EGU General Assembly 2023  

Petrus Perigrinus de Maricourt, a French physicist and mathematician wrote the first descriptions of the properties of magnets, now known as the Epistola de Magnete in 1269.  He summarized what was known at the time concerning the use of the compass, writing “that while the investigator in this subject must understand nature and not be ignorant of the celestial motions, he must also be very diligent in the use of his own hands, so that through the operation of this stone he may show wonderful effects.” (translation by J. Gimpel, 1976).  This is still true today, particularly for those of us who study the ancient magnetic field through ‘accidental’ records in geological and archaeological materials.  In this lecture I will review efforts to use ‘our own hands’ to understand the structure of the time averaged Earth’s magnetic field over the last five million years, using both directions (obtained with compasses!) and intensities.

How to cite: Tauxe, L.: Hunting the Magnetic Field, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1788, https://doi.org/10.5194/egusphere-egu23-1788, 2023.

Permeability is a key physical property across all spatial scales in the Earth’s crust and exerts significant control on the behaviour of Earth systems, with implications for natural hazards (e.g., earthquakes, slope instabilities, volcanic eruptions) and geo-resource management (e.g., geothermal energy, carbon capture and sequestration, ore deposit formation). Amongst other processes, rock-fluid interactions and the interplay between precipitation and dissolution complicates the microstructure of these materials, modifying the efficiency of fluid flow. For example, the permeability of volcanic rocks is generally controlled by the presence of pores and microfractures, but the continuum from pore-dominated to microfracture-dominated permeability is significantly perturbed by the introduction of alteration minerals that reduce the void space available to fluid flow over time. The propensity, extent, and timescales of rock alteration are therefore important factors influencing rock permeability. However, obtaining a systematic understanding of the intricate relationships between rock alteration and changes in permeability – including dissolution, transport, and redistribution of chemical compounds – is challenging. As a result, we do not fully understand how these processes modify the structure of permeable channels and over what timescales they may hamper fluid flow, limiting our ability to effectively model, for example, geothermal reservoirs or volcanic processes.

The mission of the new Rock Physics and Geofluids (RPGL) group at EPFL is to address how secondary mineral precipitation – starting with silica (SiO₂) - changes the permeability of rocks. Using a mix of rock physics, microstructural and geochemical characterization, and water-rock interaction experiments, we will quantify 1) the physical and chemical conditions promoting silica alteration under a wide range of crustal conditions, 2) how the geometry of fluid-flow pathways in rocks changes over time, 3) how these changes modify permeable flow, and 4) on what timescales these processes are active. Our goal is to establish the infrastructural, experimental, and analytical foundation needed to more broadly study the relationships between rock-fluid interactions and fluid flow, for potential application to natural hazard and geo-energy research.

How to cite: Kushnir, A.: Permeability, alteration, and microstructure: A (hopefully) coupled rock physics and geochemical approach to how rock-fluid interactions change permeability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4505, https://doi.org/10.5194/egusphere-egu23-4505, 2023.

Statistical analyses of NOAA POES data have recently evidenced electron burst losses 1.5-3.5 h before strong earthquakes in the West Pacific and 55-59 h before strong earthquakes in East Pacific. The conditional probability of a strong seismic event after an ionospheric loss event was calculated depicting possible scenarios in both areas. It presented a geohazard risk reduction initiative that can gain valuable preparation time by adopting a probabilistic short-term warning a few hours prior, especially for tsunamis in those dangerous areas. As electron losses were detected in the same region both for West and East Pacific earthquakes, the probability of a strong event in the West Pacific would be first considered and vanish in less than 4 h. Then, after considering the seismic activity, a statistical evaluation of a disastrous event for the East Pacific coast is generated, so defining a time-dependent increase in conditional probability.

How to cite: Fidani, C.: A suitable time-dependent conditional probability for Pacific strong earthquakes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-852, https://doi.org/10.5194/egusphere-egu23-852, 2023.

This is one of a series of papers in which  we investigate the Lower ionospheric variation on the occasion of intense tectonic activity.In the present paper, we investigate the TEC variations during the intense seismic activity in Thessaly, on March 2021 over Europe. The Total Electron Content (TEC) data are been provided by the  Hermes GNSS Network managed by GNSS_QC, AUTH Greece, the HxGN/SmartNet-Greece of Metrica S.A, and the EUREF Network. These data were analysed using Discrete Fourier Analysis in order to investigate the TEC turbulence. The results of this investigation indicate that the High-Frequency limit f_o of the ionospheric turbulence content, increases as aproaching the occurrence time of the earthquake, pointing to the earthquake epicenter, in accordane to our previous investigations. We conclude that the Lithosphere Atmosphere Ionosphere Coupling, LAIC, mechanism through acoustic or gravity waves could explain this phenomenology.

Keywords: Seismicity, Lower Ionosphere, Ionospheric Turbulence, Brownian Walk, Aegean area.

How to cite: Contadakis, M. E., Arabelos, D. N., Christos, P., Bitharis, S., and Scordilis, E.: Lower  Ionospheric  variation over Europe during the  tectonic activity in the area of Thessaly, Greece on March of 2021., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1592, https://doi.org/10.5194/egusphere-egu23-1592, 2023.

An instrumental array was established in southwest China for Monitoring Vibrations and Perturbations in the Lithosphere, Atmosphere and Ionosphere (MVP-LAI).  We retrieved multiple-geophysical data from the array to investigate common characteristics in LAI before earthquakes.  Broadband seismometers are utilized to monitor ground vibrations in the lithosphere.  Barometers record changes in air pressure near the Earth’s surface.  Magnetometers monitor variations in the ionospheric currents ~100 km above the Earth’s surface.  Instead of GPSTEC (Global Positioning System Total Electron Content), electromagnetic signals transmitted from the BDS (BeiDou navigation system) geostationary satellites are received by ground-based GNSS (Global Navigation Satellite System) receivers to compute TEC data.  The BDSTEC from the geostationary satellites continuously monitor changes in TECs ~350 km in altitude right over the array.  We transferred these data into the frequency domain and found that ground vibrations, air pressure, the magnetic field, and BDSTEC data share the frequency ~5×10^-3 Hz before major earthquakes.  Ground vibrations exhibit frequency characteristics of ~5×10^-3 Hz due to resonance of nature frequencies before failure of materials (i.e., dislocations of faults, and earthquakes).  Ground vibrations with frequency of ~5×10^-3 Hz persistently hit the bottom of the atmosphere that can trigger atmospheric resonance before earthquakes.  Double resonance (i.e., crustal and atmospheric resonance) provides the new way to reveal the seismo-anomalies of multiple geophysical parameters in LAI.  Double resonance would shed a light in earthquake prediction in practice once we face the major issue for efficiently retrieving resonance signals from multiple observation data. 

How to cite: Chen, C.-H., Lin, K., Zhang, X., and Gao, Y.: Double resonance before earthquakes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2087, https://doi.org/10.5194/egusphere-egu23-2087, 2023.

The ELSEM-Net (hELlenic Seismo-ElectroMagnetics Network, http://elsem-net.uniwa.gr) is a telemetric network of ground-based monitoring stations for the study of fracture-induced electromagnetic emissions. It comprises 11 telemetric stations, spanning all over Greece, and has continuously been operated for almost 30 years. In this paper we present the new, custom designed, instrumentation of the telemetric stations. Specifically, we present both the hardware and the firmware/software used, from antennae to data acquisition and data management. Finally, we present recent recordings prior to significant strong earthquakes (EQs) that have happened in Greece, as well as the obtained analysis results, using nonlinear time series analysis methods, indicating that the acquired signals embed important features associated with the impending EQ.

How to cite: Politis, D. Z., Potirakis, S. M., Malkotsis, P., Papadopoulos, N., Dimakos, D., Exarchos, M., Liadopoulos, E., Contoyiannis, Y. F., Charitopoulos, A., Kontakos, K., Doukakis, D., Koulouras, G., Melis, N., and Eftaxias, K.: New features of the ELSEM-Net electromagnetic monitoring stations network and analysis of recent data associated with strong earthquakes., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2126, https://doi.org/10.5194/egusphere-egu23-2126, 2023.

On 19 September 2021, La Palma Volcano started a VEI 3 eruption. Here we will illustrate an investigation for at least six months before the eruption with the aim of searching possible lithosphere atmosphere and ionosphere couplings.

We identify and compare the anomalies from the seismic catalogue, the geomagnetic ground observatories, the atmospheric climatological datasets, TEC maps, CSES and Swarm satellites data with respect to the volcano location and the time cumulative trends of anomalies are analyzed.

We identify a temporal migration of the seismicity from one year before the eruption at a depth of 40 km possibly associated with magma migration, firstly to a deep chamber (20-13km depth) and in the last 10 days in a shallower magma chamber. CSES-01 detects an increase in electron density at the same time as vertical ground magnetic field anomalies, very likely due to the magma uprising. A final increase of carbon monoxide 1.5 months before the eruption with unusually high values of TEC suggests the degassing of magma before the eruption associated with shallow seismicity that preceded the eruption by ten days. We identify possible different coupling mechanisms, e.g., chain of mechanical, thermal, chemical and electromagnetic phenomena, or pure electromagnetic coupling). These different lithosphere-atmosphere-ionosphere coupling mechanisms can coexist.

Our results highlight the importance of integrating several observation platforms and datasets from the ground and space (earth observation satellites) to better understand the dynamics of the processes and associated natural hazards affecting our planet.

How to cite: Marchetti, D., Zhang, H., Zhu, K., Zhima, Z., Yan, R., Shen, X., Piscini, A., Chen, W., Cheng, Y., He, X., Wang, T., Wen, J., Zhang, D., and Zhang, Y.: Possible Lithosphere Atmosphere Ionosphere Coupling before 19 September 2021 La Palma volcano eruption, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2187, https://doi.org/10.5194/egusphere-egu23-2187, 2023.

WATER’S GEOCHEMICAL COMPOSITION AS INDICATOR OF GEODYNAMIC ACTIVITY 

A.Kazarian, H. Kazarian IGN AN NAN

A detailed analysis of a long-term collection of hydro-geochemical data was carried out over a ten-year period. It revealed consistent iterations of signs of a process of earthquake preparation in this region. This preparation process has several distinct stages, which can be identified by noticeable changes in the geochemical composition of self-pouring well water. The earthquake preparation process is graphically visible and has a similar duration to the post-earthquake aftershock activity duration. The visualization of hydro-geochemical data from the pre- and post-earthquake periods for different (M> 6) earthquakes in this region shows a very similar pattern of behaviors and duration of behaviors for events of varying magnitudes and distances from the observation wells.

Changes in the main fluctuation trend of the geochemical data for helium (He) and a decrease in the standard deviation of the series for other main components appear as earthquake precursors (Na, K, HCO3, SO4, Cl, Ca, F). The detectable duration of a main shock's preparation process is approximately a year. The detailed examination of the data time series reveals a strong correlation between the overall geodynamic activity of the region and the hydrogeochemical composition of the observed wells.

The detailed analysis of earthquake activity in the region suggests a periodic nature of basic seismicity and its relationship with earthquake focal mechanisms. The obtained daily histograms for seismic activity in Armenia, Turkey, Greece, and Italy regions calculated by local time show cyclical activity patterns of 24 and 12 hours. This is consistent with variations in He and other important components in the well waters. The hypothesis and conclusion of this scientific research project are that in seismically active zones, the dynamics of hidden active tectonic processes can potentially be a priori diagnosed using this hydro-geochemical monitoring method.

How to cite: Kazarian, A. and Kazarian, A.: Water chemical composition as indicator of geodynamic activity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3596, https://doi.org/10.5194/egusphere-egu23-3596, 2023.

Three earthquakes of comparable magnitude and in different tectonic contexts occurred on 15 June 2019 (M7.2) in New Zealand (Kermadec Islands), on 6 July 2019 (M7.1) in California (Ridgecrest) and on 21 May 2021 (M7.3) in China (Maduo) (dates in UT). We applied a multiparameter - multilayer approach to lithospheric, atmospheric and ionospheric data, the latter taken from CSES  and Swarm satellites, before the mentioned large earthquakes to detect potential pre-earthquake anomalies. In all case studies, we note the following: a) similar precursor times of occurrences, confirming the Rikitake law for which the larger the earthquake magnitude the longer the anticipation time of the precursor and b) a clear acceleration of the possible precursory anomalies before each mainshock, as typical of critical systems approaching a critical state. We propose an interpretative model to take into account the chain of detected phenomena.

How to cite: De Santis, A., Campuzano, S. A., Calcara, M., Cianchini, G., D'Arcangelo, S., De Caro, M., Di Mauro, D., Fidani, C., Nardi, A., Orlando, M., Perrone, L., Piscini, A., Sabbagh, D., and Soldani, M.: Comparison of the precursory effects in lithosphere, atmosphere and ionosphere of three large earthquakes with comparable magnitude: the cases of 2019 Kermadec Islands (NZ) and Ridgecrest (USA) earthquakes and 2021 Maduo (China) earthquake, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3854, https://doi.org/10.5194/egusphere-egu23-3854, 2023.

We present a study on temporal and spatial characteristics of Thermal Radiation anomalies (TRA) and ionospheric total electron content (TEC) pre-earthquake abnormalities associated with the occurred in 2022 “anniversary” earthquakes. “Anniversary”  is a quake occurring on the same date and following the years after the main earthquake, plus or minus several days.

We studied eleven large earthquakes in four regions: i/Japan: M7.3 of 03.16.2022 and M9.0 of 03.11.2011 East Coast Honshu; ii/Mexico: M7.6 of 09.19.2022 Michoacan; M7.1 of 09.19.2017 Puebla and M8.0 of 09.19.1985 Mexico City;/iii Chile: M5.7 02.28.2022 Bio-Bio and M8.8 02.27.2010 Maule and /iv Taiwan: M6.9 of 09.18.2022 Taitung and M7.7 of 09.21.1999 Chi-Chil and M6.7 of 03.22.2022 Taitung and M6 of 03.27.2013 Nantou earthquake.

We analyzed for TRA and TEC anomalies concerning the earthquake preparation zone (EPZ). For EPZ estimates, we use Dobrovolsky et al. (1979), and Bowman et al. (1998) estimates where the EPZ radius scales exponentially with earthquake magnitude, especially from Mw ≥ 6.0 onwards, and gives an extended coverage at larger magnitudes to examine TRA and ionospheric TEC anomalies. The main goals of this study were: 1/to understand the seismotectonic conditions that preceded the earthquake re-occurrence in the same place and on the same day(s): 2/ to perform a validation study about pre-earthquake signal occurrences in the same atmospheric and solar-geophysical conditions and 3/ to understand the potential triggering mechanism. Our preliminary results show synergetic coordination between the appearance of pre-earthquake transients’ effects in the atmosphere and ionosphere (with a short time lag, from hours up to a few days). The spatial characteristics of pre-earthquake anomalies were associated with the large area but inside the preparation region estimated by Dobrovolsky-Bowman. The pre-earthquake nature of the signals in the atmosphere and ionosphere was revealed by simultaneous analysis of satellite, GPS/TEC, and Satellite Earth observations. The “anniversary” events are recognized with common pre-earthquake transient re-occurrence patterns in the atmosphere/ionosphere within EPZ, scaled to the extent of the earthquake magnitude.

How to cite: Ouzounov, D., Pulients, S., Liu, J.-Y., Hattori, K., Kafatos, M., and Taylor, P.: Transient effects in the atmosphere/ionosphere and their re-occurrence before large earthquakes. Case study for the 2022 “anniversary” events., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4399, https://doi.org/10.5194/egusphere-egu23-4399, 2023.

There has been little research on how the composition of underlying rock formation affects animal species’ distribution and abundance. The subject is worthy of consideration as, for example,  it has been shown that ultrabasic and serpentine rocks in particular can give rise to plant biodiversity hotspots with a high level of endemism. Corresponding studies of fauna are lacking. We aim to test the hypothesis that rock type affects mammal abundance and biodiversity.

Here we present a comparative analysis of the abundance of mammals and its relationship with geological composition in the area of Gorny Altai, a mountainous region in Russia.

We used GIS approaches to map the influence of rock types on mammal abundance, while holding other factors such as soil type, relief, etc. constant. The study reveals significant correlations between underlying geology and variation in mammal distribution even when other factors such as soil type, climate and vegetation are held constant.

Intrusive rocks were found to have the greatest impact on variation in mammal distribution whereas sedimentary and metamorphic rocks have almost no effect. A characteristic feature of magmatic formations is their clear geochemical specialization, i.e. certain geochemical anomalies (Fe, Cu, Au, Hg, Ag, etc.) are confined to intrusions. We suggest that geophysical fields (magnetic and electric fields) and geochemical anomalies associated with intrusive rocks may have an impact on the distribution and species composition of mammals, as well as geodynamic processes such as fault activity. This finding has implications for further research into the phenomenon of animals’ anticipatory responses to earthquakes. 

How to cite: Grant, R., Shitov, A., and Karanin, A. V.: Mammal abundance varies with geochemical specialisation in the underlying rock formations., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5813, https://doi.org/10.5194/egusphere-egu23-5813, 2023.

In this study we examine earthquakes with magnitude M>5 in the year 2022 where the epicenters are crossed by sub-ionospheric narrowband VLF/LF radio links. The study regions are Italy, Aegean area, and the Balkan Peninsula. Ideal suited for this task are paths from the transmitters TBB (26.70 kHz, Bafa, Turkey), ITS (45.90 kHz, Niscemi, Sicily, Italy), and ICV (20.27 kHz, Tavolara, Italy) to the seismo-electromagnetic receiver facility GRZ (Graz, Austria). The receiver is part of a wider network, this gives the opportunity to have multiple simultaneous crossings of an earthquake event.

We investigate electric field amplitude variations in the time span a few days around the main shock, in particular we apply the so-called night-time amplitude method. All electric field data sets have 1 sec temporal resolution. A crucial point is a certain threshold magnitude to obtain statistically significant results, but to firm up the results additional complementary investigations are necessary.

In summary, VLF/LF investigations of strong earthquakes show the complex interplay between the lithospheric events and electric field amplitude waveguide variations, multi-parametric observations in a network could be a tool to derive robust results.

How to cite: Eichelberger, H., Boudjada, M. Y., Schwingenschuh, K., Besser, B. P., Wolbang, D., Solovieva, M., Biagi, P. F., Galopeau, P., Jaffer, G., Aydogar, Ö., Schirninger, C., Muck, C., Jernej, I., and Magnes, W.: Sub-ionospheric VLF/LF waveguide variations related to magnitude M>5 earthquakes in the eastern Mediterranean area, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8521, https://doi.org/10.5194/egusphere-egu23-8521, 2023.

We report on two earthquakes (EQs) that occurred in Croatia at a distance less than 200 km from the Austrian Graz facility (15.46°E, 47.03°N). Those EQs happened on March 22 and December 29, 2020, with magnitudes of Mw5.4 and Mw6.4, respectively. The epicenters were at geographical coordinates (16.02°E, 45.87°N; 16.21°E, 45.42°N) with focuses smaller than 10 km.  Austrian Graz facility leads to detect more than fifteen VLF and LF transmitter signals (Schwingenschuh et al., 2011, Biagi et al., 2019). Transmitter ray paths cross over the EQs epicenters in particular those localised in ICV and ITS (Italy) and TBB (Turkey). We emphasize in our study on the signal fluctuations before/after the sunrise- and sunset-times, or terminator times (TTs). Transmitter amplitude signals exhibit precursor anomalies that related to EQs disturbances occurring particularly at the falling off or the growth of the ionospheric D-layer. Ground-based stations (e.g. Rozhnoi et al., 2009) and satellite observations (e.g. Zhang et al., 2020) have reported such EQs ionospheric disturbances at several occasions.

References:

Biagi et al., The INFREP Network: Present Situation and Recent Results, Open J. Earth. Research, 8, 2019. Rozhnoi et al., Anomalies in VLF radio signals prior the Abruzzo earthquake (M=6.3) on 6 April, 2009, Natural Hazards and Earth System Science, 9, 2009. Schwingenschuh et al., The Graz seismo-electromagnetic VLF facility, Nat. Hazards Earth Syst. Sci., 11, 2011. Zhang et al., Multi-experiment observations of ionospheric disturbances as precursory effects of the Indonesian Ms6.9 earthquake on August 05 2018, Remote Sens. J., 12, 2020.

How to cite: Boudjada, M. Y., Biagi, P. F., Eichelberger, H. U., Schwingenschuh, K., Galopeau, P. H. M., Hayakawa, M., Solovieva, M., Lammer, H., Voller, W., and Besser, B.: VLF transmitter signal variations as detected by Graz facility prior to Croatian earthquakes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9395, https://doi.org/10.5194/egusphere-egu23-9395, 2023.

Earthquakes are reported to relate to rupture phenomena in complex self-organizing systems. Hence, the earthquake rupture is regarded as a critical point. The preparation process of an earthquake could be considered as the crustal system approaching this critical point. Complex dynamical systems can have critical tipping points at which a sudden shift to a contrasting dynamical regime may occur; in the meantime, the time series of the systems can behave much differently. Although it is extremely challenging to predict such critical points before they are reached, work in different scientific fields is now suggesting the existence of generic early-warning signals that may indicate a wide class of systems if a critical threshold is approaching. Those precursory signals include increasing correlations and variance, varying skewness, and so on. The critical transition of a system includes spatial criticality and temporal criticality. In this study, we attempt to research the spatial and temporal criticality of the crustal system by using the self-potential (SP) signals of the Taiwan Geoelectric Monitoring System (GEMS). The GEMS network consists of 20 SP stations with an interstation distance of 50 km. We calculate the correlations of the daily signals between any two stations, which formed an adjacency matrix. Then, we estimate the connectivity density based on the adjacency matrix and compare the daily connectivity density time series with M_L ≥ 5 earthquakes. We would expect to find out high connectivity densities before a strong earthquake. This would mean that earthquake-related telluric currents flow out through the GEMS stations during the earthquake preparation process; hence, the SP signals of most stations would almost be connected. As a result, we might establish an earthquake forecasting technique using the SP data based on the concept of the critical-point theory.

How to cite: Chen, H.-J. and Chen, C.-C.: Connectivity of geoelectric network before strong earthquakes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10299, https://doi.org/10.5194/egusphere-egu23-10299, 2023.

The temporal sequences of magnitudes recorded in seismic active zones exhibit complex behavior which is associated with the wide diversity of scales of fractures sizes when an earthquake on the Earth’s crust occurs. Earthquakes can be considered to be nearly, or even critical phenomena exhibiting dynamic phase transitions, where a mainshock is the beginning of a new phase. Near the critical point is where phase transition (order-disorder) occurs, and scaling laws with long-range order correlations are produced, so that the complexity of seismicity allows earthquakes to be characterized by a more diverse and riche phenomenology. In the last years, the ideas linked to nonlinear time series analysis and complex network theory have been related. Among those ideas,  the visibility graph (VG) method has been applied to the study different complex phenomena. One of the characteristics of this method is its ability to capture dynamic properties, such as non-trivial correlations in nonstationary time series, without introducing elaborate algorithms such as detrending. Seismic processes have been of great interest and their complete understanding is still an open problem. In this work we use the VG method to study the temporal correlations in the seismic sequences monitored in three regions of the subduction zone belonging to the Cocos plate. Our analysis allows estimate persistence and the temporal correlations in the seismic activity monitored in Michoacan State, Mexican Flat Slab and Tehuantepec Isthmus, showing differences in all three.

How to cite: Ramírez-Rojas, A. and Flores-Márquez, E. L.: Correlations of the seismic activity monitored in three subduction zones belonging to Cocos plate by using the visibility graph method., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10458, https://doi.org/10.5194/egusphere-egu23-10458, 2023.

The North-South Seismic Belt of China is one of the most active seismic areas on the Chinese continent.  More than ten strong earthquakes (Ms > 6) have occurred in this region since 2010.  However, Earthquake-related conductivity anomalies are rarely reported for those earthquakes.  In this study, 3-component geomagnetic data recorded at sixty geomagnetic stations are selected to compute the Parkinson vectors to monitor the changes of conductivity before and after the earthquakes.  Considering most fluxgate magnetometers have only been installed since 2014, we concentrate on six Ms > 6 earthquakes occurred during 2014–2019.  To mitigate artificial disturbances, low noise data during the 00:00 – 5:00 LT are utilized.  We compute the background distribution and monitoring distribution using the azimuth of the Parkinson vectors at each station within six years (2014 – 2019) and a 15-day moving window, respectively.  The background distribution is subtracted from the monitoring distributions to mitigate the influences of underlying inhomogeneous tectonic structures.  The obtained difference distributions binned by 10° within 400 km from each station are superimposed during 60 days before and after the earthquake to construct integrated maps.  To analyze the potential frequency characteristics, we compute the results from low to high frequency band.  The results show that for four earthquakes, the conductivity anomalies areas appear near the epicenter 10 to 20 days before earthquakes, while the rest two earthquakes have no anomaly.  The conductivity anomalies appear at all study frequency band from 0.0005 Hz to 0.1 Hz, and significantly at 0.001 – 0.005 Hz before earthquakes.  Meanwhile, we find that the lower frequency band corresponds to larger anomalies area.  These results suggest the change of underlying conductivity near the hypocenter is a possible phenomenon for strong earthquakes, and the frequency characteristics of the seismo-conductivity anomaly during the earthquake are helpful to understand the pre-earthquake anomalous phenomena.

How to cite: Mao, Z. and Chen, C.-H.: Conductivity Anomalies before M > 6 Earthquakes in China during 2014 – 2019, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10627, https://doi.org/10.5194/egusphere-egu23-10627, 2023.

We have used 2014–2017 data from the eight receiving stations of the Japan very low frequency (VLF) monitoring network. The nighttime data of the signals of the JJI transmitter on Kyushu Island, excited VLF electromagnetic waves (EMWs) in the Earth-Ionosphere waveguide (EIWG) had been processed. The wavelet transform with a preliminary detrending, to exclude influence of daily variations, has been applied. We have observed ultra-low frequency (ULF) modulation of VLF EMW spectra in the EIWG. We therefore concluded that modulating oscillations with periods of 4 minutes belong to the acoustic branch of acoustic-gravity waves (AGWs) in the Earth–Thermosphere waveguide; modulation of VLF with periods of 6–7 minutes corresponds to global evanescent/reactive Brunt–Väisälä AGW oscillations; the oscillations with periods 20–60 min and ~3 hours may characterize evanescent/reactive Lamb gravity wave mode of AGW [1]. The appearance of the combination frequency of VLF EMW and ULF AGW is likely due to the following effects: (1) the drag of charged plasma particles by ULF AGWs jointly with the background of VLF electron density disturbances and (2) the motion of charged plasma particles in the VLF EMW field jointly with the background of ULF changes in the plasma concentration caused by AGWs.

The theory [2,3] is extended to the excitation of ionospheric Schumann resonator (SR) [4] and ionospheric Alfvén resonator (IAR) in the ULF range. It is shown that IAR oscillations with a high quality factor (for geophysical resonators) (>10) can be excited in the SR range. The features of the excited ULF and VLF modes associated with the modification of the ionosphere as a result of the powerful eruption of the Hunga-Tonga volcano are under consideration [5,6].

A ULF model of perturbations in the atmosphere-ionosphere with a boundary transition from dynamic to static limit is developed and the preliminary results of the corresponding modelling will be presented. This ensures the "recovery" of magnetostatic disturbances "lost" in most of previous models of the atmospheric electrical circuit, important for understanding the mechanisms of seismo-ionospheric coupling, volcano-ionospheric coupling and influences of the other natural hazards on the ionosphere and ionospheric monitoring of the natural hazards.

[1] Rapoport et al. Sensors 22, 10.3390/s22218191, 2022; [2] Grimalsky et al. JEMAA 2012, 4, 192-198 ; [3] Yutsis V. et al. Atmosphere 2021, 12, 801 ; [4] Nickolaenko and Rabinovich Space Res. 1982, XX, 67-88 ; [5] Astafyeva et al. GRL, 2022 ; [6] D’Arcangelo et al., Rem. Sens., 14, 3649, 2022.

This research was partially funded by the National Science Centre, Poland, grant No. 970 2022/01/3/ST10/00072

How to cite: Rapoport, Y., Reshetnyk, V., Grytsai, A., Liashchuk, A., Hayakawa, M., Grimalsky, V., Petrishchevskii, S., Krankowski, A., Błaszkiewicz, L., Flisek, P., De Santis, A., and Scotto, C.: ULF perturbations: modeling Earth-Atmosphere-Ionosphere coupling, signal processing using information entropy, determination of the electric and magnetic field components and “experiment-theory comparison“, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13172, https://doi.org/10.5194/egusphere-egu23-13172, 2023.

In order to build and implement a multi-parametric system for a time‐Dependent Assessment of Seismic Hazard (t‐DASH) the preliminary assessment of the selected parameters is required. To this aim a long-term correlation analysis - among anomalous transients and earthquake occurrence – has to be performed to establish the corresponding forecast capability and particularly the expected false-positive rate. In fact, more than the missing rate (i.e. how many earthquakes occurs in absence of specific precursors) the reliability of the forecast is much more important when the continuity of the observations cannot be guaranteed. This is the case of satellite observations in the optical band  whose continuity can be prevented by the presence of meteorological clouds. Among the others candidate parameters anomalous transients in the Earth’s emitted Thermal Radiation observed from meteorological satellites in the Thermal InfaRed band (TIR) have been since long-term proposed in the framework of a multi-parametric t-DASH system. Results achieved by RST (Robust Satellite Technique) analyses of multi-annual (more than 10 years) time series of TIR satellite images in different continents and seismic regimes, allowed to identify (isolating them from all the others possible sources) those anomalies (in the spatial/temporal domain) possibly associated to the occurrence of major earthquakes. Main lesson learnt until now can be summarized as follows:

a) Thanks to a clear definition of (Significant Sequences of TIR Anomalies (SSTAs) and well-defined validation rules, for earthquakes with magnitude greater than 4 the false positive rate is around 25% (average value over Greece, Italy, Japan, Turkey) oscillating from 7% up to 40% strongly depending on the considered region;

b) Molchan error diagram analyses gave a clear indication that a non-casual correlation exist between RST-based SSTAs and earthquake occurrence time and location;

c) SSTAs are quite rare (sporadic) with quite limited (less than 0,05% of the total investigated) alerted space-time volumes;

d) The approach based on the application of the RETIRA index (Robust Estimator of TIR Anomalies) showed some limitation related to the contextual approach that, in order to take into account of possible large scale changes of the thermal background, consider not just the TIR signal itself but its excess respect to the background (large scale spatial average of the TIR signal) introducing, this way, a strong dependence on the presence and distribution of meteorolical cloud across the scene.

In order to overcome the d) issue an alternative possibility has been investigated which can locally filter-out the contributes of occasional warming (typically associated to meteorological fronts) without the need of analyzing the TIR signal at the large-scale. In this paper RST approach is implemented by introducing the RETIRSA (Robust Estimator of TIR Slope Anomalies) devoted to identify anomalous Nocturnal TIR  Gradients in relation with the preparation phases of earthquakes. The impact in reducing the overall false-positive rates will be particularly discussed in the case of recent earthquakes occurred in Italy, Japan and California. 

