seismic activity that can allow to depict the number, size and geometry of seismogenic sources.

In this work, we focused our attention on the seismic sequence, consisting of about 230 events,  which occurred along the Central Adriatic coast, in the Conero offshore, during the 2013-2104, with a ML 4.9 mainshock located at 20 km far away from city of Ancona, the main city of Marche region. After a careful and innovative selection of the data recorded from the Italian National Seismic Network, operated by the Istituto Nazionale di Geofisica e Vulcanologia, the earthquakes were relocated according to a probabilistic approach. By the inversion of the polarity of the P-wave first arrivals, the focal mechanisms were estimated and finally the local magnitudes were re-calculated. Moreover, in order verify if there has been a migration of seismicity with the activation of different faults during the seismic sequence, the analysis of spatio-temporal evolution of the seismic sequence was performed. Preliminary results show that the seismic sequence was originated mainly at small depths (< 10 km) along NW-SE trending thrust fault structures as evidenced by fault plane solutions, consistent with NE-SW horizontal, maximum compression of the outer front of Apennines thrust belt, still active in the Central Adriatic offshore.

How to cite: Adinolfi, G. M., Battimelli, E., Amoroso, O., and Capuano, P.: Insights into seismic activity of Central Adriatic offshore (Italy) evidenced by the 2013-2014, Conero seismic sequence, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10516, https://doi.org/10.5194/egusphere-egu21-10516, 2021.

EGU21-1759 | vPICO presentations | SM1.1

Rupture process of the 7 May 2020 Mw 5.0 Tehran earthquake and its relation with the Damavand stratovolcano, and Mosha Fault

Pınar Büyükakpınar, Mohammadreza Jamalreyhani, Mehdi Rezapour, Stefanie Donner, Nima Nooshiri, Mirali Hassanzadeh, Pouria Marzban, and Behnam Maleki Asayesh

In May 2020 an earthquake with Mw 5.0 struck at ~40 km east of Tehran metropolis and ~15 km south of the Damavand stratovolcano. It was responsible for 2 casualties and 23 injured. The mainshock was preceded by a foreshock with Ml 2.9 and followed by a significant aftershock sequence, including ten events with Ml 3+. The occurrence of this event raised the question of its relation with volcanic activities and/or concern about the occurrence of larger future earthquakes in the capital of Iran. Tehran megacity is surrounded by several inner-city and adjacent active faults that correspond to high-risk seismic sources in the area. The Mosha fault with ~150 km long is one of the major active faults in central Alborz and east of Tehran. It has hosted several historical earthquakes (i.e. 1665 Mw 6.5 and 1830 Mw 7.1 earthquakes) in the vicinity of the 2020 Mw 5.0 Tehran earthquake’s hypocenter. In this study, we evaluate the seismic sequence of the Tehran earthquake and obtain the full moment tensor inversion of this event and its larger aftershocks, which is a key tool to discriminate between tectonic and volcanic earthquakes. Furthermore, we obtain a robust characterization of the finite fault model of this event applying probabilistic earthquake source inversion framework using near-field strong-motion records and broadband seismograms, with an estimation of the uncertainties of source parameters. Due to the relatively weak magnitude and deeper centroid depth (~12 km), no static surface displacement was observed in the coseismic interferograms, and modeling performed by seismic records. Focal mechanism solution from waveform inversion, with a significant double-couple component, is compatible with the orientation of the sinistral north-dipping Mosha fault at the centroid location. The finite fault model suggests that the mainshock rupture propagated towards the northwest. This directivity enhanced the peak acceleration in the direction of rupture propagation, observed in strong-motion records. The 2020 moderate magnitude earthquake with 2 casualties, highlights the necessity of high-resolution seismic monitoring in the capital of Iran, which is exposed to a risk of destructive earthquakes with more than 10 million population. Our results are important for the hazard and risk assessment, and the forthcoming earthquake early warning system development in Tehran metropolis.

How to cite: Büyükakpınar, P., Jamalreyhani, M., Rezapour, M., Donner, S., Nooshiri, N., Hassanzadeh, M., Marzban, P., and Maleki Asayesh, B.: Rupture process of the 7 May 2020 Mw 5.0 Tehran earthquake and its relation with the Damavand stratovolcano, and Mosha Fault, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1759, https://doi.org/10.5194/egusphere-egu21-1759, 2021.

EGU21-10625 | vPICO presentations | SM1.1

Analysis of background seismicity recorded at Mefite d’Ansato CO2 emission field in the framework of FURTHER project: first results.

Paola Cusano, Pierdomenico Del Gaudio, Danilo Galluzzo, Guido Gaudiosi, Anna Gervasi, Mario La Rocca, Claudio Martino, Girolamo Milano, Lucia Nardone, Simona Petrosino, Vincenzo Torello, Luciano Zuccarello, and Francesca Di Luccio

FURTHER – “The role of FlUids in the pReparaTory pHase of EaRthquakes in Southern Apennines” is an INGV Departement Strategic Project devoted to define the role of fluids in earthquake genesis. One of the target areas of the multidisciplinary study is Mefite d’Ansanto, which is the largest area of non-volcanic low temperature CO2 emission field on the Earth. In particular, Work Package 1.4 is dedicated to the application of analysis methodologies in time and frequency domains, aimed to intercept eventual variations in fluid behavior before or in correspondence of local and regional earthquakes, using recordings from the INGV National Seismic Network (IV) and local networks. For this purpose, temporary acquisition surveys have been locally deployed.

On November 20, 2020, a stand-alone seismic station equipped with a Guralp CMG40T 60s broadband sensor, was installed close to the Mefite emission field. In this study we analyze some characteristics of the local seismicity, e.g., frequency content, energy temporal pattern (RMS) and polarization (Montalbetti et al., 1970), and estimate site effects (Nakamura, 1989; http://www.geopsy.org/). Here we present the first results of the ongoing investigation of the seismic noise wavefield in the Mefite area. The temporal pattern of the retrieved seismological observables is compared with the meteorological parameters, such as temperature and rainfall, to find possible relationships with exogenous factors.

Preliminary analysis of the waveforms acquired by the stations of the (IV) have been also performed. We selected the stations inside a radius of 30 km from Mefite area to eventually retrieve  the fluid dynamics footprint in the recorded wavefield.

The identification of the wavefield and site characteristics will be useful to define the features of the next survey planned in the area.

 

References

Montalbetti, J. R., Kanasevich, E. R. (1970): Enhancement of teleseismic body phase with a polarization filter. Geophys. J. Int. 21 (2), 119–129.

Nakamura, Y. (1989). A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface, Railway Technical Research Institute, Quarterly Reports, 30 (1), 25-33.

How to cite: Cusano, P., Del Gaudio, P., Galluzzo, D., Gaudiosi, G., Gervasi, A., La Rocca, M., Martino, C., Milano, G., Nardone, L., Petrosino, S., Torello, V., Zuccarello, L., and Di Luccio, F.: Analysis of background seismicity recorded at Mefite d’Ansato CO2 emission field in the framework of FURTHER project: first results., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10625, https://doi.org/10.5194/egusphere-egu21-10625, 2021.

EGU21-15620 | vPICO presentations | SM1.1

Tectonic stress patterns along the Vrancea subcrustal zone from the inversion of focal mechanisms data

Andreea Craiu, Marius Craiu, Mariu Mihai, Elena Manea, and Alexandru Marmureanu

The Vrancea zone is an unique area with both crustal and intermediate-depth seismic activity and constitutes one of the most active seismic area in Europe.  An intense and persistent seismicity is generated between 60 and 180 km depth, within a relic slab sinking nearly vertical in the Earth’s mantle due to the increasing of the stress state within this volume. At intermediate-depths, large magnitude events are frequent, i.e. four earthquakes with moment magnitudes (Mw) >7 occurred in the last century. An unique slab geometry, likely preserved until the present, causes stress localization due to the slab bending and subsequent stress release resulting in large mantle earthquakes in the region.

In this study, we evaluate the current stress field along the Vrancea subcrustal region by computing the fault plane solutions of 422 seismic events since January 2005. The continuous development of the National Seismic Network allows us to constrain the fault plane solutions and subsequently to evaluate the current stress field.

The main style of faulting for Vrancea subcrustal events presents a predominant reverse one, with two main earthquakes categories: the first one with the nodal planes oriented NE-SW parallel with the Carpathian Arc and the second one with the nodal planes oriented NW-SE perpendicular on the Carpathian Arc. The main axis of the moment tensor may indicate a predominant compressional stress field (Tpl>450 Ppl<450). Another characteristic of  the Vrancea subcrustal zone is the tendency of the extension axis T to be almost vertical and the compression axis P being almost horizontal.

The results of stress inversion indicate a dominant reverse faulting style, with an average stress regime index of 2.9. Other tectonic regimes were observed in the present dataset as normal and strike-slip but they are retrieved for a restrained number of events.

The stress patterns obtained from formal stress inversion of focal mechanism solutions reveal many features of the current stress field that were not captured by large-scale numerical models.

How to cite: Craiu, A., Craiu, M., Mihai, M., Manea, E., and Marmureanu, A.: Tectonic stress patterns along the Vrancea subcrustal zone from the inversion of focal mechanisms data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15620, https://doi.org/10.5194/egusphere-egu21-15620, 2021.

EGU21-8209 | vPICO presentations | SM1.1

Focal mechanism of intermediate depth earthquakes in the Alboran Sea (Western Mediterranean)

Carolina López-Sánchez, Elisa Buforn, Maurizio Mattesini, Simone Cesca, Juan Vicente Cantavella, Lucia Lozano, and Agustín Udías

One of the characteristics of the seismicity in the Ibero-Maghrebian region is the occurrence of intermediate depth earthquakes (50<h<100 km), their largest concentration located at the western part of the Alboran Sea, with epicenters following an NNE-SSW alignment. In this study, we have relocated over 200 intermediate depth earthquakes (M≥3) occurred in this region in the period 2000-2020, using a non-linear probabilistic approach (NonLinLoc algorithm) together with a recent regional 3D tomography lithospheric velocity model for the Alboran-Betic Rif Zone. Maximum likelihood hypocenters confirm the NNE-SSW distribution in a depth range between 50 and 100 km. We have determined the focal mechanisms of 26 of these earthquakes with magnitudes (mb) greater than 3.9. We first derived focal mechanisms using the P-wave first motion polarity method and then performed a moment tensor inversion, using a probabilistic inversion approach based on the simultaneous fit of waveforms and amplitude spectra of P and S phases. We performed an accurate resolution study, by repeating the inversion using different 1-D velocity models and testing different moment tensor (MT) constraints: a full moment tensor, a deviatoric moment tensor and a pure double couple (DC). Misfit values are similar for different MT constraints. Most solutions have a non-DC component larger than 30%. This may be due to the tectonic complexity of the region and the use on the inversion of 1-D Earth model. The DC components obtained from the inversion show different orientations of the nodal planes. A first group of events to the northern part with epicenters inland on south Spain have horizontal tension axes in NE-SW direction. A second group of earthquakes with epicenters off-shore, but close to the Spanish coast, presents near-vertical pressure axes. The third group, formed by deeper earthquakes, with epicenters on the center of the Alboran sea have dip slip focal mechanisms of either normal or reverse motion with planes either vertical or dipping 45º plane oriented in NNE-SSW direction, approximately the same orientation as the alignment of their epicenters. The distribution of these intermediate depth earthquakes and their focal mechanisms evidence the seismotectonic complexity of the region related with a possible subduction.

How to cite: López-Sánchez, C., Buforn, E., Mattesini, M., Cesca, S., Cantavella, J. V., Lozano, L., and Udías, A.: Focal mechanism of intermediate depth earthquakes in the Alboran Sea (Western Mediterranean), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8209, https://doi.org/10.5194/egusphere-egu21-8209, 2021.

EGU21-3020 | vPICO presentations | SM1.1

Focal parameter analysis of earthquakes of the S-SE of the Iberian Peninsula (1900-1923)

Javier Fernandez Fraile, Elisa Buforn, Maurizio Mattesini, and Juan Vicente Cantavella

The aim of this study is to make a review, actualization and homogenization of the seismic parameters of the Seismic Catalogue of the National Seismic Network of Spain, which belongs to the National Geographic Institute. Our analysis focusses on the region that spans from 36.0 to 39.5° N and from 3.25° W to 1° E, which is a seismically very active region. The studied time period refers to earthquakes occurred between 1900 and 1923, where most information comes from macroseismic data and macroseismic effects.

The study begins by searching and collecting information from seismic bulletins and seismic catalogues, seismograms, seismic surveys, photographs, specific studies, historical newspapers and different digital archives. Then, the achieved information from all the different sources were reviewed and, whenever possible, the seismic parameters such as localization, seismic intensity and magnitude were recalculated.

The objective of this work is, from one hand, to establish the study methodology that allow to develop an overall review of all the earthquakes occurred in Spain from 1900 to date, and on the other hand, to provide good quality seismic data (improving the completeness and homogeneity of this seismic catalogue). Seismic data is important because it is used to make seismic hazard maps, studies of seismic risk, to update the seismic building standards and it is also used to make seismic characterization of the territory.

How to cite: Fernandez Fraile, J., Buforn, E., Mattesini, M., and Cantavella, J. V.: Focal parameter analysis of earthquakes of the S-SE of the Iberian Peninsula (1900-1923), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3020, https://doi.org/10.5194/egusphere-egu21-3020, 2021.

EGU21-3256 | vPICO presentations | SM1.1

Differences in the rupture process for very deep earthquakes at the Peru-Brazil and Peru-Bolivia borders 

Elisa Buforn, Carmen Pro, Hernando Tavera, Agustin Udias, and Maurizio Mattesini

We analyze the differences in the rupture process for twelve very deep earthquakes (h>500 km) at the Peruvian-Brazilian subduction zone. These earthquakes are produced by the contact between the Nazca and the South America Plates. We have estimated the focal mechanism from teleseismic waveforms, using the slip inversion over the rupture plane, testing rupture velocities ranging from 2.5 km/s to 4.5 km/s, and analyzing the slip distribution for each  rupture velocity. The selected 12 earthquakes have occurred in the period 1994- 2016, with magnitudes between 5.9 and 8.2 and focal depth 500- 700 km. They can be separated in two groups attending to their epicentral location. The first group is formed by 9 events located, in the Peruvian-Brazil border, with epicenters following a NNW-SSE alignment, parallel to the trench. Their focal mechanisms present solutions of normal faulting with planes oriented in NS direction, dipping about 45 degrees and with vertical pressure axis. The second group is formed by three earthquakes located to the south of the first group in northern Bolivia. Their mechanisms show dip-slip motion with a near vertical plane, oriented in NW-SE direction and the pressure axis dipping 45º to the NE. The moment rate functions correspond to single ruptures with time durations from 6s to 12s, with the exception of the large 1994 Bolivian earthquake (Mw = 8.2) which presents a complex and longer STF. The different mechanisms for the two groups of earthquakes confirm the different dip of the subducting Nazca plate at the two areas, with the steeper part at the southern one.  

How to cite: Buforn, E., Pro, C., Tavera, H., Udias, A., and Mattesini, M.: Differences in the rupture process for very deep earthquakes at the Peru-Brazil and Peru-Bolivia borders , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3256, https://doi.org/10.5194/egusphere-egu21-3256, 2021.

EGU21-3305 | vPICO presentations | SM1.1

Methods for determining focal mechanisms in laboratory experiments

Natalia Shikhova and Andrey Patonin

In laboratory experiments, acoustic emission (AE) caused by the deformation of geomaterial reflects changes in the strength and stress state of the sample. By analogy with the solution of focal mechanisms of earthquake sources, there are several methods for determining the mechanisms and types of AE sources using the amplitudes and signs of the first arrival of an elastic wave on sensors that register acoustic signals. With 16 receiving acoustic sensors, the number of polarity determinations of the incoming wave usually does not exceed 5-10, while the sign determination on some sensors is often incorrect due to the omission of the first half-period of the weak signal by the automatic registration algorithm. This reduces the reliability of determining the mechanism of the focus in laboratory tests of rocks by wellknown methods based on the distribution of signs of the first arrival of the AE wave. We propose a method for determining the directions of the axes and the values of compression and tension in the AE source. The algorithm uses information about the coordinates of events and receivers, values of amplitudes and signs of the first half-period of P-waves coming to the receivers. In this case, the model of the AE source is assumed as a quadrupole with compression and tension axes. The source-receiver distance, the directional diagram of the receiver, and the emission diagram of the source are taken into account for each of the receivers to calculate the value of displacements in the source. To test the proposed algorithm and compare it with the known methods, there was developed a program for generating an acoustic signal source of a given type with random coordinates and directions of the compression and tension axes. An array of signs and amplitudes of the first arrivals coming to the receivers was calculated from simulated data. The high efficiency of the proposed algorithm was shown. The usage of this method together with the determination of AE event types [Zang et.al., 1998] in real laboratory experiments allows us to characterize the prevailing processes of destruction during separate phases of the experiment on triaxial loading of rocks in more detail. The developed algorithm makes it possible to determine the directions of the axes and the values of compression-tension with a minimum number of signs of the arrivals of P- waves, to estimate the components of the seismic moment tensor and obtain more complete information about the mechanism of the AE source.

The work was supported partly by the mega-grant program of the Russian Federation Ministry of Science and Education under the project no. 14.W03.31.0033 and partly by the state assignment of the Ministry to IPE RAS.

How to cite: Shikhova, N. and Patonin, A.: Methods for determining focal mechanisms in laboratory experiments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3305, https://doi.org/10.5194/egusphere-egu21-3305, 2021.

EGU21-12225 | vPICO presentations | SM1.1

Moment Magnitude Estimates in the Marmara Sea Region (NW Turkey) Using Coda Wave Analysis

Berkan Özkan, Tuna Eken, Peter Gaebler, and Tuncay Taymaz

Reliable magnitude estimates of the earthquakes are of utmost important for seismic hazard studies, particularly, in tectonically active areas such as the Marmara region of NW Turkey. The region is highly populated and contains a major fault associated with destructive earthquakes. In this study we apply a coda wave modelling approach based on acoustic radiative transfer theory to calculate the source displacement spectrum, and thus to obtain moment magnitudes of small earthquakes within the Marmara region. We examine three-component waveform data extracted from local earthquakes with magnitudes 2.5 ≤ ML ≤ 5.7 recorded in a radius of 150 km. For each event in the region, an inversion is performed in several different frequency bands. Our results indicate significant similarity with the local magnitude values reported by the KOERI. Consequently, we focus on establishing a novel relation between Mw and ML in the Marmara region.

How to cite: Özkan, B., Eken, T., Gaebler, P., and Taymaz, T.: Moment Magnitude Estimates in the Marmara Sea Region (NW Turkey) Using Coda Wave Analysis, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12225, https://doi.org/10.5194/egusphere-egu21-12225, 2021.

We present a high-precision, absolute earthquake location procedure (NLL-SSST-coherence) based on waveform similarity between events and using the probabilistic, global-search NonLinLoc (NLL) location algorithm. NLL defines a posterior probability density function (PDF) in 3D space for absolute hypocenter location and invokes the equal differential-time (EDT) likelihood function which is very robust in the presence of outlier data. For NLL-SSST-coherence location we take initial NLL locations and iteratively generate smooth, 3D, source-specific, station travel-time corrections (SSST) for each station and phase type and an updated set of locations. Next, we greatly reduce absolute location, aleatoric error by combining location information across events based on waveform coherency between the events. This absolute coherency relocation is based on the concept that if the waveforms at a station for two or more events are very similar (have high coherency) up to a given frequency, then the distance separating these “multiplet” events is small relative to the seismic wavelength at that frequency. The NLL coherency relocation for a target event is a stack over 3D space of the event’s SSST location PDF and the SSST PDF’s for other similar events, each weighted by the waveform coherency between the target event and the other event. Absolute coherency relocation requires waveforms from only one or a few stations, allowing precise relocation for sparse networks, and for foreshocks and early aftershocks of a mainshock sequence or swarm before temporary stations are installed.

We apply the NLL-SSST-coherence procedure to the Mw5.8 Lone Pine CA, Mw5.7 Magna UT and Mw6.4 Monte Cristo NV earthquake sequences in 2020 and compare with other absolute and relative seismicity catalogs for these events. The NLL-SSST-coherence relocations generally show increased organization, clustering and depth resolution over other absolute location catalogs. The NLL-SSST-coherence relocations reflect well smaller scale patterns and features in relative location catalogs, with evidence of improved depth precision and accuracy over relative location results when there are no stations over or near the seismicity.

For all three western US sequences in 2020 the NLL-SSST-coherence relocations show mainly sparse clusters of seismicity. We interpret these clusters as damage zones around patches of principal mainshock slip containing few events, larger scale damage zone and splay structures around main slip patches, and background seismicity reactivated by stress changes from mainshock rupture. The Monte Cristo Range seismicity (Lomax 2020) shows two, en-echelon primary slip surfaces and surrounding, characteristic shear-crack features such as edge, wall, tip, and linking damage zones, showing that this sequence ruptured a complete shear crack system. See presentation EGU21-13447 for more details.

Lomax (2020) The 2020 Mw6.5 Monte Cristo NV earthquake: relocated seismicity shows rupture of a complete shear-crack system. Preprint: https://eartharxiv.org/repository/view/1904

 

How to cite: Lomax, A., Henry, P., and Viseur, S.: High-precision, absolute earthquake location based on waveform similarity between events and application to imaging foreshocks, fault complexity and damage zones for recent western US earthquakes., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14608, https://doi.org/10.5194/egusphere-egu21-14608, 2021.

EGU21-8035 | vPICO presentations | SM1.1

Analysis of Differences in Seismic Moment Tensors between Global Catalogs

Boris Rösler and Seth Stein

Catalogs of moment tensors form the foundation for a wide variety of studies in seismology. Despite their importance, assessing the uncertainties in the moment tensors and the quantities derived from them is difficult. To gain insight,  we compare 5000 moment tensors in catalogs of the USGS and the Global CMT Project for the period from September 2015 to December 2020. The GCMT Project generally reports larger scalar moments than the USGS, with the difference between the reported moments decreasing with magnitude. The effect of the different definitions of the scalar moment between catalogs, reflecting treatment of the non-double-couple component, is consistent with that expected. However, this effect is small and has a sign opposite to the differences in reported scalar moment. Hence the differences are intrinsic to the moment tensors in the two catalogs. The differences in the deviation from a double-couple source and in source geometry derived from the moment tensors also decrease with magnitude. The deviations from a double-couple source inferred from the two catalogs are moderately correlated, with the correlation stronger for larger deviations. However, we do not observe the expected correlation between the deviation from a double-couple source and the resulting differences in scalar moment due to the different definitions. There is essentially no correlation between the differences in source geometry, scalar moment, or fraction of the non-double-couple component, suggesting that the differences reflect aspects of the inversion rather than the source process. Despite the differences in moment tensors, the reported location and depth of the centroids are consistent between catalogs.

How to cite: Rösler, B. and Stein, S.: Analysis of Differences in Seismic Moment Tensors between Global Catalogs, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8035, https://doi.org/10.5194/egusphere-egu21-8035, 2021.

EGU21-7913 | vPICO presentations | SM1.1

A comprehensive quantification of error location uncertainties for the French earthquake catalog

Andres Felipe Peña Castro, Sophie Lambotte, Marc Grunberg, Pierre Arroucau, Jessi Mayor, Guillaume Daniel, and Jean Letort

Locating earthquakes has been a longterm problem in seismology that depends on multiple parameters like station density and spacing, azimuthal gap, velocity models, and phase pick precision. Here, we analyze the current state of the earthquake French catalog for the time period between 2010 until 2018, which we divide into different regions: the Alps, Massif Central, the West, the Pyrenees, the Grand-East and the North. We perform multiple location synthetic tests using as benchmark the earthquake catalog and the evolution of the French seismic network to quantify the improvements in 1) earthquake location through time and 2) the error locations and their uncertainties. For such endeavors, we use NonLinLoc to perform the synthetic tests varying, as input, the stations, the number of stations and phase picks, 1D velocity models and 3D velocity models, and to understand the changes in 1) earthquake hypocenters, 2) ellipsoidal errors and 3) posterior density functions. Then, we relocate the entire catalog using NonLinLoc including 3D velocity models (where available) and compare the hypocentral location differences when we relocate the catalog with 1D velocity models. Additionally, we estimate a quality factor for each of the located earthquakes and report the changes on the quality factor with the temporal evolution of the national seismic network. The resulting catalog and its associated error location will help future seismic hazard estimations in the Metropolitan French area.

How to cite: Peña Castro, A. F., Lambotte, S., Grunberg, M., Arroucau, P., Mayor, J., Daniel, G., and Letort, J.: A comprehensive quantification of error location uncertainties for the French earthquake catalog, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7913, https://doi.org/10.5194/egusphere-egu21-7913, 2021.

EGU21-5436 | vPICO presentations | SM1.1

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

Felipe Vera, Frederik Tilmann, and Joachim Saul

We present a teleseismic earthquake back-projection method parameterized with multiple arrays and combined P and pP waveforms, improving the spatiotemporal resolvability of rupture complexity. The contribution of each array to the rupture image is weighted depending on the multi-array configuration. Depth phases also contribute effectively to earthquakes at 40 km depth or deeper.

We examine 31 large earthquakes with moment magnitude greater than 7.5 from 2010-2020, which were back-projected in the 0.5-2.0 Hz band, giving access to the high-frequency rupture propagation. An algorithm estimates rupture length, directivity, and speed based on the back-projection results.

Thrust and normal earthquakes showed similar magnitude-dependent lengths and consistent subshear ruptures, while strike-slip earthquakes presented longer ruptures (relative to their magnitude) and frequently reached supershear speeds. The back-projected lengths provided scaling relations to derive high-frequency rupture lengths from moment magnitudes. The results revealed complex rupture behavior, for example, bilateral ruptures (e.g., the 2017 Mw 7.8 Komandorsky Islands earthquake), evidence of dynamic triggering by a P wave (e.g., the 2016 Mw 7.9 Solomon Islands earthquake), and encircling asperity ruptures (e.g., the 2010 Mw 7.8 Mentawai and 2015 Mw 8.4 Illapel earthquakes). The latter is particularly prevalent in subduction megathrust earthquakes, with down-dip, up-dip, double encircling, and segmented patterns. The automated choice of array weighting and the extraction of basic rupture parameters makes the approach well suited for near-real-time earthquake monitoring.

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 2021, online, 19–30 Apr 2021, EGU21-5436, https://doi.org/10.5194/egusphere-egu21-5436, 2021.

EGU21-9664 | vPICO presentations | SM1.1

A grid-based b-value approximation through Southern and Northern Norway: preliminary results 

Rodrigo Estay and Claudia Pavez

The Gutenberg – Richter’s b-value is commonly used to analyze the frequency-magnitude distribution of earthquakes, describing the proportion of small and large seismic events as the first estimation of seismic hazard. Additionally, the b-value has been used as a stress meter, giving some insights into the stress regime in different regions around the world. In this research, a grid-based spatial distribution for the b – value was estimated in three different areas of Norway: northern (74°-81° N/ 12°-26° E), southern (57°-64°N/3°-12° E), and the ridge zones of Mohns and Knipovich. For this, we used a complete catalog from the years 2000 to 2019, which was obtained from the Norwegian National Seismic Network online database. The magnitude of completeness was estimated separately for each zone both in time and space, covering a total area of ~425,000 km2. Our results show a regional variation of the mean b-value for northern (bnorth = 0.79) and southern (bsouth = 1.03) Norway, and the Ridge (bridge = 0.73), which can be interpreted in terms of the predominant stress regime in the different zones. So far, a few calculations regarding the b-value were previously done in Norway to analyze local intraplate sequences. Then, according to our knowledge, this research corresponds to the first estimation of a regional spatial variation of the b – value in the country.

How to cite: Estay, R. and Pavez, C.: A grid-based b-value approximation through Southern and Northern Norway: preliminary results , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9664, https://doi.org/10.5194/egusphere-egu21-9664, 2021.

EGU21-9249 | vPICO presentations | SM1.1

Temporal b-value variation before and after ML ≥ 6.0 Taiwan earthquakes from 2012 to 2019

Po-Yuan Chen, Sean Kuanhsiang Chen, and Yih-Min Wu

Recent studies show that earthquake b values gradually decrease before large earthquakes at the epicenters and then immediately increase after the earthquakes. Temporal b-value variations may result from crustal stress changes associated with a large earthquake. However, the physical process is rarely observed and remains unclear. Taiwan island is a young orogeny leading to frequent earthquakes with magnitudes greater than ML 6.0, which provides an excellent laboratory to examine the physical process. We calculated b-value variation before and after ML ≥ 6.0 Taiwan earthquakes at the epicenters from 2012 to 2019. The time period is based on an enhancement of earthquake detection capability from the Central Weather Bureau Seismic Network in Taiwan, which allows the magnitude of completeness (Mc) down to 1.5 in the inland region. We used a relocated earthquake catalog to precisely estimate b value and Mc by the maximum likelihood method and maximum curvature method, respectively. We designed three steps in our research. First, we calculated the b value and Mc at the epicenters of the ML ≥ 6.0 earthquakes in overall 8 years to know the background seismic activity. Based on this, second, we calculated b values and Mc per half year to test the sensitivity between the radius from epicenters (r) and the number of earthquakes with magnitudes greater than Mc (n). Finally, we will apply moving window approach with specific criteria to continuously calculate temporal b-value variations. Our results showed that spatial b values in Taiwan in overall 8 years have an average of 1.0. The b values are systematically lower in the epicenters of ML ≥ 6.0 earthquakes from 2012 to 2019. We have determined suitable r and n values for each earthquake at the epicenters and some epicenters share similar r and n values. We preliminarily observed temporal b-value decreases before the 2018 Mw 6.4 Hualien earthquake. Considering temporal b-value variation by moving windows, we aim to realize whether temporal b-value variation by a large earthquake can be frequently observed in Taiwan.

How to cite: Chen, P.-Y., Chen, S. K., and Wu, Y.-M.: Temporal b-value variation before and after ML ≥ 6.0 Taiwan earthquakes from 2012 to 2019, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9249, https://doi.org/10.5194/egusphere-egu21-9249, 2021.

EGU21-13895 | vPICO presentations | SM1.1

Correlation between earthquake b value and VP/VS ratio in Japan

Pei-Ying Wu, Sean Kuanhsiang Chen, and Yih-Min Wu

Earthquake b value is primarily controlled by differential stress in the crust. Pore pressure has also been reported influencing b value locally. In nature, the influence can only be observed in the subsurface crust by injection wells. It remains unclear whether the influence of pore pressure on b value can be observed in the scale of the entire crust. To this end, we assume that pore pressure increases proportionally with VP/VS ratio, which is derived from seismic tomography studies, to examine correlation between VP/VS ratio and b value. We investigated this correlation in Japan because it is one of the most earthquake-prone countries with dense seismic networks and high-quality earthquake catalogs. We used an earthquake catalog from the Japan Meteorological Agency from 1998 to 2011 Feb to calculate the b values in the inland region of Japan above the 30 km depth. The selected period is based on a stable completeness of magnitude (Mc) since 1998 and the strong clustering effects by the both 2011 Tohoku and 2016 Kumamoto earthquakes. We then calculated Mc and b value by maximum curvature method and maximum likelihood method, respectively, in the grids of 0.1  0.1  10 km with a radius of 30 km from the center of the grids. The b value determination requires the number of earthquakes with magnitudes greater than the Mc over 150 within the radius. For the VP/VS ratios, we used the latest data derived from the National Research Institute for Earth Science and Disaster Resilience, Japan, to resample them to the same grids as b values. We simply resampled the VP/VS ratios by either averaging them into the grids of b values, or weighting them through a triangular function to the grids center of b values in depths. We analyzed b value as a function of VP/VS ratio and binned the b values within every 0.01 VP/VS interval to calculate the means and medians for liner regressions. Our preliminarily results show that there is little correlation between entire b values and VP/VS ratios among different depth ranges (0-10 km, 10-20 km, 20-30 km). We observed a linear negative relation in the binned data at the 10-20 km depth, however, this relation is not likely observed in the other depths. It may imply that the influence of pore pressure on b value could vary with depths. We’ll calculate the b values using entire magnitude range method and compare the results to the other localized geophysical observations.

How to cite: Wu, P.-Y., Chen, S. K., and Wu, Y.-M.: Correlation between earthquake b value and VP/VS ratio in Japan, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13895, https://doi.org/10.5194/egusphere-egu21-13895, 2021.