How to cite: Tramutoli, V., Colonna, R., Filizzola, C., Genzano, N., Lisi, M., Pergola, N., and Satriano, V.: Improving RST-based analysis of long-term TIR satellite observations in relation with earthquake occurrence, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13588, https://doi.org/10.5194/egusphere-egu23-13588, 2023.

A correlation between low seismic activity and CO₂ measurements variations was observed at the Gallicano thermomineral spring, Tuscany, Italy, where an automatic monitoring multiparametric geochemical station is operative since 2003 (Pierotti et al., 2015). The above-mentioned correlation reported a time delay of about 2 days of small earthquakes with respect to CO₂ anomalies. Starting from this correlation a conditional probability of earthquake occurrence given the CO₂ anomaly detection was calculated, with a probability gain near 4 (Pierotti et al., 2022).  A statistical correlation was also calculated between rain events and CO₂ anomalies which was observed for rain vents ahead CO₂ anomalies of one days. This permitted to distinguish CO₂ anomalies due to meteorological versus tectonic activities.  Following this distinction, and subtracting the rain contribution to the CO₂ variations, a new correlation was observed between small earthquakes and CO₂ anomalies which confirmed the past results whit a better performance. The new correlation peak is better defined and concentrated in the time lag of 2 days. The p-values of both earthquake and rain to CO₂ correlations were calculated. The correspondent probability gain in an earthquake forecasting experiment, taking into account the rain events, increased from less than 4 to 4.5. 

Fidani, C. (2021). West Pacific Earthquake Forecasting Using NOAA Electron Bursts With Independent L-Shells and Ground-Based Magnetic Correlations. Front. Earth Sci. 9:673105.

Pierotti, L., Botti, F., D’Intinosante, V., Facca, G., Gherardi, F. (2015). Anomalous CO₂ content in the Gallicano thermo-mineral spring (Serchio Valley, Italy) before the 21 June 2013, Alpi Apuane earthquake (M= 5.2). Physics and Chemistry of the Earth, Parts A/B/C, 85, 131-140.

Pierotti, L., Fidani C., Facca, G., Gherardi, F. (2022). Local earthquake conditional probability based on long term CO₂ measurements. In 40^st GNGTS National Conference, Trieste, 27 - 29 June 2022.

How to cite: Pierotti, L., Fidani, C., Facca, G., and Gherardi, F.: Improving the statistical correlations between low seismic events and CO2 variations subtracting the rain contribution, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14058, https://doi.org/10.5194/egusphere-egu23-14058, 2023.

The mission of Advanced Ionospheric Probe (AIP) onboard FORMOSAT-5 (F5) satellite is to detect seismo-ionospheric precursors (SIPs) and observe ionospheric weathers.  F5/AIP plasma quantities in nighttime of 22:30 LT (local time) and the total electron content (TEC) of the global ionosphere map (GIM) are used to study SIPs of an M7.3 earthquake in the Iran-Iraq Border area on 12 November as well as two positive storms on 7 and 21-22 November 2017.  The TEC and the F5/AIP ion density/temperature anomalously increase over the epicenter area on 3-4 November (day 9-8 before the earthquake) and on the two storm days.  The anomalous TEC increase frequently appearing specifically in a small area near the epicenter day 9-8 before the earthquake indicates the SIP being observed, while those frequently occurring at worldwide high-latitudes are signatures of the two positive storms.  TEC increase anomalies most frequently appearing in the Iran-Iraq Border area on 21-22 November (day 10-9 before) is coincidently followed by an M6.1 earthquake on 1 December 2017, which again meets the temporal SIP characteristic.  The F5/AIP ion velocity uncovers that the SIPs of the two earthquakes are caused by eastward seismo-generated electric fields, and the two positive storms are due to the prompt penetration electric fields.

How to cite: Liu, J.-Y., Chang, F.-Y., Chen, Y.-I., and Chao, C.-K.: Ionospheric electric fields associated with seismo-ionospheric precursors and ionospheric storms observed by FORMOSAT-5/AIP, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17023, https://doi.org/10.5194/egusphere-egu23-17023, 2023.

Our study aims to detect anomalous seismic and geomagnetic precursor signals appearing before Vrancea, Romania medium sized earthquakes, that occurred in the last decade (2012-2022), using in the first step the visualization processing method, to identify the time lap between the two anomalies and the following earthquakes. During the study period, in Vrancea seismogenic zone there have been recorded 39 earthquakes with magnitude ML>=4.5, both at normal and intermediate depth. We have assumed that the zone of effective manifestation of the precursor deformations is a circle with the radius taken from the equation of Dobrovolsky, 1979, so the studies were done inside this zone. The Seismic data consists in seismic velocities vp and vs (vp/vs), computed from the arrivals of seismic waves at the NIEP stations situated in the earthquake preparation area. The calculations are done automatically by the Phenomenal platform https://ph.infp.ro/seismicity/data, using the corrected Romanian seismic bulletins. The seismic velocity is the geophysical property that has a key role in characterizing dynamic processes and the state of the stress around the faults, providing significant information regarding the change in tectonic regime. In the crust, velocities change before, during and after earthquakes through several mechanisms related to, for example, fault deformations, pore pressure, changes in stress state (pressure perturbation) and rebound processes.

The Geomagnetic data are obtained from Muntele Rosu (MLR) Seismological Observatory of NIEP, situated inside Vrancea seismogenic zone as primary station, and from Surlari (SUA) Geomagnetic Observatory of Intermagnet, as remote station, unaffected by medium size earthquake preparedness processes. Geomagnetic indices taken from GFZ (https://www.gfz-potsdam.de/kp-index) were used to separate the global magnetic variation from possible local seismo-electromagnetic anomalies, that might appear in a seismic area like Vrancea zone and to ensure that observed geomagnetic fluctuations are not caused by solar-terrestrial effect.

In this presentation we study the appearance of the changes of seismic propagation velocities (vp/vs) in time and the geomagnetic deviations from the normal trend before the occurrence of moderate size crustal and intermediate earthquakes from Vrancea zone, to emphasize the time span between the studied phenomena, in order to be able to find a statistical correlation between them.

Acknowledgements. This work was funded by: PN23 36 02 01/2023 SOL4RISC Nucleu Project, by MCD, Phenomenal Project PN-III-P2-2.1-PED-2019-1693, 480PED/2020 and AFROS Project PN-III-P4-ID-PCE-2020-1361, PCE/2021 supported by UEFISCDI

How to cite: Moldovan, I. A., Toader, V., Mihai, A., Borleanu, F., Petrescu, L., Placinta, A. O., and Manea, L.: Detection of correlated anomalous seismic and geomagnetic precursor signals before Vrancea moderate size earthquakes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17053, https://doi.org/10.5194/egusphere-egu23-17053, 2023.

Various preseismic electromagnetic variations have been reported so far. Oike et al. reported an increase in the number of electromagnetic pulses in the LF band for the 1995 Kobe Earthquake. However, there is a problem that the electromagnetic pulse due to lightning activity, which is a strong electromagnetic radiation source in the LF band, cannot be sufficiently distinguished from the electromagnetic pulse associated with earthquakes. At that time, it was difficult to observe waveforms with the observation equipment (especially digital measurement units), but with the development of today's ICT equipment and Internet technology, it is possible to realize an LF band broadband interferometer that can estimate the spatio-temporal sources of electromagnetic radiation. If it is an electromagnetic pulse due to lightning activity, the electromagnetic radiation source will move with the front or cloud, and if it is associated with an earthquake, the electromagnetic radiation source will be concentrated near the focal region. In this presentation, we will report the progress of the development of the LF band broadband interferometer, and the waveform analysis and pulse number variation of the nearby earthquake that occurred during the test of the interferometer element.

The developed system is a capacitive circular flat plate fast antenna, consisting of a 500 kHz low-pass filter, a 16bit AD converter, and a PC for data recording, and records 100 ms before and after the pulse waveform that exceeds the trigger level with 4 MHz sampling. The system is installed on the roof of the Faculty of Science Building No.5, Chiba University, and is conducting test observations.

First, we counted the total number of pulses recorded by the system, created an amplitude histogram, and targeted the top 15% of the pulses to investigate hourly fluctuations in the number of pulses. We calculated the average value m and standard deviation σ for the entire analysis period, and defined the anomaly in the number of pulses as m + 2σ. Next, using pulse waveforms and the mine location network blitzortung.org, waveforms (near and distant mines) caused by mine discharges were identified. In addition, we analyzed the earthquakes that occurred within 100 km of the epicenter distance and satisfied log(Es)>8 during the observation period, and investigated the relationship with the earthquakes. where Es=10^1.5M+4.8/r² (M: magnitude, r: focal distance). As a result, 4 days before the M5.0 earthquake on November 27, 2018, an abnormal increase in the number of pulses greater than m+2σ was observed, unrelated to the anti-mine. Although similar pulse waveforms did not exceed the m+2σ threshold, they were also observed prior to four other log(Es) > 8 earthquakes during the observation period, and these pulses were associated with preseismic electromagnetic waves. Possible pulse due to radiation. On the other hand, it is also possible that the pulse waveform is caused by cloud discharge, and in order to discriminate between electromagnetic radiation caused by cloud discharge and earthquake precursor electromagnetic radiation, electromagnetic radiation position determination using an interferometer and comparison with satellite images and meteorological data are required. also found to be essential.

How to cite: Hattori, K., Ohta, Y., Yoshino, C., and Imazumi, N.: Development of Broadband Interferometer System for Pre-Earthquake Electromagnetic Radiation in LF Band :Design and Performance of Antenna Elements, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17076, https://doi.org/10.5194/egusphere-egu23-17076, 2023.

The study of seismogenic faults is one of the most interesting topics in seismology.  Obtaining a more detailed image of the fault zone structure and of its properties (e.g., fluid content, permeability, lithology, rheology) is fundamental to understand how seismic ruptures originate, propagate and arrest and to study the triggering processes.  The 2009 Mw 6.1 L’Aquila seismic sequence is a perfect case study to reach this goal, thanks to the huge amount of multidisciplinary data available. 

In this study, we reprocess the high-precision large earthquake catalog available for the L’Aquila seismic sequence, focusing on the main (Paganica) seismogenic fault (about 20,000 earthquakes occurring between January-December 2009). We used cross-correlation and double-difference tomography methods to compute high-resolution (2.5 x 2.5 x 2 km grid spacing) Vp and Vp/Vs models along the fault plane. High-resolution Vp and Vp/Vs models give insights into the rheology of the Paganica fault, suggesting new ideas on earthquake generation, propagation and arrest.  

How to cite: Fonzetti, R., Valoroso, L., De Gori, P., and Chiarabba, C.: New insights into the rheology of a normal fault: the Mw6.1 2009 L’Aquila case study, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-431, https://doi.org/10.5194/egusphere-egu23-431, 2023.

Faults can act as conduits for the migration of hydrothermal fluids in the crust, affecting its mechanical behaviour and possibly leading to earthquake swarm activity. To date, there are still few constraints from the geological record on how fault-vein networks develop through time in high fluid-flux tectonic settings. Here, we describe small displacement (<1.5 m) epidote-rich fault-vein networks cutting granitoids in the exhumed Bolfin Fault Zone (Atacama Fault System, Chile). The epidote-rich sheared veins show lineated slickensides with scattered orientations and occur at the intersections with subsidiary structures in the fault damage zone. FEG-SEM cathodoluminescence (CL) reveals that magmatic quartz close to the sheared epidote-rich veins is affected by (i) thin (< 10 µm) interlaced deformation lamellae and (ii) a network of CL-dark quartz epitaxial veinlets sharply crosscutting the lamellae. EBSD maps of the deformed quartz indicate minor lattice distortion associated with the lamellae and an orientation nearly orthogonal to the c-axis. These deformation features disappear moving away into the host rock. The epidote-rich sheared veins (i) include clasts of magmatic quartz with both the deformation lamellae and the healed veinlets and (ii) show cyclic events of extensional-to-hybrid veining and localized shearing. We propose that the microstructures preserved in the quartz next to the sheared veins (i.e. deformation lamellae and epitaxial veinlets) record the high-strain rate loading associated with dynamic crack propagation and rapid micro-fracture sealing. On the other hand, the cyclic dilation and shearing within the epidote-rich veins is interpreted as the expression of a highly connected fault-vein network dominated by pore pressure oscillations leading to seismic swarm activity.

How to cite: Masoch, S., Fondriest, M., Gomila, R., Pennacchioni, G., Cembrano, J., and Di Toro, G.: Interplay between fluid flow and rock deformation in an exhumed hydrothermal fault-vein network, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-531, https://doi.org/10.5194/egusphere-egu23-531, 2023.

We propose an approach that is aimed to enrich the catalogs of acoustic emission events recorded in laboratory experiments with such parameters as seismic moment and corner frequency. Because of the difficulty of separation of direct waves in experiments performed on small samples, we use the coda waves that are composed of the reverberation of the acoustic field in the tested sample. After multiple reverberations, the resulting wavefield can be approximated as nearly homogeneously distributed over the sample and with signal amplitudes decaying exponentially in time (linearly in a logarithmic scale).

Within the framework of this model, the frequency-dependent coda amplitude at any moment of time is described as combination of a source spectra, of a decay rate combining internal attenuation with reverberation losses, and of a sensor response. One of the main difficulties with the laboratory experiments is that acoustic sensors are very difficult to calibrate and their absolute response function in most of cases remains unknown. With the simple reverberation model, the logarithms of coda amplitudes at different times and sensors and for multiple events are described by a system of linear equations that we solve in a least-square sense to find frequency-dependent coda-decay rates, relative signal spectra and sensor responses. In a next step, we compute spectral ratios between spectra of different events to eliminate the sensor responses and to estimate main source parameters such as corner frequencies and relative seismic moments.

We provide details of our data analyses technique and present time-dependent corner frequency vs relative moment diagrams for two experiments on granite of the Voronezh massif and Berea sandstone under pseudo-triaxial loading. The dependence close to the cubic that is frequently estimated for tectonic earthquakes observed on the first stages of both experiments when confining pressure steps applied to the intact rock and therefore to the pre-existing inhomogeneties. After applying axial load changes in stress-drop is being observed: with higher stress-drops prevailing in granite and lower stress-drops in sandstone. Also there is a significant difference in Gutenberg-Richter relation in these two experimental conditions observed.

How to cite: Kartseva, T. and Shapiro, N.: Coda-Based Estimation of Source Parameters of Laboratory Acoustic-Emission Events, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1330, https://doi.org/10.5194/egusphere-egu23-1330, 2023.

In 1962, the Kyona Dam was completed in a rural area 250 km southeast of Mumbai (India), primarily for hydropower generation. Since then, the area, which was essentially devoid of natural seismicity, has been affected by a sequence of moderate to large magnitude earthquakes, including the one of December 11^th, 1967 (M_L6.3, 177 casualties), the largest human-induced earthquake so far. Major earthquakes (M_L>4) are modulated by basin-filling and emptying operations, which follow the monsoon regime with peak rainfall between July and September. There are two peaks of seismicity: the first between August and September (“rapid-response”), corresponding to the rainy season, and the second in February (“delayed-response”) corresponding to the dry season. The M_L>3 earthquakes have normal to strike-slip focal mechanisms, reactivate steeply-dipping faults/fractures, and are located between 3 and 10 km depth in the granitoid Indian basement (2.7-2.6 Ga) buried beneath the 0.5-2 km thick Deccan basaltic lava flows (68-60 Ma). The temperature at hypocentral depths is estimated to be between 80 and 200°C. Especially the delayed-response seismicity implies poro-elastic effects, also related to the percolation of water from the surface to hypocentral depths. To study the seismicity of the area, a large deep drilling project was completed by the Ministry of Earth Sciences (India) which includes nine wells down to 1.5 km depth and a pilot well down to 3 km depth. Here we describe the fault rocks (mylonites, cataclasites, breccia and faults/fractures filled by epidote, quartz, chlorite and calcite veins) collected in boreholes KBH1, KBH6 and KBH7.

Visual analysis of the cores plus mineralogical, microstructural and geochemical investigations (X-ray powder diffraction; scanning electron microscope equipped with Wavelength-Dispersive X-Ray Spectroscopy) allowed us to reconstruct the sequence of deformation events. Steeply-dipping faults/fractures filled by chlorite and calcite are the last deformation event as they cut through all other structural features. We recognized eight types of chlorites based on optical properties, crosscutting relations and chemical composition. The temperature of formation of the chlorite spans from 350°C (or HT-chlorite found in the shear zones cut by the Deccan basaltic dykes), to 200°C<T<250°C (or LT-chlorite filling fault/fractures cut by calcite veins, but with uncertain crosscutting relations with Deccan basaltic dykes), and 130°C<T<135°C (or Very-LT-chlorite filling fault/fractures, which are also cut by calcite veins, and cut the Deccan basaltic dykes). LT- and Very-LT-chlorite formation temperatures were estimated with the Bourdelle & Cathelineau (2015) chlorite geothermometer. The range of 130°C<T<250°C for chlorite formation, which can be extended to lower temperatures considering that these faults/fractures are cut by calcite veins, overlaps with the one (80°C<T<200°C) estimated at the hypocentral depths of the Koyna-Warna area. Moreover, these fault/fractures found in the boreholes are hosted in steeply-dipping fault/fractures (or sub-parallel to the structures illuminated by the hypocentral distributions), and are filled by minerals precipitated from percolating fluids (i.e., consistent with the evidence of delayed-response seismicity). We conclude that the faults/fractures currently reactivated by reservoir-triggered seismicity most likely correspond to those filled by calcite and LT- to Very-LT chlorites found in the deep boreholes.

How to cite: Di Toro, G., Chiesurin, A., Spagnuolo, E., Gomila, R., and Roy, S.: Fault rocks associated with the reservoir-triggered seismicity of the Koyna-Warna area (India), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2537, https://doi.org/10.5194/egusphere-egu23-2537, 2023.

In order to make seismic hazard estimates, it is necessary to assume some distribution of the number of earthquakes of a certain magnitude, i.e. a Gutenberg-Richter distribution. This is true for both natural seismicity as well as induced seismicity, but in both cases the number of historical earthquakes at the tail end of the distribution (i.e. the largest ones) is limited and often the maximum possible magnitude is unknown. In contrast, in a laboratory setting the maximum size of an unstable slip event (“stick-slip” or laboratory earthquake) is controlled by the size of the sample and the imposed stress. In our rotary shear apparatus, we can theoretically achieve unlimited fault displacement which allows us to produce earthquake-like distributions with thousands to tens of thousands event.

In this presentation, I will present results from experiments on simulated fault gouges which show unstable frictional behaviour at room temperature conditions. The results show that the event size distribution can change spontaneously, without any changes in the boundary conditions. Observations of fault gouge material after the experiment, suggest that wear of the granular material generates alternative surfaces for slip, which changes the macroscopic behaviour. Interestingly, the change in event size distribution is reversable, presumably because the fine-grained layers become disturbed with ongoing shear.

In an attempt to simulate the macroscopic behaviour, we have, for the first time, measured the rate-and-state frictional (RSF) properties on single grain contacts. Using these values in a numerical model for seismic slip (so-called “seismic cycle simulator”), we obtain maximum stress drops that are comparable to those obtained in the experiments, but with some differences. The differences are most likely due to the fact that the grains in our simulated fault are affected wear in previous slip events, which should change their RSF parameters. In addition, the normal stress at each individual grain contact is unknown in the experiment and could vary significantly from event to event.  This latter difference between model and experiment can be overcome by using a discrete element method with contact-scale RSF properties to simulate slip.

How to cite: Niemeijer, A. R., Korkolis, E., Mishra, T., Elbertsen, R., Jop, B., and Pires de Vasconselos, I.: Microstructural controls on seismicity distribution in simulated fault gouges, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2911, https://doi.org/10.5194/egusphere-egu23-2911, 2023.

The elastic medium that hosts several, multi-scale, faults could be regarded as an energy reservoir that is charged by the far field stress rate and discharged by friction dissipation during earthquake slips on the faults.  In this study, we carefully analyze the energy budget variations that occur throughout a synthetic, 2D plane-strain, earthquake cycle on a fault system comprising of a main fault surrounded by a hierarchy of off-fault slip planes/fractures. We evaluate the rate of kinetic energy variation, stress power across the continuum, far field power supply, and the dissipation due to the rate-and-state friction on the faults given a spectrum of slips ranging from slow-slip to rapid ruptures. We study how the medium's energy budget evolves after these components have been determined.  Additionally, we compute the dissipation rate for a variety of slip rates to determine the contributions of so-called slow-slip events, low-frequency earthquakes (LFEs), and tremors to this budget. We also evaluate the share of off-fault fractures to determine their energetic role during earthquake cycles.

How to cite: Kheirdast, N., Almakari, M., Villafuerte, C., Thomas, M. Y., and Bhat, H. S.: Energy budget of quasi-dynamic earthquake cycle, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3982, https://doi.org/10.5194/egusphere-egu23-3982, 2023.

Investigating clusters of events and geophysical screening often provides limited constraints on fault geometries. This imaging issue prevents the integration of realistic fault zone geometry into earthquake studies, which can affect our capacity to evaluate the role of pre-existing faults on seismicity. This study describes a modelling strategy accounting for realistic fault zone geometries. Our approach uses stochastic methods underpinned by quantitative fault zone parameterization, followed by an assessment of seismicity from simulations of rupture dynamics controlled by fault geometry. This method is used to investigate the role of fault maturity on seismicity for two case studies, including seismicity associated with the reactivation of a pre-existing fault network due to hydraulic fracturing in Harrison County, Ohio, from 2013 to 2015, as well as the natural seismicity associated with the Yushu-Ganzi left-lateral strike-slip fault system in central-eastern Tibet. In the Harrison County case, we analyze the effect of vertical variability in fault maturity and show how more mature faults in the deeper crystalline basement generate higher magnitude seismicity than shallow, immature faults in younger sedimentary rocks. In the Yushu-Ganzi case study, we show how lateral variability in structural maturity, arising from long-term fault propagation and strain rates, leads to different seismicity on individual fault segments. Our findings indicate that fault geometry determines seismic patterns, with rupture length controlled by fault zone geometry rather than fault lengths, and favour the adoption of a structural geological perspective for the integration of realistic fault geometry into seismicity prediction strategies.

How to cite: Roche, V., van der Baan, M., and Walsh, J.: Modelling seismicity based on fault geometry: maximum magnitudes and magnitude-frequency distributions., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5033, https://doi.org/10.5194/egusphere-egu23-5033, 2023.

The seismogenic zone is commonly defined as the portion of the Earth's upper crust where most earthquakes nucleate. According to seismological data, the seismogenic interval is typically located between 5 and 35 km depth. However, shallow seismicity, with earthquake hypocentral depths < 5 km, has been reported in several tectonic settings. Although less studied, such shallow seismic sources represent potential treats and deserve to be thoroughly investigated and included in seismic hazard evaluations.

For this purpose, we present the results of a multidisciplinary study focusing on faults affecting high-porosity fluvio-deltaic, sandstone-dominated deposits belonging to the Pliocene-Pleistocene succession of the Crotone Basin, South Italy. The studied fault zone is well exposed along the Vitravo Creek canyon, has a maximum displacement of ~50 m, and is characterized by an indurated, sharp master slip surface. The fault footwall displays an 8-10 m-wide deformation band-dominated damage zone with deformation bands occurring both as clusters and as single structures. The frequency of deformation bands increases towards the master slip surface. Approaching the master slip surface, a 1.5 m-thick mixing zone occurs, where strong tectonic mixing affected the sandstone strata with different grain size and thickness. The fault core consists of ~1 m-wide, calcite-cemented cataclastic volume and hosts a wealth of fault-parallel deformation bands and subsidiary slip surfaces. Due to its selective cementation, the fault core stems in strong positive relief compared to the host high-porosity sandstone. The hanging wall block is characterized by a dense network of thin deformation bands with diminishing frequency away from the fault surface. Along the master slip surface, at the top of the indurated fault core, a 1-2 cm-thick dark gouge layer is present. The gouge is persistent throughout all the fault exposure, and has been injected in the underlying, fractured cemented fault core. Microstructural analysis of the gouge reveals a strong cataclastic grain size reduction along thin (< 1 mm) slip zones alternated with portions showing lens-shaped (resembling S-C) fabric. XRD analysis of the < 2 µm grain-size fraction of the gouge layer displays short-ordered illite-smectite mixed layers which support deformation temperatures in the 100-120°C range. XRD analysis performed on clay fraction from the fault core, next to the dark gouge layer, indicates temperatures lower than 50-60°C, consistent with the expected shallow burial conditions. Following this, the anomalous temperature rise recorded within the dark gouge layer is suggested to be produced by frictional heating during coseismic deformation. We conclude that the microstructural observations, grain size, and XRD data provide a line of evidence supporting the occurrence of coseismic deformation affecting high-porosity granular materials at near surface conditions and could help in better evaluation and risk assessment of seismically active faults.

How to cite: Pizzati, M., Lieta, N., Torabi, A., Aldega, L., Storti, F., and Balsamo, F.: Evidence for coseismic slip preserved in high-porosity sandstone at very shallow burial conditions (Crotone Basin, Italy), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6468, https://doi.org/10.5194/egusphere-egu23-6468, 2023.

We propose a numerical model of laboratory earthquake cycle inspired by a set of experiments performed on a triaxial apparatus on sawcut Carrara marble samples. The model couples two representations of rock matter: rock is essentially represented as an elastic continuum, except in the vicinity of the sliding interface, where a discrete representation is employed. This allows to simulate in a single framework the storage and release of strain energy in the bulk of the sample and in the loading system, the damage of rock due to sliding, and the progressive production of a granular gouge layer in the interface. After independent calibration, we find that the tribosystem spontaneously evolves towards a stick-slip sliding regime, mimicking in a satisfactory way the behaviour observed in the lab. The model offers insights on complex phenomena which are out of reach in experiments. This includes the variability in space and time of the fields of stress and effective friction along the fault, the progressive thickening of the damaged region of rock around the interface, and the build-up of a granular layer of gouge accommodating shear. We present in detail several typical sliding events, we illustrate the fault heterogeneity, and we analyse quantitatively the damage rate in the numerical samples.

How to cite: Mollon, G., Aubry, J., and Schubnel, A.: On-fault damage evolution in laboratory earthquakes: a numerical perspective on fault complexity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6517, https://doi.org/10.5194/egusphere-egu23-6517, 2023.

Faults are complex systems embedded in an evolving medium fractured by seismic ruptures. This off-fault damage zone is shown to be thermo-hydro-mechano-chemically coupled to the main fault plane by a growing number of studies. Yet, off-fault medium is still, for the most part modelled as a purely elastic -- hence passive -- medium. Using a micromechanical  model we investigate the depth variation of dynamically triggered off-fault damage and its counter-impact on earthquake slip dynamics. We show that if the damage zone becomes narrower with depth, it is also denser and thus, unlike what is commonly believed, remains an energy sink even at depth. The results are in agreement with the complementary model by Okubo et al., 2019. In contrast to study cited above, our model accounts for the dynamics changes of elastic moduli related to crack growth. This lead to the dynamic creation of low-velocity zone that can trapped seismic waves and further impact the earthquake dynamics, even at greater depth. We therefore claim that the intertwined dynamics between the main fault plane and its surrounding medium must be including along the all seismogenic.

How to cite: Ferry, R., Thomas, M., and Jeandet, L.: Depth dependence of coseismic off-fault damage, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6974, https://doi.org/10.5194/egusphere-egu23-6974, 2023.

Surface rupture produced by earthquakes can pose great threat on near-surface infrastructures and elevate damages. Accessing the potential of surface rupture along faults is critical to mitigating such hazards. It is commonly suggested that earthquakes with Mw>6.5 will break the surface. However, there are events with much smaller magnitudes rupturing the ground. To understand the potential controlling mechanisms, we first collect source parameters for earthquakes with  and  surface-breaching events in seismically active regions including west China, North America, Europe, Taiwan, Japan, and Iran. For strike-slip and normal events, almost all earthquakes with magnitudes over 6.7 broke the surface. In contrast, buried and surface-breaching events co-exist with moderate magnitude (6.0-6.7). For reverse events, there is no clear magnitude boundary, as thrust buried events can be quite large due to the downdip size of the seismogenic zone. The relocated hypocenter depths for moderate-to-large events are concentrated at depth of 5 -20 km with no systematic difference between buried and surface-breaching ruptures. Differently, all  surface-breaching events occurred at very shallow depths (<5 km). We also conduct dynamic rupture simulations and propose two conceptual models to explain whether or not ruptures may break the surface. The first model represents a fault with a continuous but heterogeneous seismogenic zone (velocity-weakening) that can hold moderate-to-large earthquakes. In this case, ruptures need to overcome the shallow velocity-strengthening zone (VS) with certain energy sink to reach the surface. Therefore, a thinner shallow RS zone and a higher stress drop of the earthquake can promote surface rupture, consistent with our observations. However, ruptures nucleating from different locations on heterogeneous faults may lead to different surface rupture patterns and final magnitudes, shedding lights on the diverse behaviors among moderate earthquakes. The second model is for small surface-breaching earthquakes. Those events are supposed to occur on shallow isolated velocity-weakening patches, consistent with the fact that usually no large earthquakes have been reported on the same fault zones. Such asperities may be formed on bodies with high-strength materials, leading to energetic ruptures with intense stress release. Our study contributes to the understanding of the surface rupture behaviors references for assessing near-surface damage in future earthquakes.

How to cite: Yao, S., Yang, H., and Tang, Z.: Investigating relationships between surface rupture and multiple source parameters of earthquakes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7142, https://doi.org/10.5194/egusphere-egu23-7142, 2023.

The 2016 Amatrice, Italy earthquake sequence occurred on a normal fault system in the Central Apennines, and contained over 1,300 M>=3 earthquakes. With ~140 permanent or temporary seismic stations directly above the seismic activity, the sequence has been exceptionally well recorded. Starting from a deep learning-based catalogue of earthquake hypocentres (~900,000 re-located events from ~15 million seismic phases; Tan et al., 2021), we use a convolutional neural network classifier to predict P-wave first motion polarities, from which we compile a deep catalogue of earthquake focal mechanisms. The catalogue consists of >56'000 focal mechanisms, about 8'000 of which have nodal plane uncertainties below 25 degrees.

In contrast to existing, conventional focal mechanism catalogues, the deep catalogue samples almost the entire study region, and almost the entire magnitude range (~M0-M5), although nodal plane uncertainties generally tend to increase with decreasing magnitude. We use the focal mechanism catalogue to study the kinematics of the Amatrice earthquake sequence, to test the hypothesis of a coseismic rotation of the static stress field by large and small events, and to analyse the complexity of the stress field before, during and after the earthquake sequence.

How to cite: Meier, M.-A., Lanza, F., and Martinez-Garzon, P.: A deep catalogue of 56k focal mechanisms for the 2016 Amatrice, Italy earthquake sequence, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7167, https://doi.org/10.5194/egusphere-egu23-7167, 2023.