EGU21-3511 | vPICO presentations | SM1.1

Direct P-Wave Travel Time Tomography of the Middle East

Susini Desilva, Ebru Bozdag, Guust Nolet, Rengin Gok, Ahmed Ali, and Yahya Tarabulsi

High-resolution seismic images of the crust and mantle beneath regions of complex surface geological structures are necessary to gain insights on the underlying geodynamical processes. One such region embodying various plate boundary motions and intraplate deformations is the Middle East, and consequently the region is prone to significant seismic activity. Hence a tomographic investigation using a more recent and reliable data set is vital in understanding the ongoing complicated deformation process driven by the African, Arabian and Eurasian plates. The purpose of our study is to retrieve a detailed  model of the crust and mantle beneath the Middle Eastern region using teleseismic P arrival times from the ISC-EHB bulletin (Engdahl et al., 1998).

Starting with AK135 as the reference model we invert for tomographic models of compressional wavespeed perturbations down to lower mantle depths in an area bounded by longitudes 22E–66E and latitudes 8N–48N.  The data set used in this study consists of regionally observed P-phase arrival times from over 1000 global events from 1996–2016 culminating in a larger dataset than other similar studies. Selection of a reliable data, ray tracing, preconditioning and inversion steps are carried out using the BD-soft software suite (https://www.geoazur.fr/GLOBALSEIS/Soft.html).

Preliminary inversion results are consistent with the previous regional tomographic studies. In checkerboard tests, cell sizes as low as ∼ 2.8° × 2.8° ( ∼ 240 × 240 km at surface) are generally well recovered down to a 1000 km depth beneath the Anatolian plateau where we currently have the densest coverage. Additionally the Caucasus region and northern parts of the Iranian plateau shows good recovery of ±4% Vp perturbation amplitudes at depths ∼ 70 – 135 km. There is fair recovery for a minimum cell size of ∼ 2.8° × 2.8° beneath the Iranian Plateau, Zagros mountain region, Persian gulf, and northeast Iraq, along with quite good recovery of cell amplitudes towards the Anatolian-Caucasus region at depth ranges 380 – 430 km, 650 – 700 km, and around 950 km. Tomographic inversions unveil a low P velocity zone stretching from the Afar region to Sinai Peninsula consistent with S wave velocity observations of a similar feature by Chang and van der Lee 2011.

We are able to further improve coverage especially down to lithospheric depths within the Arabian peninsula using first arrival times measured from waveform data collected from regional networks. Addition of first arrival time delays from waveforms highlights a prominent low velocity in the tomographic inversions beneath the volcanic fields of western Saudi Arabia. Our ultimate goal is to perform full-waveform inversion of the region constrained by the constructed P-wave model.

How to cite: Desilva, S., Bozdag, E., Nolet, G., Gok, R., Ali, A., and Tarabulsi, Y.: Direct P-Wave Travel Time Tomography of the Middle East, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3511, https://doi.org/10.5194/egusphere-egu21-3511, 2021.

EGU21-507 | vPICO presentations | SM1.1

Quantifying Intrinsic and Extrinsic Contributions to Elastic Anisotropy Observed in Seismic Tomography Models

John Keith Magali, Thomas Bodin, Navid Hedjazian, Yanick Ricard, and Yann Capdeville

Large-scale seismic anisotropy inferred from seismic observations has been loosely interpreted either in terms of intrinsic anisotropy due to Crystallographic Preferred Orientation (CPO) development of mantle minerals or extrinsic anisotropy due to rock-scale Shape Preferred Orientation (SPO). The coexistence of both contributions misconstrues the origins of seismic anisotropy observed in seismic tomography models. It is thus essential to discriminate CPO from SPO in the effective anisotropy of an upscaled/homogenized medium, that is, the best possible elastic model recovered using finite-frequency seismic data assuming perfect data coverage. In this work, we investigate the effects of upscaling an intrinsically-anisotropic and highly-heterogeneous Earth's mantle. The problem is applied to a 2-D marble cake model of the mantle with a binary composition in the presence of CPO obtained from a micro-mechanical model. We compute the long-wavelength effective equivalent of this mantle model using the 3D non-periodic elastic homogenization technique. Our numerical findings predict that overall, upscaling purely intrinsically anisotropic medium amounts to the convection-scale averaging of CPO. As a result, it always underestimates the anisotropy, and may only be overestimated due to the additive extrinsic anisotropy from SPO. Finally, we show analytically (in 1D) and numerically (in 2D) that the full effective radial anisotropy ξ* is approximately just the product of the effective intrinsic radial anisotropy ξ*CPO and the extrinsic radial anisotropy ξSPO:

ξ= ξ*CPO × ξSPO

Based on the above relation, it is imperative to homogenize a texture evolution model first before drawing interpretations from existing anisotropic tomography models. Such a scaling law can therefore be used as a constraint to better estimate the separate contributions of CPO and SPO from the effective anisotropy observed in tomographic models.

How to cite: Magali, J. K., Bodin, T., Hedjazian, N., Ricard, Y., and Capdeville, Y.: Quantifying Intrinsic and Extrinsic Contributions to Elastic Anisotropy Observed in Seismic Tomography Models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-507, https://doi.org/10.5194/egusphere-egu21-507, 2021.

EGU21-9852 | vPICO presentations | SM1.1

A new 3-D mantle density model from recent normal-mode measurements

Rûna van Tent, Arwen Deuss, Andreas Fichtner, Lars Gebraad, Simon Schneider, and Jeannot Trampert

Constraints on the 3-D density structure of Earth’s mantle provide important insights into the nature of seismically observed features, such as the Large Low Shear Velocity Provinces (LLSVPs) in the lower mantle under Africa and the Pacific. The only seismic data directly sensitive to density variations throughout the entire mantle are normal modes: whole Earth oscillations that are induced by large earthquakes (Mw > 7.5). However, their sensitivity to density is weak compared to the sensitivity to velocity and different studies have presented conflicting density models of the lower mantle. For example, Ishii & Tromp (1999) and Trampert et al. (2004) have found that the LLSVPs have a larger density than the surrounding mantle, while Koelemeijer et al. (2017) used additional Stoneley-mode observations, which are particularly sensitive to the core-mantle boundary region, to show that the LLSVPs have a lower density. Recently, Lau et al. (2017) have used tidal tomography to show that Earth's body tides prefer dense LLSVPs.

A large number of new normal-mode splitting function measurements has become available since the last density models of the entire mantle were published. Here, we show the models from our inversion of these recent data and compare our results to previous studies. We find areas of high as well as low density at the base of the LLSVPs and we find that inside the LLSVPs density varies on a smaller scale than velocity, indicating the presence of compositionally distinct material. In fact, we find low correlations between the density and velocity structure throughout the entire mantle, revealing that compositional variations are required at all depths inside the mantle.

How to cite: van Tent, R., Deuss, A., Fichtner, A., Gebraad, L., Schneider, S., and Trampert, J.: A new 3-D mantle density model from recent normal-mode measurements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9852, https://doi.org/10.5194/egusphere-egu21-9852, 2021.

EGU21-2829 | vPICO presentations | SM1.1

Global observations of 3D mantle attenuation using normal modes 

Sujania Talavera-Soza and Arwen Deuss

Seismic tomographic models based solely on wave velocities are unable to distinguish between a temperature or compositional origin for Earth’s 3D structure variations, such as the Large Low Shear Velocity Provinces (LLSVPs) beneath the lower mantle of Africa and the Pacific. Seismic attenuation or damping is able able to provide additional information that may help to unravel the origin of the LLSVPs, which is fundamental to understand mantle convection evolution. For example, a thermal origin for the LLSVPs will point to them being short-lived anomalies, whereas a compositional origin will point to them being long-lived, forming mantle 'anchors' and influencing the pattern of mantle convection for a large part of Earth’s history. Seismic attenuation is able to make that distinction, because it is directly sensitive to temperature variations. So far, global 3D attenuation models have only been available for the upper mantle, with only two regional body waves studies exploring the lower mantle (Lawrence and Wysession, 2006; Hwang and Ritsema, 2011).
Here, we use normal mode data to measure elastic splitting functions (dependent on velocity and density) and anelastic splitting functions (dependent on attenuation). The advantage of normal modes is that they allow us to include focussing and scattering due to the velocity structure without the need for approximations, because we measure the elastic splitting function jointly with the anelastic splitting function. In our measurements for upper mantle sensi- tive modes, we find anti-correlation between the elastic and anelastic splitting functions, suggesting a thermal origin for low velocity spreading ridges, and agreeing with previous studies. On the other hand, for lower mantle sensitive modes, we find correlation, suggesting the averagely attenuating LLSVPs are surrounded by strongly attenuating regions potentially due to the presence of post-perovskite.

How to cite: Talavera-Soza, S. and Deuss, A.: Global observations of 3D mantle attenuation using normal modes , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2829, https://doi.org/10.5194/egusphere-egu21-2829, 2021.

EGU21-4843 | vPICO presentations | SM1.1

Lg wave propagation and attenuation characteristics of the NW Himalaya

Kaushik Kumar Pradhan and Supriyo Mitra

Lg waves are formed by the superposition of shear waves trapped within the crustal waveguide and are the most destructive at regional distances. Excitation of Lg waves, its propagation and lateral variability determine the intensity of ground shaking from regional earthquakes. Spatial decay of spectral amplitude of Lg waves have been used to quantify the attenuation characteristics of the crust. In this study we use regional waveform data from the Jammu and Kashmir Seismological NETwork (JAKSNET) to study Lg wave propagation across the Indian Peninsula, Himalaya, Tibetan Plateau and Hindu Kush regions. We compute Lg/Sn wave ratio to distinguish regions with efficient Lg propagation from those with Lg blockage. These results are categorised using earthquake magnitude and depth to study Lg wave excitation and propagation across these varying geological terrains. We further use the two-station method to study Lg wave quality factor and its frequency dependence for the NW Himalaya. Seismograms recorded at two stations of the network, which are aligned within 15 degrees of the event, are used for analysis. The spectral ratio of Lg wave amplitude recorded at the two stations will be used to estimate the Q (quality factor) as a function of frequency. This will provide Q0 along all inter-station paths, which will then be combined to form Q0 tomography maps for the region. Checkerboard tests will be performed to estimate the resolution of the tomographic maps and accordingly the results will be interpreted.

How to cite: Pradhan, K. K. and Mitra, S.: Lg wave propagation and attenuation characteristics of the NW Himalaya, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4843, https://doi.org/10.5194/egusphere-egu21-4843, 2021.

In recent years, several types of Machine Learning (ML) methods have been employed by Earth scientists to extract patterns and structures from multi-dimensional feature spaces. In this regard, images of the mantle obtained by different seismic tomography (ST) models are diverse datasets with varying structures due to their different theoretical approximations and input data. In this work, we apply an unsupervised ML method, K-means clustering, on ST models to explore their similarities and differences to improve our physical understanding of the Earth’s interior. The K-means clustering method requires ST models to be standardized in a three-dimensional domain. For this purpose, we implement a weighted average technique to resample ST models to radial structural zones with uniform horizontal grid resolutions. However, the homogenized ST models still have 103-104 parameters, which need to be distilled into a small number of summary features. Feature selection is thus a key part of this study: features should be independent from unphysical effects of inversion choices (e.g., the damping factor) and should instead capture the essence of the geological structure. Preliminary results obtained using the center of mass as the attribute to represent the longest wavelength part of the mantle structure show that P-wave and S-wave models do not cluster separately. Therefore, compositional anomalies do not play an essential role at these spatial scales. We plan to expand our analysis by including more summary attributes from both the spatial as well as the frequency domain. 

How to cite: Rahimzadeh Bajgiran, M. and Colli, L.: Applying cluster analysis to seismic tomography models: Uncovering similarities and differences in the spatial and spectral domains, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5940, https://doi.org/10.5194/egusphere-egu21-5940, 2021.

Seismic attenuation provides valuable information about the structure of the crust. For the analysis of seismic attenuation in the central part of the Leipzig-Regensburg fault zone in Germany, where numerous areas of intracontinental earthquake swarms are located, we use 18 of the region's strongest earthquakes from the period 2008 to 2019 with a magnitude between 1.4 and 3.0 in the frequency range between 3 and 34 Hz. Two different methods were used to determine the frequency-dependent scattering and the intrinsic attenuation on one hand and to compare the two methods with respect to their results on the other hand. Both methods, the Multiple Lapse Time Windows Analysis (MLTWA) and the Qopen method use the acoustic radiative transfer theory for forward modelling to generate synthetic data and fit them to the observed data. As a by-product of Qopen, we also obtain the energy site amplifications of the stations used in the inversion, as well as the estimated moment magnitudes of the inverted earthquakes. In addition, factors that influence the inversion were investigated. Different combinations of inversion parameters were tested for the MLTWA, as well as the influence of the window length on the result of Qopen. The results from both methods provide similar results within their error bars, with intrinsic attenuation being stronger than scattering and overall, rather low attenuation values compared to other regions.

How to cite: van Laaten, M., Eulenfeld, T., and Wegler, U.: Seismic attenuation analysis in the central part of the Leipzig-Regensburg fault zone using the Multiple Lapse Time Window Analysis and Qopen, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11829, https://doi.org/10.5194/egusphere-egu21-11829, 2021.

EGU21-8183 | vPICO presentations | SM1.1

Crustal shear wave blockage in and around the Eastern Alps from the 2016 Alland earthquake

Petr Spacek, Pavel Zacherle, Götz Bokelman, Sven Schippkus, Rita Meurers, Jana Pazdírková, and the AlpArray Working Group

Earthquakes in the Eastern Alps are characterized by strongly elongated isoseismals, documenting significantly more efficient propagation of seismic waves towards the foreland (F) than into the orogen (O). In an effort to understand this phenomenon we analysed the local to regional wavefield of a single earthquake with ML4.2 / mb3.6 and epicenter WSW of Vienna (Alland) using instrumental data with unprecedented dense coverage (including AlpArray) and rich macroseismic observations. This earthquake with characteristic asymmetry of isoseismals and with the source located in the basement of the European plate just beneath the frontal thin-skinned thrust of the Penninic units is considered a representative example of the stronger historical and potential future earthquakes from this regionally important seismogenic source area. The analysis of macroseismic intensities and PGA, PGV and spectral content within time windows tied to Sg+Lg wavetrains and other interpreted phases indicates a very good match of smoothed high precision instrumental and high resolution macroseismic wavefields, which allows their joint interpretation. In the F-direction, a very small decrease of intensity and PGA values at an epicentral distance range between 30-50 km and 130-180 km is well approximated by intensity prediction equations derived for central and eastern North America. On the other hand, a sudden drop of respective values is observed at a distance of 20-30 km in the O-direction, correlating with the seismically active fault zone of Mur-Mürz line. The geographic distribution of regional distance-corrected PGA perturbations (dPGA) reveals several well-defined domains with internally limited variance whose boundaries partly correlate with known major geologic structures. Special attention has been paid to description of contrasts between the Foreland domain (Bohemian Massif + autochthonous sediments), the North Alpine domain (between the frontal thrust and Mur-Mürz line + its WSW continuation, i.e. close to southern limits of stable European plate) and the South Alpine domain (south of the former to the southern limits of the region of interest at latitude 46.2°N). The ratio of mean dPGA values observed in these three neighbouring domains is 1.00 : 0.27 : 0.05, respectively. Furthermore, significant contrast between the three domains is observed in terms of spectral content. High frequency signal above 10Hz is characteristic for the Foreland domain and strongly reduced in the South Alpine domain, suggesting that the structures related to the margin of stable European plate act here as an efficient high-cut frequency filter. While map isolines of high frequency spectral amplitude are strongly elongated in F-direction, in agreement with PGA and macroseismic intensity, for frequencies below ~5Hz the isolines of spectral amplitude are quasi-isometric around the epicenter at least within distance of ~120 km. Combination of several mechanisms is considered to explain the wave energy propagation, including intrinsic attenuation at fault zones, blockage at waveguide inhomogeneities and Q(f) contrasts between the crustal domains. Numerous other interesting observations from the whole region including the Carpathian and Pannonian domains, demonstrate the strong potential of densely sampled earthquake wavefields for studies of crustal structure and seismic hazard in the generally low-rate seismicity areas.

How to cite: Spacek, P., Zacherle, P., Bokelman, G., Schippkus, S., Meurers, R., Pazdírková, J., and AlpArray Working Group, T.: Crustal shear wave blockage in and around the Eastern Alps from the 2016 Alland earthquake, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8183, https://doi.org/10.5194/egusphere-egu21-8183, 2021.