The reactivation of faults can occur when the effective stresses acting on them are perturbed. In some cases man-made changes in effective stress can result in fault reactivation that can have enormous impacts including loss of integrity of underground storage facilities. Current practical methods for making this assessment are generally based on shear stresses calculated over fault surfaces. Depending on the absolute or relative magnitudes of these resolved shear stresses, which depend primarily on the local orientation of the fault surface relative to the regional stress field, different faults or parts of faults are said to be closer or further from the Coulomb failure envelope and are therefore more likely to slip due to changes in effective stress. This proximity to failure/slip is referred to as the slip tendency or reactivation tendency.

Although the slip/reactivation tendency approach is firmly grounded in Coulomb theory and laboratory experiments, there may be issues applying it to the reactivation of irregular fault surfaces. A key assumption of the approach is that an area of a fault surface can be treated in isolation so that the slip tendency can be evaluated once its orientation and frictional properties are known. However, it is well established that fault surfaces are not planar but often have highly irregular geometries and fault rock distributions so that the likelihood that a particular part of a fault will reactivate must also depend, not only on the properties at that point but also on adjacent areas of the fault surface.

To investigate the significance of fault surface irregularity for the evaluation of fault slip/reactivation tendency, we conduct numerical modelling of fault reactivation resulting from an increase in pore pressure within a normal faulting stress regime. The modelling employs a form of the Discrete Element method that uses rigid blocks. This approach provides for both accurate representation of the geometry and frictional properties of the irregular slip surfaces and also failure in the surrounding wall-rock and is capable of modelling the variety of ways in which slip may initiate on, or adjacent to an irregular fault.

How to cite: Aleksans, J., Childs, C., and Schöpfer, M.: The impact of fault surface 3D geometry on risking fault reactivation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7327, https://doi.org/10.5194/egusphere-egu23-7327, 2023.

Earthquake nucleation has been understood as controlled by the frictional properties of fault zones. Mature fault zones host abrasive wear products, such as gouges, which result from the frictional sliding occurring in successive slip events. Shear localization in fault gouges is strongly dependent on, among others, fault mineralogical composition and grain size distribution, originating a wide variety of microstructural textures that may be related to different types of fault motion from aseismic creep, slow earthquakes to fast slip events. Additionally, within a fault, one can encounter different stages of maturity, ranging from an incipient and poorly-developed fault zone (i.e. discontinuous and thin gouge layer) to a mature fault zone that has experienced a lot of wear from previous sliding events (i.e. well-developed gouge layer). The localization of deformation within a mature gouge layer has been identified as possibly responsible for mechanical weakening and as an indicator of a change in stability within the fault.

To gain insights on the role of grain size distribution, and thus fault maturity, in slip behavior and fault rheology, we performed friction experiments on quartz fault gouge in a double direct shear configuration using a biaxial apparatus (BRAVA at INGV in Rome, Italy). The experiments were performed at a constant normal stress of 40 MPa and under 100% humidity.  We investigated different sliding velocities, from 10 µm/s to 1 mm/s, to assess time-dependent physical processes. Different bi-disperse mixtures of quartz were sheared to reproduce different initial grain size distributions within the fault (F110, average grain size  and Min-u-sil, average grain size ). Samples were carefully collected at the end of the experiments to prepare thin sections for microstructural analyses.

A first set of experiments was performed increasing the proportion between smaller and larger particles within a homogeneous blend. The friction evolves from a strain-hardening behavior for a sample with only F110 to a slip-weakening one for the one with only Min-u-sil. The difference in rheology is observable in the analyzed microstructures. Particularly, the two end members clearly show comminution and localization along boundary shear planes, whereas mixtures of the two sizes of particles only present a more diffused deformation. In the second set of experiments, we sheared gouges with a horizontal layering of the two grain sizes and observed different behaviors in terms of friction and rheology. These layered gouges present strain hardening behavior, with a strengthening part corresponding to the material of the layer in contact with the sliding block and a steady-state part with slightly higher friction than for the homogeneous mixtures.

These results give important information on the connection between grain size distribution, shear localization, and the resulting fault slip behavior. In this context, the proportion between small/large particles and their distribution and percentages within the fault plays an important role in controlling fault rheology. We also complete our knowledge by using Discrete Element Method, simulating gouge sliding with different grain scale properties (size, distribution, cementation…), and observing a detailed evolution of shear localizations.

How to cite: Casas, N., Giorgetti, C., Collettini, C., and Scuderi, M. M.: Frictional behavior and rheology of bi-disperse quartz gouge mixtures, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7594, https://doi.org/10.5194/egusphere-egu23-7594, 2023.

Major earthquake ruptures occur predominantly in thrust faults producing devastating events and tsunamis such as the 2011 Mw 9.0 Tohoku earthquake, the 2004 Mw 9.2 Sumatra earthquake and the 1999 Mw 7.7 Chi-Chi earthquake. Understanding the mechanics of earthquakes in thrust faults and the effect of the free surface is thus crucial to explain their large shallow slip, their asymmetric ground motion and their damage patterns surrounding the fault and the free surface. In this work, we carry out 2D dynamic rupture simulations on thrust faults to accurately characterize a possible unclamping effect, its responsible physical mechanism, and to produce dynamically activated off-fault fracture networks. To conduct the simulations, we use the software tool based on the Combined Finite-Discrete Element Method (FDEM), HOSSedu, developed by Los Alamos National Laboratory. Our dynamic rupture models in an elastic medium confirm that unclamping occurs in thrust faults and increases significantly as the rupture reaches the free surface and for the fault models with lower dip-angles. We show that this is a consequence of the torque mechanism induced in the hanging wall, and the release of this torque when the rupture reaches the free surface produces a “flapping” in the toe of the wedge where the most significant unclamping (possibly leading to fault opening) is taking place. Our results indicate that the free surface produces a considerable reduction of the compressive normal stress when the rupture is propagating up-dip that facilitates the extension and the amount of slip close to the trench as observed for large thrust earthquakes.This significant normal stress change is reflected in the orientation of the principal stresses before and after the rupture, where under certain conditions, the greatest principal stress changes from subhorizontal to almost vertical leading to a post-rupture tensional stress state in the hanging wall that has been confirmed by observations of recent in-situ, seismological and geodetic studies. Finally, we investigate whether this dramatic normal stress reduction stands when we allow for the activation of coseismic off-fault damage and explore its role in the rupture dynamics, the near-field deformation and radiation patterns.

How to cite: Villafuerte, C., Okubo, K., Rougier, E., Madariaga, R., and Bhat, H. S.: Dynamics and radiation of thrust earthquakes with coseismic off-fault damage, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7849, https://doi.org/10.5194/egusphere-egu23-7849, 2023.

Catastrophic failure in brittle, porous materials initiates when structural damage, in the form of smaller-scale fractures, localises along an emergent failure plane or 'fault' in a transition from stable crack growth to dynamic rupture. Due to the extremely rapid nature of this critical transition, the precise micro-mechanisms involved are poorly understood and difficult to capture. However, these mechanisms are crucial drivers for devastating phenomena such as earthquakes, including induced seismicity, landslides and volcanic eruptions, as well as large-scale infrastructure collapse. Here we observe these micro-mechanisms directly by controlling the rate of micro-fracturing events to slow down the transition in a unique triaxial deformation experiment that combines acoustic monitoring with contemporaneous in-situ x-ray imaging of the microstructure. The results [1] provide the first integrated picture of how damage and associated micro-seismic events emerge and evolve together during localisation and failure and allow us to ground truth some previous inferences from mechanical and seismic monitoring alone. They also highlight where such inferences miss important kinematically-governed grain-scale mechanisms prior to and during shear failure.

The evolving damage imaged in the 3D x-ray volumes and local strain fields undergoes a breakdown sequence involving several stages: (i) self-organised exploration of candidate shear zones close to peak stress, (ii) spontaneous tensile failure of individual grains due to point loading and pore-emanating fractures within an emergent and localised shear zone, validating many inferences from acoustic emissions monitoring, (iii) formation of a proto-cataclasite due to grain rotation and fragmentation, highlighting both the control of grain size on failure and the relative importance of aseismic mechanisms such as crack rotation in accommodating bulk shear deformation. Dilation and shear strain remain strongly correlated both spatially and temporally throughout sample weakening, confirming the existence of a cohesive zone, but with crack damage distributed throughout the shear zone rather than concentrated solely in a breakdown zone at the propagating front of a pre-existing discontinuity.

Contrary to common assumption, we find seismic amplitude is not correlated with local imaged strain; large local strain often occurs with small acoustic emissions, and vice versa. The seismic strain partition coefficient is very low overall and locally highly variable. Local strain is therefore predominantly aseismic, explained in part by grain/crack rotation along the emergent shear zone. The shear fracture energy calculated from local dilation and shear strain on the fault is half of that inferred from the bulk deformation, with a smaller critical slip distance, indicating that less energy is required for local breakdown in the shear zone compared with models of uniform slip.

This improvement in process-based understanding holds out the prospect of reducing systematic errors in forecasting system-sized catastrophic failure in a variety of applications.

[1] Cartwright-Taylor et al. 2022, Nature Communications 13, 6169, https://doi.org/10.1038/s41467-022-33855-z

How to cite: Cartwright-Taylor, A., Mangriotis, M.-D., Main, I. G., Butler, I. B., Fusseis, F., Ling, M., Andò, E., Curtis, A., Bell, A. F., Crippen, A., Rizzo, R. E., Marti, S., Leung, D. D. V., and Magdysyuk, O. V.: Micromechanics of damage localisation and shear failure of a porous rock: sound and vision, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7933, https://doi.org/10.5194/egusphere-egu23-7933, 2023.

Gas production from the Groningen gas field in the northeast of the Netherlands causes compaction and induced seismicity within the reservoir and overlying/underlying lithologies. Recent earthquake localization studies show that seismicity dominantly occurs on complex normal fault systems that juxtapose lithologies of contrasting mechanical properties. However, little is known about the effects of along-fault heterogeneity on the frictional behaviour of these faults. This study aims at understanding how material mixing and clay-smearing in fault gouges affects the mechanical strength and stability of faults that juxtapose contrasting lithologies (e.g. clay-rich and quartz-rich) by performing friction experiments.

Velocity stepping tests are performed on homogeneously mixed and spatially segmented simulated fault gouges, within a rotary shear configuration. Experiments are performed under normal stresses ranging between 2.5 and 10 MPa and imposed velocities ranging between 10 and 1000 µm/s. The rotary shear configuration allows for the large shear-displacements (>145 mm in our experiments) required to study the effects of lithology mixing. Simulated gouges are saturated with DI-water and subsequently sheared under drained conditions. Because low-permeability clay-rich materials promote the build-up of local pressure transients, a specially designed piston with four installed pressure transducers is used to monitor fluid pressures in the vicinity of the simulated fault gouges.

The mechanical data on segmented gouges show an evolution in frictional strength, characterized by a phase of strong displacement-weakening followed by displacement-strengthening. The frictional stability strongly increases with shear-displacement, comprising a transition from velocity-weakening to velocity-strengthening. Microstructural analysis of the sheared gouges provides evidence for the development of clay-smears and strain-localization within localized shear bands, explaining the evolution in frictional stability and the initial phase of shear-weakening. However, the dilatation observed at large displacements suggests that the quartz-rich gouge is incorporated within the clay smear. This incorporation is confirmed by microstructural analysis of the clay smear and provides a mechanism responsible for the observed strengthening at large shear-displacements. Monitoring of local pore fluid pressures shows that segmented gouges are more susceptible to pressure transients, depending on the initial distribution of high porosity sandstone gouges and low permeability claystone gouges.

This study shows that the frictional strength and stability of spatially heterogeneous gouges highly depends on the amount of shear-displacement. The frictional strength is characterized by subsequent phases of displacement-weakening and strengthening, whereas the frictional stability only increases with shear-displacement. This eventually leads to relatively strong but also frictionally stable faults at large displacements. The results have important implications for modelling earthquake nucleation,  propagation, and arrest and apply to faults in geological settings that exhibit induced seismicity, like the Groningen gas field, but are also relevant for tectonically active faults located elsewhere.

How to cite: Arts, J., Niemeijer, A., Drury, M., Willingshofer, E., and Matenco, L.: Mechanical and microstructural characterization of spatially heterogenous simulated fault gouges, derived from the Groningen gas field stratigraphy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8163, https://doi.org/10.5194/egusphere-egu23-8163, 2023.

frictional melting and thermal pressurization are commonly proposed to reduce dynamic shear resistance along a fault during earthquake propagation. The key factor on triggering either thermal pressurization or frictional melting may be the hydraulic properties of surrounding rock. Observations in Taiwan Chelungpu-fault drilling project (TCDP) Hole-A and Hole-B suggest that frictional melting and thermal pressurization occurred along the fault during the Mw 7.6 Chi-Chi earthquake, but the underlying process is still unclear. Here, we present the microstructural observation in experimental and natural fault gouge, the mechanical data at seismic rate and mineralogical characteristics. Results show that amorphous material only occurred at drained condition. Taken together, these results imply that the difference between Hole-A and Hole-B is attributed to the drainage.

How to cite: Wu, W.-J., Kuo, L.-W., Kuo, C.-W., Wu, W.-H., and Sheu, H.: Frictional melting and thermal pressurization during seismic slip controlled by drainage, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8389, https://doi.org/10.5194/egusphere-egu23-8389, 2023.

Preparatory earthquake processes such as slow preparatory slip (preslip) are connected to variations in frictional strength linked to frictional instabilities and appear in various scales across the Earth’s crust. For dry and bare surfaces, the fault surface characteristics affect the contact conditions. These conditions are established through asperities, which are topographical heights where the normal stress concentrates, imposing variations in fault strength. The effect of surface conditions on preslip can be studied in the laboratory where fault surface characteristics can be identified. Developing a more refined understanding of features controlling preslip (e.g., roughness) will lead to more realistic models describing frictional stability. In this study, we performed a triaxial test at sequentially increasing confining pressure steps (P_c = 60, 80, 100 MPa) on a saw-cut sample of Carrara Marble in dry and unlubricated conditions. Two types of technologies were used to study this frictional response in space and time: (1) an array of acoustic emission sensors monitored localized precursory seismicity and (2) quasi-static deformation in the fault-parallel strain was monitored using distributed strain sensing (DSS) with fiber optics. The differential stress was also measured throughout and allowed us to study the onset of frictional weakening/strengthening. In the first confining pressure step (P_c = 60 MPa), a single stick-slip event was observed with an associated 43 MPa static stress drop. In the subsequent confining pressure steps of P_c = 80 and 100 MPa, even though the normal stress on the fault was increased, no stick-slip events were observed, and the fault smoothly transitioned to sliding with smaller magnitude stress drops of 3 and 4 MPa, respectively. That suggests that a change in the frictional nature of the interface was incurred during the first rupture at P_c = 60 MPa. The high-density DSS array displayed a significant heterogeneous distribution of fault-parallel strain in time and space and experienced sudden reorganization at various phases of the experiment. Due to the high spatial resolution, DSS allowed us to investigate local deviations from an elastic response attributed to inelastic processes. A larger amount of local strain accumulation was needed to produce a stick-slip instability. At higher normal stress on the pre-ruptured fault, this level of locking was not possible in the subsequent confining pressure steps. Dissipative inelastic deformation was attributed to local frictional weakening that resulted in non-uniform preslip. Furthermore, priori measurements of contact pressure heterogeneities were obtained using a pressure sensitive film. These results showed regions of lower normal stress along the fault that correlated with regions that incurred more anelastic response on the DSS array. Post-mortem contact pressure measurements showed clear changes in the normal stress distribution that correlated to visual damage and wear. We believe that this contributed to the fault's inability to lock as before and mitigate dynamic rupture. Our results provide more insight into potential mechanisms controlling preslip distribution leading to dynamic and quasi-static frictional weakening.

How to cite: Michail, S., Selvadurai, P. A., Cebry, S. B. L., Salazar Vásquez, A. F., Bianchi, P., Rast, M., Madonna, C., and Wiemer, S.: Laboratory Observations linking Fault Surface Characteristics to Preparatory Earthquake Processes and Fault Stability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8634, https://doi.org/10.5194/egusphere-egu23-8634, 2023.

During earthquake propagation, a shock wave damages rocks at the rupture tip, creating numerous microfractures and altering the mechanical properties of fault zone rocks. This damage, which occurs dynamically at the millisecond time scale, controls rock strength during earthquake slip that occurs in the wake of rupture propagation. How the presence of water and the initial porosity of the rock control damage during high strain rate deformation remains an open question. We have performed a series of shock experiments using a split Hopkinson pressure bar apparatus installed at the European Synchrotron Radiation Facility. Using two ultra-fast cameras synchronized with the X-ray bunches of the synchrotron; we imaged deformation with microsecond time resolution on centimetre-scale core samples during shock wave damage. We deformed dry and water saturated low porosity Westerly granite and porous Berea sandstone samples. Several samples were surrounded by a thin aluminium jacket allowing recovering them after deformation and image them using X-ray microtomography with micrometre spatial resolution. Results confirm previous studies that have shown that rock pulverization occurs above a threshold strain rate produced by the shock wave. Water saturated samples are consistently weaker than dry samples as they pulverize under lower peak stress. Analyses of rock microstructure acquired using the ultrafast cameras and X-ray microtomography data shed light on the micro-mechanisms of damage production. Either the entire sample pulverized (Westerly granite) or a compaction of the sample occurred before shear zones were dynamically produced (Berea sandstone). These results demonstrate fundamental differences in dynamic damage production in crystalline and porous dry and wet rocks. Our data unravel mechanisms of gouge production before any significant slip has occurred on a fault, which control the shear strength during earthquake slip.

How to cite: Renard, F., Cordonnier, B., Doan, M.-L., Fondriest, M., Lukic, B., and Prastyani, E.: Dynamic damage in dry and wet rocks monitored by ultra-fast synchrotron imaging, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8778, https://doi.org/10.5194/egusphere-egu23-8778, 2023.

Understanding the mechanical and geochemical processes of fault rock development is a key clue into the understanding of fault healing rates. Fault healing rate β – the change of the static friction coefficient (Δμ) with log time (β = μ₀ + Δμ/log(1+t_hold /t_cutoff)) – is a significant parameter in the seismic cycle, controlling the storage of the elastic strain energy in the fault wall rocks and allowing earthquakes to repeatedly occur in pre-existing faults.

Fault healing is investigated with slide-hold-slide (SHS) experiments aimed at reproducing the seismic cycle. However, most of these experiments have been conducted under room conditions, while natural earthquakes nucleate at temperatures T > 150°C and in presence of pressurized fluids. Under these conditions, fluid-rock interaction (reaction kinetics, pressure-solution transfer, sub-critical crack growth, etc.) may impact severely on β and on the magnitude of Δμ.

In this study, motivated by the evidence of intense fluid-rock interaction in exhumed seismogenic faults hosted in the continental crust (Gomila et al., 2021, G³), we performed SHS experiments in a rotary shear apparatus equipped with a dedicated hydrothermal vessel. The goal is to investigate (1) the tribochemical processes and healing behavior of gouge-bearing faults made of granodiorite and, (2) explore how the mechanical properties and healing rates evolve with fault maturity (e.g., fault displacement, duration of fluid-rock interaction).

For the simulated gouge samples (grain size < 75 µm), three set of experiment of SHS were conducted, the first with run-in duration of 500s, whereas the 2^nd and 3^rd with 5000s, and geochemically contrasted against a non-sheared sample. The fluid (deionized water) saturated gouges were kept under an effective normal stress (σ_n^eff) of 10 MPa, a fixed temperature T of 300°C and a constant pore fluid pressure P_f of 25 MPa, and they were slid for ca. 15 mm and 60 mm at a slip rate of 10 µm/s. Hold periods between slip events ranged from 3s to 10000s (1^st and 2^nd experiments) and from 3s to 300000s (3^rd), to investigate the dependence of β and the underlying tribochemical processes with both cumulative slip and duration of the experiment.

Under these hydrothermal conditions, Δμ first increased with holding time (β value of ca. 2.0x10^-2 , independently of run-in duration) and then decreased (β = -3.6x10^-2, β = -3.0x10^-2 and β = -2.6x10^-2, for the 1^st, 2^nd and 3^rd experiment, respectively). Bulk XRF analyses on sheared samples show an enrichment of TiO₂, MgO and P₂O₅, while a loss of MnO and CaO oxides with respect to the non-sheared sample. Detailed SEM-EDS analyses show a main mineral loss of biotite and quartz within the main slip zone.

This suggest that under hydrothermal conditions, total shear displacement and duration of the fluid-rock interaction enhance mineral reactions that promote negative healing rates (β < 0) in faults during the seismic cycle. This would imply that during the life-span of an evolving fault, as it matures, it would be possible to (1) lower the fault yield strength due to and increasing fluid-rock interaction, henceforth (2) increase the recurrence but decrease the intensity of the seismic activity.

How to cite: Gomila, R., Feng, W., and Di Toro, G.: Fault-healing and tribochemical processes in granodiorite under hydrothermal conditions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9600, https://doi.org/10.5194/egusphere-egu23-9600, 2023.

Fault frictional healing Δμ controls the storage of the elastic strain energy in the fault wall rocks and the re-occurrence of earthquakes in pre-existing faults. In the last 40 years, fault healing has been investigated with laboratory slide-hold-slide (SHS) experiments aimed at reproducing the seismic cycle. Experiments performed with different rocks types (e.g., granite, limestone, basalt) revealed that (1) Δμ increases with hold time t_h and, (2) the frictional healing rate β =Δμ/log t_h >0. This increase in fault frictional strength with t_h is interpreted as due to the increase (1) in the real area of contact or (2) of chemical bond strength. However, most of these experiments were conducted under room conditions, whereas natural earthquakes generally nucleate at ambient temperatures T  >150℃ and in the presence of pressurized fluids. Under these ambient conditions, fluid-assisted and thermally-activated processes (pressure-solution transfer, stress corrosion, etc.) may impact on the magnitude of Δμ and on β.

In this study, SHS experiments were performed on gabbro-built gouges (grain size <88 mm) in a rotary shear machine equipped with a pressurized vessel to explore frictional healing under hydrothermal conditions. All experiments were conducted at a constant effective normal stress (σ_eff =50MPa), and temperature (T) ranging from 25 to 400 ℃  under dry or pore fluid (deionized H₂O) pressure (P_f=30 MPa) conditions. In the SHS sequence, the imposed slip velocity was V=10 μm/s, and hold time t_h varied from 3 to 10000 s. For each experiment, two SHS sequences separated by a slip displacement interval of 40 mm were conducted.

Under dry conditions at all tested temperatures and under hydrothermal conditions but at T  <100℃, Δμ increases with t_h, consistent with previous experiments. Moreover, the Δμ and β values in the 2^nd SHS sequence are slightly higher than those in the 1^st sequence, possibly due to the smaller grain size at the larger displacement that promotes fault healing. By contrast, in the experiments performed under hydrothermal conditions but T >200℃, Δμ decreases and β switches to negative values (<0) when the hold is longer than a threshold hold time. In detail, at T=300℃: β= 0.0161±0.0017 for holds <300s and -0.0074±0.0043 for holds >300s, and at T=400℃: β= 0.0057±0.0020 for holds <100s and -0.0227±0.0042 for holds >100s.

The underlying mechanism responsible for the decrease in Δμ and the transition from β > 0 to β < 0 with the hold time, which could result in the transition from seismic to aseismic fault behavior in nature, is still poorly understood. However, high-resolution microstructural analyses conducted by scanning electron microscopy on experimental fault products rule out the formation of weak minerals (e.g., clays) in the gouge layer.  Consequently, the weakening of the fault is probably related to the decrease in bond strength at the asperity contacts.

The experimental data presented here suggest that fault healing of natural faults is controlled by the feedback of multiple physico-chemical processes associated with the slip history and type of fluid-rock interaction under hydrothermal conditions.

How to cite: Feng, W., Yao, L., Gomila, R., Ma, S., and Di Toro, G.: Healing of gabbro-built faults under hydrothermal conditions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9616, https://doi.org/10.5194/egusphere-egu23-9616, 2023.

Slip along pre-existing faults in the Earth’s crust occurs whenever the shear stress resolved on the fault plane overcomes fault frictional strength, potentially generating catastrophic earthquakes. The coupling between shear stress and normal stress during fault loading depends on 1) the orientation of the fault within the stress field and 2) the tectonic setting. In compressional settings, a load-strengthening path occurs because along thrust faults the increase in shear stress is coupled with an increase in effective normal stress. On the contrary, in extensional settings, the increase in shear stress is coupled with a decrease in effective normal stress, resulting in load-weakening paths for normal faults.

Analytical approaches to evaluate the potential for fault reactivation are generally based on the assumption that faults are ideal planes, characterized by zero thickness and constant friction, embedded in homogeneous isotropic elastic media. However, natural faults typically host thick fault cores and highly fractured damage zones, which can compact or dilate under different loading paths (i.e., different coupling between normal and shear stress). In addition, in most laboratory friction experiments, the fault is loaded under constant or increasing normal stress and at optimal orientation for reactivation. Here, we present laboratory experiments simulating reactivation of thick gouge-bearing faults that experienced different loading paths.

Our results show that the differential stress required for reactivation strongly differs from theoretical predictions, and unfavourably oriented faults appear systematically weaker, especially when a thick gouge layer is present. Before reactivation fault zone compacts in load-strengthening paths whereas dilation is observed in load-weakening path. Upon fault reactivation at comparable normal stress, load-strengthening promotes stable creep  whereas load-weakening results in accelerated slip. Our study highlights the importance of fault thickness and loading path in fault hydromechanical coupling and stability with significant implications for fluid circulation within fault zones and earthquake mechanics.

How to cite: Giorgetti, C., Violay, M., and Collettini, C.: The role of loading path on fault reactivation: a laboratory perspective, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10016, https://doi.org/10.5194/egusphere-egu23-10016, 2023.

Although the physical mechanism of earthquake nucleation processes and the link with foreshocks are under debate, foreshocks are still considered as the most reliable earthquake precursors. Investigating the temporal and spatial evolution of foreshock sequences with high resolution and monitoring b-values in real time may shed light on these key issues. Many foreshock and aftershock sequences accompanying moderate mainshocks have been reported in the west of Yunnan Province, China, such as the 2016 Yunlong M 5.1 and 2021 Yangbi Ms 6.4 earthquake sequences. The recently improved coverage of seismic network in western Yunnan provides the opportunity to investigate how the foreshock sequence evolved and establish the temporal transient in b values. To find missing earthquakes and built more comprehensive earthquake catalogs, we carried out earthquake detection using the matched-filter detector. We used events in the standard catalog of China Earthquake Networks Center as templates to scan through continuous waveforms 3-6 months before and after the main shock. We then estimated the b-value and its temporal changes based on the newly developed catalogs. An obvious reduction in b-values before the major earthquake is observed in both the 2016 Yunlong and 2021 Yangbi sequences. We also found that the scattered spatial pattern of foreshocks exhibits a cascading manner and does not support the hypothesis of slow slip driving nucleation of mainshocks.

How to cite: Zhu, G. and Yang, H.: Foreshocks preceding moderate earthquakes in Western Yunnan, China, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10314, https://doi.org/10.5194/egusphere-egu23-10314, 2023.

The constitutive behavior of faults is central to many interconnected aspects of earthquake science, from fault dynamics to induced seismicity, to seismic hazards characterization. Yet, a friction law applicable to the range of temperatures found in the brittle crust and upper mantle is still missing. In particular, rocks often exhibit a transition from steady-state velocity-strengthening at room temperature to velocity-weakening in warmer conditions that is poorly understood. Here, we investigate the effect of competing healing mechanisms on the evolution of frictional resistance in a physical model of rate-, state-, and temperature-dependent friction. The yield strength for fault slip depends on the real area of contact, which is modulated by the competition between the growth and erosion of interfacial micro-asperities. Incorporating multiple healing mechanisms and rock-forming minerals with different thermodynamic properties allows a transition of the velocity- and temperature-dependence of friction at steady-state with varying temperatures. We explain the mechanical data for granite, pyroxene, amphibole, shale, and natural fault gouges with activation energies and stress power exponent for weakening of 10-50 kJ/mol and 55-150, respectively, compatible with subcritical crack growth and inter-granular flow in the active slip zone. Activation energies for the time-dependent healing process in the range 90-130 kJ/mol in dry conditions and 20-65 kJ/mol in wet conditions indicate the prominence of viscoelastic collapse of micro-asperities in the absence of water and of pressure-solution creep, crack healing, and cementation when assisted by pore fluids. 

How to cite: Barbot, S.: A rate-, state-, and temperature-dependent friction law with competing healing mechanisms, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10779, https://doi.org/10.5194/egusphere-egu23-10779, 2023.

Many large and damaging earthquakes on mature faults in the Earth’s crust propagate along layers of rock gouge, the fine granular material produced by comminution during sliding. Characterizing gouge rheology is of paramount importance to improve our understanding of earthquake physics, as friction controls key processes of earthquakes, including nucleation, propagation and arrest and how damaging they can be.  In this work, we characterize friction evolution in rock gouge layers during the propagation of dynamic ruptures in a laboratory setting. The experimental setup features a hybrid configuration with a specimen made of an analog material and a rock gouge layer embedded along the interface. This configuration allows us to trigger dynamic ruptures due to the lower shear modulus of the analogue material while at the same time study the gouge frictional behavior during spontaneously evolving dynamic events. Ruptures are captured by the use of digital image correlation coupled with ultrahigh-speed photography. Our measurements reveal dramatic friction variations, with the gouge layer initially displaying strengthening behavior and inhibiting earthquake rupture propagation. However, the gouge layer later features dramatic frictional strength losses, and hosts rupture re-nucleation enabled by dynamic stressing and marked friction weakening at higher slip velocities. Our measurements of the weakening and strengthening behavior of friction in fine rock gouge illustrate the strong dependence of their rheology on slip velocity and related processes, including shear heating, localization/delocalization of shear, and dilation/compaction of the granular shear layer.

How to cite: Rubino, V., Rosakis, A., and Lapusta, N.: Dynamic weakening and rupture re-nucleation in rock gouge, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11776, https://doi.org/10.5194/egusphere-egu23-11776, 2023.

Fault zones consist of one or more fault cores sandwiched by a damage zone surrounded by less deformed wall rocks. Most of the deformation is accommodated in the fault core through slip along one or more principal slipping zones. The thickness of fault cores (mm to m) and individual slipping zones (µm to dm) increases with fault slip displacement. In particular, small-displacement or immature faults have such thin slip zones that resemble bare rock surfaces. When exhumed from <5-6 km depth, slip zones are made by poorly cohesive fault gouges.