EGU21-13043 | vPICO presentations | SM1.1

The Samos Mw6.9 event: Damage investigation in the town of Vathy incorporating a stochastic finite-fault source with site and structural information

Georgia Giannaraki, Zafeiria Roumelioti, and Nikolaos S. Melis

The stochastic method is applied for the finite-fault modelling of strong ground motion from the October 30, 2020, shallow earthquake of Mw=6.9 which occurred offshore the northern part of Samos island in the Aegean Sea, Greece. The earthquake resulted to several human casualties and many injuries, to considerable infrastructure damage in Samos island and Western Turkey, especially in the city of Izmir, and a tsunami affecting both the Greek and Turkish coast. We focus this research on reproducing the ground motion field and damage pattern observed in Vathy, the capital of Samos Island. Different source representations, based on preliminary finite-fault slip distribution models, are tested against their capability to reproduce the two acceleration records available in Vathy. Site effects are incorporated in our modelling in the form of empirical amplification factors assigned according to a Vs30 distribution for the Samos island, which we constructed based on local geology and terrain-based proxies and on the Vs profiles at the sites of the two permanent accelerometric stations. The analysis further focuses on the empirical assessment of structural vulnerability for an estimated exposure model per building block in Vathy, which suffered structural damage due to the mainshock, mainly to a number of old and monumental buildings. The estimated exposure model in Vathy, when combined with the synthetic ground motion derived from the validated stochastic model, provides results in good agreement with available macroseismic intensities and damage reports. Our results contribute to better understanding the observed spatial distribution of damage in Vathy with respect to variations in the quality of buildings, the foundation soil and the frequency content of the excitation motion as radiated from the seismic source. The usefulness of our validated stochastic model is further demonstrated through blind predictions at sites of considerable earthquake effects, at which no record of the Mw=6.9 earthquake is available, such as in the town of Karlovasi in Samos and in the port of Chios Island.

How to cite: Giannaraki, G., Roumelioti, Z., and Melis, N. S.: The Samos Mw6.9 event: Damage investigation in the town of Vathy incorporating a stochastic finite-fault source with site and structural information, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13043, https://doi.org/10.5194/egusphere-egu21-13043, 2021.

EGU21-8564 | vPICO presentations | SM1.1

High-frequency strong ground-motion simulation for the 2016 Mw 7.0 Kumamoto earthquake

Javier Ojeda, Sebastian Arriola, Christian Flores, Cristian Otarola, and Sergio Ruiz

The 2016 Kumamoto earthquake Mw 7.0 occurred in Japan reveal a multisegment shallow fault rupture that was well recorded by the KiK-net stations in accelerographs placed inside boreholes and on the surface. The numerous damaged buildings due to this earthquake reflect the critical implications for seismic hazard estimation and improvement of earthquake-resistant design for a shallower event. Herewe generate synthetic accelerograms at high frequencies implementing a stochastic method that allow us to simulate horizontal and vertical strong ground-motion accelerograms in azimuthal well-distributed stations. We included multisegment finite fault geometries estimated by independent authors as input for source model. From each sub-fault we calculated the incident and azimuthal angles arriving at each seismic station, we determined free surface effect, energy partition, radiation pattern and dynamic frequency corner for sources effect. Besideswe adopted region-specific attenuation parameters such as geometrical spreading and anelastic attenuation for path effect, and site effect parameters such as generic amplifications, soil amplification transfer functions for body waves, and high-frequency attenuation kappa filter. Our simulated acceleration time series show similarities in time and frequency with the observed records in the frequency band between 1 – 10 Hz. We obtained a good agreement between peak ground accelerations for both horizontal and vertical components, and we reproduce the amplitude and attenuation trend for the horizontal component of the GMPE models in the region. Finallywe are capable to simulate the high-frequency band of engineering interest using physics-based parameters to improve our knowledge about the sourcepath, and site effect and their impact on a seismic hazard assessment in earthquake-prone regions.

How to cite: Ojeda, J., Arriola, S., Flores, C., Otarola, C., and Ruiz, S.: High-frequency strong ground-motion simulation for the 2016 Mw 7.0 Kumamoto earthquake, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8564, https://doi.org/10.5194/egusphere-egu21-8564, 2021.

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 earthquakes and blasts of quarries occurred between 2015 and 2020 in the Mecsek Mountains in southern Hungary within 120 km to MORH and KOVH stations.

The objective of this study was to determine the linear discrimination line between the two classes earthquakes and explosions. We investigated the effectiveness of P/S amplitude ratios using filtered waveforms at different ranges of frequencies. We applied waveform cross-correlation to build correlation matrices at both stations 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.

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 compare the discrimination parameters and capability of both stations to identify the explosions in analyzed quarries that were misclassified as earthquakes in the Hungarian National Bulletins.

How to cite: Kiszely, M., Süle, B., and Bondár, I.: Discrimination of earthquakes and quarry blasts in Mecsek Mountain region (Hungary) and its vicinity by using a linear discrimination function , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3092, https://doi.org/10.5194/egusphere-egu21-3092, 2021.

EGU21-14253 | vPICO presentations | SM1.1

Estimation Of High-Frequency Attenuation Parameter (Kappa) For Hard Rock Stations Of Turkish Strong Motion Network

Atınç Özgün Akgün, Zeynep Gülerce, and Atilla Arda Özacar

Site-specific decay in the Fourier Amplitude Spectrum (FAS) at high frequencies, a.k.a. the zero-distance kappa (κ0), is frequently used in seismic analysis of critical infrastructure; especially for the host-to-target adjustment of the design spectrum and the site response analysis. The zero-distance kappa value for hard rock sites is more crucial but harder to constrain because the amount of strong-motion stations on hard-rock sites is limited in the global datasets. The objective of this study is to calculate the zero-distance kappa value for the hard rock strong-motion stations operated by the Disaster and Emergency Presidency of Turkey (AFAD). For this purpose, 6463 recordings from 22 strong-motion stations with measured average shear wave velocities at the first 30 meters (VS30) higher than 740m/s and having at least 100 records have been analyzed. The slope of the decay in the S-wave portion of the FAS (kappa) at high frequencies is determined for a carefully selected and record-specific frequency range. Variation of the kappa with epicentral distance is evaluated to determine the median zero-distance kappa and its uncertainty for each recording station. Estimated median zero-distance kappa values vary between 0.01s to 0.06s and are consistent with the limited amount of previously published data. Only a weak reduction in median zero-distance kappa is observed with increasing VS30 and a rather large scatter in kappa for the same VS30 values is observed. More robust results might be attained by isolating the site amplification effects of weak surficial layers and subcategorization based on available geological and geographical information.

How to cite: Akgün, A. Ö., Gülerce, Z., and Özacar, A. A.: Estimation Of High-Frequency Attenuation Parameter (Kappa) For Hard Rock Stations Of Turkish Strong Motion Network, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14253, https://doi.org/10.5194/egusphere-egu21-14253, 2021.

EGU21-8781 | vPICO presentations | SM1.1

Seismic shaking scenarios for city of Zagreb, Croatia

Helena Latečki, Josip Stipčević, and Irene Molinari

In order to assess the seismic shaking levels, following the strong Zagreb March 22nd 2020 earthquake, we compute broadband seismograms using a hybrid technique. In a hybrid technique, low frequency (LF, f < 1 Hz) and high frequency (HF, f = 1–10 Hz) seismograms are obtained separately and then merged into a single time series. The LF part of seismogram is computed using a deterministic approach while for the HF part, we adopt the semi-stochastic method following the work of Graves and Pitarka (2010). For the purposes of the simulation, we also assemble the 3D velocity and density model of the crust for the city of Zagreb and its surrounding region. The model consists of a detailed description of the main geologic structures that are observed in the upper crust and is embedded within a greater regional EPCrust crustal model (Molinari and Morelli, 2011). To test and evaluate its performance, we apply the hybrid technique to the Zagreb March 22nd 2020 Mw = 5.3 event and four smaller (3.0 < Mw < 5.0) events. We compare the measured seismograms with the synthetic data and validate our results by assessing the goodness of fit for the peak ground velocity values and the shaking duration. Furthermore, since the 1880 Mw = 6.2 historic earthquake significantly contributes to the hazard assessment for the wider Zagreb area, we compute synthetic seismograms for this event at two different hypocenter locations. We calculate broadband waveforms on a dense grid of points and from these we plot the shakemaps to determine if the main expected ground-motion features are well-represented by our approach. Lastly, due to the events that occured in the Petrinja epicentral area at the end of 2020, we decided to extend our 3D model to cover the area of interest. We will present the preliminary results of the simulation for the December 29th 2020 Mw = 6.4 strong earthquake, as well as our plans for further research.

How to cite: Latečki, H., Stipčević, J., and Molinari, I.: Seismic shaking scenarios for city of Zagreb, Croatia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8781, https://doi.org/10.5194/egusphere-egu21-8781, 2021.

EGU21-14796 | vPICO presentations | SM1.1

The TURNkey TB4 Achaia Array: Bridging School and Citizen Seismology through Earthquake Alerting

Nikolaos S. Melis, Eustratios Liadopoulos, Georgia Giannaraki, Ioannis Kalogeras, and Konstantinos Boukouras

An extended strong motion array comprised mainly of low cost sensors has been deployed in the Achaia region: the Patras city and the Aigion, Kalavrita towns, Greece. It combines: 4 standard accelerometric stations operated by the National Observatory of Athens, Institute of Geodynamics (NOA), 15 P-Alert MEMS acceleration devices, already deployed and operated in public sector buildings, schools and private dwellings (the Patras P-Alert Array) and 40 Raspberry Shake 4D sensors, which are deployed through the newly established Test Bed 4 region (TB4) for the H2020 financed TURNkey project. Principal aim, in an operational approach, to estimate rapidly the intensity of a felt event in a highly populated urban environment and inform local Civil Protection Agencies and through them the final responders and the general public. Moreover, the deployment of these low cost sensors, especially in schools of the Achaia region, aims to involve the pupils/students, in primary and secondary education, towards exploring School Seismology exercises, in a region where strong felt earthquakes are very frequent. Simple exercises in class, using the recorded data after a felt event have been completed such as: locating the event, estimating the magnitude, show the distribution of max PGA values in the region etc. Taking advantage of the school – local community link, the resilience increase has been already demonstrated in the local communities through happenings, popularized seminars and local press postings. A connection with the Municipalities and the Communal public sector allows the expansion of the citizen involvement (Citizen Seismology) through the use of dedicated smartphone app (i.e. LastQuake@EMSC). Citizens are informed and also pass their felt experience. This allows improved estimation and distribution of the shaking in a second phase, useful for Civil Protection Agencies. The increase of the resilience and public awareness are under monitoring with the collaboration of local media. All data will be also used as input to a TURNkey under development central platform, serving as an EEW system, mainly focusing to schools in an application for the TB4 project region in Greece.

How to cite: Melis, N. S., Liadopoulos, E., Giannaraki, G., Kalogeras, I., and Boukouras, K.: The TURNkey TB4 Achaia Array: Bridging School and Citizen Seismology through Earthquake Alerting, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14796, https://doi.org/10.5194/egusphere-egu21-14796, 2021.

EGU21-6771 | vPICO presentations | SM1.1

Current status of design basis earthquake level for nuclear power plant sites in several countries

Hoseon Choi and Seung Gyu Hyun

According to strict criteria step by step for site selection, design, construction and operation, the seismic safety of nuclear power plant (NPP) sites in South Korea are secured by considering design basis earthquake (DBE) level capable of withstanding the maximum ground motions that can occur on the site. Therefore, it is intended to summarize DBE level and its evaluation details for NPP sites in several countries.

Similar but different terms are used for DBE from country to country, i.e. safe shutdown earthquake (SSE), design earthquake (DE), SL2, Ss, and maximum calculated earthquake (MCE). They may differ when applied to actual seismic design process, and only refer to approximate comparisons. This script used DBE as a representative term, and DBE level was based on horizontal values.

The DBE level of NPP sites depends on seismic activity of the area. Japan and Western United States, where earthquakes occur more frequently than South Korea, have high DBE values. The DBE level of NPP sites in South Korea has been confirmed to be similar or higher compared to that of Central and Eastern Unites Sates and Europe, which have similar seismic activity.

How to cite: Choi, H. and Hyun, S. G.: Current status of design basis earthquake level for nuclear power plant sites in several countries, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6771, https://doi.org/10.5194/egusphere-egu21-6771, 2021.

Consistent and monochromatic signals appear as sharp peaks in frequency spectra or as continuous lines in spectrograms on many permanent and temporary seismic stations in Central Europe, especially in South-Eastern Germany, Austria and the Czech Republic. Similar observations have already puzzled the seismic community more than 20 years ago. Here we report on new observations of such monochromatic seismic signals within a 1 – 10 Hz range across central Europe using the dense AlpArray network.

We identify several monochromatic signals on both permanent and temporary stations. The respective frequencies of e.g. 1.72 Hz, 2.08 Hz, 2.77 Hz or 4.16 Hz are generally stable even over long time spans (months to years). Strikingly, all such signals at any given station show identical and simultaneous short-term (minutes to days) frequency variations of up to 0.4% of the central frequency. These variations precisely correspond to fluctuations of the frequency of the European electric power network, which is regulated to 50 Hz +/- 0.4%. In fact, all persistent seismic signals that follow this behavior have frequencies of 50 Hz / n with n being an integer number (50 Hz / 29 = 1.72 Hz, 50 Hz / 24 = 2.08 Hz, 50 Hz / 18 = 2.77 Hz, 50 Hz / 12 = 4.16 Hz). We show that if the frequency of an observed spectral line is an integer fraction of the power network frequency (and only in that case) it will perfectly follow the fluctuations of the power network. This obviously raises questions about the nature of the signal itself, in particular if it is of seismic or maybe electro-magnetic origin.

We confirm that the signals are of seismic origin and we have identified water turbines inside river power plants as the source. The observed frequencies correspond well to reported rotation frequencies of water turbines at several different river power plants in Southern Germany and Austria. The seismic signals may propagate to almost 100 km from the corresponding plant. We analyze the spatial distribution of signal amplitudes for a selected river power plant in Austria, and show that it is similar to expected isolines of seismic shaking for an earthquake in the region.

Knowing the source of those exotic signals potentially enables us to use them for seismo-tectonic purposes. The long-term (several years) stability and the permanent availability (24h operation of water turbines) render them very interesting sources e.g. for studying temporal seismic velocity variations in the shallow crust.

How to cite: Fuchs, F., Bokelmann, G., and Working Group, A.: Persistent monochromatic seismic signals across central Europe: AlpArray data indicate a man-made seismic source for regional wave propagation studies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11008, https://doi.org/10.5194/egusphere-egu21-11008, 2021.

EGU21-14708 | vPICO presentations | SM1.1

Orientation of broadband seismographs in the Kashmir Himalaya: Effect on vector-based studies

Ramees Mir, Imtiyaz Parvez, and Vinod Gaur

We used regional as well as global Rayleigh wave signals (source-receiver distance: 5°-175°; M≥ 6, Depth ≤ 150 km) recorded at 12 broadband seismic stations in northwestern Himalaya to compute arrival angles of surface waves at each station, assuming orthogonality of the horizontal components, and error-free levelling of the instrument. The average of all measurements at a station with cross-correlation values > 0.8, between Hilbert transformed vertical and radial components, was interpreted as the degree of misalignment of the horizontal components in a geographic frame of reference.

Out of the 12 station data used in this analysis, 3 were found to have instrument misorientation errors between 5° and 10° w.r.t geographic north, 2 between 10° and 15° and the remaining 7 < 5°. The number of measurements at each of these stations ranged from 75 to 331, with 11 stations having more than 90 measurements, assuring high reliability. We also analysed data from two nearby broadband instruments located in Ladakh Himalaya. One of these (LEH) with 46 measurements showed a misorientation error of 14.87°±4.87° and the other (HNL) with 48 showed an error of 0.75°±3.48°. Since misorientation errors based on less than 90 data elements are considered to be unstable, these were not used for further analysis.

We evaluated the effect of seismograph misorientations on the inverted solutions for P-wave receiver functions (RFs) and core-refracted shear waves (SKS). The errors in Moho depths and those of other intra-crustal features were within ±2 km for instrument misorientations of up to ~15°, that is close to the resolution errors. But, the SKS results, notably the azimuths of the fast component, were, found to be quite sensitive to instrument misalignment. For example, a ~14° error in orientation was found to cause a shift of up to 20° in the calculated azimuth of the fast component. Corrections of misorientation errors in both cases showed reduction of variance in the inverted solutions.