Several laboratory experimental configurations aim to reproduce the deformation processes activated during seismic slip episodes. In the laboratory, the slip zone is represented as the interaction volume of two bare rock surfaces (i.e., immature faults) or as a mm-thick gouge layer (i.e., more mature faults). Most studies have focused on the frictional behavior of gouge layers or bare rocks during single seismic events, and only a few on the mechanical and microstructural evolution of a gouge layer subjected to multiple events of seismic slip (e.g., Smith et al., 2015). Here, we present rotary-shear friction experiments that reproduce seismic slip on both gouge layers and bare rocks derived from calcite-rich marble. The aim of this study is to analyze the frictional evolution of a gouge layer undergoing multiple seismic slip pulses: four trapezoidal slip pulses at 1 m/s for 1 m of slip, with hold time of 120 s between each pulse. Moreover, we compare this evolution with one of bare rocks of the same material but slid only once at 1 m/s for a total slip higher than 1 m. Experiments were performed at normal stress of 10, 20, and 30 MPa under room humidity conditions.

Our experimental results show that despite the static and dynamic friction coefficients are higher in the gouge layer than in the bare rock experiments, the frictional work to achieve the dynamic friction decreases at each seismic slip pulse in the gouge experiments and is comparable with the bare rock one after the second pulse. High-resolution scanning electron microscope investigations of the sheared gouge layers show that in the first two slip pulses most of the frictional work is spent on (1) strain localization into newly-formed slip zones bounded by continuous ultra-smooth surfaces and, (2) grain size reduction, sintering and compaction (i.e., porosity reduction) within the bulk gouge layer. However, after the second pulse, the slip is localized in one or more well-developed slip zones bounded by ultra-smooth surfaces, that cut through the compacted gouge layer, and the mechanical behavior is similar to that of bare rocks.

Carbonate-bearing fault zones are common seismogenic sources in the Mediterranean area (e.g. 2009 L'Aquila Mw6.3 and 1981 Corinth M6.6 earthquakes). In a series of subsequent seismic slip events, it is shown that the evolution of a gouge layer in carbonate-bearing fault rocks tends to produce a similar mechanical behaviour of bare rocks although the volumetric distribution of strain is significantly different. Importantly, the energy spent by apparently different mechanical processes is eventually similar.

How to cite: Cornelio, C., Aretusini, S., Spagnuolo, E., Di Toro, G., and Cocco, M.: Frictional evolution of gouge-bearing faults during multiple seismic slip velocity pulses, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14284, https://doi.org/10.5194/egusphere-egu23-14284, 2023.

In this paper, based on the model of thermal pressurization, we present a new way for the emergence of rate and state phenomenology (RSF, friction law) during the earthquake cycle. In the framework of fault mechanics, the common physical mechanism for the RSF phenomenology is slip and plastic deformation at the asperity contacts. We show that the fundamental physical mechanism of thermal pressurization together with viscosity inside the fault can also reproduce rate and state phenomenology.

More specifically, in our numerical analyses we model frictional weakening during large seismic slip due to thermal pressurization inside the fault. We introduce thermo-hydro-mechanical couplings to model thermal pressurization and a first order micromorphic Cosserat continuum, in order to avoid mesh dependence of the numerical results. Moreover, we introduce viscosity in the form of strain rate hardening. When we perform velocity stepping analyses, our numerical findings show that friction presents, initial peak over-strength and frictional oscillations around a residual value (see Figure). Our results, deriving from fundamental modeling assumptions, exhibit rate and state phenomenology, without the need to introduce the physical mechanism of slip at the asperity contacts.

Keywords: THM couplings; Viscosity; Cosserat continuum; Tribology; Earthquakes

How to cite: Stathas, A. and Stefanou, I.: Viscosity and thermal pressurization during large seismic slip lead to rateand state phenomenology, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14607, https://doi.org/10.5194/egusphere-egu23-14607, 2023.

In the framework of empirically-derived rate- and state- friction (RSF) laws, friction constitutive parameters a, b, and D_c  (and further sets of state parameters) are obtained from inverse modelling of laboratory data on the assumption that steady-state conditions are reached following the velocity steps. This method also includes removing any slip-dependent linear trends in friction by linear regression when steady-state conditions are considered to be achieved. The choice of where linear detrending, thereby where to assume the attainment of steady-state friction conditions is therefore key for a correct retrieval of the modelled RSF parameters and their consequent use in modelling of earthquake nucleation. Nonetheless, to date this procedure is still user-dependent and as such, RSF outputs may differ ceteris paribus.

To better elucidate the detrimental consequences of an incorrect assumption of steady-state friction conditions in RSF analysis, in this study synthetic velocity steps were generated with superimposed random Gaussian noise, characterized by increasing characteristic slip distances in the second set of state variables, D_c2,from 0 to 500 µm. In each velocity step, steady-state conditions were assumed starting at progressively larger displacements with respect to the occurrence of the velocity jump. This means that the arbitrarily chosen “steady-state” may or may not correspond to the true steady-state conditions. To retrieve RSF parameters, a slip window of constant size (i.e., 100 µm) was applied from the selected “steady-state” point onwards to remove any linear trend in friction, implying that the remainder of the velocity step beyond the slip window is also at steady-state. During each RSF analysis, the slope calculated from linear regression within the 100 µm long slip window after the velocity steps is systematically compared with the slope computed from linear regression prior to the velocity steps.

Our results show that:

• while a, b₁ and D_c1 are essentially constant regardless of the choices of steady-state and equal to the true values used to generate the synthetic velocity steps, b₂ and D_c2 may significantly differ if D_c2 is commensurate with the whole displacement window that contains the velocity step;
• all modelled RSF parameters coincide with the true ones when the ratio of the slopes before and after the velocity steps approach unity; this observation can be regarded as a proxy for the achievement of the steady-state conditions and becomes increasingly relevant with larger D_c2.

Based on such evidence, we developed a routine that automates the above described work flow, providing a systematic and reproducible technique to determine steady-state friction and thus return the correct RSF parameters. Furthermore, this novel procedure determines the optimal minimum slip window size to remove slip-dependent linear trends in friction and alerts the user when steady-state is not reached within a given step length and hence when D_c2 and b₂ cannot be properly determined with experimental data.

How to cite: Giacomel, P., Faulkner, D., Lambert, V., and Allen, M.: A novel automated procedure for determining steady-state friction conditions in the context of rate- and state- friction analysis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15563, https://doi.org/10.5194/egusphere-egu23-15563, 2023.

In addition to regular earthquakes, observations of spatiotemporally complex slip events have multiplied over the last decades. These slip events range along different time scales: from creep , slow slip events to LFEs and tremors. At present, these events are generally interpreted by imposed frictional heterogeneities along the fault plane. However, fault systems are geometrically complex in nature over different scales. We aim in this work to investigate the role of “realistic” fault geometry on the dynamics of slip events. We consider a fault system in a 2D quasi-dynamic setting. The fault system consists of a main self-similar rough fault, surrounded by a dense network of off-fault fractures. All fractures are frictionally homogeneous (rate weakening) and can potentially undergo dynamic slip. We aim to understand how the deformation in the volume is accomodated by the off-fault damage zone and the main fault. What fraction of the “supplied” moment rate is hosted by the off-fault fractures during an earthquake cycle?

How to cite: Bhat, H., Almakari, M., Kheirdast, N., Villafuerte, C., and Thomas, M.: Fault zone complexity naturally produces the full slip spectrum: Insights from numerical models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16290, https://doi.org/10.5194/egusphere-egu23-16290, 2023.

The question "what arrests an earthquake rupture?" sits at the heart of any potential prediction of earthquake magnitude. Here, we present a one-dimensional, thin-elastic-strip, minimal model, to illuminate the basic physical parameters that control the arrest of large ruptures. The generic formulation of the model allows for wrapping various earthquake arrest scenarios into the variations of two dimensionless variables, valid for both in-plane and antiplane shear loading. Our continuum model is equivalent to the standard Burridge-Knopoff model, with an added characteristic length scale, that corresponds to either the thickness of the damage zone for strike-slip faults or to the thickness of the downward moving plate for subduction settings. We simulate the propagation and arrest of frictional ruptures and present closed-form expressions to predict rupture arrest under different conditions. Our generic model illuminates the different energy budget that mediates crack- and pulse-like rupture propagation and arrest. Despite its simplicity, this minimal model is able to reproduce several salient features of natural earthquakes that are still debated (e.g. various arrest scenarios, stable pulse-like rupture, back-propagating front, asymmetric slip profiles).

How to cite: Barras, F., Thøgersen, K., Aharonov, E., and Renard, F.: How do earthquakes stop? Insights from a minimal model of frictional rupture, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16320, https://doi.org/10.5194/egusphere-egu23-16320, 2023.

Pore fluids are ubiquitous throughout the lithosphere and are commonly cited as a major factor producing slow slip and complex modes of tectonic faulting. Here, we investigate the role of pore pressure on slow slip and the frictional stability transition and find that the mode of fault slip is largely unaffected by pore pressure once we account for effective stress. Ambient temperature experiments are done on synthetic fault gouge composed of quartz powder with a median grain size of 10μm with an average permeability of  8E-17m² – 6E-18m² from shear strains 0 - 26. We conduct constant velocity experiments at 20MPa σ_n’, with P_p/σ_n’ratios of λ from 0.05 to 0.28. Under these conditions, dilatancy strengthening is minimal and we find that slip rate dependent changes in the critical rate of frictional weakening are sufficient to explain slow slip.

How to cite: Affinito, R., Elsworth, D., and Marone, C.: The Stability Transition from Stable to Unstable Frictional Slip with Finite Pore Pressure, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16777, https://doi.org/10.5194/egusphere-egu23-16777, 2023.

The deformation and failure processes of rocks under stress are primarily induced by microcracking. Detecting this micro-interaction phenomenon before the ultimate failure has paramount importance for predicting the post-failure rock damage characteristics. In this study, we aim to quantify the evolution of microcracking through fractal analyses of scanning electron microscope (SEM) images, captured from three different rock types subjected to uniaxial loading at various stress levels. In terms of uniaxial compressive (UCS) and tensile strength (UTS) values, the rocks range from the strongest to the weakest as being diabase, ignimbrite, and marble, respectively.  All rock samples are uniaxially loaded up to critical stress thresholds as crack initiation (σ_ci), crack damage (σ_cd), and peak stress (σ_p) levels, considering their pre-defined characteristic stress-strain curves. Using the box-counting technique, the fractal dimension values (D_B) of cracking intensity, induced by loading are determined for all these three stages. Here, it should be noted that higher fractal dimensions represent more intense microcracking according to the fractal theory. The results show that the D_B values are increasing with the increasing amount of microcracks and the greatest D_B values are calculated for Diabase due to its highest strength ratio (UCS/UTS). Although the marble has the weakest strength values, it presents a higher D_B value than that of ignimbrite (D_Bmarble = 1.215 and D_Bignimbrite = 1.133) once the σ_cd stress threshold is reached. Furthermore, the D_Bmarble value is also greater than the D_Bignimbrite value for the σ_p stress level. It is because marble has a higher UCS/UTS ratio than the ratio of ignimbrite. Our results highlight the important role of rock texture on brittleness which exerts a primary control on fractal dimensions (D_B). A decrease in volumetric rigidity is more dramatic in marble than in ignimbrite with incremental loading. The insights provide a better understanding of the microcracking process that leads to macro-scale deformations in rock engineering.

How to cite: Dinç Göğüş, Ö., Avşar, E., Develi, K., and Çalık, A.: Progressive failure characteristics of different rock types through fractal analysis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-254, https://doi.org/10.5194/egusphere-egu23-254, 2023.

The pore size and the distribution of individual or connected pores contribute to the porosity in a rock which is closely related to rock weathering degree and rock strength. The chemical reaction is normally higher for the larger specific surface area which is closely related to the pore size distribution in a rock. The variation of pore size distribution in sedimentary rocks from Gyeongsan basin in Korea was determined by the laboratory artificial acceleration weathering experiment using peristatic pumps. The pore size distribution of rock specimens was measured by the nitrogen gas adsorption method using BELSORP-max II of Microtrac MRB. The pore characteristics were measured on the outer surface and the innermost part of rock samples to determine the variation of pore size distribution since the outer surface was directly affected by weathering processes while the innermost part was not. The high-purity nitrogen gas is used to evaluate the pore size distribution with different methods such as BET, BJH, and HK. The overall pore volume and size have been increased by the weathering experiment for the tested sedimentary rocks-sandstone, conglomerate, and shale. The increase of macropore in sandstone by weathering experiment leads mainly to the increase in pore volume, while the rise of micropore and mesopore in conglomerate drives the increase of pore volume. 

Acknowledgments

This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education(NRF-2020R1F1A107576412).

How to cite: Woo, I.: Pore Size Redistribution by Laboratory Weathering Tests on Sedimentary rocks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1518, https://doi.org/10.5194/egusphere-egu23-1518, 2023.

A zone of significant high-amplitude magnetic anomalies is observed without a comparable gravity high along the Cascadia margin and is spatially correlated with the low-velocity fore-arc mantle wedge, which is understood to be serpentinized fore-arc mantle and is further considered to be the main source of the high-amplitude magnetic anomalies. To test this concept, the magnetization-density ratio (MDR) is estimated along the Cascadia margin to highlight the physical characteristics of serpentinization (reduce density and increase in magnetization). Interestingly, high MDR values are found only in central Oregon, where slab dehydration and fore-arc mantle serpentinization (50%-60% serpentinization) are inferred in conjunction with sparse seismicity. This result may indicate either a poorly serpentinized fore-arc mantle or that the fore-arc mantle is deeper than the Curie temperature isotherm for magnetite in northern and southern Cascadia. This finding means that magnetic anomaly highs and serpentinized fore-arc mantle may not be completely positively related in subduction zones.

How to cite: Doo, W.-B. and Wang, H.-F.: Relationship between the high-amplitude magnetic anomalies and serpentinized fore-arc mantle in the Cascadia subduction zone, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1727, https://doi.org/10.5194/egusphere-egu23-1727, 2023.

Understanding compaction localization in porous limestone in the laboratory is significantly more challenging than in sandstone because of the lack of consistent acoustic emission activity in carbonate samples. Previous studied have therefore relied on X-ray Computed Tomography imaging (CT). The first unambiguous evidence of compaction band development in limestone was provided by Huang et al. (2019), who performed synchrotron in situ CT imaging during shear-enhanced compaction in a sample of Leitha limestone. This sample was deformed in the HADES rig at the European Synchrotron Radiation Facility, in dry conditions and at a confining pressure of 20 MPa. In this study, we analysed this data set using Digital Volume Correlation (DVC). Not only could we use DVC to characterize quantitatively the spatiotemporal development of displacement and strain, we were also able to compare with direct observations to assess the stress-induced damage in multiple scales. Our new results confirm that inelastic compaction occurred in two stages in Leitha limestone: macropore collapse first and then sequential growth of compaction bands. In the pore collapse stage, DVC reveals complex and heterogeneous grain-scale strains, implying significant heterogeneity in the internal stress field. Such complexity is to be accounted for if one were to connect micromechanical and continuum models. At higher stresses, we have obtained further quantitative constraints on the spatial distribution of volumetric and shear strain during the growth of compaction bands. Our results demonstrate that compaction banding in Leitha limestone can be analysed as a bifurcation phenomenon, that would typically occur preferentially in zones of high porosity. The displacement field inferred from DVC revealed that the bands showed mostly normal displacement discontinuities, as expected for compaction bands. DVC analysis also gave more constraints on band geometric attributes. Analysis of the autocorrelation function for the strain suggested that the decay and rebound of the autocorrelation as a function of the axial separation may provide proxies for the mean width and spacing of compaction bands. The 2D autocorrelation function on the band planes also provides relevant clues on the complex sequential growths of the compaction bands.

How to cite: Baud, P., Meng, F., Huang, L., and Wong, T.: Compaction localization in 4D imaged by X-ray Computed Tomography and Digital Volume Correlation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2397, https://doi.org/10.5194/egusphere-egu23-2397, 2023.

Heterogeneities control rock properties, especially hydraulic and geophysical properties. Complex systems typically include multiple porosities at embedded scales, from the micro/meso cracks and pores to geological macro-fractures and karsts. This complex network play coupled roles and introduces difficulties in the characterization of the whole formation.

In order to constrain these coupled effects, we use seismic to acoustic data to characterize a multi-scale double porosity network and to understand the corresponding flow and mechanical properties of a shallow aquifer reservoir. The study focusses on the platform “Observatoire des transferts dans la Zone Non-Saturée” (O-ZNS, Orléans, France), an artificial excavation in the karstified and fractured limestone formation of Beauce aquifer. It is composed by an exceptional well (20 m-depth, 4 m-diameter) surrounded by 8 cored boreholes.

Two seismic refraction profiles crossing the O-ZNS site were carried out to determine P-wave velocities. The profiles delineated three main geological units: (i) a clayey soil (0-2 m), (ii) a weathered and karstified limestone layer (2-7 m), and (iii) massive limestone down to the underlying Molasse du Gâtinais layer at a depth of 25 m. In consistence with the lithological log, a thin layer of more massive limestone is highlighted around 5 m-depth. In addition, we also observed that the increase in P-wave velocity slows down after 15 m. This effect is consistent with the increasing fracture density and karst development observed on the direct log imagery and on the well 3D scan. In the massive thin limestone layer of 5 m-depth, the interpreted relative crack density is low, around 0.08. However, in the last layer from 15 to 20m-depth, the relative crack density is much more important, even so discrepant, with maximal values around 0.4.

In parallel to large scale field investigation, mechanical tests and elastic wave velocities have been measured on representative core samples. A strong discrepancy is observed, whatever the property. For example, at 16 m-depth, P-wave velocities are distributed from 3,650 to 5,700 m.s^-1 and the corresponding mechanical parameter of crack density ranges from 0 to 0.5. In addition, extreme values of crack density, above 1 are observed around 19 m-depth. These large discrepancies and crack density values are consistent with mechanical behavior and microstructure observation made directly on core samples, even though some samples are more porous than cracked and the distinction need to be kept. Samples are then classified through image processing in three categories: the porous ones, the cracked ones, and the mixed ones allowing to discuss and organize the heterogeneity distribution of the O-ZNS.

To complete the study, an intermediate characterization is running on metric blocs sampled at different depth from the O-ZNS well. Their analyses include 3D external scans at high resolution (as for the surface of O-ZNS well) and P-wave velocity measurements at intermediate frequencies. These blocs are then iterativelly, cut into smaller blocs and re-characterized in order to obtain the distribution of heterogeneity size and characteristics with depth targeting the determination of a REV (Relative Elementary Volume) for future modeling developments.

How to cite: Mallet, C., Laurent, G., and Azaroual, M.: Seismic to acoustic characterization of geomechanical and microstructural properties of a vadose zone heterogeneous limestone formation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3096, https://doi.org/10.5194/egusphere-egu23-3096, 2023.

The presence of water causes a dramatic reduction of the strength of most rocks. Under compressive stress conditions, fracture mechanics models show the strength of a rock sample is in particular controlled by frictional parameters and the fracture toughness of the material. Previous studies suggested that these parameters could change significantly in the presence of water, but there is a paucity of data quantifying this. Here, we report fracture toughness, frictional and uniaxial compression tests performed on five sandstones and five limestones under dry and water-saturated conditions, that provide new insight into the mechanical influence of water on sedimentary rock strength. Our new data showed that on both sandstones and limestones, the presence of water causes a reduction of both the fracture toughness (from 0 to 50%) and the static friction coefficient (from 0 to 40%), suggesting that water weakening in these sedimentary rocks is mostly due to a reduction of these two parameters under the relatively high strain rate conditions investigated here. While for sandstone we found a reduction of the Uniaxial Compressive Stress between 0 to 35%, it was less variable in limestone, in most cases around 40%. The measured fracture toughness and frictional parameters were then introduced into two well-known micro-mechanical models (the pore-emanating cracks model and the wing crack model), which provide simple theoretical expressions for the Uniaxial Compressive Strength. We found that the predicted water-weakening based on our toughness and friction parameter measurements is in overall agreement with our strength measurements on dry and wet samples.

How to cite: Violay, M., Noel, C., and Baud, P.: Effect of water on sandstone and limestone, fracture toughness, frictional parameters and brittle strength., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4578, https://doi.org/10.5194/egusphere-egu23-4578, 2023.

Recent industrial processes that involve injection of fluids, such as geothermal stimulation, disposal of waste water from hydraulic fracturing and carbon sequestration, have induced seismicity that has caused concern and resulted in discontinuation of the activity. Although field observations are the ultimate test of the effects of pore fluid on failure, their interpretation is complicated by heterogeneity of hydrologic and mechanical structure, and pumping and loading history. In particular circumstances, well-designed field tests can overcome some of these limitations. Laboratory experiments, despite their limited size and time scales, provide a more controlled environment that can yield an understanding of fundamental processes. Simple models that simulate the experiments can assess whether the mechanisms included in the models are sufficient to describe well the response or more complex formulations are needed. In addition, simulations can extend results for parameter values and loading programs beyond those achievable in experiments and aid in extrapolation to field applications.

This work uses a spring-block model and rate and state friction to simulate experiments conducted in a double direct shear apparatus on simulated carbonate fault gouge (Scuderi et al., EPSL, 2017) and on a shale bearing rock (Scuderi and Collettini, JGR, 2018). Both sets of experiments used the same loading protocol and injected pore fluid under creep conditions. When velocity strengthening rate and state friction is used to simulate the experiments on the simulated carbonate fault gouge the results agree well with the observed onset of tertiary creep in the experiment. Thus, the simulation reinforces the observation that pore fluid injection can induce rapid slip even when the friction relation is velocity strengthening. The rate and state framework provides an interpretation alternative to the standard one of the Mohr's circle moving to the left as pressure increases. In the rate and state framework, the friction coefficient must increase with pore pressure increase. The shale has a very low nominal friction coefficient (0.28) and is much more velocity strengthening than the carbonate. The simulation agrees with the observations that increases in pore pressure induce an increase in slip velocity but the magnitudes reach only about 100 µm/s by the end of the experiment. The simulation predicts reasonably well the times at which representative values of the slip velocity and displacement occur but the overall agreement of simulation and observation is not as good as for the carbonate. Mechanisms other than rate and state friction, for example, direct dependence of the friction coefficient on slip and porosity changes, may be significant.

How to cite: Rudnicki, J.: Rate and State Simulation of Two Experiments with Pore Fluid Injection Under Creep Conditions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4620, https://doi.org/10.5194/egusphere-egu23-4620, 2023.

Earthquakes introduce long-lasting transient mechanical damage in the subsurface that can take years to recover to a new elastic steady-state. The associated transient perturbation of the elastic moduli can cause postseismic hazards such as enhanced landsliding.  This dynamics is linked to relaxation, a phenomenon observed in a wide class of materials after straining perturbations. In this study, we analyze the successive effect of two large earthquakes (the 2017 Mw7.7 Tocopilla and the 2014  Mw8.2 Iquique earthquakes) on ground properties through the monitoring of seismic velocity from ambient noise interferometry in the Atacama desert in Chile. The absence of rainfall in this area allows study of the mechanical state of the subsurface by limiting the potential effect of variations in groundwater content. We show that relaxation timescales are a function of the current state of the subsurface when perturbed by earthquakes, rather than ground shaking intensity. Our study highlights the predictability of earthquake damage dynamics in the Earth's near-surface and potentially other materials. We propose to reconcile this paradigm with existing physical frameworks by considering the superposition of different populations of damaged contacts. 

How to cite: Illien, L., Turowski, J. M., Sens-Schönfelder, C., Berenfeld, C., and Hovius, N.: Predictable healing rates in near-surface materials after earthquake damage in Chile, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5402, https://doi.org/10.5194/egusphere-egu23-5402, 2023.

Rocks are very much susceptible to deformation in tension, especially under elevated temperatures. Therefore, the study of the dynamic tensile behaviour of rock exposed to high temperature is highly significant to understand the tensile deformation behaviour in dynamic loading conditions which will be proved useful in a variety of engineering problems such as quantifying the blast load impact in fire affected underground/opencast coal mine regions; assessment of ground subsidence due to coalmine fire coupled with blast loading etc. The Jharia coalfield region, known as the coal capital of India, is affected by pervasive underground coalmine fire for decades resulting in small to large-scale surface fracturing. So,the present study focuses on the effect of high temperature on dynamic tensile behaviour and its relation with micro-mineralogical properties of subsurface coal-bearing sandstone samples from a fire-affected mine.  To achieve the objective, the prepared samples were kept in the furnace for 24h with a heating rate of 5°C/min and then allowed to cool down naturally within the furnace. Samples were divided into nine groups based on the thermal treatment at 25 °C, 100 °C, 200 °C, 300 °C, 400 °C, 500 °C, 600 °C, 700 °C, and 800 °C. Using the Split-Hopkinson Pressure Bar (SHPB), the indirect dynamic tensile strength was measured for each group. Based on the obtained results, the indirect dynamic tensile strength of heat-treated specimens is characterized into three zones; viz.: Zone 1 (25-400°C), Zone 2 (400-600°C) and Zone 3 (600-800°C). In zone 1, an increase in average indirect dynamic tensile strength is observed with elevated temperature. However, in zone 2, a sharp decreasing trend in indirect dynamic tensile strength was observed with increasing temperature. This zone is characterised by a progressive increase in thermal cracks and porosity which is possibly the prime reason for a sharp transition in thermal properties. An overall reduction in indirect dynamic tensile strength is observed within zone 3, however, the rate of reduction is gentle. The plasticity that occurred due to high temperature was responsible for a slow rate of reduction in indirect dynamic tensile strength.

How to cite: Tripathi, A., Khan, M. M., K. Singh, A., and Pain, A.: Dynamic tensile behaviour of Barakar sandstone under high-temperature conditions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5667, https://doi.org/10.5194/egusphere-egu23-5667, 2023.

At the nanoscale, fracturing creates surface area and flow pathways, which control the rates of fluid-rock interactions. However, how fractures form at the nanoscale remains enigmatic. Here, we implement molecular dynamics simulations to reproduce fracture propagation in quartz and basalt. These simulations require large systems and long simulation times and are therefore currently depending on interatomic potentials. In the recent years, machine learning approaches have been established as a way to fit interatomic potentials, where the potentials are trained with quantum-mechanical data obtained from ab initio molecular dynamics simulations. We have developed machine-learned interatomic potentials for silica and basalt that allow using molecular dynamics simulations to simulate fracture propagation at the nanoscale. The interatomic potentials reproduce the mechanical properties of bulk silica and basalt sand have also been trained to account for fracture propagation. First, we trained a potential on silica to verify the fitting procedure, and then we used the same procedure to train an interatomic potential for basalt. By training the potential with water and carbon dioxide as fluids, we aim to study how a dynamic fracture damage basaltic glass and how the water and carbon dioxide enter these fractures in the wake of rupture. Our results are relevant for carbon mineralization where a coupling between dissolution of the basalt and precipitation of carbonate minerals can lead to nanofracturing of the rock.

How to cite: Guren, M. G., Sveinsson, H. A., Malthe-Sørenssen, A., Caracas, R., and Renard, F.: Machine-learned interatomic potentials for modelling nanoscale fracturing in silica and basalt, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6256, https://doi.org/10.5194/egusphere-egu23-6256, 2023.

Stromboli volcano, located in the north-easternmost island of the Aeolian archipelago (Southern Italy) and well known for its persistent volcanic activity, has experienced at least four sector collapses over the past 13 thousand years. The most recent activity resulted in the formation of the Sciara del Fuoco (SDF) horseshoe-shaped depression and a tectonic strain field believed to have promoted flank collapses and formed a NE / SW trending weakness zone across the SDF and the western sector of the island. The tectonic strain field interplayed with dyking and fracturing appears to control the episodes of instability and the onset of slip surfaces. This study presents new data identifying areas of damage that could promote fracturing via remote sensing and rock friction measurements taken on rocks around the SDF and the coupled “weak” zone. We have carried out a multiscale approach by integrating satellite and microscale observations with frictional tests carried out in triaxial configuration on cm scale slabs.

Key units have been sampled on the field (Paleostromboli, Vancori and Neostromboli) with reference to SDF and the weak zone. Direct-shear tests in triaxial configuration were carried out to explore the frictional and seismic properties using rectangular basalt slabs at 5 – 15 MPa confining pressure in dry and saturated conditions, while recording acoustic emissions (AE) via two Piezo-Electric Transducers. The sliding velocity was changed to acquire rate and state friction parameters (RSF). Preliminary results show a variation in the friction coefficient (m) between 0.55 and 0.9 with a general m decrease with increasing confining pressure and saturation. RSF parameters a-b (0.1 < a-b < 0.1) and steady state friction coefficient (m_ss) (0.6 < m_ss < 0.9) are controlled by changing sliding velocity, confinement and by the physical properties of each unit, in particular the porosity.  AE key attributes, such amplitude, frequency and duration and their evolution confirm the relation to sliding velocity, confinement and porosity. Ongoing post mortem SEM analysis are aiming to assess the impact that textural features, such porosity, crystal distribution and glass groundmass for the different units have on the evolution of crack damage and their control on the frictional properties. Quantitative crack density analysis will be carried out using the Matlab tool box FracPaQ on the microstructures to quantify fractures properties and highlight which mechanical features (for example crystals or pores) control the development of asperities/stress concentration. This finding can be related to the field scale fracture density analysis, providing quantitative support for the identification of structurally weak zones across the SDF and constraint the mechanical behaviour of the fractured zones prone to instability.

How to cite: Alcock, T., Vinciguerra, S., and Benson, P.: Multiscale analysis of physical rock properties at Stromboli Volcano: what controls the frictional properties?   , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6624, https://doi.org/10.5194/egusphere-egu23-6624, 2023.

How to implement rock bridges and rock bridge failure in slope stability analysis is an ongoing discussion within the rock mechanic and landslide community. Although there has been intensive research over several decades, there is still a lack of knowledge on how to measure intact rock bridges on rock slopes, how to quantify their impact on rock mass strength, and how they affect the initial failure mechanism. Therefore, we present the analysis of a rock fall case study located in the alpine environment of southern Salzburg (Austria), where a rock slope composed of a polymetamorphic rock mass hosted three rock fall events in the year 2019. The primary aim of this study is the reconstruction of the multiphase failure event and the investigation of the influence of the discontinuity network with its intact rock bridges on the initial failure mechanism.

In our study, we performed a detailed reconstruction of the rock fall process by helicopter-borne event documentation. Moreover, we identified the rock fall failure mechanism by analysing a video capturing the first rock fall event.

Furthermore, we developed a high-resolution digital surface model of the complex post-failure topography by unmanned aerial vehicle photogrammetry (UAV-P) with real-time kinematics (RTK). Based on this model, we map the location, orientation and persistence of pre-existing discontinuities and identify failed intact rock bridges on the rupture surface of the unstable rock slope.

Additionally, we conducted point load and direct shear tests in the rock mechanic laboratory. We applied the former on block specimens to derive the uniaxial compressive strength of the intact rock. The latter allowed us to estimate the Mohr-Coulomb shear strength properties of intact rock and of failure planes, which formed sub-parallel to foliation planes in course of the test procedure.

After the third rock fall event of 2019, a ground-based interferometric synthetic aperture radar (GbInSAR) was installed for 166 days to monitor the actual deformation of the rock slope. We analysed the obtained deformation data at mm resolution to detect zones of ongoing slope movements.

Finally, we integrate the topographical and geological model, the structural inventory, and the geomechanical properties into a 2D numerical model based on the distinct element method (UDEC). We use Voronoi tessellation to allow the development of any failure path within intact rock bridges. By varying the persistence of pre-existing discontinuities and the shear-strength properties of rock bridges, we study the impact of rock bridge location, spatial distribution, and strength on the initial failure mechanisms of the rock slope. We validated the distinct element model by comparing its outcome with the essential characteristics of the rock fall observed in the event reconstruction and deformation monitoring.

By this integrated approach of methods applied to a polyphase rock fall process, we show that the initial rock fall failure mechanism is sensitive to the spatial distribution of rock bridges and their assigned shear strength properties.

How to cite: Gerstner, R., Kuschel, E., Fey, C., Voit, K., Valentin, G., and Zangerl, C.: Rock bridge control on the failure mechanism of a rock fall in a metamorphic rock mass, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6698, https://doi.org/10.5194/egusphere-egu23-6698, 2023.

As an active mountain building, Taiwan Island is deduced from the oblique collision between the Eurasian and the Philippine Sea plate, and its northern part and offshore region are under post-collision collapse. The magmatism induced from the post-collision collapse therefore distributes in the northern Taiwan and its offshore area. A series of submarine volcanoes and igneous rock isles are rooted in the area. For investigation of the volcanic and igneous arrangement, we have collected the magnetic data over the past few decades to combine and compile a map of regional magnetic anomalies. A pronounced magnetic high largely dominates the area of most submarine volcanoes and extends eastward, while the adjacent areas to the north and west are lower. To better understand the magnetic features for the submarine volcanic area, the magnetization for an equivalent magnetic layer thickness was calculated. The result shows that a high magnetization concentrated on the SV7 and extends northwestward that could be a magnetic dipole combining with its northeastern low part. To the southward, the submarine volcanoes SV1, SV3-SV6 locate between this high and another low magnetization. We also applied the enhanced analytic signal technique from the same magnetic data to evaluate the magnetic source strength distribution. Except for the SV2, SV5 and SV6, the 0^th degree of enhanced analytical signal shows that most signal high concentrated on the submarine volcanic areas. For higher degree of enhanced analytical signal, the highest magnitude focus on the Ks and SV1, and PV, SV3 and SV4 are slightly minor.

How to cite: Lo, C.-L., Hsu, S.-K., Lin, S.-S., Tsai, C.-H., Doo, W.-B., Chen, S.-C., and Su, P.-J.: Geomagnetic characteristics of submarine volcanic area off the coast of northern Taiwan, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7718, https://doi.org/10.5194/egusphere-egu23-7718, 2023.

The propagation of micro-cracks will cause the change of rock infrared radiation (IR) information, which provides the possibility to study the rock damage behavior and failure precursor using IR. In this paper, a new quantitative characterization method of rock damage evolution using IR is proposed. Firstly, the maximum classes square error and median filter methods are used to separate the temperature increment caused by crack development in IR images. On this basis, a new index, Damage Infrared Energy Response (DIER), is proposed to describe the crack evolution state and recognize the failure precursor of rock. It is found that the change characteristics of DIER and Acoustic Emission (AE) count are consistent: the DIER remains at the level in the compaction and elastic stages, rises gradually in the stable crack propagation stage, and increases sharply in the unstable crack propagation stage and fluctuates with the appearance of AE count “peak”. The change characteristics of DIER in unstable crack propagation stages can be regarded as the failure precursors of rock, about 84.10% of peak stress. Then, according to the continuum damage mechanics theory, the DIER is used to establish a theoretical characterization of the damage variable for rock, which can accurately describe the damage evolution process of the rock under uniaxial compression. The research results can provide experimental and theoretical support for monitoring slope and rock engineering stability by IR.

How to cite: Liu, W. and Jaboyedoff, M.: Theoretical damage characterization and failure precursor recognition of the rock under uniaxial compression using infrared radiation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7857, https://doi.org/10.5194/egusphere-egu23-7857, 2023.

In rocks and concrete, dynamic excitation leads to a fast softening of the material, followed by a slower recovery process where the material recovers part of its initial stiffness as a logarithmic function of time. This requires us to exit the convenient framework of time independent elastic properties, linear or not, and investigate non-classical, non-linear elastic behavior.

These phenomena can be observed during seismic events in affected infrastructure as well as in the subsurface. Since the transient material changes are not restricted to elastic parameters but also affect hydraulic and electric parameters as well as material strength, as documented for instance by long lasting changes in landslide rates, it is of major interest to characterize the softening and recovery phases. It may help us gain more insight in hazard prediction from both a geological and engineering perspective.

The underlying physics behind those non-classical, non-linear effects, sometimes referred to as “Nonlinear Mesoscopic Elasticity”, are not agreed upon. There is a lack of experiments that would allow us to discriminate between the existing models.: we aim to contribute to filling that knowledge gap.

Our experiments are made on a sample of Bentheim sandstone, initially dry and then fully saturated, in a triaxial cell. We subject the sample to loading and holding cycles in the microstrain range, while also varying confining pressure and pore pressure. Active acoustic measurements during those loading cycles with an array of 14 piezoelectric sensors allow us to monitor relative velocity changes during the experiment by using Coda Wave Interferometry (CWI).

We observe the dynamic softening as well as the recovery processes in the sample during repeated loading phases of different durations. We find that characteristics of the observed velocity changes vary depending on the observed sensor combination, indicating spatial variability of the response, as well as depending on the lapse time and frequency content of the acoustic measurements that we perform the CWI on.

These experiments serve to estimate the exact capabilities of our experimental setup in terms of signal quality, signal stability and lapse time dependent decorrelation of coda waves. We expect our results to inform a future series of similar but more refined experiments addressing the pore pressure dependence of the non-classical response of rocks.

How to cite: Asnar, M., Sens-Schönfelder, C., Bonnelye, A., and Dresen, G.: Non-classical, non-linear elasticity in rocks: experiments in a triaxial cell with pore pressure control, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8050, https://doi.org/10.5194/egusphere-egu23-8050, 2023.

Parametric analysis of laboratory Acoustic Emission (AE) during rock deformation laboratory experiments has revealed periodic trends and precursory behaviour of the rupture source, as crack damage nucleates, it grows and coalesces into a fault zone. Due to the heterogeneity of rocks and the different effective pressures, finding a full prediction of rupture mechanisms is still an open goal.

4x10cm cylindrical samples of Alzo granite were triaxially deformed at confining pressures of 5-40 MPa, while AE are recorded by an array of twelve 1MHz Piezo-Electric Transducers. AE are then post-processed to derive attributes and parameters. We aim to identify what are our most important parameters, and more interestingly, when they are most relevant for predicting when the rock will fail.

Time Delay Neural Networks (TDNN) have shown promise in forecasting failure when using AE-derived parameters. We trained a TDNN with 5 key parameters: 1) AE event rate, i.e. the number of events obtained during the incremental deformation (strain); 2) AE amplitude, i.e. maximum amplitude of S-waves, 3) AE source mechanisms inferred by the source radiation patterns to categorize events and obtain source orientations of mixed-mode type mechanisms; 4) Seismic scattering, i.e. the ratio between the low frequency (LF, 50-500 kHz) and high frequency (HF, 500-1000 kHz) peak delay (PD) values for individual AE and 5) Bulk elastic S-wave velocity measured at intervals throughout the experiment along the ray-paths created by transmitters and receivers. As each parameter investigates a specific mechanical aspect, taken together they provide information on deformation, fracturing and the evolving state of the background medium as failure is approached. These timeseries are then classified by the TDNN as variations in stress and strain (target parameters).

We are currently assessing the importance of individual parameters by omitting one at a time from the training routine. The more important the omitted parameter, the larger the misfit will be when comparing the network output and the target timeseries. The omission analysis determines what are the most important parameters to use when training a neural network to predict dynamic failure. Results are strongly dependent on the methods used to define the training parameters, but several trends are emerging. Event rate and amplitude differently influence predictions of stress and strain. Event rate appears relevant only in the early deformation phases, while amplitude seems much more significant during the coalescence/propagation phase. Seismic scattering and source mechanisms also show an early relevance, interpreted as due 1) to the breakup of low frequency surface waves as microcracks begin to coalesce and 2) bursts of tensile events in the enucleation phase and an increase at ~80% UCS, likely related to the crack propagation. Similarly, there is a clear pivot in the importance of seismic velocity during the early stage, but it emerges a progressive increase ~40% UCS whose origin is unclear. We are currently determining if these variations are directly related to the mechanics of the fault zone or are simply an artifact of the processing.

How to cite: Vinciguerra, S., King, T., Adinolfi, G. M., and Benson, P.: Using AE based Machine Learning Approaches to Forecast Rupture during Rock Deformation Laboratory Experiments, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8188, https://doi.org/10.5194/egusphere-egu23-8188, 2023.

We present a novel apparatus designed to investigate the mechanical and chemical processes active during the nucleation and the subsequent propagation of a seismic rupture. The earthquake is experimentally represented by the sudden frictional sliding of two blocks caused by either: i) the passage of a rupture front from a nearby seismogenic source at prescribed slip velocity, or ii) by the sudden release of strain energy cumulated during the slow (tectonic) loading stage preceding the nucleation of seismic rupture. M.E.E.R.A. (Mechanics of Earthquake and Extended Rupture Apparatus) is a biaxial horizontal machine installed at the laboratories of the Istituto Nazionale di Geofisica e Vulcanologia of Rome (Italy) thanks to a grant funded by the Italian Dipartimento di Protezione Civile. MEERA works on two blocks sized 320x80x50 mm³ put in frictional contact under a normal load up to 30 MPa. Blocks can be either rocks or analogue materials. The normal load and the shear stress are supplied by 6 hydraulic piston cylinders. One piston applies the tangential force up to 150 kN and up to 40 mm/s of slip rate. The other 5 cylinders modulate the normal force on the 320 x 50 mm² contact surface. The 6 pistons are mounted on a rigid stainless-steel vessel that can be closed by a top built in plexiglass, which enables the environmental chamber for fluid confinement. The plexiglass top resists up to 6 MPa of fluid pressure exerted and controlled by using two ISCO pumps.  MEERA is designed following the outline described in McLaskey and Yamashita (2017) and introduces three novelties: the control in displacement and displacement rate of the tangential piston up to 1kS/s; the environmental chamber; the rigid stainless-steel frame. MEERA is designed to study how the tectonic loading of a frictional interface composed of natural rocks determine the stress state and shear stress evolution governing seismogenic processes. To this end, the simulated fault in MEERA is equipped with acoustic sensors, strain gauges, optical fibers and high velocity cameras to measure and constrain rupture nucleation processes and earthquake source parameters, including directivity and rupture velocity, the dynamics of seismic ruptures and the earthquake energy budget at different scales. We aim at comparing the laboratory observations and the signals collected by MEERA with those collected by the newly developed on-fault observatory of the ERC FEAR project in the Bedretto Underground Laboratory for Geosciences and Geoenergies (BULGG, Swiss Alps) to provide novel insights in earthquake mechanics.

How to cite: Spagnuolo, E., Cornelio, C., Aretusini, S., Pozzi, G., Cocco, M., Selvadurai, P., and Di Stefano, G.: A novel apparatus to study the mechano-chemical processes active during the nucleation and propagation of earthquakes (MEERA)., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8807, https://doi.org/10.5194/egusphere-egu23-8807, 2023.

Morphology of rock joints (faults, fractures) has been recognized as the key factor controlling their mechanical behaviour, including the pre-peak and post peak phases as well as dilatancy. It also appears playing a significant role in two mechanical behaviours that have been observed on natural fracture during shearing: (i) joint dilation, or (ii) joint closure in association with asperities crushing, and rock matrix plastic deformation. We examine how (i & ii) occur in the joint, discussing their relationship with normal stress, joint morphology and intact matrix mechanical properties. To do this, two innovative methodologies based on 3DP technologies using a sand and phenolic binder on one side and a polymer (PA12) and binder jetting technology on the other one are applied to built fractures in a weak matrix, and in a strong matrix respectively. Joint surface roughness are built as fractal property with a self–affine replication, in accordance with natural observations. Results of direct shear tests under constant normal stress reveal that the mechanical behavior of the joints is first controlled by the mechanical parameters of the material (UCS/σ_n ratio), then by the joint geometry. In the case where the UCS/σ_n ratio is high (>40) corresponding to a strong material compares to the mechanical solicitation, no significant damage is notice on the joint and the maximal dilation angle is controlled by the steepest angles of the shorter wavelength asperities, which may only represent a small percentage of the surface roughness. In a the case of a weak material the joint behaviour is more complex, and is controlled by a specific range of asperities sizes. Three behaviours were observed depending on the applied normal stress: (i) at low normal stress the larger wavelengths asperities cause dilation since they are not sheared off; (ii) at normal stress over 40% of the UCS value, tensile and/or slip cracks were observed around those asperities, leading to their crushing and beheading; (iii) at normal intermediate stress, the two mechanisms were conjointly observed. In the second case (ii) joint closure is observed and the permeability increases in the surrounding matrix. Those results implies that the UCS/σ_n ratio plays an key role in fault shear behaviour, off-fault damage propagation and fluid circulation.

How to cite: Conin, M., AbiAad, E., Deck, O., and Jaber, J.: Investigation of fault behaviour during shear process for weak and strong materials using 3D printing technologies, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9823, https://doi.org/10.5194/egusphere-egu23-9823, 2023.

Fatigue and damage accumulation in granitoids are classical, but poorly characterised, rock mechanics problems. In order to explore these phenomena, we consider colliding curling stones as a rock physics experiment. Curling stones are made using granitoids from either Ailsa Craig (Scotland) or Trevor (North Wales), which are chosen for their uniformity, strength, and durability. During a curling game, stones are slid over an ice sheet and made to collide along a circumferential striking band. From a rock physics perspective, the collision of curling stones can be modelled as unconfined uniaxial compression of two convex surfaces under well defined boundary conditions. A curling stone experiences about 2900 collisions per season and is played for 10-15 years before refurbishment, which provides a unique long-term opportunity to study fatigue and damage accumulation under cyclic loading.

Here, we first determine the stress magnitudes and strain rates of head-on curling stone impacts using a series of on-ice experiments involving a high speed camera and pressure-sensitive films. We then characterise the observed damage that these collisions produce on the centimetre and micrometre scale using photogrammetry, synchrotron microtomography, optical microscopy, and backscattered electron imaging. We show that during each impact, a curling stone is stressed to at least 300-680 MPa (for a maximum-velocity scenario of 2.9±0.1 ms-1), which exceeds the unconfined compressive strength of the rocks (232-395 MPa; Nichol, 2001, J. Gemm. 27/5). Over its lifetime, a curling stone thus experiences thousands of impacts that will cause damage. The strain rates of these impacts (24±4 s-1) most closely resemble seismic magnitudes, suggesting that the impacts are dynamic in nature. This is supported by the type of damage observed in aged curling stones: (1) Hertzian cone fractures, (2) ejection of rock powder during collisions, and (3) minor spalling microcracks. Most samples show damage being confined to macroscopic Hertzian cone fractures and their immediate collet zones in the relatively narrow striking band. Within the striking band, the circumferential density of cone fractures is limited to about 2-2.5 fractures/cm. Surprisingly, damage does not appear to extend beyond about 3-5 cm into the stones along a radial direction.

Our observations allow us to formulate a model for damage evolution in curling stones. We infer that high-velocity/high-stress impacts initiate cone fractures up to a specific spatial density. As they mature over repeated impacts in the same regions of the striking band, cone fractures progressively propagate and coarsen with subsequent collisions, concentrating and channelling the accumulating damage. This damage geometry is surprisingly effective in shielding the rest of the stones from the reaching critical stress levels for damage. Our findings are significant for applications where rocks are exposed to large numbers of high-stress impacts and suggest that a relatively narrow damage zone can dampen even high-impact stresses over a relatively moderate network of fractures.

How to cite: Leung, D., Fusseis, F., and Butler, I.: Where curling stones collide with rock physics: Cyclical damage accumulation and fatigue in granitoids, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10164, https://doi.org/10.5194/egusphere-egu23-10164, 2023.

Deep-seated landslides may become long-term creeping or transform to catastrophic landslides. Both serious threat to mountainous roads, villages, tourist area, and reservoir areas, which belongs to long-term and extensive effects. Many historical catastrophic landslides have caused the devastating disasters, such as Tsaoling landslide induced by the 1999 Chichi Earthquake, the Hsiaolin village landslide induced by extreme rain of the 2009 Typhoon Morakot, and the long-term large-scale landslides (creeping) of Lushan and Lishan have affected on the environment of the adjacent areas for decades or even longer. In Taiwan, there are many regions of widespread dip slope landform with potential planar failures, study area ranges the right bank of Chishan River from south to north (~35km in length). Most potential planar failure areas had delineated, and several platforms or gentle surfaces on the slope represent the deposits of paleo planar slides or old landslides in study area. But few cases of the geomorphologic evolution are investigated. However, it is difficult to estimate the slope stability of potential planar failures without geomorphologic evolution model. Therefore, the main purpose of this study is to use field investigation and topographic analysis to establish engineering geological models, then propose the geomorphologic evolution model for evaluation of slope stability. The methods of this study include: (1) identifies the microstructure of landslides by high-resolution LiDAR data, (2) performs the geological investigation to verify topographic interpretation and records occurrences of outcrops, (3) collects historical orthoimages to evaluate the activity of slope, (4) use high-precision aerial photography to establish digital surface model and analysis point cloud data to obtains the discontinuous plane state, and (5) the failure mechanism would be analyzed by the stereographic projection. In study area, 160 platforms are identified and area ranges from 220 m² to 82386 m². The largest two platforms, Tianziding (TZD) and Mujiliao (MZL) platforms, are investigated by field survey and drone. The attitudes of interbeds under TZD platform are surveyed along gullies, which is obvious gentler than the strata of dip slope. The front of MZL platform occurred slope failure during 2016 Typhoon Megi, therefore, the exposed rock mass of platform can be identified as deposits of planar slide. The preliminary results can infer that numerous paleo planar slides are exist in study area and the geological profiles of TZD and MZL are plotted. The structures of rock masses and attitudes of discontinuities by field survey and point cloud analysis need to interpreted carefully. Then, the geological model and geomorphological evolution can be proposed. 

How to cite: Yang, C.-M., Chuang, I.-L., and Zhang, E.-L.: Re-failure and geomorphological evolution of paleo planar slide, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10226, https://doi.org/10.5194/egusphere-egu23-10226, 2023.

At present, a reliable method for forecasting earthquakes has not been developed yet, as the physical mechanisms that generate them are very complex and still not completely understood. To overcome the difficulties of retrieving direct observations and measurements in the field, here we employ laboratory and numerical models to investigate and better understand strain localization preceding mainshocks.

We perform a failure test on an intact and dry sample of Berea sandstone confined at 20 MPa with a triaxial machine (LabQuake). Employing in-house developed, conical-type and fully calibrated piezo-electric transducers (PZT), we are able to investigate the acoustic emission (AE) clouds by relocating the single events and by computing their focal mechanisms and scalar seismic moments. The PZT sensors are also used actively to allow for the construction of inhomogeneous and anisotropic velocity models. We further employ distributed strain sensing (DSS) with optical fibers to capture the heterogeneous spatial distribution of the surface strain by gluing the fibers on the sample surface. We observe AE clustering in two regions located at the top and bottom of the rock specimen throughout the majority of the experiment. As the test approaches the main failure, AE localize at one side of the sample in the lower half before obliquely propagating upwards by forming a macro-fracture. Surface strain heterogeneities are detected during the experiment, and regions of higher extensional strain correlate in time and space with rock volumes experiencing high AE activity. Numerical simulations, which are conducted using a two-dimensional continuum-based and fully coupled seismo-hydro-mechanical poro-visco-elasto-plastic modelling tool (H-MEC), are validated with both AE and DSS data. The combination of laboratory and numerical investigations allows us to individuate and study physical mechanisms (e.g., visco-plastic compaction of pores and shear banding) that explain the processes responsible for both surface strain concentration and the generated AE clouds. These findings suggest that the deformation in the interior of the sample is mainly occurring inelastically and is localized along an obliquely forming shear band. We estimate the partitioning between seismic and total deformation to be ~0.09 % and this effectively confirms the previous evidences related to the irreversible localization of strain within the rock specimen. 

How to cite: Bianchi, P., Selvadurai, P. A., Salazar Vásquez, A., Dal Zilio, L., Madonna, C., Gerya, T., and Wiemer, S.: Evidence of Strain Localization Preceding Rock Failure: Insights From Laboratory and Physics-Based Poroelastic Models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11447, https://doi.org/10.5194/egusphere-egu23-11447, 2023.

Talc is an important product of several hydration and dehydration reactions in deep faults and subduction zones. The unique weakness of talc along its basal planes makes it an essential component in understanding various fault slip behaviors (e.g., episodic vs continuous slip, seismic vs aseismic) or realistic geodynamic models. A recent experimental study by Boneh et al. (2023) on talc mechanical behavior at high P-T conditions highlighted: (i) talc’s low friction coefficient under all conditions (<0.14), with thermal weakening down to µ~0.01 at 700 °C. (ii) Grain-scale microstructures demonstrate a component of fracturing and microcracking under all conditions tested. And (iii) pressure-dependence of talc strength decreases at higher temperatures, where there is also a greater tendency for localization. A vital part of depicting mineral rheology is the understanding of their underlying mechanisms of deformation associated with the observed bulk mechanical and microstructural behavior. To reveal the underlying deformation mechanism/s we analyzed the deformed samples through high-resolution transmission electron microscopy (TEM) at Utrecht university of samples prepared using a focus ion beam (FIB). Five talc samples were examined – an undeformed sample, and samples deformed at 400, 600, and 700°C under 1 GPa, and at 400 °C under 1.5 GPa.

Seven FIB lamellae sampled areas adjacent to the main fracture (if exists) or high damage zones. The starting material shows talc flakes with a thickness of ~100-400 nm without a sample-scale preferred alignment. The sample deformed under 400°C and 1.5 GPa exhibits distributed deformation with opening cracks along talc basal planes and pervasive kinking normal to the basal planes. The sample deformed at 400°C and lower pressure (1.0 GPa) exhibits thin lamination (~50 nm) well oriented with the orientation of the main fracture plane. The sample deformed at 600°C exhibits crystal delamination along the basal cleavage (forming grain fragments <10 nm in width) along the main fracture. The sample deformed at 700°C exhibits more areas of high damage, possibly due to the similar basal-cleavage delamination. A key incentive is to relate the observed nano-scale crystal defects with the bulk mechanical behavior and with processes that might promote the localization of deformation. Pressure-dependent strength can be accounted for by kinking and kinking-induced porosity while thermal weakening can be related to temperature-dependent mobility of crystal defects leading to delamination along the basal cleavage. We will discuss possible physical mechanisms of talc deformation and the prospect of extrapolating the mechanical behavior of talc achieved at the lab to the range of conditions expected in natural settings.

How to cite: Boneh, Y., Ohl, M., Plümper, O., Hirth, G., and Peč, M.: The Weakest Link – Revealing the microphysical deformation mechanisms of talc under P-T conditions associated with fault creep and slow slip events, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11449, https://doi.org/10.5194/egusphere-egu23-11449, 2023.

The Universal Distinct Element Code (UDEC) has been rising much prevalence and is a validated technique applied to simulations in varied branches of geotechnics. Nevertheless, the characterization of water-weakening effects on a rock model remains elusive, resulting in a poor constraint referring to water-induced simulations by UDEC. In this context, previous research has been attempting to conduct an intuitive link between modelling parameters and saturation degrees, S_r, to implement a water-weakening process in UDEC, leading to a detrimental potential devoid of the basic logic that modelling parameters determine macroscopic properties of a rock model in contrast to dominance by S_r owing to the discrepancy in a physical sense and spatial scale. To fill in this gap, a new methodology coupled with a parametric study is first proposed with procedures that macroscopic properties of actual rock with different S_r are input into parametric relations to acquire predicted modelling parameters, which will be sequentially calibrated and adjusted until simulations are in line with actual tests. Utilizing this methodology, water-weakening effects on macroscopic properties, mechanical behaviours, and failure configurations of numerical models in UDEC are commensurate with tested ones to the utmost with noticeable computational expediency harnessing the benefit of the parametric study, indicating the feasibility and simplicity of the methodology. Thus, in the implementation of a water-weakening process in UDEC, we suggest converting an embarking from an intuitive link between modelling parameters and S_r into a parametric analysis to determine modelling parameters according to macroscopic properties under different S_r.

How to cite: Bu, F., Jaboyedoff, M., Derron, M.-H., and Xue, L.: Parametric study to characterize water-weakening effects in UDEC, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12979, https://doi.org/10.5194/egusphere-egu23-12979, 2023.

Neural networks have proved to be able to capture the relevant and informative features of a wide variety of data types and predict the desired output for different regression or classification problems. Finding a mapping between materials’ structure and a given physical property of those systems is an example of a problem that could be approached with machine learning methods like neural networks. Especially when we are dealing with systems with a very large design space where using classical computational methods like molecular dynamics can be very time and resource-consuming for the study of a very large number of systems, a well-trained neural network can be greatly faster and more efficient for computing the relevant properties. In this work, we study α-quartz crystals with one porous layer with simplex noise as the shape of porosity. Simplex noise is a gradient based procedural algorithm that can produce irregular geometries with surface morphology resembling what is observed in nature. The property that we want the neural network to learn is the yield stress of these systems under both shear and tensile deformation. Molecular dynamics simulations are used for a randomly selected sample of possible structures in order to generate the ground truth to be used as the training data. We employ deep convolutional neural networks (CNN) which are commonly used when dealing with image or image-like data since the input data for the problem in hand is a binary 2-D structure of the porous layer of the systems. The trained model is compared with a basic polynomial fit of stress versus porosity. The trained CNN is successful in predicting the yield stress of a system based on the geometry of that given system, with lower variability and higher precision compared to the base polynomial regression method. The saliency maps created with the trained model show the model to be successful in capturing the physics of the problem when compared with the stress fields calculated using molecular dynamics simulations. This method of modeling materials can be further developed for the inverse design of structures with desired properties without the need for a huge number of simulations on a wide domain of possible systems.

How to cite: Najafi, F., Andersen Sveinsson, H., Dreierstad, C., and Malthe-Sørenssen, A.: Modeling the relationship between mechanical yield stress and material geometry using convolutional neural networks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13293, https://doi.org/10.5194/egusphere-egu23-13293, 2023.

Static friction is the force required to impose sliding on a rested body. The force depends on material properties and external factors such as normal pressure and temperature, but also a time dependent component is important. The frictional aging effect is at origination of the difference between static and dynamic friction, and is also believed to be responsible for the velocity-weakening of sliding friction. Despite immense effort, how microscopic processes affect the macroscopic aging is still not fully understood. We have performed molecular dynamics simulations where we demonstrate that high surface diffusion may provoke rapid contact area growth of an asperity-substrate interface, inducing a strong frictional aging effect. This mechanism differs from elastic and plastic creep in the sense that it occurs even at no normal pressure. The growth of contact area was found to be nearly logarithmic due to an exponentially decaying diffusivity. Furthermore, when applying a normal stress the aging effect is enhanced due to plastic creep. Our work suggests a new explanation of the logarithmic nature of aging and helps bridging the gap between empirical macroscale friction laws and the microscale behavior. While aging due to plastic and elastic creep is well-known and incorporated into most friction laws, diffusion aging has yet to be considered. The ultimate goal is to design or redesign friction laws taking the microscopic behavior into account and conceivably improve the accuracy of the laws. In the long term, this may contribute to improved earthquake forecast.

How to cite: Nordhagen, E. M.: Atomic scale frictional aging in silicon carbide due to diffusion and creep, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13723, https://doi.org/10.5194/egusphere-egu23-13723, 2023.

Ischia (Southern Italy) is a volcanic island of the Phlegrean Volcanic District that was recently affected by multiple geological hazards, including floodings, landslides, rockfall and earthquakes.

In this study, rockfall stability is analysed, assuming as a case study a 400m-wide cliff made of Green Tuff and located on the western area of Mt. Epomeo. The two outcrops studied are located at 280 and 420 m a.s.l., above the site of Frassitelli, Forio d’Ischia, which is an area of high residential, touristic and agricultural importance. The former is a high-angle outcrop affected by tens of meters-long faults, whereas the latter is characterised by high-dip pinnacles.

We analysed the fracture systems affecting the examined formation to compute the kinematic analysis of the potential rupture mechanisms and to perform numerical simulations of potential rockfall scenarios. The data acquisition was carried out by means of classical geological field surveys and structural analysis on Virtual Outcrop Models (VOM) obtained from images acquired by drones. The VOMs were analysed with ‘CloudCompare v2.10.2’ and ‘OpenPlot’ software. The former allowed the automatic digitalisation of the exposed discontinuities by applying the ‘Facets’ plugin, based on a least-square fitting algorithm (Fernández, 2005). ‘OpenPlot’ enabled the extraction of the geostructural information from the VOM, by computing the best-fit planes of the polylines manually drawn along the interference between the geological surface and the outcrop topography (Tavani et al., 2011).

The measured and the extracted features were classified following their attitude. Three main sets were defined, striking N-S, NW-SE and NE-SW. The fracture dataset was used to perform a kinematic analysis with ‘DIPS’ software on the surface discontinuities extracted from ‘Facets’ plugin. The 'wedge sliding' resulted the most critical potential rupture mechanism to occur on the analysed outcrops. Successively, numerical simulations of rockfall scenarios were computed based on the acquired structural information. The latter permitted us to identify the maximum run out of the potential blocks and draw some consideration on the rockfall hazard of the area.

How to cite: Massaro, L., Rauseo, F., De Falco, M., Marino, E., Forte, G., and Santo, A.: Characterization of rockfall mechanisms and run-out in active volcano-tectonic areas: a case study from Ischia Island, Southern Italy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14394, https://doi.org/10.5194/egusphere-egu23-14394, 2023.

Pressure, temperature, and infilling fluids influence the petrophysical properties and the associated damaging processes of rocks at all scales. Moreover, each fluid-rock system possesses peculiar mechanical behaviours being these particularly complex in carbonate rocks hosting fluids. In this work, we analyze the laboratory results of deformed clean and hydrocarbon-filled limestones under varying pressure and temperature, providing links between recorded physical properties (seismic velocity), fluid behavior, and damaging. We focus on carbonate-bearing reservoir (Bolognano Formation) rocks, sampled in the Majella massif (Central Italy) that represents a very good analogue for buried carbonate reservoirs. This reservoir is composed by calcarenites with connected porosity of about 20% saturated by hydrocarbon in the solid state at the outcrop conditions. We performed hydrostatic, triaxial and true-triaxial deformation tests up to a temperature of 100º C and a confining pressure up to 100 MPa on both clean and naturally hydrocarbon-filled limestone samples. Results show increasing seismic velocity and Young’s modulus with increasing confining pressures for both clean and saturated samples as expected. However, different results are observed when the temperature is increased. At low temperatures saturated samples show larger seismic velocity and rigidity with respect to clean samples whilst at higher temperatures the opposite occurs. In particular, when temperature is rised up to 100º C the Young’s modulus of the saturated samples dramatically decreases, being this coupled by a clear volume reduction even during hydrostatic tests (no differential stress applied). Accordingly, microstructural observations highlight grain crushing related to a large amount of randomly distributed cracks within saturated samples. On the contrary, clean samples are characterized by few microfractures, pointing out the primary role played by liquid hydrocarbons. These observations are in good agreement with meso and microstructural features observed on outcropping hydrocarbon-filled carbonate-bearing faults. The presence of fluid hydrocarbons (high temperature) severely weakens the rock promoting fracturing whilst at lower temperature the presence of solid hydrocarbons increases the mechanical properties of hydrocarbon-bearing rocks. These observations have a large impact for the petrophysical characterization of reservoirs and for the understanding microscale to mesoscale mechanisms of deformation and fluids movement along deformed rock volumes.