How to cite: Mir, R., Parvez, I., and Gaur, V.: Orientation of broadband seismographs in the Kashmir Himalaya: Effect on vector-based studies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14708, https://doi.org/10.5194/egusphere-egu21-14708, 2021.

SM2.1 – Sensing ground translation, rotation, and strain - instrumentation, theory and applications

EGU21-859 | vPICO presentations | SM2.1

Visualizing the Seismic Wavefield with AlpArray

On Ki Angel Ling, Simon Stähler, Domenico Giardini, and the AlpArray Working Group

The AlpArray Seismic Network (AASN) is a large-scale multidisciplinary seismic network in Europe that consists of over 600 3-component (3C) broadband stations with mean inter-station distance of 30-40km. This dense array allows the recording of the seismic wave propagation of distant earthquakes at a resolution of typical body and surface waves.

By animating the spatially-dense seismic recordings of the AASN, we can visualize seismic waves propagating across the European Alps as a function of space and time. Our 3C ground motion animations illustrate the full spatial-temporal evolution of global body and surface waves and demonstrates how a dense array allows the transformation from translation measurements at single stations to spatial gradients of the wavefield at the surface, capturing both small- and large-scale wave propagation phenomena. The addition of travel-time estimation, ray path illustration, and array-specific information such as slowness vector of incoming waves facilitate identification of seismic phases and their arrival-angle deviations. We will highlight some interesting observations of different seismic wave types in the animations of a few example teleseismic events during the course of the AASN between 2016-2019. Application for future research and education will also be discussed.

How to cite: Ling, O. K. A., Stähler, S., Giardini, D., and Group, T. A. W.: Visualizing the Seismic Wavefield with AlpArray, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-859, https://doi.org/10.5194/egusphere-egu21-859, 2021.

EGU21-8817 | vPICO presentations | SM2.1

Spectral ratio comparison between translation and rotational records from induced seismic events.

Dariusz Nawrocki, Maciej Mendecki, and Lesław Teper

The seismic observations of the rotational signals are a field of seismology that is constantly developed. The recent research concerns sensors technology and its potential application in seismic tests. This study presents the results of a comparative analysis of rotational and translational seismic records using the horizontal-to-vertical spectral ratio (HVSR) method. In terms of transitional signal ratio, we have used the name of HVSR, but in terms of rotational component spectra, we have introduced a torsion-to-rocking spectral ratio (TRSR) which corresponds to horizontal rotation spectrum to vertical rotation spectrum. It has to be noticed that rotation in the horizontal axes has a vertical character and rotation in the vertical axis has a horizontal character.

The comparison was carried out between velocity signals of translational and rotational records, as well as, between acceleration signals respectively. All seismic data were recorded by two independent sensors: the rotational seismometer and translational accelerometer at the Imielin station, located in the Upper Silesia Coal Basin (USCB), Poland. The seismic data composed of three-component seismic waveforms related to 56 recorded tremors which were located up to 1,5 km from the seismic station and they resulted from the coal extractions carried out in the neighboring coal mines. The rotational acceleration was obtained by numerical differentiation and the translational velocity was produced by numerical integration.

The conducted spectral analyses allowed to estimate the range of frequency in which the rotational HVSR and the corresponded translational HVSR are comparable. The analysis of HVSR/TRSR curves (in the selected frequency range of 1Hz to 10Hz) showed a strong correlation between the spectral ratios for the velocity signals (translational and rotational) in the frequency range of 1Hz to 2Hz. Respectively, the comparison of the accelerometer signals indicated the correlation between HVSR/TRSR curves in the frequency range of 1Hz to 3Hz. Moreover, both of the TRSR (for velocity and acceleration) showed additional maxima in the same frequency range of 3Hz to 5Hz. These relatively high-frequency maxima did not correspond to translational spectra.  

How to cite: Nawrocki, D., Mendecki, M., and Teper, L.: Spectral ratio comparison between translation and rotational records from induced seismic events., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8817, https://doi.org/10.5194/egusphere-egu21-8817, 2021.

EGU21-6446 | vPICO presentations | SM2.1

Simulation of High-Frequency Rotational Motion in a Two-Dimensional Laterally Heterogeneous Half-Space

Ivan Lokmer, Varun Kumar Singla, and John McCloskey

The seismic waves responsible for vibrating civil engineering structures undergo interference, focusing, scattering, and diffraction by the inhomogeneous medium encountered along the source-to-site propagation path. The subsurface heterogeneities at a site can particularly alter the local seismic wave field and amplify the ground rotations, thereby increasing the seismic hazard. The conventional techniques to carry out full wave field simulations (such as finite-difference or spectral finite element methods) at high frequencies (e.g., 15 Hz) are computationally expensive, particularly when the size of the heterogeneities is small (e.g., <100 m). This study proposes an alternative technique that is based on the first-order perturbation theory for wave propagation. In this technique, the total wave field due to a particular source is obtained as a superposition of the ‘mean’ and ‘scattered’ wave fields. Whereas the ‘mean’ wave field is the response of the background (i.e., heterogeneity-free) medium due to the given source, the ‘scattered’ wave is the response of the background medium excited by fictitious body forces. For a two-dimensional laterally heterogeneous elastic medium, these body forces can be conveniently evaluated as a function of the material properties of the heterogeneities and the mean wave field. Since the problem of simulating high-frequency rotations in a laterally heterogeneous medium reduces to that of calculating rotations in the background medium subjected to the (1) given seismic source and (2) body forces that mathematically replace the small-scale heterogeneities, the original problem can be easily solved in a computationally accurate and efficient manner by using the classical (analytical) wavenumber-integration method. The workflow is illustrated for the case of a laterally heterogenous layer embedded in a homogeneous half-space excited by plane body-waves.

How to cite: Lokmer, I., Singla, V. K., and McCloskey, J.: Simulation of High-Frequency Rotational Motion in a Two-Dimensional Laterally Heterogeneous Half-Space, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6446, https://doi.org/10.5194/egusphere-egu21-6446, 2021.

EGU21-13490 | vPICO presentations | SM2.1

Democratizing and Densifying Low Noise Long Period Broadband Stations

Geoffrey Bainbridge, Valarie Hamilton, and Timothy Parker

The Streckeisen STS-1 set a very high performance and lasting broadband (VBB) sensor standard that has been hard to match by other instruments, but these sensors also required a very careful emplacement and shielding from environmental changes and conditions, along with the high costs of ensuring the conditions for this level of instrument performance.  Recent developments have demonstrated equivalent and broader bandwidth sensors that enable deploying these types of sensors in most any terrestrial environment.  These new instruments, in many types of form factors, all magnetically shielded, open up new opportunities for continuing and expanding these VBB observations, democratizing the observations of these long period signals and opening up the possibilities of better performance through deep boreholes and observations of less developed sites that have harsher environmental conditions, along with recapitalizations of sites where STS-1s are no longer supported.   We will describe recent testing results of Trillium 360 GSN vault, borehole, and posthole sensors as well as the Horizon 360 from many observatories and new potential use cases, some in polar environments that were impractical until now, and discuss development of the new Horizon 360 OBS.

How to cite: Bainbridge, G., Hamilton, V., and Parker, T.: Democratizing and Densifying Low Noise Long Period Broadband Stations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13490, https://doi.org/10.5194/egusphere-egu21-13490, 2021.

EGU21-12443 | vPICO presentations | SM2.1

On the use of Distributed Acoustic Sensing for seismic divergence and curl estimations

Pascal Edme, Patrick Paitz, David Sollberger, Tjeerd Kiers, Vincent Perron, Cedric Schmelzbach, Andreas Fichtner, and Johan O.A. Robertsson

Distributed Acoustic Sensing (DAS) is becoming an established tool for seismological and geophysical applications. DAS is based on Rayleigh scattering of light pulses conveyed in fibre optic cables, enabling unprecedented strain rate measurements over kilometers with spatial resolution of less than a meter. The low cost, logistically easy deployment, and the broadband sensitivity make it a very attractive technology to investigate an increasing number of man-made or natural phenomena.

One key restriction however is that DAS collects axial strain rate instead of the vector of ground motion, resulting in a poor sensitivity to broadside events like (at the surface) vertically incident waves or surface waves impinging perpendicular to the cable. Helically wound cables partially mitigate the issue but still do not provide omni-directional response as the typical vertical component of seismometers or geophones.

The present study is about the potential of using unconventional DAS cable layouts to replace and/or complement traditional sensors. We investigate the possibility of estimating the divergence and the vertical rotational components of the wavefield from cables deployed in a square or circular shape. The impact of the size of the arrangement as well as that of the interrogation gauge length is discussed.  Real data are shown and the results suggest that DAS has the potential to offer additional seismic component(s) useful for wave type identification and separation for example.

How to cite: Edme, P., Paitz, P., Sollberger, D., Kiers, T., Perron, V., Schmelzbach, C., Fichtner, A., and Robertsson, J. O. A.: On the use of Distributed Acoustic Sensing for seismic divergence and curl estimations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12443, https://doi.org/10.5194/egusphere-egu21-12443, 2021.

EGU21-8438 | vPICO presentations | SM2.1

The GIOTTO Project - Building Monitoring with 6DoF Sensors

Felix Bernauer, Louisa Murray-Bergquist, Felix Strobel, Joachim Wassermann, Heiner Igel, Eva P.S. Eibl, Cun-Man Liao, Ernst Niederleithinger, Sneha Singh, and Celine Hadziioannou

Characterizing earthquake induced building damage in an efficient, automated and non-
invasive way is a crucial support for the decision on further usability of critical infras-
tructure. In the GIOTTO project (Gebäudeschwingungen: kombinierte Zustandsanalyse
mit innovativem Sensorkonzept) we propose to use 6 degrees of freedom sensors (6DoF)
to monitor the complete movement of a building structure in three rotational and three
translational degrees of freedom. On one side, we develop 6DoF sensor networks for
strong motion building monitoring on the basis of 20 inertial measurement units (IMU50
by iXblue, France) originally designed as north-finding gyroscopes, on the other side we
incorporate the new observable of rotational ground motions into the concept of coda wave
interferometry for continuous real-time structural health monitoring. In this contribution
we show first results (1) from laboratory experiments for sensor performance characteriza-
tion as well as (2) from a 6DoF active source experiment at a horizontal 24 m long concrete
beam (the BLEIB test structure hosted by the Bundesanstalt für Materialforschung und
-prüfung, south of Berlin, Germany).

How to cite: Bernauer, F., Murray-Bergquist, L., Strobel, F., Wassermann, J., Igel, H., Eibl, E. P. S., Liao, C.-M., Niederleithinger, E., Singh, S., and Hadziioannou, C.: The GIOTTO Project - Building Monitoring with 6DoF Sensors, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8438, https://doi.org/10.5194/egusphere-egu21-8438, 2021.

A new type of portable six-component seismometer is invented and used in the development of a seismic imaging method for shallow subsurface anomaly detection. This new six-component seismometer contains a mini-MEMS-array for acceleration and rotational velocity measurements. The self-noise for acceleration measurement is about 8 µg/√Hz, and the self-noise for rotational velocity measurement is about 5 µrad/s/√Hz. The frequency band is DC-1000 Hz. Different from the traditional seismic imaging methods that require the deployment of an array of either one-component or three-component seismometers, our imaging method is established based on the data recorded at individual six-component seismometer. Because the rotational field (i.e., the curl field) gives information about the spatial gradient of a seismic wavefield, so the translational field together with the rotational field can be used to derive the frequency-dependent velocity (i.e., dispersion) of the formation right beneath a seismic station. This single station velocity inversion approach delivers localized subsurface velocity information, making it suitable for imaging of small-scale underground anomalies. Especially, the Rayleigh wave dispersion is used in our method as Rayleigh wave is generally the dominant signal in surface seismic data. An underground velocity model can be immediately constructed by consolidating the dispersion curves derived from individual receivers. In our study, we first demonstrate the accuracy of our imaging method through numerical modeling of various scenarios of subsurface anomalies and then conduct an experiment to further verify the performance of our self-invented six-component seismometer and the field applicability of our imaging method.

How to cite: Fang, X.: Application of a new type of six-component seismometer for underground anomaly detection, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8551, https://doi.org/10.5194/egusphere-egu21-8551, 2021.

EGU21-10927 | vPICO presentations | SM2.1

Performance of a rotational sensor at Etna, Italy focusing on back-azimuth estimations of volcano-seismic events

Martina Rosskopf, Eva P. S. Eibl, Gilda Currenti, Philippe Jousset, Joachim Wassermann, Daniel Vollmer, Graziano Larocca, Daniele Pellegrino, Mario Pulvirenti, and Danilo Contrafatto

The field of rotational seismology has only recently emerged. Portable 3 component rotational sensors are commercially available since a few years which opens the pathway for a first use in volcano-seismology. The combination of rotational and translational components of the wavefield allows identifying and filtering for specific seismic wave types, estimating the back azimuth of an earthquake, and calculating local seismic phase velocities.

Our work focuses on back-azimuth calculations of volcano-tectonic and long-period events detected at Etna volcano in Italy. Therefore, a continuous full seismic wavefield of 30 days was recorded by a BlueSeis-3A, the first portable rotational sensor, and a broadband Trillium Compact seismometer located next to each other at Mount Etna in August and September of 2019. In this study, we applied two methods for back-azimuth calculations. The first one is based on the similarity of the vertical rotation rate to the horizontal acceleration and the second one uses a polarization analysis from the two horizontal components of the rotation rate. The estimated back-azimuths for volcano-tectonic events were compared to theoretical back-azimuths based on the INGV event catalog and the long-period event back-azimuths were analyzed for their dominant directions. We discuss the quality of our back azimuths with respect to event locations and evaluate the sensitivity and benefits of the rotational sensor focusing on volcano-seismic events on Etna regarding the signal to noise ratios, locations, distances, and magnitudes.

How to cite: Rosskopf, M., Eibl, E. P. S., Currenti, G., Jousset, P., Wassermann, J., Vollmer, D., Larocca, G., Pellegrino, D., Pulvirenti, M., and Contrafatto, D.: Performance of a rotational sensor at Etna, Italy focusing on back-azimuth estimations of volcano-seismic events, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10927, https://doi.org/10.5194/egusphere-egu21-10927, 2021.

EGU21-16524 | vPICO presentations | SM2.1

Short Review of True-North Alignment Method on the Field Demonstrating the benefits of the use of an Optical Gyrocompass

Frédéric Guattari, Pierrick Auregan, Elliot de Toldi, Theo Laudat, and Laurent Mattio

To install a seismometer with a properly defined orientation - inside a vault or into a borehole - as a single station including various instruments or as a part of an array - an ‘adequate’ tool and an ‘absolute’ reference are needed.

In the past, and sometimes it persists nowadays, magnetic North have been used as a reference for Z-orientation of seismic station. Several studies have extensively measured the orientation error that have been made with this method, using an optical gyrocompass providing True-North as a reference, and their work will be summarized here.

In these studies, optical Gyrocompass is said to be the good solution, even if it is too heavy, expensive, and difficult to export. This paper will explain how iXblue has overcome these limitations to design the new-born Seistans Optical Gyrocompass.

Moreover, to aim True-North with a reliable accuracy is not the only think you need to do on the field. The method to transfer the North-line from the gyrocompass to the instrument to aligned must not induce errors that ruined the accuracy obtained using state-of-the-art gyrocompass. So an exhaustive study of the different ways to transfer the orientation from the compass to the aligned sensor will be presented, and corresponding added uncertainty will be evaluated, which is a good way to promote good practice on the field.

Finally, some figures will be gathered and shared from literature to quantify the precision needed for the alignment of a seismic sensor. There are today so few papers about this important matter that it is worth to spread their information.

How to cite: Guattari, F., Auregan, P., de Toldi, E., Laudat, T., and Mattio, L.: Short Review of True-North Alignment Method on the Field Demonstrating the benefits of the use of an Optical Gyrocompass, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16524, https://doi.org/10.5194/egusphere-egu21-16524, 2021.