How to cite: Trippetta, F., Ruggieri, R., Motra, H. B., and Collettini, C.: Mechanical behavior of porous carbonates as a function of pressure, temperature, and fluid content from laboratory experiments and correlation with larger scale structures, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14779, https://doi.org/10.5194/egusphere-egu23-14779, 2023.

The reliable assessment of the fracturing state of rock masses is a fundamental step towards the evaluation of their geomechanical quality and the quantification of their hydraulic and mechanical properties. Traditional field discontinuity mapping techniques remain fundamental to collect statistical populations of discontinuity attributes and characterize rock mass structure and quality. However, point-like field surveys are strongly biased by scale and orientation. The development of 3D surveys allowed to partly overcome this problem by providing high-resolution point clouds. These allow a robust characterization of fracture geometry but require significant mapping efforts. Here we proposed a quantitative contactless approach to rock mass fracturing assessment by the use of Infrared Thermography (IRT).

IRT is increasingly used in rock-mechanics to characterize rock porosity/fracturing and to monitor rock mass stability, by measuring the thermal response of rock materials to heating or cooling. However, existing IRT applications to the geomechanical study of rock masses are mostly qualitative and lacking sound theoretical and experimental foundations. Starting from the laboratory scale, studying the thermal behaviour of rock samples with different fracture degrees, we propose a quantitative approach to quantify rock mass fracturing, that combines IRT rock temperature monitoring during cooling with the quantification of different descriptors of fracturing state suitable for different analysis scales (laboratory vs in situ).

As a field laboratory we used the Mount Gorsa porphyry quarry (Trento, North Italy), characterized by a homogeneous rock type but strongly variable fracturing states related by complex structurally-controlled and progressive slope damage processes.

We performed a field campaign on quarry front making a) Geometrical UAV surveys and b) field Geological Strength Index (GSI) evaluation on typical spots, c) we carried out IRT monitoring during night cooling using FLIRT1020 thermal camera.

During data processing d) thermal data acquired were corrected by environmental effects (blue sky radiation, slope inclination etc.) adopting original and ad hoc calibrated filters to skim the thermal response from geometrical and external biases. Finally we try to find a correlation between the thermal response of rock-mass outcrop to their quality index.

Our results support the possibility to upscale the analysis to field conditions in order to account for the radiative characteristics of natural environments, the limitations of the technique and upscaling issues typical of fractured rock-mass, taking into account that fracturing metrics (used in laboratory phase) at rock-mass scale, should influence block size distributions, which is fundamental in the evaluation of quality indices, e.g the Geological Strength Index (GSI) widely used in engineering applications.

Emphasising all these issues, the goal of our work is to investigate the relationship between the thermal response of rock mass quality index, through an experimental method developed at laboratory scale and upscaled to in situ conditions.

How to cite: Franzosi, F., Crippa, C., Garzonio, R., Casiraghi, S., Derron, M.-H., Jaboyedoff, M., and Agliardi, F.: In situ assessment of rock mass fracturing using infrared thermography, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14869, https://doi.org/10.5194/egusphere-egu23-14869, 2023.

3D printing is a rapidly evolving technology that has proven useful for a wide variety of disciplines and industries. However, knowledge of its applicability to the fields of rock and soil physics remains limited. 3D printing allows the design of samples with any desired microstructural composition, enabling independent control of properties such as pore space fabric, size, and density; a feat impossible to accomplish with naturally occurring geomaterials. The use of 3D printed samples is therefore highly attractive for relating the effective properties of heterogeneous materials to their microstructural arrangement, a key subject in the fields of rock and soil physics.

This study aims to characterize the physical properties of 3D printed materials (i.e., elasticity parameters, porosity, permeability) and evaluate whether they are suitable to be used as proxies for heterogeneous geomaterials. Two distinct 3D printing technologies were employed for this purpose: the Fused Deposition Modelling (FDM), and Stereolithography (SLA) methods. The FDM method constructs 3D objects by superposing layers of polymer-based filament through a heated nozzle, whereas the SLA method, also known as resin 3D printing, uses a laser light source to cure liquid resin into hardened plastic.

Samples with a variety of pore shapes (sphere, needle, penny shaped), sizes, and pore densities were designed and printed as cylindrical samples of 25 mm diameter and 62.5 mm height. Samples were then subjected to uniaxial compression to measure their effective elastic parameters (elasticity modulus and Poisson’s ratio), and these measurements were compared with theoretical predictions. Preliminary results indicate that the FDM printing method is inadequate for representing a heterogeneous solid composed of an isotropic matrix and void space, due to the intrinsically anisotropic fabric resulting from the layer-by-layer printing method. Additionally, samples with a porous microstructure appear to be effectively stiffer than the intact material, which is attributed to enhanced material sintering surrounding the edges of the void spaces. On the other hand, SLA printing appears to hold more promise and be able to represent a composite material composed of an isotropic matrix with a heterogeneous void space. Further measurements need to be made to confirm these preliminary findings, and this work is currently in progress. 

How to cite: Adamus, F., Stanton-Yonge, A., Mitchell, T., Healy, D., and Meredith, P.: Physical properties of 3D printed materials and their applicability as proxies for heterogeneous geomaterials, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16739, https://doi.org/10.5194/egusphere-egu23-16739, 2023.

Electrical resistivity measurements of Fe-5wt%Ni were made in-situ under pressures of 2-5 GPa and temperatures up to 2000 K in a cubic-anvil press. The thermal conductivity was calculated from the measured electrical resistivity data using the Wiedemann–Franz law. Comparison of these data with previous studies on pure Fe and Fe-10wt%Ni shows that a change in the Ni content within the range 0-10wt% Ni has no significant effect on electrical resistivity of Fe alloys.

The thermal conductivity values of Fe-5wt%Ni from this study, was used to calculate the adiabatic heat flux in Vesta’s core. Vesta is of interest because the remnant magnetism in eucrites dated at 3.69Ga, reveals it possessed an internally generated dynamo (Fu et al., 2012). Comparing the estimated adiabatic core heat flux of ~331 MW at the top of Vesta’s core to the range of estimated heat flux through the CMB of 1.5–78 GW, we infer that the mechanism stirring Vesta’s liquid outer core to generate its surface magnetic field tens of millions of years in its early history was thermal convection.

How to cite: Orole, O., Yong, W., and Secco, R.: Thermal Convection in Vesta’s Core from Experimentally-Based Conductive Heat Flow Estimates, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-479, https://doi.org/10.5194/egusphere-egu23-479, 2023.

ABSTRACT

In this talk, I will introduce the methods developed in my group, especially the machine learning and graph theory aided crystal structure prediction method (MAGUS) [1]. In addition, I will show some applications of these methods combined with first-principles calculations, for instance, the predictions of new possible compounds (helium-water, helium-ammonia, helium-methane, helium-silica, silica-water, etc) in the interior of giant planets or exoplanets, and their exotic new states under planetary high-pressure and high-temperature conditions (superionic state, plastic state, and their coexistence) [2-6]. These new compounds and their states may have some important implications for giant planets, including demixing, magnetic field, erosion of the rocky core, etc.

REFERENCE

• Kang Xia et al., “A novel superhard tungsten nitride predicted by machine-learning accelerated crystal structure search”, Sci. Bull. 63, 817 (2018).
• Cong Liu et al., “Mixed coordination silica at megabar pressure”, Phys. Rev. Lett. 126, 035701 (2021).
• Cong Liu et al., “Multiple superionic states in helium-water compounds”, Nature Physics 15, 1065 (2019).
• Cong Liu et al., “Plastic and Superionic Helium Ammonia Compounds under High Pressure and High Temperature”, Phys. Rev. X 10, 021007 (2020).
• Hao Gao et al., “Coexistence of plastic and partially diffusive phases in a helium-methane compound”, Natl. Sci. Rev. 7, 1540 (2020).
• Hao Gao et al., “Superionic Silica-Water and Silica-Hydrogen Compounds in the Deep Interiors of Uranus and Neptune”, Phys. Rev. Lett. 128, 035702 (2022).

How to cite: Sun, J.: Unexpected new compounds and their states in the interior of giant planets predicted from first-principles calculations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3333, https://doi.org/10.5194/egusphere-egu23-3333, 2023.

Iron is considered to be the main component of the Earth's core. Substantial efforts have been made to understand its phase diagram and physical properties at extreme conditions. However, it remains debated about how the atoms in solid iron are arranged at Earth's core conditions, where possible candidates include hexagonal close-packed (hcp), body-centred cubic (bcc), and face-centred cubic (fcc) structures. As crystal structure and physical properties are closely related, there is also a significant uncertainty in the properties of Earth's core, such as elasticity, heat conductivity, and density, making the accurate interpretation of seismic observations difficult. Here we aim to study the phase stability of solid iron at Earth's core conditions. For this, a deep-learning interatomic potential was developed with ab initio accuracy but is more cost-effective. To further check the performance of such potential, we examine the elastic and plastic behaviour of hcp iron and the effects of structural defects at inner core conditions [1]. We then compute the Gibbs free energy of the bcc, fcc, hcp and liquid phases by performing large-scale molecular dynamics simulations. The calculated free energy allows for determining the phase stability of solid iron in Earth's core.

[1] Li, Z., & Scandolo, S. (2022). Elasticity and viscosity of hcp iron at Earth's inner core conditions from machine learning-based large-scale atomistic simulations. Geophysical Research Letters, 49, e2022GL101161.

How to cite: Li, Z. and Scandolo, S.: Phase diagram of pure iron in Earth's core from deep learning, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11982, https://doi.org/10.5194/egusphere-egu23-11982, 2023.

Pyroxene minerals are key to understand the structure and composition of Earth and rocky exoplanets interiors. Nevertheless, the full details of the MgSiO₃ phase diagram still remain unclear, in particular in the high temperature region. Protoenstatite (PEn) is one of the HT polymorphs of MgSiO₃ pyroxene, having stability range from ~1200 to 1800 K at ambient pressure. Its importance has been recognized by many authors, since PEn is regarded as a precursor phase of low-clinoenstatite (LP-CEn)/orthoenstatite (OEn) intergrowths in some cometary samples [1] and in calcium-aluminum-rich inclusions (CAIs) from CV3 chondrites [2]. Moreover, PEn is the liquidus phase of pyroxene in the MgO-SiO₂ binary system and may have played a role in gas solar nebula condensation processes [3]. Very little is known about the thermodynamics and phase relations of protoenstatite. This is due for the most part to its unquenchable nature, meaning that even if PEn can be synthetized at high temperature conditions, it doesn’t preserve at ambient conditions since it very rapidly reverts either to OEn or LP-CEn. The difficulty to perform measurements on samples of PEn prevents to obtain complete information on its thermodynamic properties, which are in turn fundamental for the investigation of phase equilibria of this mineral. In that sense, ab initio calculations based on quantum-mechanical theory are one of the most reliable methods available to obtain information on thermodynamics and phase relations of minerals at HT conditions.   

We present a DFT based ab initio B3LYP computational study on MgSiO₃ PEn. All the relevant thermophysical and thermodynamic properties of PEn (e.g. heat capacity, vibrational entropy, thermal expansion, EoS) have been calculated in the framework of the quasi-harmonic approximation (QHA) by a full phonon dispersion calculation. This allowed to obtain original insights into protoenstatite thermodynamics and enabled to retrieve a complete set of physically consistent thermodynamic properties, that are in good agreement with the very few experimental data currently available [4].  The computed properties have been tested by predicting relevant phase equilibria involving PEn up to melting conditions, in particular the OEn – PEn phase transition. The P-T location of the phase boundary and its Clapeyron slope (dP/dT = 2.04 MPa/K) are consistent with previous pyston-cylinder experiments ([5],[6]). Theoretical modelling of the melting curve of MgSiO₃ polymorphs reveals a change of the melting behavior from incongruent to congruent due to the onset of the OEn – PEn transition in the phase diagram.

[1] Schmitz, S., and Brenker, F.E (2008) Astrophys. J., 681, L105-L108.

[2] Che, S., and Brearley, A.J. (2021) Geochim. Cosmochim. Acta, 296, 131-151.

[3] Nagahara, H.. (2018) Rev. Mineral. Geochem., 84, 461-497.

[4] Thiéblot, L., Téqui, C., and Richet, P. (1999) Am. Mineral., 84, 848-855.

[5] Boyd, F. R., England, J. L., and Davis, B. T. (1964) J. Geophys. Res., 69, 2101-2109.

[6] Chen, C. H., and Presnall, D. C. (1975). Am. Mineral., 60, 398-406.

How to cite: La Fortezza, M. and Belmonte, D.: Ab initio thermodynamics and phase stability of MgSiO3 pyroxene polymorphs: new insights on protoenstatite, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12045, https://doi.org/10.5194/egusphere-egu23-12045, 2023.

Mars’ mantle dynamical history has certainly been dominated by a stagnant-lid regime, with limited mixing and homogenization. Accordingly, the chemical and mineralogical signatures of early processes, including the crystallization of a primitive magma ocean, are overall well preserved on Mars. The major geological structures visible at its surface are the remains of an intense ancient volcanism, not so dissimilar from the large igneous provinces found on Earth at very old ages (several million/billion years).

Current models used to determine the mantle thermal evolution and the crustal extraction heavily relies on melting properties of materials expected to form the Martian mantle, which, however are poorly known. In particular, the fact that the Martian mantle is probably richer in iron than the terrestrial mantle has a direct impact on the solidus and liquidus and on the chemistry of the magmas that can be produced at different pressures. Thus, the study of Martian volcanism and thermal history requires a precise understanding of the melting properties of the mantle (solidus, liquidus and extent of melting) as a function of pressure and temperature. Studies in literature are scant, mainly address the solidus, and are limited to analysis of recovered samples, missing in situ diagnostics.

To address this problem, we studied the solid-liquid melting relations and, more generally, the melting diagram for a mineralogical assemblage model of mantle composition, by high-pressure and high-temperature experiments in multi anvil press performed at the PSICHE beamline of the SOLEIL synchrotron. We determined the solidus and the liquidus of the investigated rock at pressures up to 12 GPa by complementary in-situ diagnostics (X-ray diffraction and falling sphere technic). The obtained solidus and liquidus are well lower (difference >200K), especially at the highest investigated pressures, compared to previous studies, with strong implications for the origin of volcanism and notably the crystallization of the magma ocean. Furthermore, our experiments provide important data to refine the extent of melting (Φ), modal proportion and the chemistry of all the different phases present between the solidus and the liquidus at different conditions (P, T, Φ).

Altogether, these new results are critical to constrain models of thermal evolution and crust extraction and formation, as well as to address the evolution of the magmatism and volcanism at the Mars surface since 3.5 Ga. Finally, depending on different parameters, such as the thickness of the crust or the concentration of radioactive elements, the estimated areotherm could cross the solidus and lead to partial melting of the mantle, especially close to the core-mantle boundary, where a high extent of melting could be reached.

How to cite: Pierru, R., Dominijanni, S., Parisiadis, P., Andriambariarijaona, L., Zhao, B., Blanchard, I., Guignot, N., King, A., Badro, J., and Antonangeli, D.: Melting properties and melting phase relations of the Martian mantle from in-situ measurements on iron-rich mineralogical assemblages, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12231, https://doi.org/10.5194/egusphere-egu23-12231, 2023.

The internal structure of Mercury holds key information regarding the planet’s formation and its peculiar magnetic field. Waiting for incoming observations by BepiColombo, current knowledge of the interior structure of Mercury relies primarily on geodetic and surface chemistry data collected by MESSENGER. Results from spectral and compositional analysis supplemented by cosmochemical evidence indicate that light elements such as S, and Si are most likely alloyed to Fe in Mercury’s core. This notion is further supported by the very reducing redox conditions (from -2.6 to -7.3 log units below Fe-FeO oxygen buffer) predicted to occur during the planet’s differentiation that argue for significant quantities of Si and S partitioned into metallic iron. Thus, it is of primary importance to determine the Fe-Si-S phase diagram and to understand the high pressure and high temperature properties and thermodynamic behavior of Fe-Si-S alloys at conditions directly relevant for Mercury’s core. Very recently the binary Fe-FeSi phase diagram has been established at Mercury’s core conditions, but phase and melting relations in the Fe-Si-S ternary system still are poorly constrained, in particular at the relatively low pressures and temperatures relevant for Mercury’s core.

To address this issue, we performed angular dispersive powder X-ray diffraction experiments in laser-heated diamond anvil cells on selected composition in the Fe-Si-S system (i.e., Fe-4S-6Si, Fe-16S-6Si, Fe-4S-12Si, and Fe-16S-12Si, all in wt. %) at the P02.2 Extreme Conditions beamline at DESY Synchrotron facility (Germany). For all compositions, eutectic melting and subsolidus phase relations were investigated up to about 45 GPa. Ex situ chemical analysis of the recovered run products were performed at the IMPMC laboratory on the extracted FIB thin sections cut throughout the heated spots.

Here we will present preliminary results on the eutectic melting and Fe-Si-S phase relations as a function of pressure, temperature and composition, with specific focus to the conditions expected within the core of Mercury.

How to cite: Dominijanni, S., Anzellini, S., Kurnosov, A., Morard, G., Boccato, S., Fedotenko, T., and Antonangeli, D.: Melting and subsolidus phase relations of Fe-Si-S alloys at Mercury’s core conditions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13353, https://doi.org/10.5194/egusphere-egu23-13353, 2023.

Fostered by third-generation synchrotron sources, experimental studies of physical and chemical properties of liquids at high pressure and temperatures are constantly pushed towards more and more extreme conditions, with applications ranging from Earth and planetary science, to material science, to fundamental physics.

In the last 20 years, many efforts have been dedicated to the development of a method to obtain structural information from the X-ray diffuse scattering signal of a liquid [1], allowing, for instance, to improve our understanding of the structure and evolution of deep planetary interiors. However, while data collection protocols are by now quite advanced and overall comparable across beamlines worldwide, data analysis largely differs depending on user and employed codes. To answer to the need of a unified data analysis tool for liquids and amorphous systems, we developed Amorpheus [2].

Amorpheus is an open-source, versatile, free and easy-to-use software for the analysis of X-ray diffuse scattering signal, allowing to perform a customizable analysis of a large amount of data and to invert for the density.  Available on GitHub [3] it is fully accessible by the community. This software has been tested on data collected with DAC and with large volume presses and it is well adapted for the analysis of liquid metals and alloys, as well as of amorphous systems. Here we will present and discuss selected examples of data analysis performed by Amorpheus in order to determine local structure and density of liquid iron binary and ternary alloys at planetary core conditions.

[1] Eggert JH, Weck G, Loubeyre P, Mezouar M. Quantitative structure factor and density measurements of high-pressure fluids in diamond anvil cells by x-ray diffraction: Argon and water. Phys Rev B. 2002;65(17):174105. doi:10.1103/PhysRevB.65.174105

[2] Boccato S, Garino Y, Morard G, et al. Amorpheus: a Python-based software for the treatment of X-ray scattering data of amorphous and liquid systems. High Press Res. 2022;42(1):69-93. doi:10.1080/08957959.2022.2032032

[3] https://github.com/CelluleProjet/Amorpheus

How to cite: Boccato, S., Morard, G., Garino, Y., Sanloup, C., Zhao, B., Morand, M., and Antonangeli, D.: Analysis of diffuse scattering from liquid and amorphous samples: protocols and software, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14735, https://doi.org/10.5194/egusphere-egu23-14735, 2023.

Transient creep controls the behavior of Earth’s mantle at human timescales. Transient creep occurs during postseismic creep, glacial isostatic adjustment, and tidal deformation of planetary interiors experiencing large tidal stresses, such as Jupiter’s moon Io or various identified exoplanets. Unfortunately, laboratory data of transient creep of olivine, the most abundant mineral in Earth’s upper mantle, remain limited, and at present we lack the microphysical understanding of transient creep required to extrapolate experimental data to geological grain sizes and time scales. Several mechanisms for transient creep have been proposed, both intergranular mechanisms such as plastic anisotropy and elastically or diffusionally-assisted grain boundary sliding, and intragranular mechanisms including long-range dislocation interactions and various other dislocation damping mechanisms. Each mechanism produces distinct rheological behavior, presenting a hurdle for modeling geodynamic processes occurring on timescales of hours to years.

To distinguish among the various proposed microphysical mechanisms for transient creep, we performed compressional load-reduction experiments on cylindrical, isostatically hot-pressed aggregates of San Carlos olivine in a gas-medium Paterson apparatus at confining pressures of 300 MPa and temperatures of 1200°C. Samples were subjected to a constant differential stress of 200 MPa, which resulted in a steady-state strain rate of ~10^-5 s^-1. After steady state was achieved, the samples were subjected to a near-instantaneous load reduction of 10–70% of the original load. 

For load reductions exceeding ~50%, the samples exhibited a period of transient anelastic relaxation with zero or negative strain rate before continuing to strain at a positive strain rate lower than the previous steady state. The duration of relaxation increased with the magnitude of the load reduction. Multiple load reductions from the same steady-state strain rate were performed during a single experiment to test for reproducibility. 

We interpret our results to indicate that, at these conditions, the backstress stored in polycrystalline olivine is approximately half of the differential stress applied to the material. The magnitude of backstress is compatible with long-range dislocation interactions on the [100](010) and [100](001) or [001](100) slip systems previously observed for single crystals of olivine. If transient creep is controlled by such dislocation interactions then it may be inappropriate to apply the traditional Burgers rheology based exclusively on intergranular dissipation processes or power-law flow laws calibrated for steady-state creep to model transient creep and transient viscosity evolution in the upper mantle.

How to cite: Hein, D., Hansen, L., and Dillman, A.: Backstresses in polycrystalline olivine and implications for transient deformation of the mantle, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15457, https://doi.org/10.5194/egusphere-egu23-15457, 2023.

Our understanding on how opaque materials respond to extreme compression and high temperatures heavily relies on x-ray techniques. While X-ray diffraction is a powerful approach, some properties such as viscosity are experimentally not accessible with such approaches.  Application of x-ray imaging has been widely applied in extreme pressure and temperature conditions using multi-anvil presses, but obtaining sufficient space and time resolution for similar experiments using diamond anvil cells, at commensurately higher pressure and temperature conditions, have been challenging. We present recent developments in synchrotron X-ray imaging at the ECB beamline at PETRA III, DESY used in combination with laser heated DAC. Simultaneous X-ray imaging and diffraction allows for deeper insight in properties of materials under high pressure as well as the direct correspondence of phase transition diagnostics.  We present a case study of bismuth in the laser-heated diamond anvil cell showing detection of solid-solid and solid-liquid transitions and explore how X-ray imaging can be used to determine viscosity of molten bismuth under pressure.

How to cite: Massani, B., Husband, R., Campbell, D., Sneed, D., Jenei, Z., Liermann, H.-P., McWilliams, S., and O'Bannon, E.: Laser-melting Bismuth - A case study for X-ray imaging at high pressure and temperature, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15584, https://doi.org/10.5194/egusphere-egu23-15584, 2023.

Large scale dynamics within the Earth are the result of cooling. Heat is transported towards the surface by large scale convection in the mantle and in the core, and by conduction across the thermal boundary layers at the core–mantle boundary and the lithosphere. There is a range of estimates for the thermal transport properties, e.g. thermal conductivity (k) in the lower mantle ranges between 4 and 16 W/m K, [i] resulting from a lack of consensus on how to represent the pressure (and temperature) dependence of k; different models yield very different extrapolations.[ii]

A three-pronged approach is here established to study thermal conductivity of deep earth minerals at CMB conditions.

(i) Generating high-pressure and high-temperature states of matter in a diamond anvil cell (DAC) and resolving crystallographic changes in the sample via powder XRD. HED at European XFEL is the only facility at present that has sufficiently high X-ray energy coupled with MHz pulse trains to perform time resolved measurements of heat flow in high pressure samples heated by XFEL pulse trains. The femtosecond FEL pulses generate a unique thermal disturbance in bulk matter at a definitive time point, providing an idealized starting point for thermal relaxation. The AGIPD detector at HED allows for easier determination of relaxation dynamics and heat flow.

 (ii) Powder XRD analysis can be carried out by utilising the different timescales of XRD and X-ray absorption, whereby XRD is immediate and occurs before any subsequent unit-cell expansion due to X-ray absorption. The first X-ray pulse is used to collect a diffraction image of the unexcited state of the sample. The next X-ray pulse probes the heated state of the sample 222 ns after first excitation (at 4.5 MHz), before heating the sample again and the step is sequentially repeated.

(iii) Finite element modelling studies allow the determination of thermal parameters such as thermal conductivity and expansivity. Utilising volume change with temperature in a sample, which can be extracted from the diffraction data, a primary model can be made. Temperature dependent thermal conductivity is fitted to the data. Beam energetic data is integrated into the Finite element modelling to dynamically model the fluctuations in the intensity of energy pulses.

How to cite: Younes, Z., Massai, B., Liermann, H., Konopkova, Z., Husband, R., Prescher, C., Sanchez, C., Jaisle, N., and McWilliams, S.: Thermal conductivity of deep earth minerals using high pressure-temperature time-resolved powder X-ray diffraction at European XFEL, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17152, https://doi.org/10.5194/egusphere-egu23-17152, 2023.

Rock masses are often criss-crossed by generations of discontinuities, including veins, fractures, and joints. The presence of fractures and joints can increase rock mass permeability and decrease rock mass strength. However, fluid flow within rock masses can result in secondary mineral precipitation within these spaces. Secondary mineralisation can reduce permeability, with important consequences for fluid flow in systems that rely on discontinuity-dominated permeable networks. Here we investigate if variably sealed joints can be reactivated during deformation and the role joint reactivation plays on permeability. We deformed 20 mm in diameter by 40 mm long cores of un-jointed and jointed (variably sealed) bedded sandstones. Samples were cored such that their dominant structural feature (i.e., bedding or joint) was oriented parallel, perpendicular, or at approximately 30° to the sample axis. We find that the permeability of the undeformed samples is sensitive to the presence and orientation of bedding. In jointed samples, well-sealed joints can act as barriers to fluid flow, but partially filled joints neither inhibit nor promote fluid flow with respect to their joint-free counterparts. While all rocks in this study deformed in the brittle regime under triaxial deformation conditions, the location of the experimentally induced fractures depends on the extent to which joints are sealed. The mineralisation that fills well-sealed joints also permeates the surrounding sandstone matrix, locally reducing porosity and forming a cohesive bond between the joint-fill and the host-rock that increases rock strength: experimentally induced fractures do not exploit pre-existing joint surfaces in these samples. By contrast, strain is localised on the joint surface in samples containing partially sealed joints and the strength of these samples is lower than their un-jointed counterparts. The permeability of all samples increased after deformation, but permeability increase was largest in samples with pre-existing, poorly filled joints. We conclude that partially sealed joints act as planes of weakness within rock masses and that their reactivation can result in significant permeability increase. Well-sealed joints, however, may locally increase rock strength and never become reactivated during deformation: consequently, these joints may never re-contribute to the permeability of a rock mass. These observations provide insight into how fluid flow in the crust may evolve, with possible implications for how these systems weather over time.

How to cite: Kushnir, A., Heap, M., Baud, P., Reuschlé, T., and Schmittbuhl, J.: Reactivating sealed joints: rock strength reduction and permeability enhancement … sometimes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3676, https://doi.org/10.5194/egusphere-egu23-3676, 2023.

Previous investigations of the compressive failure of porous rocks under true triaxial compression have focused on the brittle faulting regime. These studies have underscored the dependence of the peak stress state on the interplay of the three principal stresses. In comparison, there is a paucity of systematic investigations of ductile failure under true triaxial compression. In this study we selected Bleurswiller sandstone, which has been extensively investigated in relation to the brittle-ductile transition under conventional triaxial compression at room temperature. Experiments were conducted in Wuhan on water-saturated samples with the size of 100mm×50mm×50mm at the minimum and intermediate principal stresses ranging up to 70 MPa and 170 MPa, respectively. Previous conventional tests have shown that the initial yield points of Bleurswiller sandstone fall on a linear cap relating the differential and mean stresses. Our new data show that initial yielding under true triaxial loading at a fixed Lode angle is also characterized by a Mises effective shear stress that decreases linearly with increasing mean stress, in agreement with the prediction of an elastic-plastic pore collapse model. Subsequent yielding was manifested by various degrees of strain hardening, that would culminate in a spectrum of failure modes (high-angle shear bands, conjugate shear bands, compaction bands, distributed cataclastic flow). The 3D complexity and geometric attributes of these failure modes have been characterized by X-ray CT imaging of the failed samples.   

How to cite: Meng, F., Shi, L., Hall, S., Baud, P., and Wong, T.: Inelastic compaction and failure mode of Bleurswiller sandstone under true triaxial compression, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4110, https://doi.org/10.5194/egusphere-egu23-4110, 2023.

With increasing depth, crustal rocks gradually transition in deformation type, from being brittle/cataclastic to being crystal plastic, as well as in deformation mode, from being localized (faults, shear zones) to being ductile (homogeneous flow). This transitional layer, commonly referred to as brittle-ductile transition (BDT) has recently become the focus of economic development with the advent of Superhot Rock geotherm Reservoirs (SHR). Superhot Rock geothermal projects (e.g., Japan Beyond-Brittle Project, Iceland Deep Drilling Project, and Newberry Volcano) seek to extract heat from geothermal reservoirs where water reaches a supercritical state (≥ 400 °C). These could multiply the power output of geothermal plants by a factor ten, a progress that is critical in the context of the climate crisis.

However, SHR reservoirs are generally localized at the BDT in semibrittle rocks (rocks deforming through a mixture of brittle and crystal plastic processes) which hydraulic properties are poorly understood.

Here, we report experiments conducted in TARGET, a newly designed gas-confining triaxial apparatus located at EPFL, CH. We deformed cylindrical cores of Lanhelin granite of dimension 40 x 20 mm at a confining pressure of 100 MPa and temperatures ranging from 200 to 800°C and a strain rate of 10^-6 s^-1. While deforming, sample permeability was recorded using the pore pressure oscillation method with an oscillation amplitude of 5 MPa and a period of 2400 s.