EGU21-16525 | vPICO presentations | SM2.1

From prototypes to first production units: blueSeis-1C Fiber Optical Gyroscope industrial solution for broadband ground motion rotation measurement at the best self-noise

Kevin Gautier, Pierrick Auregan, Theo Laudat, and Frédéric Guattari

SM2.2 – Advances in fibre-optic technologies for geophysical applications

EGU21-6445 | vPICO presentations | SM2.2 | Highlight

Towards multi-method geophysical sensing on submarine cables 

Zhongwen Zhan, Mattia Cantono, Jorge Castellanos, Miguel González Herráez, Zhensheng Jia, Valey Kamalov, Hugo Martins, Antonio Mecozzi, Rafael Müller, Zhichao Shen, Ethan Williams, and Shuang Yin

The oceans present a major gap in geophysical instrumentation, hindering fundamental research on submarine earthquakes and the Earth’s interior structure, as well as effective earthquake and tsunami warning for offshore events. Emerging fiber-optic sensing technologies that can leverage submarine telecommunication cables present an new opportunity in filling the data gap. Marra et al. (2018) turned a 96 km long submarine cable into a sensitive seismic sensor using ultra-stable laser interferometry of a round-tripped signal. Another technology, Distributed Acoustic Sensing (DAS), interrogates intrinsic Rayleigh backscattering and converts tens of kilometers of dedicated fiber into thousands of seismic strainmeters on the seafloor (e.g., Lindsey et al., 2019; Sladen et al., 2019; Williams et al., 2019; Spica et al., 2020). Zhan et al. (2021) successfully sensed seismic and water waves over a 10,000 km long submarine cable connecting Los Angeles and Valparaiso, by monitoring the polarization of regular optical telecommunication channels. However, these new technologies have substantially different levels of sensitivity, coverage, spatial resolution, and scalability. In this talk, we advocate that strategic combinations of the different sensing techniques (including conventional geophysical networks) are necessary to provide the broadest coverage of the seafloor while making high-fidelity, physically interpretable measurements. Strategic collaborations between the geophysics community and telecommunication community without burdening the telecomm operation (e.g., by multiplexing or using regular telecom signals) will be critical to the long term success.

 

Marra, G., C. Clivati, R. Luckett, A. Tampellini, J. Kronjäger, L. Wright, A. Mura, F. Levi, S. Robinson, A. Xuereb, B. Baptie, D. Calonico, 2018. Ultrastable laser interferometry for earthquake detection with terrestrial and submarine cables. Science, eaat4458.

Lindsey, N.J., T. C. Dawe, J. B. Ajo-Franklin, 2019. Illuminating seafloor faults and ocean dynamics with dark fiber distributed acoustic sensing. Science. 366, 1103–1107.

Sladen, A., D. Rivet, J. P. Ampuero, L. De Barros, Y. Hello, G. Calbris, P. Lamare, 2019. Distributed sensing of earthquakes and ocean-solid Earth interactions on seafloor telecom cables. Nat Commun. 10, 5777.

Spica, Z.J., Nishida, K., Akuhara, T., Pétrélis, F., Shinohara, M. and Yamada, T., 2020. Marine Sediment Characterized by Ocean‐Bottom Fiber‐Optic Seismology. Geophysical Research Letters, 47(16), p.e2020GL088360.

Williams, E.F., M. R. Fernández-Ruiz, R. Magalhaes, R. Vanthillo, Z. Zhan, M. González-Herráez, H. F. Martins, 2019. Distributed sensing of microseisms and teleseisms with submarine dark fibers. Nat Commun. 10, 5778.

Zhan, Z., M. Cantono, V. Kamalov, A. Mecozzi, R. Muller, S. Yin, J.C. Castellanos, 2021. Optical polarization-based seismic and water wave sensing on transoceanic cables. Science, in press.

How to cite: Zhan, Z., Cantono, M., Castellanos, J., González Herráez, M., Jia, Z., Kamalov, V., Martins, H., Mecozzi, A., Müller, R., Shen, Z., Williams, E., and Yin, S.: Towards multi-method geophysical sensing on submarine cables , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6445, https://doi.org/10.5194/egusphere-egu21-6445, 2021.

EGU21-2498 | vPICO presentations | SM2.2

First deployment of a 6-km long fiber-optic strain cable and a seafloor geodetic network, across an active submarine fault (offshore Catania, Sicily): The FOCUS experiment

Marc-Andre Gutscher, Jean-Yves Royer, Shane Murphy, Frauke Klingelhoefer, Giovanni Barreca, Arnaud Gaillot, Lionel Quetel, Giorgio Riccobene, Salvatore Aurnia, Philippe Jousset, Charles Poitou, and Viorel Ciausu

For the first time, a 6-km long fiber-optic strain cable was deployed across an active fault on the seafloor with the aim to monitor possible tectonic movement using laser reflectometry, 25 km offshore Catania Sicily (an urban area of 1 million people). Brillouin Optical Time Domain Reflectometry (BOTDR) is commonly used for structural health monitoring (bridges, dams, etc.) and under ideal conditions, can measure small strains (10-6) along a fiber-optic cable, across very large distances (10 - 200 km), with a spatial resolution of 10 - 50 m. The FocusX1 expedition, (6-21 October 2020) onboard the R/V Pourquoi Pas? was the first experiment of the European funded FOCUS project (ERC Advanced Grant). We first performed micro-bathymetric mapping and a video camera survey using the ROV Victor6000 to select the best path for the cable track and for deployment sites for eight seafloor geodetic stations. Next we connected a custom designed 6-km long fiber-optic cable (manufactured by Nexans Norway) to the TSS (Test Site South) seafloor observatory in 2100 m water depth operated by INFN-LNS (Italian National Physics Institute) via a new Y-junction frame and cable-end module. Cable deployment was performed by means of 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 cable track crosses the North Alfeo Fault at four locations. Laser reflectometry measurements began on 18 October 2020 and are being calibrated by a 3 - 4 year deployment of eight seafloor geodetic instruments (Canopus acoustic beacons manufactured by iXblue) deployed on 15 October 2020. During a future marine expedition, tentatively scheduled for early 2022 (FocusX2) a passive seismological experiment is planned to record regional seismicity. This will involve deployment of a temporary network of Ocean Bottom Seismometers (OBS) 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 slipping event types along the North Alfeo Fault (e.g. - creep, slow-slip, rupture). A long-term goal of the project is the development of dual-use telecom cables with industry partners.

How to cite: Gutscher, M.-A., Royer, J.-Y., Murphy, S., Klingelhoefer, F., Barreca, G., Gaillot, A., Quetel, L., Riccobene, G., Aurnia, S., Jousset, P., Poitou, C., and Ciausu, V.: First deployment of a 6-km long fiber-optic strain cable and a seafloor geodetic network, across an active submarine fault (offshore Catania, Sicily): The FOCUS experiment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2498, https://doi.org/10.5194/egusphere-egu21-2498, 2021.

EGU21-5042 | vPICO presentations | SM2.2

The Potential of DAS on Underwater Fiber Optic Cables for Deep-Sea Current Monitoring 

Daniel Mata Flores, Jean-Paul Ampuero, Diego Mercerat, Anthony Sladen, and Diane Rivet

Distributed Acoustic Sensing (DAS) enables the use of existing underwater telecommunication cables as multi-sensor arrays, allowing for detailed study of the seismic wavefield. Since underwater telecommunication cables were not deployed for seismological investigations, the coupling between the cable and the seafloor varies, dramatically reducing the usefulness of poorly coupled cable segments for seismological research. In particular, underwater cables include segments that are suspended in the water column across seafloor valleys or other bathymetry irregularities. Here, we propose that ocean bottom currents may be studied by monitoring the vibrations of suspended cable segments. We analyze DAS-strain recordings on three dark fibers deployed in the Mediterranean Sea. Several cable segments, presumably suspended, feature high-amplitude signals with harmonic spectra as expected from a theoretical model of in-plane vibration of hanging cables. The spatial shape of the vibration modes are determined by filtering and stacking. Their comparison to theory allows constraining the attenuation of longitudinal waves propagating along the cable in the non-suspended sections. The vibration frequencies change over time scales of tens of minutes. Assuming that oscillations of suspended sections are driven by deep sea currents, the temporal fluctuations of the vibration frequencies are related to changes of the cables tension which, in turn, are related to the drag force induced on the suspended cable by the shedding of Karman vortex. On this basis, we propose a method to infer changes of deep sea current speeds from the changes of fundamental frequency of cable vibrations. Submarine optical reconnaissance campaigns and controlled smaller-scale experiments are planned to validate the approach. The work aims at demonstrating the potential of using suspended telecommunication cables to monitor and investigate marine currents in deep ocean environments.

How to cite: Mata Flores, D., Ampuero, J.-P., Mercerat, D., Sladen, A., and Rivet, D.: The Potential of DAS on Underwater Fiber Optic Cables for Deep-Sea Current Monitoring , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5042, https://doi.org/10.5194/egusphere-egu21-5042, 2021.

EGU21-7404 | vPICO presentations | SM2.2

Measuring floating ice thickness with optical fibers and DAS, a test case study on a frozen moutain lake.

Olivier Coutant, Ludovic Moreau, Pierre Boué, Eric Larose, and Arnaud Cimolino

Accurate monitoring of floating ice thickness is an important safety issue for northern countries where lakes, fjords, and coasts are covered with ice in winter, and used by people to travel. For example in Finland, 15-20 fatal accidents occur every year due to ice-related drowning. We have explored the potential of fiber optics to measure the propagation of seismic waves guided in the ice layer, in order to infer its thickness via the inversion of the dispersion curves. An optical fiber was deployed on a frozen lake at Lacs Roberts (2400m) above Grenoble and we measured with a DAS the signal generated by active sources (hammer) and ambient noise. We demonstrate that we can retrieve the ice thickness. This monitoring method could be of interest since the deployment of a fiber on ice is quite simple (e.g. using a drone) compared to other techniques for ice thickness estimation such as seismic survey or manual drilling.

How to cite: Coutant, O., Moreau, L., Boué, P., Larose, E., and Cimolino, A.: Measuring floating ice thickness with optical fibers and DAS, a test case study on a frozen moutain lake., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7404, https://doi.org/10.5194/egusphere-egu21-7404, 2021.

EGU21-8284 | vPICO presentations | SM2.2

A variety of surface waves in ocean-bottom DAS records

Zack Spica, Loïc Viens, Jorge Castillo Castellanos, Takeshi Akuhara, Kiwamu Nishida, Masanao Shinohara, and Tomoaki Yamada

Distributed acoustic sensing (DAS) can transform existing telecommunication fiber-optic cables into arrays of thousands of sensors, enabling meter-scale recordings over tens of kilometers. Recently, DAS has demonstrated its utility for many seismological applications onshore. However, the use of offshore cables for seismic exploration and monitoring is still in its infancy.
In this work, we introduce some new results and observations obtained from a fiber-optic cable offshore the coast of Sanriku, Japan. In particular, we focus on surface wave retrieved from various signals and show that ocean-bottom DAS can be used to extract dispersion curves (DC) over a wide range of frequencies. We show that multi-mode DC can be easily extracted from ambient seismo-acoustic noise cross-correlation functions or F-K analysis. Moderate magnitude earthquakes also contain multiple surface-wave packets that are buried within their coda. Fully-coupled 3-D numerical simulations suggest that these low-amplitude signals originate from the continuous reverberations of the acoustic waves in the ocean layer. 

How to cite: Spica, Z., Viens, L., Castillo Castellanos, J., Akuhara, T., Nishida, K., Shinohara, M., and Yamada, T.: A variety of surface waves in ocean-bottom DAS records, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8284, https://doi.org/10.5194/egusphere-egu21-8284, 2021.

EGU21-2502 | vPICO presentations | SM2.2

A self-supervised Deep Learning approach for improving signal coherence in Distributed Acoustic Sensing

Martijn van den Ende, Itzhak Lior, Jean Paul Ampuero, Anthony Sladen, and Cédric Richard

Fibre-optic Distributed Acoustic Sensing (DAS) is an emerging technology for vibration measurements with numerous applications in seismic signal analysis as well as in monitoring of urban and marine environments, including microseismicity detection, ambient noise tomography, traffic density monitoring, and maritime vessel tracking. A major advantage of DAS is its ability to turn fibre-optic cables into large and dense seismic arrays. As a cornerstone of seismic array analysis, beamforming relies on the relative arrival times of coherent signals along the optical fibre array to estimate the direction-of-arrival of the signals, and can hence be used to locate earthquakes as well as moving acoustic sources (e.g. maritime vessels). Naturally, this technique can only be applied to signals that are sufficiently coherent in space and time, and so beamforming benefits from signal processing methods that enhance the signal-to-noise ratio of the spatio-temporally coherent signal components. DAS measurements often suffer from waveform incoherence, and processing submarine DAS data is particularly challenging.

In this work, we adopt a self-supervised deep learning algorithm to extract locally-coherent signal components. Owing to the similarity of coherent signals along a DAS system, one can predict the coherent part of the signal at a given channel based on the signals recorded at other channels, referred to as "J-invariance". Following the recent approach proposed by Batson & Royer (2019), we leverage the J-invariant property of earthquake signals recorded by a submarine fibre-optic cable. A U-net auto-encoder is trained to reconstruct the earthquake waveforms recorded at one channel based on the waveforms recorded at neighbouring channels. Repeating this procedure for every measurement location along the cable yields a J-invariant reconstruction of the dataset that maximises the local coherence of the data. When we apply standard beamforming techniques to the output of the deep learning model, we indeed obtain higher-fidelity estimates of the direction-of-arrival of the seismic waves, and spurious solutions resulting from a lack of waveform coherence and local seismic scattering are suppressed.

While the present application focuses on earthquake signals, the deep learning method is completely general, self-supervised, and directly applicable to other DAS-recorded signals. This approach facilitates the analysis of signals with low signal-to-noise ratio that are spatio-temporally coherent, and can work in tandem with existing time-series analysis techniques.

References:
Batson J., Royer L. (2019), "Noise2Self: Blind Denoising by Self-Supervision", Proceedings of the 36th International Conference on Machine Learning (ICML), Long Beach, California

How to cite: van den Ende, M., Lior, I., Ampuero, J. P., Sladen, A., and Richard, C.: A self-supervised Deep Learning approach for improving signal coherence in Distributed Acoustic Sensing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2502, https://doi.org/10.5194/egusphere-egu21-2502, 2021.

EGU21-7869 | vPICO presentations | SM2.2

Parameterizing the gains in earthquake monitoring using submarine optical fiber telecom cables

Luis Matias, Fernando Carrilho, Vasco Sá, Manfred Niehus, Carlos Corela, and Yasser Omar

The need for submarine observatories to monitor offshore tectonic sources that can generate destructive earthquakes and tsunamis is widely recognized but the requirements of real-time communications and cost has hindered its Implementation. Only very few dedicated cables with sensors are in operation today. If the dozens of commercial telecommunication submarine cables that are deployed every year were instrumented, they could revolutionize the offshore earthquake monitoring. These cables, named as SMART (Science Monitoring And Reliable) have been advocated by the JTF of United Nations (Joint Task Force) for nearly a decade but none has been deployed today. However, there are several identified projects that should become the first pilots worldwide.

Fiber optic research have shown that the cable itself can be used as strain meters and useful for seismic monitoring.

One technology is DAS, Distributed Acoustic Sensing. DAS uses a single dedicated portion of (dark) fiber on a submarine cable, with a length about ~100 km. It can be modelled as a distributed strain sensor, with localization ability of a few meters. The DAS signal using OTDR (optical time domain reflectometry) and signal phase detection measures the fiber strain and record earthquakes with a resolution like broadband seismic sensors.

Another technology is LI (Laser Interferometry). LI may use a dark fiber or a single telecom wavelength channel in an optical fiber pair with commercial traffic, thousands km long. It relies on frequency stable laser sources and coherent detection. LI detects the changes of fiber optical transmission parameters over the whole cable. Using recording instruments on both ends, the arrival point of the first seismic waves is determined, and the azimuth to the epicenter estimated.

This work proposes and applies one methodology to assess the gain in earthquake source information using any of the three cable sensor technologies mentioned, against a background scenario that includes only land stations. We use a Monte-Carlo simulation to allow for picking uncertainties, local and regional variations of propagation velocity models. We parametrize the gain in information by measuring the epicenter uncertainty ellipse and the focal depth variability.

The proposed methodology is applied to the NE Atlantic domain, SW Iberia and the Azores archipelago, an area where the relative motion of the Nubia, Eurasia and North America plates can generate large and destructive earthquakes and tsunamis.

While the inclusion in the monitoring network of SMART observatories, placed inside cable repeaters, spaced every ±70 km, is straightforward, the use of DAS and LI is not. For DAS and LI we consider that observations can be decimated to virtual seismic stations every 5 km and 1 km respectively. To avoid using a set of very close stations, we implement different station selection algorithms.