Lanhelin granite transitions from being localized with the formation of a sample scale fracture to being ductile between 600 and 800°C. In the localized regime, samples have an ultimate strength of around 600 to 650 MPa. In this regime, permeability initially slightly decreases upon loading from its initial value of 10^-20 m² before increasing with continued deformation. Permeability eventually plateaus upon sample failure and remains constant with further deformation. In the localized regime, permeability increase never exceed 2x10^-19 m^2. In the ductile regime, sample strength is halved and, past the initial decrease upon loading, permeability increases monotically by more than an order of magnitude.

We interpret these data has being the result of sample bulk controlling the sample permeability. In our localized experiments, the fracture never connected the ends of the rock core but would concentrate all of the strain after nucleation, limiting permeability improvement by micro-cracking in the bulk. In the ductile regime, since no localization occurs, bulk permeability of the rock would continuously improve with strain. These results bear important implications for the engineering of permeability in semibrittle reservoirs as well as for the understanding of hydrothermal circulation in the continental crust.

How to cite: Meyer, G. and Violay, M.: Permeability of Lanhelin granite through the brittle-ductile transition., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4576, https://doi.org/10.5194/egusphere-egu23-4576, 2023.

Water penetration, changes in the groundwater level and moisture content changes can affect the physical and chemical properties of coal in an open pit mine. Water levels in open coal pit mines can vary throughout the year, resulting in a number of wet and dry cycles for brown coal. Wet and dry cycles occurring throughout the year can affect the mechanical strength of the stone's microstructure and macroscopic structure. Loss of strength can have severe negative impacts if such rock is integral component in landform design. Until now, no research has been conducted on the effects of wet and dry loading cycles on brown coal. This study investigates the effect of wet and dry cycles on brown coal's strength by conducting a series of unconfined compressive strength (UCS) laboratory tests. For this purpose, nine laboratory samples with dimensions of 38 x 76 cm were prepared. Samples were placed inside distilled water chambers in a temperature-controlled environment. Afterwards, the samples were subjected to unconfined compressive strength (UCS) tests following 0, and 3 cycles of wet and dry conditions. The results of the UCS test show that as the number of wetting and drying cycles increased, the UCS of the samples decreased from 2150 to 330 kPa after three cycles of wetting and drying. In addition, the results indicate that the elastic modulus of brown coal has decreased from 10500 to 1200 kPa. Also, the Poisson ratio decreased from 0.34 to 0.27. This study confirms the importance of paying attention to the wet and dry cycles in brown coal mines.

How to cite: Baghbani, A., Baumgartl, T., and Filipovic, V.: Effects of Wetting and Drying Cycles on Strength of Latrobe Valley Brown Coal, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4804, https://doi.org/10.5194/egusphere-egu23-4804, 2023.

Rockwall erosion by rockfall processes proceed at rates between 0.05 ±0.03 to 14.4 mm a^-1 (Draebing et al., 2022) and are key agents of alpine landscape evolution. Previous studies suggest that frost weathering is a major contributing process to alpine rockwall erosion (Draebing and Mayer, 2021). Frost weathering occurs primarily by frost cracking driven by ice segregation, but our current process understanding is based on studies focusing on high-porosity low-strength rocks. However, rock types forming alpine rockwalls are characterized by crack-dominated porosity and high rock strength, therefore, it is unclear how past findings from low-strength rocks apply in these settings. In this study, we will perform laboratory ice segregation tests on rock samples with different saturation levels and fracture density to quantify their influence on frost cracking efficacy.

We used Wetterstein limestone rock samples in laboratory experiments and exposed rocks to realistic-rockwall freezing conditions while monitoring acoustic emissions as a proxy for cracking. To differentiate triggers of cracking, we modelled ice pressures and thermal stresses. We tested the influence of (i) saturation (low versus full initial saturation), (ii) crack density (0.4 versus 0.6 % rock porosity), and (iii) temperature range (-10 to 0°C) on the efficacy of ice segregation.

(i)  Our data showed that the efficacy of ice segregation is not controlled by initial water content in alpine rocks. These results suggest that water available at depth within alpine rock masses can rapidly travel along fractures to form ice lenses near the rock surface.

 (ii) Crack density has a direct impact on the elastic properties of rocks, which shifts the stress threshold for crack propagation. A fractured rock with high crack density is less prone to ice segregation as its lower brittleness increases the critical fracture toughness.

(iii) Our data revealed temperature patterns promoting ice segregation with highest rates of frost cracking at temperatures between -10 and -7 °C in high strength Wetterstein limestone.

We conclude that frost cracking efficacy in high alpine environments is more impacted by temperatures than by initial rock moisture, which potentially results in more rockfall at colder north- than warmer south-facing rockwalls.

Draebing, D., Mayer, T., Jacobs, B., and McColl, S. T.: Alpine rockwall erosion patterns follow elevation-dependent climate trajectories, Communications Earth & Environment, 3, 21, https://doi.org/10.1038/s43247-022-00348-2, 2022.

Draebing, D., and Mayer, T.: Topographic and geologic controls on frost cracking in Alpine rockwalls, Journal of Geophysical Research: Earth Surface, 126, e2021JF006163, https://doi.org/10.1029/2021JF006163, 2021.

How to cite: Mayer, T., Eppes, M., and Draebing, D.: Quantifying controlling factors of ice segregation in alpine rocks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5126, https://doi.org/10.5194/egusphere-egu23-5126, 2023.

The growth of rock fractures enables physical and chemical rock erosion, sets the pace for infrastructure-, rockfall- and landslide-hazards, and influences rock hydrologic, and therefore chemical, processes. Although fracture growth ultimately lowers rock strength, when rocks are subject to stresses lower than that magnitude, laboratory experiments indicate that the growth of fractures can counterintuitively make rock more resilient to subsequent fracture growth through ‘stress memory’ or ‘fatigue-limit’ phenomena such as the Kaiser effect.  Thus, over geologic time scales, all other things being equal, fracturing rates may decrease, which would have important implications for understanding and interpreting a wide range of landscape evolution processes. To date, however, there have been few if any data explicitly showing that fracture-resilience feedback phenomena arise naturally in subaerially exposed rock.

Here we test for a natural stress memory in two ~25 cm diameter boulders for which we have 1-4 years of known environmental exposure history.  The granite boulders were collected from an unvegetated bar in an ephemeral channel issuing from the south flank of the San Bernardino Mountains, California. As such, we infer that natural abrasion in the channel had removed any major cracks or heterogeneities, effectively ‘resetting’ the rock to a relatively pristine state. The rocks were left on the ground in full sun exposure for 1 and 3 years respectively in humid temperate North Carolina and semi-arid temperate New Mexico, USA. Per-minute rock surface and environmental conditions and cracking (using acoustic emissions) were monitored. Prior work (Eppes et al., 2016 & 2020) indicates that thermal stresses were the primary driver of cracking in the rocks during these time periods. The boulders were then cut in half, and 20x40mm cores were collected from various locations within the rock interior, in duplicate and triplicate for locations of varying distance to the rock exterior. We measured core porosity and P-wave velocity in the cores as proxies for initial rock crack composition, as well as thermal conductivity. We then subjected sets of cores collected at different distances from the rock exterior to increasing magnitudes and number of thermal stress cycles in a temperature-monitored oven, beginning with our best approximation of those matching the maximum stresses leading to observed cracking during the 1 and 3 year observation periods. Our preliminary results reveal that initial crack characteristics vary as a function of distance from rock exterior, as might be expected due to the different magnitudes of thermal stresses experienced within these locations within the rock. Thus, we hypothesize that areas starting with the highest porosities and lowest velocities will experience less change following heating cycles than those parts of the rock with few inferred fractures. We hope that these data will help elucidate mechanisms and feedbacks of natural rock fracturing phenomena that occur over geologic time scales.

How to cite: Eppes, M.-C., Heap, M., Baud, P., Bonami, T., Dahlquist, M., Keanini, R., LaCroix, C., Rasmussen, M., Rinehart, A., El Alaoui, Y., and Windenberger, A.: Testing natural fracture growth-fracturing resilience feedbacks in rock, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5158, https://doi.org/10.5194/egusphere-egu23-5158, 2023.

Fractures in rock are ubiquitous; from cold dry planetary bodies to the hottest, wettest climates on Earth, and from km-scale tectonic fractures deep in Earth’s crust to microcracks in surficial rocks. Yet, many of these fractures propagate progressively over geologic timescales, making their development complex and enigmatic. Therefore, to measure how fractures have developed in rocks exposed at Earth’s surface over millennia, and how this consequently changes rock physical properties, we collected ten ~25 cm diameter granitic boulders from two sites in the Eastern Sierra, California, USA. The boulders were deposited on the surface of alluvial terraces and fans during geologically instantaneous glacial and alluvial events at different times since about 148ka BP, then the depositional surfaces were subsequently abandoned. The chronosequences of geomorphic surfaces provide a natural laboratory in which rocks of consistent lithology have been exposed to similar environmental conditions for different lengths of time, allowing us to compare rock property evolution on the order of 0 to 10⁵ years of environmental exposure; an approach that allows us to better understand and characterize mechanical weathering processes, especially long-term changes in rock fracturing. Note that fresh (time-zero) rocks in this study are represented by boulders found within active channels, and that the measured changes in rocks with longer exposure times are interpreted by comparison with the fresh rocks. Focusing only on similarly sized boulders removes any ambiguities in tectonic and exhumation history that might arise in outcrop samples, thus ensuring that rocks from each site have experienced similar stress conditions; namely those restricted to the environment.

We performed laboratory measurements on 10 granitic boulders (four from Lundy Canyon, with exposure ages of ~0 to ~148 ka; six from Shepherd Creek, with exposure ages of ~0 to ~117 ka) to quantify how rock physical properties changed as a function of environmental exposure age. We measured key parameters commonly used as proxies for crack damage, including porosity, compressional wave velocity (Vp), and shear wave velocity (Vs). We hypothesize that changes in crack damage are likely to affect rock mechanical properties, so we also measured tensile strength, uniaxial compressive strength (UCS), and Young’s modulus (E). We find that all measured parameters evolve as a function of exposure age, with systematic increases in porosity, and systematic decreases in Vp, Vs, tensile strength, UCS, and E. For example, porosity increases from 0.5 – 1.0 % in the fresh rock to 2.6 – 3.2 % in the oldest rocks. We interpret these changes as reflecting progressive subcritical crack growth that arises due to ubiquitous, but relatively low magnitude, environmental stresses continuously acting on the boulders, as opposed to differences inherited before their erosion from bedrock.

Apart from demonstrating the importance of environmentally driven cracking in rock weathering, these observations of progressive crack damage accumulation also have significant implications for the interpretation of any measurements made on rocks exposed at Earth’s surface, even if the age of exposure is relatively short compared to the age of the geologic deposit itself.

How to cite: Meredith, P., Yuan, Y., Rasmussen, M., Hofer Apostolidis, K., Nara, Y., Webb, P., Mitchell, T., Xu, T., Keanini, R., Mushkin, A., Shaanan, U., Dahlquist, M., Rinehart, A., and Eppes, M.: Evidence for increase in crack damage in rocks with duration of exposure at Earth’s surface., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7101, https://doi.org/10.5194/egusphere-egu23-7101, 2023.

Geomorphology context: Earth surface scientists have long posited what controls bedrock to soil conversion rates, which we can now test (assuming steady state) using cosmogenic nuclides. Additionally, near-surface geophysics allows us to image the near-surface with increasing fidelity, such that weathering states from ‘fresh’ bedrock to weathered rock to soil can be inferred over hillslope scales. Current models do not always match field data, and we are yet unable to predict soil thickness. Pedologist Hans Jenny's five factors of soil formation (climate, organisms, topography, parent material, and time) complement the factors geomorphologists presume drive soil production rates (and thus thickness). Geomorphology considers soil production rates from the top-down - whereas the existing soil thickness controls the efficacy of climate and organisms in converting bedrock into disaggregated material, and climate, topography and organisms control the transport efficiency necessary to remove soil - thus keeping the boundary between rock and soil thin enough for more top-down weathering. In most settings, we have few observations of in situ physical weathering. Weathering mechanisms (e.g., thermal, ice segregation, wind-driven tree sway, plant water uptake) are cyclic over brief (seconds to minutes), diurnal, or seasonal cycles. Almost all bring water to the crack network. Unlike traditional laboratory experiment conditions, surface rock is buffered by a soil layer and is subject to disturbance agents that can remove loose fragments - thus modifying the stress state and the crack network. 

Rock physics context: Laboratory experiments to date only consider bare rock. While frost weathering has a rich history of physical experimentation, we know of no other physical experiments that directly test near-surface weathering conditions specifically. While all near-surface rock is to some degree broken by tectonics, the journey to the surface, or contraction cooling; a threshold density of cracks is necessary for cracks to intersect significantly. Because crack growth rate is a function of the crack length and eventually, degree of stress accommodation due to increasing porosity, crack growth in non-uniform over time and thus physical weathering is non-uniform even if conditions remain constant. In its simplest form, considering only mechanical sources of damage, the 'Kaiser effect' suggests that under conditions of cyclic loading, cracking happens only when the previous maximum stress is exceeded. However, in natural environments, each cycle of opening refreshes water at the crack tip, allowing chemical damage to accrue, and for fracture propagation. Most progressive rock failure experiments are run monotonically, with the fracture under a consistent loading, or with rapid, cyclic loading—neither replicating conditions experienced in the natural world necessary to estimate material property change through time.

Interdisciplinary context: Geomorphologists and soil scientists have generally ignored factors governing fracture propagation, and rock physicists, focused on index properties and detailed process understanding, have not simulated relevant field conditions. Here, we explore such as above in asking if and how the non-uniform nature of subcritical cracking may be a first order control on soil production and bedrock landscapes, and if so, what experiments exist or are needed to arrive at a new type of soil production function?

How to cite: Marshall, J., Rinehart, A., Eppes, M. C., and Meredith, P.: Should we? Can we? apply experimental rock physics knowledge to reconsidering soil production functions?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7443, https://doi.org/10.5194/egusphere-egu23-7443, 2023.

We present a new structural model of the Theistareykir high-temperature geothermal field, located in NE Iceland within a volcanically active rift zone. Various interdisciplinary geoscientific methods are applied and cross-analyzed. We use remote sensing data, structural and geological surface and subsurface mapping, drone surveys, borehole images, seismicity, potential field, and CO₂ surface emissions data. This composite data modelling approach aims to highlight primary fault zones and fractured intervals at the surface by assigning fault types and their spatial orientation. The compiled surface and subsurface datasets were used for 3D fault projections and the delineation of fault-block compartments. Fault plane solutions from earthquakes supported fault property assignments, slip direction, and stress-field orientations. Surface fault segments and fracture intensity maps highlight areas of relay ramping, fault damage, and accommodation zones in between the NW-SE, NE-SW to N-S striking fault systems of the Theistareykir rift segment. The fault systems are intersected by WNW-ESE striking embryonic transfer zones that form boundaries between rift valley graben segments south and within the Theistareykir geothermal field area. These WNW-ESE transfer zones accommodate the differential and oblique opening of the rift zone, which overall follows the right-lateral opening direction of the region south of the Husavik-Flatey transform fault. Our structural model of the Theistareykir geothermal field area is subdivided into six structural domains that form fault block compartments, with varying degrees of faulting and fracturing, reflecting the different quality of hydraulic connectivity across the field.

How to cite: Blischke, A., Hjartardóttir, Á. R., Gudnason, E. Á., Ágústsdóttir, T., Benediktsdóttir, Á., Vilhjálmsson, A. M., Þorsteinsdóttir, U., Einarsson, G. M., Óladóttir, A. A., and Mortensen, A. K.: 3D structural mapping of the Theistareykir geothermal field, NE-Iceland: Fault and fracture connectivity within a volcanically active rift zone., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8009, https://doi.org/10.5194/egusphere-egu23-8009, 2023.

We perform laboratory experiments of decametric scale using a novel triplet apparatus (Fig. 1) that allows (a) to reproduce earthquake-like instabilities and (b) to prevent them by active fluid pressure adjustment. The dynamic earthquake events are prevented using high-end, robust stabilizing controllers that stabilize the system even in the absence of knowledge about its friction, elasticity and other complex phenomena that are hard to quantify in practice.

Two scenarios are investigated experimentally. In the first scenario, the system is loaded close to its instability point and then fluid is injected in order to provoke a seismic event. We observe how the controller automatically adjusts the fluid pressure in order to prevent such instabilities and immobilize the system. In the second scenario, the controller adjusts the fluid pressure automatically in order to drive the system in a new stable equilibrium of lower energy in an aseismic manner. Despite the inherent unstable behavior of the system, uncertainties related to friction, elasticity and multiphysics couplings, the earthquake-like events are avoided and controlled. We expect our methodology to inspire earthquake mitigation strategies regarding anthropogenic and/or natural seismicity.

References

[1] Stefanou, I. (2019). Controlling Anthropogenic and Natural Seismicity: Insights From Active Stabilization of the Spring‐Slider Model. Journal of Geophysical Research: Solid Earth, 124(8), 8786–8802. https://doi.org/10.1029/2019JB017847
[2] Tzortzopoulos G., Braun P., Stefanou I. (2021), Absorbent Porous Paper Reveals How Earthquakes Could be Mitigated, Geophysical Research Letters 48. https://doi.org/10.1029/2020GL090792.
[3] Stefanou, I., Tzortzopoulos, G. (2022). Preventing instabilities and inducing controlled, slow-slip in frictionally unstable systems. Journal of Geophysical Research: Solid Earth. https://doi.org/10.1029/2021JB023410
[4] Gutiérrez-Oribio D., Tzortzopoulos G., Stefanou I., Plestan F. (2022). Earthquake Control: An Emerging Application for Robust Control. Theory and Experimental Tests. http://arxiv.org/abs/2203.00296
[5] Papachristos, E., Stefanou, I. (2022), Controlling earthquake-like instabilities using artificial intelligence. http://arxiv.org/abs/2104.13180.
[6] Gutiérrez-Oribio D., Stefanou I., Plestan F. (2022). Passivity-based Control of a Frictional Underactuated Mechanical System: Application to Earthquake Prevention. https://arxiv.org/abs/2207.07181

Fig.1: Experiments of decametric scale using a novel triplet apparatus for preventing earthquake-like instabilities

How to cite: Stefanou, I., Tzortzopoulos, G., Braun, P., and Gutierrez-Oribio, D.: Controlling earthquake-like instabilities in the laboratory, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8589, https://doi.org/10.5194/egusphere-egu23-8589, 2023.

Deformation bands, or tabular zones of localised strain, are a common manifestation of deformation in upper crustal sedimentary rocks. Any mining or energy-related engineering applications must consider the possibility of reactivating these pre-existing failure planes because doing so can cause seismicity and compartmentalise the reservoir. However, there has only been a small amount of research done on laboratory-induced deformation in rocks with natural deformation features.

On a low porosity bioclastic calcarenite from the Cotiella Basin, Spanish Pyrenees, our current experimental work aims to capture, for the first time to our knowledge, the dominant failure mechanisms during the reactivation of natural deformation bands oriented at different angles to the principal stress direction. At the I12-JEEP beamline at the synchrotron facility of Diamond Light Source, UK, we carried out triaxial compression experiments using a modified version of the Mjolnir cell used by Cartwright-Taylor et al., (2022) to examine how these highly heterogeneous rocks respond to additional mechanical deformation. During the deformation experiments, 4D (time and space) x-ray tomography images (8 m voxel size resolution) were acquired. We tested confining pressures between 10 MPa and 30 MPa.

The mechanical data demonstrate that the existence of natural deformation features within the tested samples weakens the material. For instance, solid samples of the host rock subjected to the same confining pressures had higher peak differential stresses. Additionally, our findings demonstrate that new deformation bands form as their angle, θ, to σ1 increases, while the reactivation of pre-exiting deformation bands in this low porosity carbonate only occurs for dipping angles close to 70^o. The spatio-temporal relationships between the naturally occurring and laboratory-induced deformation bands and fractures were investigated using time-resolved x-ray tomography and Digital Volume Correlation (DVC). Volumetric and shear strain fields were calculated using the SPAM software (Stamati et al., 2020). The orientation of the recently formed failure planes is influenced by the orientation of the pre-existing bands, as well as their width and the presence (or absence) of porosity along their length. Additionally, pre-existing secondary deformation features found in the tested material trigger additional mechanical damage that either promotes the development or deflects the new failure planes.

References

Cartwright-Taylor et al. 2022, Nature Communications 13, 6169, https://doi.org/10.1038/s41467-022-33855-z

Stamati et al. 2020, Journal of Open Source Software, 5(51), 2286, https://doi.org/10.21105/joss.02286

How to cite: Charalampidou, E.-M., Taxopoulou, M.-E., Beaudoin, N., Aubourg, C., Cartwright-Taylor, A., Butler, I., Atwood, R., and Michalik, S.: Failure mechanisms in low-porosity carbonate rocks during the reactivation of deformation bands with various orientations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9609, https://doi.org/10.5194/egusphere-egu23-9609, 2023.

Human intervention in subsurface geoenergy systems, such as fluid injection, can lead to induced seismicity. Particularly in geothermal systems where faults or fractures serve as fluid pathways, fault reactivation is a significant risk. Therefore, we must elucidate under which stress conditions faults become reactivated. The geometry of fault planes evolves as a function of fault displacement. Mature faults (i.e. with 10-100 m displacement) are most likely gouge-filled due to material weathering during movement. This study investigates the Lower Carboniferous system and specifically the Dinantian formation. This specific formation is particularly interesting for deep geothermal energy in the Netherlands, Belgium and Germany but can serve as a proxy for fractured carbonate deep geothermal reservoirs worldwide. The Dinantian carbonates exhibit pre-existing fractures which mainly contribute to rock permeability. However, there is little or no knowledge of the fault geometry and filling material. Subsequently, it is essential to investigate a spectrum of carbonate fault geometries and gouge material to deliver fault stability conditions. 

Here, we aim to experimentally characterise fault strength through all stages of their temporal evolution, from bare rock to highly strained fault gouges at a range of normal stresses. All experiments are performed at room temperature, using de-ionised (DI) water as pore fluid. The bare rock surfaces are gouge-free saw-cut samples, loaded into a Hoek cell embedded in a 500 kN uniaxial loading machine. These experiments are performed at a range of confining pressure from 10 to 50 MPa, corresponding to normal stresses from 60 to 90 MPa and undrained conditions. We determined the critical range of axial, shear and normal stress values per experiment at which fault reactivation was initiated. We used a rotary shear apparatus to increase fault gouge maturity through the shearing of a simulated gouge material in drained conditions. In a single experiment and sample, we changed the normal stress with a protocol of 2-4-6-8-10-8-6-4-2 MPa. For every stress interval, we performed a slide-hold-slide procedure, where the slide-hold times were 10-10-10-100-10-1000-10 sec and the velocity was 20-0-20-0-20-0-20 μm/sec respectively. After each hold time, the reactivation leads to a different peak shear stress. By characterizing the different peak stresses we can quantify the evolution of critical shear stress as a function of fault inactivity time. We used the reactivation stresses for both experimental types to calculate the intercept and angle of the reactivation envelopes in a Mohr-Coulomb context using linear regression, which corresponds to the cohesion and friction coefficient of the laboratory faults. 

Our preliminary results for the rotary shear experiment show that mature carbonate laboratory faults exhibit a cohesion of <1 MPa and friction coefficient up to 0.65 under wet conditions. Moreover, the cohesion of the fault decreases as a function of healing time and the friction coefficient increases. Future plans include the investigation of fluid chemistry on the reactivation envelope. To conclude, we aspire to give insight to the operators on how to safely design the geothermal injection and production schemes accounting for the geomechanical constraints.

How to cite: Kane, E., Pluymakers, A., and Niemeijer, A.: Reactivation envelopes of immature and mature faults of Dinantian carbonates targeted for geothermal energy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11236, https://doi.org/10.5194/egusphere-egu23-11236, 2023.

Germany's geothermal potential outside the known geothermal provinces has been little investigated so far, as it does not involve deep sedimentary basins. The crystalline basement offers a yet untapped potential for producing geothermal energy with enhanced geothermal systems (EGS). In the research alliance "Geothermal Alliance Bavaria", which is funded by the Bavarian State Ministry of Science, we are investigating the potential for EGS in northern Bavaria.

A region in Franconia with a geothermal anomaly has already been delimited and is the focus of further investigations. A granite body covered by several kilometers of sediments was identified as the source of this geothermal anomaly. For a more detailed investigation of the hydraulic conditions in a fault zone-controlled granite reservoir, a surface analogue was found in a granite quarry in the Fichtelgebirge in northeastern Bavaria.

The quarry is transected by a fault zone and shows a narrow fracture network at the surface. A testfield consisting of 15 wells with depths between 15 and 25 m was set up in the quarry to analyse the influence of a fault zone-controlled fracture network on the hydraulic permeability. A photogrammetric model, surface geophysical measurements and borehole geophysics have been carried out to record the fracture network in detail. The influence of the fracture network on the hydraulic permeability is to be determined by various hydraulic tests.

Slug and pulse tests show variable, but overall low hydraulic permeabilities in the individual boreholes with values between 10^-7 – 10^-10 m/s. Slightly higher permeabilities assumingly correlate with more prominent fractures or fracture zones detected in image logs and several geophysical logs. Double packer tests on selected fractures/fracture zones will determine single fracture permeabilities in order to clarify which fractures and whether certain fracture properties mainly influence the hydraulic permeability. Furthermore, these double packer tests are intended to identify connectivity between individual wells through specific fractures or fracture zones.

In a further step, hydraulic packer tests will be used to determine fracture opening pressures and the stress field.

First results of hydraulic tests evaluated so far will be presented and the influence of recorded fracture properties of single fractures and fracture zones on the observed hydraulic permeability will be presented and discussed.

How to cite: Hähnel, L., Bauer, W., and Stollhofen, H.: Influence of fractures and their properties on hydraulic permeability in a fault zone-controlled fractured granite – basic scientific research for an EGS feasibility study, Northern Bavaria, Germany, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12406, https://doi.org/10.5194/egusphere-egu23-12406, 2023.

Keywords: geothermal, high resolution, fault stability, induced seismicity.

High resolution predictions of three-dimensional subsurface stress changes are required for the assessment of geothermal operations with respect to fault stability and the potential risk for induced seismicity. The effects of long-term cooling on reactivation and seismicity potential of faults near a geothermal doublet require quantification and management for safe and effective application of geothermal energy. This work presents a detailed analysis of the sensitivity for fault reactivation and induced seismicity based on different scenarios for reservoir characteristics and production parameters. To this end, analytical solutions are used as well as a TNO-proprietary tool known as MACRIS (Mechanical Analysis of Complex Reservoir for Induced Seismicity) (Van Wees et al., 2019) that allows for both poro- and thermo-elastic stress evaluations in structurally complex (i.e. highly faulted) reservoirs. The stress evaluations take as input the pressure and thermal field of the reservoir and over- and underburden which are obtained from the Open Porous Media (OPM) Flow reservoir simulator (Rasmussen et al., 2021). In this workflow, high resolution stress change solutions at the faults are available.

The workflow has been applied to a high resolution three-dimensional reservoir model, including over- and underburden rock, marked by a single fault. Key elements in the dynamic and mechanical behaviour of the reservoir are varied, along with different production scenarios. Simulated stress evolutions in MACRIS and alternative analytical solutions show a predominant sensitivity for fault reactivation to the thermo-elastic parameters, i.e. the Young’s modulus and thermal expansion coefficient. Furthermore, in cooling reservoirs, the intersection area of the cold-water volume in direct contact with the fault plane is shown to be the main driver for fault reactivation and subsequent seismic potential.

References

Rasmussen, A.F., Sandve, T.H., Bao, K., Lauser, A., Hove, J., Skaflestad, B., … and Thune, A. (2021). The Open Porous Media Flow reservoir simulator, Computers and Mathematics with Applications, 81, 159-185.

Van Wees, J.D., Pluymaekers, M., Osinga, S., Fokker, P. A., van Thienen-Visser, K., Orlic, B., Wassing, B. B. T., Hegen, D., and Candela, T. (2019). 3-D mechanical analysis of complex reservoirs: a novel mesh-free approach, Geophysical Journal International, 219 (2), 1118-1130.

How to cite: Marelis, A., van Wees, J.-D., and Beekman, F.: A sensitivity analysis of stress changes related to geothermal direct heat production in clastic reservoirs and potential for fault reactivation and seismicity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14219, https://doi.org/10.5194/egusphere-egu23-14219, 2023.

The creation of fractures in boreholes by hydraulic stimulation is of importance for enhancing fluid transport in the context of geothermal reservoirs. Detecting whether a fracture is created and evaluating its capacity to transport fluid are usually performed by locating the acoustic emissions generated during the rock failure and by comparing the hydraulic properties before and after this type of hydraulic stimulation using pumping tests, respectively. However, free oscillations, exerted by rapid changes in pumping parameters, can be used as well to detect the existence of a fracture and to evaluate its permeability. The diagnostic properties of free pressure oscillations are their spectral components, i.e., frequency and damping coefficient. The hydraulic system, which includes technical equipment such as tubes and hoses, and the rock formation, which can be tight or leaky. In a tight system, the free pressure oscillations are attenuated by the viscous interaction of the fluid and the borehole wall and the local coupling of the fluid compression and deformation of the borehole due to the pressure variation. In a leaky system, the attenuation of free pressure oscillations includes fluid exchange between the borehole and the hydraulic conduits of the rock. We developed an analytical solution starting from the dispersion relation of fluid-flow waves in a tight borehole by accounting for the fluid exchange as a modified boundary condition. Deviation of spectral components of observed oscillation from the analytical solution for a tight borehole is an indication that the free pressure oscillations contain information of the hydraulic properties of the penetrated formation. The oscillations typically last for tens of seconds, which allows assessing the success of the stimulation operation on a near-real-time basis. We analyzed the characteristics of free operations recorded in boreholes during several hydraulic stimulation campaigns. We investigated the evolution of the spectral components in the course of the stimulation and with changes in mean injection pressure and obtained transmissivity values that favorably compare to the results of conventional analyses.

How to cite: Jimenez Martinez, V. A. and Renner, J.: Discerning fracturing and constraining hydraulic properties from the characteristics of free pressure oscillations in boreholes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14681, https://doi.org/10.5194/egusphere-egu23-14681, 2023.

Deep heat mining requires activation of slip on pre-existing geological discontinuities and the creation of hydraulically conductive fracture networks. Fluid injection or diffusion of ground waters can rise the fluid pressure near pre-existing fractures and faults, which may induce frictional slip. The fracturing process depends strongly on the initial stress conditions and rupture planes orientation. It is known that vertical stress is varying linearly with depth whereas horizontal stresses are likely not to exhibit linear dependence. Nevertheless, within certain length scales, one may assume linear relations for all stress tensor components.

In the previous study [1], it was shown that for a planar rupture which is propagating due to fluid injection under a constant overpressure in the absence of stress gradient, the solution is self-similar and depends only on one dimensionless parameter which determines two limiting regimes. The first so-called "critically-stressed limit” designates that the fault is initially close to failure, whereas the “marginally pressurized limit” represents the case when the fluid pressure is “just sufficient” to activate the fault. One of the main features of the solution in the uniform stress case is that the rupture tips are propagating symmetrically.