The investigation presented in this work was conducted by LEA, Listening to the Earth under the Atlantic, a partnership between IT, IPMA and IDL. One of the main objectives of LEA is to promote research, development, training and outreach on geophysical and oceanographic phenomena using submarine cables, fostering its applications to Science and Civil Protection.

How to cite: Matias, L., Carrilho, F., Sá, V., Niehus, M., Corela, C., and Omar, Y.: Parameterizing the gains in earthquake monitoring using submarine optical fiber telecom cables, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7869, https://doi.org/10.5194/egusphere-egu21-7869, 2021.

EGU21-7601 | vPICO presentations | SM2.2

Strain to Ground Motion Conversion of DAS Data for Earthquake Magnitude and Stress Drop Determination

Itzhak Lior, Anthony Sladen, Diego Mercerat, Jean-Paul Ampuero, Diane Rivet, and Serge Sambolian

The use of Distributed Acoustic Sensing (DAS) presents unique advantages for earthquake monitoring compared with standard seismic networks: spatially dense measurements adapted for harsh environments and designed for remote operation. However, the ability to determine earthquake source parameters using DAS is yet to be fully established. In particular, resolving the magnitude and stress drop, is a fundamental objective for seismic monitoring and earthquake early warning. To apply existing methods for source parameter estimation to DAS signals, they must first be converted from strain to ground motions. This conversion can be achieved using the waves’ apparent phase velocity, which varies for different seismic phases ranging from fast body-waves to slow surface- and scattered-waves. To facilitate this conversion and improve its reliability, an algorithm for slowness determination is presented, based on the local slant-stack transform. This approach yields a unique slowness value at each time instance of a DAS time-series. The ability to convert strain-rate signals to ground accelerations is validated using simulated data and applied to several earthquakes recorded by dark fibers of three ocean-bottom telecommunication cables in the Mediterranean Sea. The conversion emphasizes fast body-waves compared to slow scattered-waves and ambient noise, and is robust even in the presence of correlated noise and varying wave propagation directions. Good agreement is found between source parameters determined using converted DAS waveforms and on-land seismometers for both P- and S-wave records. The demonstrated ability to resolve source parameters using P-waves on horizontal ocean-bottom fibers is key for the implementation of DAS based earthquake early warning, which will significantly improve hazard mitigation capabilities for offshore and tsunami earthquakes.

How to cite: Lior, I., Sladen, A., Mercerat, D., Ampuero, J.-P., Rivet, D., and Sambolian, S.: Strain to Ground Motion Conversion of DAS Data for Earthquake Magnitude and Stress Drop Determination, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7601, https://doi.org/10.5194/egusphere-egu21-7601, 2021.

EGU21-7856 | vPICO presentations | SM2.2

Leveraging coherent wave field analysis and deep learning in fiber-optic seismology

Benjamin Schwarz, Korbinian Sager, Philippe Jousset, Gilda Currenti, Charlotte Krawczyk, and Victor Tsai

Fiber-optic cables form an integral part of modern telecommunications infrastructure and are ubiquitous in particular in regions where dedicated seismic instrumentation is traditionally sparse or lacking entirely. Fiber-optic seismology promises to enable affordable and time-extended observations of earth and environmental processes at an unprecedented temporal and spatial resolution. The method’s unique potential for combined large-N and large-T observations implies intriguing opportunities but also significant challenges in terms of data storage, data handling and computation.

Our goal is to enable real-time data enhancement, rapid signal detection and wave field characterization without the need for time-demanding user interaction. We therefore combine coherent wave field analysis, an optics-inspired processing framework developed in controlled-source seismology, with state-of-the-art deep convolutional neural network (CNN) architectures commonly used in visual perception. While conventional deep learning strategies have to rely on manually labeled or purely synthetic training datasets, coherent wave field analysis labels field data based on physical principles and enables large-scale and purely data-driven training of the CNN models. The shear amount of data already recorded in various settings makes artificial data generation by numerical modeling superfluous – a task that is often constrained by incomplete knowledge of the embedding medium and an insufficient description of processes at or close to the surface, which are challenging to capture in integrated simulations.

Applications to extensive field datasets acquired with dark-fiber infrastructure at a geothermal field in SW Iceland and in a town at the flank of Mt Etna, Italy, reveal that the suggested framework generalizes well across different observational scales and environments, and sheds new light on the origin of a broad range of physically distinct wave fields that can be sensed with fiber-optic technology. Owing to the real-time applicability with affordable computing infrastructure, our analysis lends itself well to rapid on-the-fly data enhancement, wave field separation and compression strategies, thereby promising to have a positive impact on the full processing chain currently in use in fiber-optic seismology.

How to cite: Schwarz, B., Sager, K., Jousset, P., Currenti, G., Krawczyk, C., and Tsai, V.: Leveraging coherent wave field analysis and deep learning in fiber-optic seismology, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7856, https://doi.org/10.5194/egusphere-egu21-7856, 2021.

EGU21-7927 | vPICO presentations | SM2.2

Combining Distributed Acoustic Sensing and Beamforming in a Volcanic Environment on Mount Meager, British Columbia.

Sara Klaasen, Patrick Paitz, Jan Dettmer, and Andreas Fichtner

We present one of the first applications of Distributed Acoustic Sensing (DAS) in a volcanic environment. The goals are twofold: First, we want to examine the feasibility of DAS in such a remote and extreme environment, and second, we search for active volcanic signals of Mount Meager in British Columbia (Canada). 

The Mount Meager massif is an active volcanic complex that is estimated to have the largest geothermal potential in Canada and caused its largest recorded landslide in 2010. We installed a 3-km long fibre-optic cable at 2000 m elevation that crosses the ridge of Mount Meager and traverses the uppermost part of a glacier, yielding continuous measurements from 19 September to 17 October 2019.

We identify ~30 low-frequency (0.01-1 Hz) and 3000 high-frequency (5-45 Hz) events. The low-frequency events are not correlated with microseismic ocean or atmospheric noise sources and volcanic tremor remains a plausible origin. The frequency-power distribution of the high-frequency events indicates a natural origin, and beamforming on these events reveals distinct event clusters, predominantly in the direction of the main peaks of the volcanic complex. Numerical examples show that we can apply conventional beamforming to the data, and that the results are improved by taking the signal-to-noise ratio of individual channels into account.

The increased data quantity of DAS can outweigh the limitations due to the lower quality of individual channels in these hazardous and remote environments. We conclude that DAS is a promising tool in this setting that warrants further development.

How to cite: Klaasen, S., Paitz, P., Dettmer, J., and Fichtner, A.: Combining Distributed Acoustic Sensing and Beamforming in a Volcanic Environment on Mount Meager, British Columbia., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7927, https://doi.org/10.5194/egusphere-egu21-7927, 2021.

EGU21-3858 | vPICO presentations | SM2.2

Beamforming Reliability of DAS Ambient Noise Data and Wave Modes Identification

Yumin Zhao and Yunyue Elita Li

Ambient noise generated by the anthropological activities in the urban environments may contain both Rayleigh and Love waves. Due to the differences in the physics of Rayleigh and Love waves, a pre-knowledge of the wave modes in the cross-correlogram is essential for an accurate inversion of the subsurface velocity model. Several studies (Martin and Biondi, 2017; Martin et al., 2017; Luo et al., 2020) demonstrated that only Rayleigh waves can be extracted by cross-correlation if the virtual source is colinear with the DAS array based on the assumption that the ambient noise sources are random and uniformly distributed. However, in realistic cases, ambient noise sources may come from a certain direction (e.g., Dou et al., 2017; Zhang et al., 2019). Moreover, the source propagation direction should be resolved and used to correct the apparent dispersion curves. Zhao et al. (2020) and van den Ende et al. (2020) proposed that beamforming results are not always reliable due to the measurements of DAS.

Based on the synthetic DAS ambient noise data recorded by a near “L” shape array (Source-West corner of the Stanford DAS-1 array), we prove that beamforming can resolve the source direction when the ambient sources are mainly coming from one direction. Two important processing procedures are that: check the polarity in the data and apply polarity flip on one part of the data; apply amplitude normalization on the data if strong amplitude difference exits in the data. Based on the source direction, the coordinate of the DAS array, and amplitude ratio of the data recorded by the two segments of the DAS array, we propose an inversion method to calculate the amplitude ratio of the Rayleigh and Love waves generated by the ambient sources.

We apply the method to two 100-second DAS ambient noise data recorded by the Stanford DAS-1 array. We first resolve the source propagation direction from the two data. The results indicate that the ambient noise in the data were mainly generated by the motor vehicles running on the Campus Drive in the northwest of the array. Then we invert for the Rayleigh and Love waves amplitude ratio using the proposed method. The ratios for the two data are 0.2 and 0.13, respectively. The results suggest that the ambient noise generated by motor vehicles running on the northwest corner of the Campus Drive mainly contain Love waves.

How to cite: Zhao, Y. and Li, Y. E.: Beamforming Reliability of DAS Ambient Noise Data and Wave Modes Identification, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3858, https://doi.org/10.5194/egusphere-egu21-3858, 2021.

EGU21-12216 | vPICO presentations | SM2.2

Wave-selective beamforming with Distributed Acoustic Sensing

Daniel C. Bowden, Sara Klaasen, Eileen Martin, Patrick Paitz, and Andreas Fichtner

As fibre-optic DAS deployments become more common, researchers are turning to tried-and-true methods of locating or characterizing seismic sources such as beamforming. However, the strain measurement from DAS intrinsically carries its own sensitivities to both wave type and polarization (Martin et al. 2018, Paitz 2020 doctoral thesis). Additionally, a measurement along a conventional fibre-optic cable only provides one component of motion, and so certain azimuths may be blind to certain types of seismic sources, unless the cable layout can be designed to be oriented in multiple directions.

In this work, we explore the development and application of a beamforming algorithm that explicitly searches for multiple wavetypes. This builds on 3-component beamforming or Matched Field Processing (MFP) algorithms by Riahi et al. (2013), and Gal et al. (2018), where in addition to gridsearching over possible source azimuths, a distinct gridsearch is performed for each possible wavetype of interest. This does not solve the problem that a given cable orientation might be less sensitive to certain directions, but at least an array-response function can be robustly defined for each type of seismic excitation. This might help further distinguish whether beamforming observations are dominated by primary sources or by secondary scattering (van der Ende and Ampuero, 2020 preprint).

Much of this work uses analytic theory and synthetic examples. Time permitting, the enhanced algorithm will also be applied to data from the Mt. Meager experiment to explore its feasibility and efficacy with real data (EGU contribution from Klaasen et. al, 2021).

How to cite: Bowden, D. C., Klaasen, S., Martin, E., Paitz, P., and Fichtner, A.: Wave-selective beamforming with Distributed Acoustic Sensing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12216, https://doi.org/10.5194/egusphere-egu21-12216, 2021.

EGU21-10943 | vPICO presentations | SM2.2

Signal analysis between DAS and geophones in a vertical borehole from active and passive sources

Marius Paul Isken, Torsten Dahm, Sebastian Heimann, Christopher Wollin, Matthias Ohrnberger, Thomas Reinsch, and Charlotte M. Krawczyk

We present an analysis and qualitative comparison between acoustic data recorded on a distributed acoustic sensing (DAS) instrument (Silixa iDAS, version 2) and a three-component geophone chain colocated in a 400 m deep ICDP borehole in the magmatically active Vogtland area, Germany. A tight buffer single-mode fiber optic cable with a structured surface was installed and cemented behind casing down to total depth of the well. Additionally, a vertical array of 10 Hz geophones is suspended within the borehole.At the surface, further geophones were installed to shape a permanent three-dimensional seismic array. For this experiment the DAS system sampled strain-rate data at 10 m gauge length and 1 m spacing, yielding a high-resolution image of the wave field. Both seismic systems recorded data for 24 hours at 1 kHz sampling rate.

Within these 24 hours of recording, we shot a vertical seismic profile (VSP) with a 300 kg heavy and 2.4 m tall drop weight source moving up to a distance of 400 m away from the wellhead. Furthermore, passive seismic events at local and regional distances were recorded.

We compare the signal quality between the DAS system and the calibrated three-component geophones using the active and passive signals, to determine the sensitivity, signal-to-noise ratios and frequency response. Further we investigate the noise characteristics of both systems in this natural and remote environment, and evaluate the feasibility of borehole DAS behind casing for micro-earthquake monitoring. We give an outlook how dense DAS data can be utilized for VSP experiments with the aim to develop methods for fault detection and characterisation for application in DAS data recorded at the surface.

How to cite: Isken, M. P., Dahm, T., Heimann, S., Wollin, C., Ohrnberger, M., Reinsch, T., and Krawczyk, C. M.: Signal analysis between DAS and geophones in a vertical borehole from active and passive sources, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10943, https://doi.org/10.5194/egusphere-egu21-10943, 2021.

EGU21-7868 | vPICO presentations | SM2.2

Urban DAS recording of a vibroseismic campaign with a 21km-long dark fibre in Potsdam, Germany

CharLotte Krawczyk, Christopher Wollin, Stefan Lüth, Martin Lipus, Christian Cunow, Ariane Siebert, Philippe Jousset, and Sven Fuchs

The de-carbonization strategy of the city of Potsdam, Germany, incorporates the utilization of its geothermal potential.  As a first step of developing a deep geothermal project for district heating, an urban seismic exploration campaign of the Stadtwerke Potsdam took place in December 2020 in the city centre of Potsdam.  Since urban measurements are often difficult to setup and a low-footprint alternative is sought for, we supplemented the contractor-performed Vibroseis survey along three profiles by distributed acoustic sensing (DAS).  In close cooperation with the municipal utilities, we interrogated a 21 km-long dark telecommunication fibre whose trajectory followed the seismic lines as close as possible.  This was accompanied by a network of 15 three-component geophones for further control and research.

In this contribution we present the data set, the approach for geo-referencing the fibre, and first results regarding DAS recording capabilities of vibroseismic signals in an urban environment.  Following the paradigm that the high density of telecommunication networks in urban areas may facilitate the exploration of the often insufficiently known local geology, we strive to further shed light on the possibilities of their employment for urban exploration.  In this respect we aim at tackling the question of the accuracy of fibre localization, recording sensitivity and range of active stimulation.

How to cite: Krawczyk, C., Wollin, C., Lüth, S., Lipus, M., Cunow, C., Siebert, A., Jousset, P., and Fuchs, S.: Urban DAS recording of a vibroseismic campaign with a 21km-long dark fibre in Potsdam, Germany, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7868, https://doi.org/10.5194/egusphere-egu21-7868, 2021.

EGU21-14860 | vPICO presentations | SM2.2

Field Test of 12 serially connected FBG accelerometer parallelly with the vertical sensor of 4.5 -Hz geophones.

Aarathy Ezhuthupally Reghuprasad, Alberto Godio, Davide Luca Janner, Chiara Colombero, and Diego Franco

Fibre Bragg Grating (FBG) sensors are widely used for measuring vibrations in the fields like seismology and civil engineering. FBG sensors possess several advantages when compared to the traditional vibration sensors like immunity to electromagnetic interference, multiplexing, miniature size, higher sensitivity. Highly sensitive systems are required for capturing the seismic vibrations with low magnitude of acceleration. In this work a cost-effective cantilever based FBG accelerometer is developed. The structure is modelled using the software Solid Works and fabricated with PLA by 3D printing. Finally, a comparison test was carried out by serially connecting 12 FBG accelerometers parallelly to common vertical 4.5-Hz geophones outside the lab environment. Hammer shots were acquired along the tested line and the experimental results from both the systems were analysed and compared. The FBG system demonstrated here is suitable for seismic field acquisitions with potential applications to seismic refraction surveys, surface-wave analyses and passive seismic recordings.

 

How to cite: Ezhuthupally Reghuprasad, A., Godio, A., Janner, D. L., Colombero, C., and Franco, D.: Field Test of 12 serially connected FBG accelerometer parallelly with the vertical sensor of 4.5 -Hz geophones., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14860, https://doi.org/10.5194/egusphere-egu21-14860, 2021.

SM2.3 – Improving seismic networks operations from site scouting to waveform services and products

EGU21-6119 | vPICO presentations | SM2.3

ORFEUS Services and Activities to Promote Observational Seismology in Europe and beyond

Carlo Cauzzi, Jarek Bieńkowski, Susana Custódio, Sebastiano D'Amico, Christos Evangelidis, Philippe Guéguen, Christian Haberland, Florian Haslinger, Giovanni Lanzano, Lars Ottemöller, Stephane Rondenay, Reinoud Sleeman, and Angelo Strollo

ORFEUS (Observatories and Research Facilities for European Seismology; http://orfeus-eu.org/) is a non-profit foundation that promotes observational seismology in the Euro-Mediterranean area through the collection, archival and distribution of seismic waveform data, metadata, and closely related services and products. The data and services are collected or developed at national level by more than 60 contributing Institutions in Pan-Europe and further enhanced, 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 FAIR 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 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/). New emerging groups within ORFEUS are focused on mobile pools and computational seismology. ORFEUS services currently provide access to the waveforms acquired by ~ 14,500 stations, including dense temporary experiments, with strong emphasis on open, high-quality data. Contributing to ORFEUS data archives means benefitting from long-term archival, state-of-the-art quality control, improved access, increased usage, and community participation. Access to data and products is ensured through state-of-the-art information and communication technologies, with strong emphasis on federated web services that considerably improve seamless user access to data gathered and/or distributed by the various ORFEUS institutions. Web services also facilitate the automation of downstream products. Particular attention is paid to adopting clear policies and licenses, 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), which comprises European Earth scientists with expertise on a broad range of disciplines. All ORFEUS services are developed in coordination with EPOS and are largely integrated in the EPOS Data Access Portal, as ORFEUS is one of the founding Parties and fundamental contributors of the EPOS Thematic Core Service for Seismology (https://www.epos-eu.org/tcs/seismology). In this contribution, we selectively present the activities of ORFEUS, with the main aims of facilitating seismological data discovery and encouraging open data sharing and integration, as well as promoting best practice in observational seismology.

How to cite: Cauzzi, C., Bieńkowski, J., Custódio, S., D'Amico, S., Evangelidis, C., Guéguen, P., Haberland, C., Haslinger, F., Lanzano, G., Ottemöller, L., Rondenay, S., Sleeman, R., and Strollo, A.: ORFEUS Services and Activities to Promote Observational Seismology in Europe and beyond, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6119, https://doi.org/10.5194/egusphere-egu21-6119, 2021.

EGU21-15558 | vPICO presentations | SM2.3

The EIDA federator – a one-stop access to EIDA seismic data holdings

Philipp Kaestli, Daniel Armbruster, and The EIDA Technical Committee

With the setup of EIDA (the European Integrated Data Archive https://www.orfeus-eu.org/data/eida/) in the framework of ORFEUS, and the implementation of FDSN-standardized web services, seismic waveform data and instrumentation metadata of most seismic networks and data centers in Europe became accessible in a homogeneous way. EIDA has augmented this with the WFcatalog service for waveform quality metadata, and a routing service to find out which data center offers data of which network, region, and type. However, while a distributed data archive has clear advantages for maintenance and quality control of the holdings, it complicates the life of researchers who wish to collect data archived across different data centers. To tackle this, EIDA has implemented the “federator” as a one-stop transparent gateway service to access the entire data holdings of EIDA.

To its users the federator acts just like a standard FDSN dataselect, station, or EIDA WFcatalog service, except for the fact that it can (due to a fully qualified internal routing cache) directly answer data requests on virtual networks.

Technically, the federator fulfills a user request by decomposing it into single stream epoch requests targeted at a single data center, collecting them, and re-assemble them to a single result.

This implementation has several technical advantages:

  • It avoids response size limitations of EIDA member services, reducing limitations to those imposed by assembling cache space of the federator instance itself.
  • It allows easy merging of partial responses using request sorting and concatenation, and reducing needs to interpret them. This reduces computational needs of the federator and allows high throughput of parallel user requests.
  • It reduces the variability of requests to end member services. Thus, the federator can implement a reverse loopback cache and protect end node services from delivering redundant information and reducing their load.
  • As partial results are quick, and delivered in small subunits, they can be streamed to the user more or less continuously, avoiding both service timeouts and throughput bottlenecks.

The advantage of having a one-stop data access for entire EIDA still comes with some limitations and shortcomings. Having requests which ultimately map to a single data center performed by the federator can be slower by that data center directly. FDSN-defined standard error codes sent by end member services have limited utility as they refer to a part of the request only. Finally, the federator currently does not provide access to restricted data.

Nevertheless, we believe that the one-stop data access compensates these shortcomings in many use cases.

Further documentation of the service is available with ORFEUS at http://www.orfeus-eu.org/data/eida/nodes/FEDERATOR/

How to cite: Kaestli, P., Armbruster, D., and EIDA Technical Committee, T.: The EIDA federator – a one-stop access to EIDA seismic data holdings, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15558, https://doi.org/10.5194/egusphere-egu21-15558, 2021.

EGU21-5889 | vPICO presentations | SM2.3

Waveform quality checks at German networks

Klaus Stammler

Recently a set of quality control procedures have been implemented at the data center of the BGR (Seismic Survey of Germany). Goal is to identify unusual deviations in amplitude, timing and waveform caused by data and metadata errors. One of the strategies applied is to evaluate long term observations of seismic noise at specific frequencies at many stations. Particularly at lower frequencies this analysis is quite sensitive to amplitude changes. Also useful is the characterization of station sites by looking at anthropogenic noise patterns in a frequency range of 4-14 Hz. The sites show fundamental differences when looking at daily and weekly noise patterns and some also have specific responses to local wind. Changes in the noise patterns indicate changes in the environment or uncompensated hardware or metadata changes. Furthermore, correlations of teleseismic signals reveal  possible inconsistencies in waveform shape, travel time residuals and amplitudes within the station set. When applied systematically a statistical  analysis of the correlation parameters indicates long term deviations in these three observables. Finally, a formal check of the transfer function given in the metadata is implemented to identify wrong settings in the normalization and illegal specifications in the poles and zeros (conjugate complex pairs and negative real part at poles). These implemented measures help us to keep our data at a high quality level and to react quickly on the occurrence of  hardware and metadata errors.

 

How to cite: Stammler, K.: Waveform quality checks at German networks, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5889, https://doi.org/10.5194/egusphere-egu21-5889, 2021.

EGU21-12436 | vPICO presentations | SM2.3

Improving data, metadata and service quality within Résif-SI

Jonathan Schaeffer, Fabien Engels, Marc Grunberg, Christophe Maron, Constanza Pardo, Jean-Marie Saurel, David Wolyniec, Patrick Arnoul, Philippe Bollard, Jérôme Touvier, Jérôme Chèze, Fabrice Peix, and Catherine Pequegnat

Résif, the French seismological and geodetic network, was launched in 2009 in an effort to develop, modernize, and centralize geophysical observation of the Earth’s interior. This French research infrastructure uses both permanent and mobile instrument networks for continuous seismological, geodetic and gravimetric measurements.

Résif-SI is the Information System that manages, validates and distributes seismological data from Résif.

The construction of Résif-SI has lead to a federated organisation gathering several data and metadata producers (Nodes) and a national Seismological Data Centre.

The Résif Seismological Data Centre is one of 19 global centres distributing data and metadata in formats and using protocols which comply with International Federation of Digital Seismograph Networks (FDSN) standards. It is also one of the eleven nodes in EIDA, the European virtual data centre and seismic data portal in the European Plate Observing System (EPOS) framework.

Inside Résif-SI, each Node has it's specificities and dedicated procedures in order to manage and validate the data and metadata workflow from the station instruments to the Résif Seismological Data Centre.

To meet the expectations and needs of the end user in terms of data quality, metadata consistency and service availability, Résif-SI operates a complex set of quality enhancement operations.

This contribution will present the quality expectations that are in the core of Résif-SI, and show the methods and tools that help us meeting the expectations, and that could be of interest for the rest of the community.

We will then list some of our quality improvement projets and the expected results.

How to cite: Schaeffer, J., Engels, F., Grunberg, M., Maron, C., Pardo, C., Saurel, J.-M., Wolyniec, D., Arnoul, P., Bollard, P., Touvier, J., Chèze, J., Peix, F., and Pequegnat, C.: Improving data, metadata and service quality within Résif-SI, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12436, https://doi.org/10.5194/egusphere-egu21-12436, 2021.

EGU21-263 | vPICO presentations | SM2.3

Continuous Improvement in the Performance and Operations of the Global Seismographic Network (GSN)

Katrin Hafner, Dave Wilson, Rob Mellors, and Pete Davis

The decades long recordings of high-quality open data from the Global Seismographic Network have facilitated studies of earth structure and earthquake processes, as well as monitoring of earthquakes and explosions worldwide.  These data have also enabled a wide range of transformative, cross-disciplinary research that far exceeded the original expectations and design goals of the network, including studies of slow earthquakes, landslides, the Earth’s “hum”, glacial earthquakes, sea-state, climate change, and induced seismicity. 

The GSN continues to produce high quality waveform data, metadata, and multiple data quality metrics such as timing quality and noise levels.   This requires encouraging equipment vendors to develop modern instrumentation, upgrading the stations with new seismic sensors and infrastructure, implementing consistent and well documented calibrations, and monitoring of noise performance.    A Design Goals working group is convening to evaluate how well the GSN has met its original 1985 and 2002 goals, as well as how the network should evolve in order to be able to meet the requirements for enabling new research and monitoring capabilities.   

In collaboration with GEOFON and GEOSCOPE the GSN is also reviewing the current global distribution and performance of very broadband and broadband stations that comprise these three networks.  We are working to exchange our expertise and experience about new technologies and deployment techniques, and to identify regions where we could collaborate to make operations more efficient, where current efforts are overlapping or where we have similar needs for relocating stations. 

How to cite: Hafner, K., Wilson, D., Mellors, R., and Davis, P.: Continuous Improvement in the Performance and Operations of the Global Seismographic Network (GSN), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-263, https://doi.org/10.5194/egusphere-egu21-263, 2021.

EGU21-8510 | vPICO presentations | SM2.3

The national seismic network for the Maltese islands: Update 2021

Pauline Galea, Matthew Agius, George Bozionelos, Sebastiano D'Amico, and Daniela Farrugia

The Maltese islands are a small country 15 km wide by 30 km long located about 100 km south of Sicily, Italy. Since 2015 Malta has set up a national seismic network. The primary aim of this network is to monitor in real-time and to locate more accurately the seismicity close to the islands and the seismicity in the Sicily Channel, offshore between Sicily, Tunisia and Libya. This Channel presents a range of interesting and complex tectonic processes that have developed in response to various regional stress fields mainly as a result of the collision between the African plate with Europe. The Maltese islands are known to have been affected by a number of earthquakes originating in the Channel, with some of these events estimated to be very close to the islands.

The seismotectonic characteristics of the Sicily channel, particularly south of the Maltese islands, is not well understood. This situation is being partially addressed through an increase in the number of seismic stations on the Maltese archipelago. The Malta Seismic Network (FDSN code ML), managed by the Seismic Monitoring and Research Group, within the Department of Geosciences, University of Malta, currently comprises 8 broadband, 3-component stations over an area slightly exceeding 300 km2. We present a technical description of the MSN including quality control tests such as spectral analysis (Power Spectral Density and HVSR), station orientations and timings as well as examples of local and regional earthquakes recorded on the network. We describe the upgrades to real-time data transmission and archiving, and automated epicentre location for continuous seismic monitoring using the local network amalgamated with a virtual seismic network to monitor the seismicity in the extended Mediterranean region. Such a dense national network, besides improving epicentral location in the Sicily Channel, is providing valuable information on microearthquake activity known to occur in close proximity to the islands, which has been very difficult to study in the past. It also provides an important tool for analysing site response and site amplification related to underlying geology, which constitutes a major component of seismic hazard analysis on the islands. Furthermore, the increase in seismic stations to the seismic monitoring system provides more robust earthquake estimates for the tsunami monitoring/simulation system.

Funding for stations was provided by Interreg Italia-Malta projects (SIMIT and SIMIT-THARSY, Codes B1-2.19/11 and C1-3.2-57) and by Transport Malta.

How to cite: Galea, P., Agius, M., Bozionelos, G., D'Amico, S., and Farrugia, D.: The national seismic network for the Maltese islands: Update 2021, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8510, https://doi.org/10.5194/egusphere-egu21-8510, 2021.

EGU21-6195 | vPICO presentations | SM2.3

Residual analysis of large strong-motion flatfiles as a tool for detecting data error and anomalies

Claudia Mascandola, Giovanni Lanzano, and Francesca Pacor

The rapid increase of seismic waveforms, due to the increment of seismic stations and continuous real-time streaming to data centres, leads to the need for automatic procedures aimed at supporting data processing and data quality control. In this study, we propose a semi-automatic procedure for the consistency check of large strong-motion datasets, classifying the anomalies observed on the residuals analysis and identifying the possible causes.

The data collected in the strong-motion databases are usually arranged as parametric tables (called flatfiles), used to disseminate the Intensity Measures (IMs) and the associated metadata of the processed waveforms. This is the current practice for the ITalian ACcelerometric Archive (ITACA, D’Amico et al., 2020) and Engineering Strong Motion (ESM; Lanzano et al. 2019a) databases. The adopted criteria for flatfile compilation are designed to collect IMs and related metadata in a uniform, updated, and traceable way, with the aim of providing datasets useful to develop Ground Motion Models (GMMs) for Probabilistic Seismic Hazard Assessment (PSHA) and engineering applications. Therefore, the consistency check of the flatfiles is a crucial task to improve the quality of the products provided by the waveform services.

The proposed procedure is based on the residual distributions obtained from ad-hoc ground motion prediction equations for the ordinates of the 5% damped acceleration response spectra. In this study, we focus on the active shallow crust events in ITACA, considering the ITA18 ground motion model (Lanzano et al., 2019b) as a reference for Italy. The total residuals, computed as logarithm difference between observations and predictions, are decomposed in between-event, between-station and event-and-station corrected residuals by applying a mixed-effect regression (Bates et al., 2015). This is the common practice for the (partial) removal of the ergodic assumption in empirical GMMs (e.g., Stafford 2014), where the contribution of the systematic corrective effects of event and station on aleatory variability are identified and shifted to the epistemic uncertainty. Afterward, the proposed procedure is applied to raise a warning in case of anomalous residual values. Warnings are provided when the normalized residuals exceed a certain threshold, in three ranges of periods (i.e., 0.01-0.15 s, 0.15-1 s, 1-5 s). The causes of warnings may be several and may concern the event, the site, the waveform, or a combination of them. Among the possible sources of anomalous trends, the more common are: preliminary or inaccurate event localization or magnitude, wrong soil category assigned based on proxies, misleading tectonic regime assigned to the earthquake, and fault directivity that may cause strong-ground motion amplification in certain directions. Warnings may also raise for peculiarities in the site-response (e.g., large amplifications/de-amplifications at certain frequency-bands) and to the occurrence of near-source effects in the waveforms (see Pacor et al., 2018). Based on the raised warnings, a decision tree classifier is developed to identify the common anomaly sources and to support the consistency check of the semi-automatic procedure.

This study may help to enhance the waveform services and related products, besides reducing the variability of ground motion models and guiding decisions for site characterization studies and network maintenance.

How to cite: Mascandola, C., Lanzano, G., and Pacor, F.: Residual analysis of large strong-motion flatfiles as a tool for detecting data error and anomalies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6195, https://doi.org/10.5194/egusphere-egu21-6195, 2021.

EGU21-459 | vPICO presentations | SM2.3

Evaluation of seismic sensor orientations in the full moment tensor inversion results

Mohammadreza Jamalreyhani, Mehdi Rezapour, and Pınar Büyükakpınar

Three-component seismograms recorded by seismic sensors are momentous data to study the source mechanism of earthquakes. Correct orientation of sensors relative to the true north is important for the waveform inversion techniques. Yet, the non-precise orientation of horizontal components of seismic sensors has been reported in many seismic networks worldwide. In this study, we evaluated the effect of sensor misorientations (deviations from the true north) on time-domain moment tensor inversion, relying on the recent sensor orientation studies on broadband seismic networks in Iran and Turkey. We selected several well-recorded countrywide local and regional moderate magnitude earthquakes, which are associated with the tectonic events, in the time period of 2012-2019. We calculated the moment tensor inversion of those events before and after applying the orientation correction using a Bayesian bootstrap-based probabilistic method. This leads to reaching the uncertainties and trade-offs of parameters and helps to stabilize the inversion. Our analysis shows that in the presence of misoriented sensors, an approximate solution is achievable. However, this includes the remarkable uncertainties in inverted parameters and makes the reliable determination of the moment tensor’s elements challenging. We also found an additional significant non-double couple component while using the misoriented radial and transverse components. Results show that the misfit and uncertainties decrease significantly when sensor orientation correction applied. We suggest that the evaluation of metad