In our work, we investigate how linear stress gradient acting initially on the fault affects the shear rupture growth, namely, how it breaks the symmetry of the rupture propagation. The problem couples quasi-static elastic equilibrium and fluid flow on the fault plane via a Coulomb shear failure criterion with a constant friction coefficient. From a scaling analysis, it is shown that the problem is governed by two dimensionless parameters, T_o (similar to the one found in [1]) and dimensionless time. Parameter T_o is the ratio between the initial distance to failure and the strength of injection [1] calculated at the injection point. T_o determines two propagation regimes similar to those found in [1] (critically stressed and marginally pressurized limits). Dimensionless time parameter determines symmetric and asymmetric propagation periods and encapsulates the information about stress-gradient values. At early times, the solution is similar to the homogeneous stress case and the rupture stays symmetrical. At times near the characteristic time of each regime, the non-uniform in-situ stress distribution makes the rupture to propagate asymmetrically. We investigate the transition time for each limiting regime and compare it with real field observations. Our solution can also provide a benchmark for numerical solvers.

REFERENCES

[1] Viesca, R., 2021 Self-similar fault slip in response to fluid injection, Journal of Fluid Mechanics, vol. 928

How to cite: Fakhretdinova, R., Sáez, A., and Lecampion, B.: The effect of in-situ linear stress gradient on the frictional shear rupture growth in 2D., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15244, https://doi.org/10.5194/egusphere-egu23-15244, 2023.

Environmental effects on cracking have important implications in geoengineering applications, such as mechanized tunnel construction. The time required and the cost of excavation could be reduced by employing techniques such as special cutting fluids that lower the strength of the rock.

At first contact, the cutting tool acts as an indenter, penetrating the rock surface and causing distributed brittle damage and subsequently localized deformation. We performed indentation tests using blunt indenters, mimicking a section of a standard cutting disk, on unconfined cylindrical Anroechter sandstone specimens using a servo-hydraulic press. The loading rate was varied over two orders of magnitude. A first set of experiments was performed at room temperature and humidity; in a second, specimens had their top surface wetted with water during indentation. Preliminary results show that in the presence of water, the peak indentation force is reduced by approximately 13%. The short duration of the tests (a few minutes) and the relatively low porosity of the sample material (approximately 10%) suggest a fast-acting weakening mechanism. Currently, we are focusing our efforts on understanding the effect of wetting fluid chemistry on peak indentation pressure and exploring the interplay with loading rate.

How to cite: Korkolis, E. and Renner, J.: Experimental investigation of environmentally affected cracking during indentation testing of Anroechter sandstone, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15452, https://doi.org/10.5194/egusphere-egu23-15452, 2023.

Brittle creep in rock that results from time-dependent subcritical crack growth often plays a fundamental role in the emergence of precursory phenomena of impending catastrophic events in the upper crust. Laboratory has been used to investigate time-dependent cracking in brittle rocks, and signals of acoustic emission (AE) and X-ray tomography were employed as proxies for damage accumulation, which increase non-linearly towards failure (Heap et al., 2009; Renard et al., 2020). Despite these studies, the evolution of damage in real time and especially the potential impact of strain localization on dynamic critical transition of failure yet are understudied.

We study brittle creep in a dry Herrnholz granite (with an initial porosity of 2.2%) under triaxial stress conditions. The test procedures consisted of (i) confining the sample to P_c =10 MPa and (ii) then applying and holding a differential stress σ_d = 234 MPa. This stress was held constant and a standard creep response was observed exhibiting a clear trimodal behavior that culminated with the formation of a shear fracture and catastrophic failure of the sample. We used the distributed strain sensing (DSS) fiber optic technology to obtain local estimations of strain and calculate volumetric strain on the surface of the sample using an interpolation strategy. During the primary creep phase, deformation mapped with DSS was found to be sparsely distributed in the form of volumetric deformation in general uniform throughout the sample and expressed on the surface. The transient acceleration of creep (creep burst) was only identified in local strain measurements near the final faulting position and occurred during the steady-state creep phase. This clearly indicates that strain began to localize around the ultimate location of fracture, which was also confirmed by the postmortem 3D optical scanning.

Using this better understanding of progressive strain localization, we searched for indications of damage evolution and critical behavior. During the creep phases, changes in certain properties of DSS array were examined for potential precursory signatures. We analyzed the statistics of damage rate and incremental strain and detected a significant breaking of scaling during the creep phase which led to a critical interpretation of fracture. Prior to the critical point, creep bursts correlated with the nucleation and growth of the main fault, which likely indicates the onset of scaling divergence where damage began to self-organize toward failure. These results show that strain localization which drives the fracture development can be captured by DSS technology and the brittle creep processes in Herrnholz granite follow a critical point transition which can be attributed to a self-adjustment of local strains after creep burst.

References:

Heap, M. J., Baud, P., Meredith, P. G., Bell, A. F., & Main, I. G. (2009). Time-dependent brittle creep in Darley Dale sandstone. Journal of Geophysical Research, 114(B7), B07203. https://doi.org/10.1029/2008JB006212

Renard, F., Kandula, N., McBeck, J., & Cordonnier, B. (2020). Creep burst coincident with faulting in marble observed in 4‐D synchrotron X‐ray imaging triaxial compression experiments. Journal of Geophysical Research: Solid Earth, 125, e2020JB020354. https://doi.org/ 10.1029/2020JB020354

How to cite: Chen, H., Selvadurai, P. A., Vasquez, A. S., Bianchi, P., Lei, Q., Madonna, C., and Wiemer, S.: Illumination of Damage and Critical Transition During Time-Dependent Deformation in Herrnholz Granite Using Distributed Strain Sensing, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16084, https://doi.org/10.5194/egusphere-egu23-16084, 2023.

Hydraulic stimulation of pre-existing fractures is used in deep geothermal development in order to increase reservoir permeability and achieve economical flow rates – with mixed success [1, 2]. Although the primary idea is to shear dilate these pre-existing discontinuities via injection, in a number of field tests [3], a large increase of permeability is only observed when fracture opening has been reached (sometimes denoted as hydraulic jacking). Shearing of pre-existing discontinuities can also occur during more traditional hydraulic fracturing operations in oil and gas reservoirs, either by direct fluid pressurization or via stress transfer from the main fractures. In this contribution, we discuss the development of a robust numerical solver for the modeling of the fluid-driven growth of a shear crack along frictional discontinuities, accounting for shear-induced dilatancy as well as possible transition to opening hydraulic fracturing. An elasto-plastic constitutive relation with a non-associated flow rule is used to model the frictional and cohesive behavior of the pre-existing discontinuity with or without slip-dependent frictional properties. A fully coupled hydro-mechanical solver is developed for this class of problem. It combines a boundary element discretization of the fracture(s) for the solution of the quasi-static elastic equilibrium of the rock mass with a finite element discretization of the width-averaged fluid mass conservation and momentum in the fractures. Using implicit time-stepping, the resulting non-linear system of coupled equations is solved via a Newton-Raphson procedure using the consistent tangent elasto-plastic operator obtained from the local integration of the interfacial constitutive relation via a predictor-corrector scheme. We present a number of stringent verification problems for strictly frictional as well as strictly hydraulic fracturing conditions. We then investigate the evolution of both the shear and opening front in terms of the properties of the pre-existing discontinuities (friction and dilatancy), the in-situ and injection conditions [4, 5, 6]. We highlight relevant conditions associated with deep geothermal reservoirs, and discuss the occurrence of different propagation regimes from purely frictional to hydraulic fracturing type.

References

[1] R. Jung. EGS - Goodbye or Back to the Future. In ISRM International Conference for Effective and Sustainable Hydraulic Fracturing. International Society for Rock Mechanics, 2013.

[2] M. W. McClure and R. N. Horne. An investigation of stimulation mechanisms in Enhanced Geothermal Systems. Int. J. Rock Mech. Min. Sci., 72:242–260, 2014.

[3] Y. Guglielmi, C. Nussbaum, P. Jeanne, J. Rutqvist, F. Cappa, and J. Birkholzer. Complexity of fault rupture and fluid leakage in shale: Insights from a controlled fault activation experiment. Journal of Geophysical Research: Solid Earth, 2020.

[4] K. Hayashi and H. Abe. Opening of a fault and resulting slip due to injection of fluid for the extraction of geothermal heat. Journal of Geophysical Research: Solid Earth, 87(B2):1049–1054, 1982.

[5] A. Sáez, B. Lecampion, P. Bhattacharya, and R. Viesca. Three-dimensional fluid-driven stable frictional ruptures. J. Mech. Phys. Sol., 160:104754, 2022.

[6] E. Detournay. Mechanics of hydraulic fractures. Annual Review of Fluid Mechanics, 48:311–339, 2016.

How to cite: Lecampion, B., Sáez, A., Fakhretdinova, R., and Gupta, A.: Development of a robust numerical simulator for mixed shear and opening modes fluid driven fracture propagation along pre-existing discontinuities, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16563, https://doi.org/10.5194/egusphere-egu23-16563, 2023.

Understanding the poromechanical response of fractured rock is necessary for applications ranging from hydraulic stimulation of the subsurface to teleseismic impulses from earthquakes that may reactivate faults or otherwise breach reservoir seals. We describe laboratory experiments that seek to decouple the role of fracture interface stiffness and fracture infill (sediment transport) in the hydraulic and elastodyamic properties of fractured rock. Experiments are conducted on multiple samples of Westerly granite with different uniform roughness (silicon carbide grit-roughened or milled) that were loaded under triaxial stresses in a pressure vessel while permeability evolution is measured from the flow-through of deionized water. In some experiments, thin layers of synthetic wear (gouge) material are added to the fracture interface to simulate mature, sheared, fractures whose poromechanical response is dominated by clogging and unclogging of pore throats. Oscillations of pore pressure and normal stress are applied at amplitudes ranging from 0.2 to 1 MPa at 1Hz. The experiments also consider the influence of fracture aperture with effective stress perturbations applied at normal stresses ranging from 5 to 20 MPa (reducing aperture with increasing effective normal stress). Before, during, and after the dynamic stressing, an array of piezoelectric transducers (PZTs) continuously transmits and receives ultrasonic pulses across the fracture to monitor the evolution of fracture stiffness and fluid transport due to the dynamic stressing. These allow evaluation of stress-induced changes in transmitted ultrasonic wave velocity and amplitude to estimate the contact acoustic nonlinearity of the fracture interface concurrent with permeability evolution. We compare the results for samples with and without synthetic wear (gouge) material to understand the role and evolution of fracture stiffness and clogging-unclogging mechanisms in pore throats of porous and fractured media.

How to cite: Wood, C., Ke, C.-Y., Rivière, J., Elsworth, D., Marone, C., and Shokouhi, P.: Decoupling the poromechanics of particle remobilization and interface stiffness of dynamically stressed tensile fractured rock, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17555, https://doi.org/10.5194/egusphere-egu23-17555, 2023.

Fractures are ubiquitous through out the Earth's upper crust and dominate the mechanical and hydraulic properties of the affected rock masses. Indeed, open fractures act as fluid conduits and, commonly, flow is controlled by larger fractures, which are, in turn, likely to be connected to smaller ones. Therefore, fracture characterization is of paramount importance for many pertinent applications, such as geothermal energy production, CO₂ sequestration, nuclear waste storage, and hydrocarbon exploration. Seismic reflection methods are useful tools for fracture characterization due to the generally high reflectivity that large fractures exhibit as a consequence of their strong mechanical contrast with the embedding intact background. The magnitude of this mechanical contrast is known to be strongly affected by fracture-to-background wave-induced fluid pressure diffusion (FPD). Conversely, the FPD effects associated with secondary connected fractures remain so far unexplored. We investigate the influence of FPD on the normal compliance and on the vertical incidence PP reflectivity of a large fracture that is hydraulically connected to smaller fractures. To this end, we use several models that consist of an infinite horizontal main fracture connected to multiple vertical secondary fractures of finite length. This fracture system is embedded in impermeable background. The individual models differ only with regard to the geometrical (e.g., length and aperture), and physical properties (e.g., permeability and bulk modulus) of the secondary fractures. For comparison, we also calculate the normal compliance and the reflectivity of an isolated infinite horizontal fracture. To assess the changes of fracture compliance due to FPD, we perform a vertical compressional oscillatory test over samples of the aforementioned models that include part of the fracture system and the embedding background. This test simulates the FPD effects that a vertically propagating P-wave generates between the main and secondary fractures. Specifically, the wave produces a pressure increase in the horizontal fracture that equilibrates as fluid flows into the secondary vertical fractures. Based on this oscillatory test, we compute the average of the vertical components of strain and stress over the main fracture, which we use to estimate its normal compliance. We then proceed to calculate the PP reflectivity at normal incidence using its inferred P-wave modulus. Our results show that both the compliance and the PP reflectivity of the main fracture increase as much as two-orders of magnitude in response to the presence of secondary fractures. We also find that the physical and geometrical properties of the secondary connected fractures have an influence on the normal compliance and reflectivity of the main fracture.

How to cite: Sotelo, E., Rubino, J. G., Barbosa, N. D., Solazzi, S. G., and Holliger, K.: Seismic reflectivity of fractures: the impact of secondary connected fractures, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1059, https://doi.org/10.5194/egusphere-egu23-1059, 2023.

Abstract：The bauxite reservoir of the new type Taiyuan Formation in Zhengning area, southwest of Ordos Basin, is affected by the karst palaeogeomorphology, and its thickness varies greatly.In order to systematically study bauxite, a new type of reservoir, based on core observation, microscopic thin section, high-pressure mercury injection, low-temperature nitrogen adsorption and other experimental methods, the petrological characteristics and pore structure characteristics of bauxite reservoir were studied, which further verified the significance of reservoir exploration.The results show that: (1) the upper and lower parts of the reservoir are bauxite mudstones, and the middle part is argillaceous bauxite. The relatively developed dissolution pores are the main storage space of bauxite; (2) The bauxite minerals of Taiyuan Formation are mainly composed of aluminum minerals and clay minerals. The main minerals are diaspore, kaolinite, illite and chlorite; (3) Bauxite reservoir space is mainly composed of intragranular dissolved pores, matrix dissolved pores, intergranular dissolved pores, intergranular pores and microcracks, with the pore size mainly between 20 and 200 μm； The pore diameter of the main throat of the reservoir is 150 nm~4μm. The pore structure is good, mercury removal efficiency is high, and the overall pore throat is mainly submicron to micrometer; The average physical porosity of the reservoir is 10.6%, and the average permeability is 4.04×10^-3μm. Greater than 0.3×10^-3μm 36% of them are above m, and the reservoir conditions are good.The research results provide a basis for bauxite gas exploration in Ordos Basin.

How to cite: Li, H. Y.: Bauxite Reservoir Characteristics of Taiyuan Formation in Zhengning Area of Southwest Ordos Basin, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1115, https://doi.org/10.5194/egusphere-egu23-1115, 2023.

The capillary force shows great potential to improve the recovery of shale oil and gas reservoirs through spontaneous imbibition. However, the mechanism of capillary force on shale oil migration and its controlling factors are still unclear. By NMR, low-temperature nitrogen adsorption, high-pressure mercury injection and other experimental means, this work attempts to investigate the role of capillary force in improving shale oil recovery. The results show that the nuclear magnetic resonance T₂ spectra obtained through spontaneous imbibition can be divided into three types, and the shale oil recovery can reach 38.72% - 65.52%, which is mainly contributed by the first peak (P1). The water imbibition and oil imbibition experiments were carried out on samples of the same size, and the dynamic wettability index of the samples with the spontaneous imbibition time was calculated. It was found that type 1 shale is mainly lipophilic, type 2 and type 3 samples are mainly hydrophilic, the P1 of three types of shale is hydrophilic to neutral, and the water imbibition volume of the three samples was greater than the oil imbibition volume. In addition, by comparing the relationship between pore throats and pores and combining the structural characteristics of samples, three typical types of pore throats are summarized. Finally, through a comprehensive study on the wettability, pore structure of shale and shale oil recovery , it is concluded that water can drive oil droplets in micropores or pore throats (P1) to enter the mesopore (P2), and then the mesopore (P2) transmits the oil to the fractures by transfering pressure difference, and the oil-water distribution pattern before and after spontaneous imbibition under the effect of capillary force is summarized to provide theoretical basis for shale oil exploration. and development.

How to cite: Yin, N.: Shale oil mobility and pore size-associated wettability under capillary pressures, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1152, https://doi.org/10.5194/egusphere-egu23-1152, 2023.

Solid organic matter (SOM) is an important component of natural sediments and plays a crucial role in providing substrate for microbial reactions and the degradation of contaminants in soil and groundwater. Knowledge about its distribution in the subsurface is crucial for the delineation of potential hotspots of microbial activity. The subsurface is, however, difficult to access, limiting our ability to reliably delineate the spatially heterogeneous distribution of SOM. Recently, the geophysical method induced polarization (IP) has been shown to be a potentially promising mapping tool, able to detect the presence of SOM. However, the mechanisms controlling IP signals in the presence of SOM are not (yet) well understood, with a handful of studies highlighting inconclusive results (Katona et al., 2021; Mellage et al., 2022; Ponziani et al., 2012; Schwartz & Furman, 2014). Moreover, a non-negligible contribution of polarization from the organic matrix can yield signals that may cause misinterpretation of other petro-physical relationships in unconsolidated sediments.

In this study, we measured the spectral IP (SIP) response of aquifer sediment cores (2 – 8 m depth) collected from an alluvial floodplain aquifer in southwest Germany. The total organic carbon (TOC) content in the cores and the cation exchange capacity (CEC) exhibit a positive correlation with the magnitude of polarization (i.e. imaginary conductivity). In addition, strong differences in the frequency dependence of the IP measurements as a function of TOC fraction were observed for the otherwise calcareous matrix devoid of other strongly polarizing mineral phases (e.g. pyrite or clay minerals). While the CEC at the site is strongly dominated by the amount of SOM, polarization is more strongly linked to SOM than CEC. We hypothesize that the weaker correlation between SOM and CEC highlights the contribution of poorly understood charge storage mechanisms within the polydisperse organic matrix that differ from polarization at mineral surfaces. Ongoing experiments with artificial soil mixtures of calcitic sand and varying fractions of peat, under controlled conditions (i.e. constant electrical conductivity of the pore fluid), will help to shed light on the controls behind our field-derived relationships. We expect that our combined field and laboratory investigations will provide insights into the petro-, or rather, organo-physical relationship between SOM and the imaginary conductivity, and thus contribute to a conceptualization of the underlying polarization mechanisms in organic matrices.

References

Katona, T., Gilfedder, B. S., Frei, S., Bücker, M., & Flores Orozco, A. (2021). High-resolution induced polarization imaging of biogeochemical carbon-turnover hot spots in a peatland. Biogeosciences, 18(13), 4039–4058.

Mellage, A., Zakai, G., Efrati, B., Pagel, H., & Schwartz, N. (2022). Paraquat sorption- and organic matter-induced modifications of soil spectral induced polarization (SIP) signals. Geophysical Journal International, 229(2), 1422–1433. https://doi.org/10.1093/gji/ggab531

Ponziani, M., Slob, E. C., Vanhala, H., & Ngan-Tillard, D. (2012). Influence of physical and chemical properties on the low-frequency complex conductivity of peat. Near Surface Geophysics, 10(6), 491–501. https://doi.org/10.3997/1873-0604.2011037

Schwartz, N., & Furman, A. (2014). On the spectral induced polarization signature of soil organic matter. Geophysical Journal International, 200(1), 589–595. https://doi.org/10.1093/gji/ggu410

How to cite: Strobel, C., Dörrich, M., Cirpka, O. A., Huisman, J. A., and Mellage, A.: Organic matter matters - The imaginary conductivity of sediments rich in solid organic carbon, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1838, https://doi.org/10.5194/egusphere-egu23-1838, 2023.

Access to top research equipment facilitates top research. However, the research equipment needed may not always be available within individual institutes, while access to external facilities may not in all cases be affordable. This restricts the research that any individual can do and hampers scientific breakthroughs, particularly across disciplines. To overcome this limitation, a collaborative infrastructure network was initiated: EPOS-NL (European Plate Observing System- Netherlands). EPOS-NL provides free-of-charge access to geophysical labs at Utrecht University and Delft University of Technology, both in the Netherlands, for research within rock physics, analogue modelling of tectonic processes, X-ray tomography and microscopy. These labs include capabilities for among others: A) Mechanical and transport testing at crustal stress, temperature and chemistry conditions; B) Analogue tectonic modelling, including dynamic model imaging in 2D and 3D; C) X-ray tomography at sub-µm resolution; and D) A correlative workflow for imaging and microchemical mapping, down to nm resolution. As such, these labs can provide you with the means and expertise for your research into the physical behavior of the Earth’s crust and upper mantle.

Access to EPOS-NL can be requested by applying to a bi-annual call, posted on www.EPOS-NL.nl. This involves submitting a short (1-2 page) research proposal. Research proposals are reviewed on the basis of feasibility and excellence, but generally have a high chance of success (~80% in previous rounds). Interested? Have a look on the EPOS-NL website – and apply!

How to cite: Wessels, R. and Pijnenburg, R.: Access for free: How to get free-of-charge access to Dutch Earth scientific research labs through EPOS-NL, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2834, https://doi.org/10.5194/egusphere-egu23-2834, 2023.

In a context of energy transition and water resources crisis, studying the fluid flow in the critical zone appears to be a major issue. The O-ZNS (Observatory of transfers in the Vadose Zone, Orleans, France) site has been designed for the development of innovative tools that can characterize and monitor the dynamics of the vadose zone (VZ). The geological structure of this VZ is composed mainly by a lacustrine limestone formation located between 10 and 20 m-deep, characterized by multiscale heterogeneities (facies variations, presence of cracks, fractures, pores, cavities and karstification). In order to predict fluid flow, heat transfer, and aquifer recharge through this VZ, the limestone heterogeneities have to be integrated into geological concepts and numerical models.

This study is a key part of the O-ZNS project, as it aims at (i) understanding and classifying the microstructural and petrophysical properties at laboratory scale; (ii) predicting these properties through quantitative geophysical parameters and; (iii) developing new geophysical interpretations through coupled approaches.

From well logs analysis of O-ZNS site, we collected limestone samples from four main facies (with four samples per facies). We performed a state-of-the-art petrophysical characterization including connected and total porosity, density, and permeability measurements. Then we carried out acoustic measurements on dry and water-saturated plugs (2.5 and 4 cm diameter) with P- and S-waves at two frequencies 0.5 and 1 MHz.

The measurement results show a large dispersion of the petrophysical properties. For example, connected porosity ranges from 4 to 12 %, and density from 2,3 to 2,5 g/cm³. This dispersion of petrophysical properties is interpreted in terms of heterogeneity of the type of porosity (micro to cm pore size, presence of cracks and fracture) and mineralogy. However, it appears that the deepest facies (located at the aquifer level) is more homogenous and shows the highest porosity. This is consistent with directly observed (micro)structure from 3D sample and well scans.

Acoustic velocity results show coherent values for fractured limestone rocks. The different facies show dispersion, such as Vp varying from 4950 to 5600 m/s for the shallowest facies at 9 m-deep. Here also, the deepest facies appears to be the most homogeneous with the lowest velocities (around 4875 m/s). Thus, velocities are consistent with the petrophysical measurements and one can draw a simple relationship between the porosity, density and acoustic velocities. However, other petroacoustic relationships are necessary to better discriminate between each facies and therefore predict their microstructure and transport properties.

The following step of this work is to add electric measurements and develop petro-acoustico-electrical models and enhance our capacity to upscale these properties from the laboratory to the field.

How to cite: Yacouba, A. N., Mallet, C., Deparis, J., Leroy, P., Laurent, G., Azaoural, M., and Jougnot, D.: Petroacoustic characterization of fractured and weathered limestone from the O-ZNS Critical Zone Observatory, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3097, https://doi.org/10.5194/egusphere-egu23-3097, 2023.

Fractures are ubiquitous throughout the Earth’s upper crust and represent localized zones of mechanical weakness as well as preferential pathways for fluid flow. Correspondingly, their detection and characterization is vital for a wide range of pertinent applications in geological, civil, and environmental engineering, hydrocarbon exploration, nuclear waste and carbon dioxide storage, as well as geothermal energy production. Particularly important mechanical characteristics of fractures are their normal and shear compliances, which relate the displacement perpendicular and parallel to the fracture plane, respectively, to the corresponding components of the prevailing stress tensor. Based on the linear slip model, previous works developed a phase delay method to estimate the normal compliance of individual fractures using the P-wave first-arrivals in full-waveform sonic (FWS) log data. This approach is viable for a quasi-normal incidence scenario of the sonic wavefield. However, the conditions under which this technique remains valid at oblique P- and S-wave incidence angles as well as the role played by the combined effects of the normal and shear compliances remains enigmatic. To alleviate this problem, we have extended the phase delay technique to allow for non-normally-incident P- and S-waves. In addition to improving the accuracy of the normal compliance estimates with respect to the results computed under a normal incidence assumption, this method allows for a simultaneous estimation of the normal and shear compliances. The proposed approach has been validated through analytical tests and numerical simulations of wave propagation in a hard-rock-type borehole environment intersected by a single fracture with dip angles of 0, 30, and 40 degrees with regard to the horizontal. For fracture compliance values typical of mesoscale fractures (10^-14 to 10^-12 m/Pa), the effects associated with oblique incidence become significant for dip angles larger than 50 and 30 degrees for P- and S-waves, respectively. However, our results also demonstrate that the normal incidence assumption can produce similar errors at even lower fracture dip angles in the presence of larger fracture compliance values and/or shear-to-normal compliance ratios. Finally, we apply the method to observed FWS data acquired in granitic rocks where the considered boreholes intersect fractures at a range of oblique angles. Direct in-situ estimates of compliances for discrete individual fractures are scarce, but essential to bridge the scale gap between laboratory estimates and input data for reservoir scale models. While recent studies show the feasibility of estimating normal compliances from FWS data, this study aims to explore whether and to what extent this approach can be practically extended to shear compliances and to the corresponding shear-to-normal compliance ratios.

How to cite: Zhou, Z., Caspari, E., Barbosa, N. D., Favino, M., and Holliger, K.: Estimation of normal and shear compliance for inclined fractures from full-waveform sonic log data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3347, https://doi.org/10.5194/egusphere-egu23-3347, 2023.

Field geological studies have revealed the heterogeneous structure of fault zones down to the sub-metric scale due to the juxtaposition of rocks presenting distinct deformation intensity and physical-transport properties. However, such internal variability is not generally resolved by most seismic tomography techniques due to spatial resolution limits. Quantifying the heterogeneous internal structure of fault zones is fundamental to understand their mechanical and hydrological characteristics. In this sense, determining seismic wave velocities and related physical properties (elastic moduli, porosity and fracture intensity) within fault zones, at different observational scales, is crucial.

Here, the near-surface velocity structure of two active seismogenic fault zones located in the Central Apennines of Italy was quantified at different length scales, from laboratory measurements of ultrasonic velocities (rock samples of few centimeters, 1 MHz source) to high-resolution first-arrival seismic tomography (spatial resolution of few meters). Detailed structural mapping was conducted within the Vado di Corno and Monte Marine fault zones, two NW-SE trending structures with length of ~ 15 km and up to 1.5 km of extensional displacement. Distinct structural units separated by fault strands were recognized in the fault zone footwall blocks cutting Mesozoic dolomitic carbonates: (i) fault core cataclastic units, (ii) breccia unit, (iii) high-strain damage zone, (iv) low-strain damage zone. The single units were systematically sampled along transects orthogonal to the average strike of the faults and characterized in the laboratory in terms of directional P and S ultrasonic wave velocities, porosity and microstructures. The fault core cataclastic units were significantly “slower” (V_P = 4.5±0.4 kms^-1, V_S = 2.7±0.2 kms^-1) compared to the damage zone units (V_P = 5.6±0.6 kms^-1, V_S = 3.2±0.3 kms^-1) at short length scales (i.e. few centimeters). A general negative correlation between ultrasonic velocity and porosity was observed, with some variability within the fault core mostly related to the textural maturity (clast/matrix volume ratio) of the fault rocks and the degree of pore space sealing by calcite cements.

Multiple P- and S-wave high-resolution seismic profiles (length 90-116 m, geophone spacing 1-1.5 m) were acquired across the two fault zones at different structural sites, moving from the principal fault surface into the outer damage zone. The derived first-arrival tomography models highlighted fault-bounded rock bodies with distinct velocities and characterized by geometries which well compared with those deduced from the structural mapping. At the larger length scale investigated by the active seismic survey, relatively “fast” fault core units (V_P ≤ 3.0 kms^-1, V_S ≤ 1.8 kms^-1) and very “slow” high-strain damage zones (V_P < 1.6 kms^-1, V_S < 1 kms^-1) were recognized. These velocity ranges were significantly different from those determined in the laboratory on small samples. This apparent discrepancy could be reconciled using an effective medium approach, considering the effect of mesoscale fractures density and size distributions affecting each structural unit.

This combined study highlighted the high petrophysical variability of carbonate-hosted fault zones, with structural units characterized by sharp contacts and different velocity scaling. In particular, the persistence of compliant high-strain damage zones at shallow depth might strongly affect near-surface deformation.

How to cite: Fondriest, M., Vassallo, M., Garambois, S., Mitchell, T. M., Giuseppe, D. G., Doan, M.-L., and Voisin, C.: The heterogeneous near-surface velocity structure of carbonate-hosted seismogenic fault zones investigated at different length scales: from ultrasonic measurements to subsurface seismic tomography, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3376, https://doi.org/10.5194/egusphere-egu23-3376, 2023.

Pore pressure has a major influence on the effective stress and thus on the mechanical behaviour and the physical (elastic and transport) properties of microcracked rocks. In the field, in-situ measurements of pore-pressure is difficult outside of local measurements around boreholes. Yet, fluid migration is observed ubiquitously in the continental crust, whether in fault zones or in volcanic geothermal areas. In particular, pore pressure perturbations change the effective stress, which may lead to microseismic activity. This may also occur in conventional reservoirs, the storage of CO₂ or deep geothermal energy extraction.

In this study, we focus -in the laboratory- on the hydro-mechanical behavior of thermally treated Westerly granite and naturally microcracked Etna basalt samples (40 mm in diameter and 80 mm in length). The goal is to determine the pore pressure distribution and diffusion laws under different pore pressure gradients. First, classical (constant flow method) permeability measurements under small pore pressure gradient (1 MPa over the length of the sample) were carried out as a function of increasing confining pressures Pc (up to 70 MPa). The results show that permeability of samples varies exponentially with effective pressure, which is expected for cracks-porous rocks. The pressure sensitivity factor for permeability was then deduced to be of the order of 0.011~0.057 MPa^-1.

In a second step, permeability was measured at high (70 MPa) confining pressure, under large pore-pressure gradients (up to 60 MPa). During this part of the experiments, pore pressure was measured along the sample using newly developed fluid pressure sensors (with an absolute accuracy of +/-1MPa). Under small pore pressure gradient (2.5 MPa), our results show that the pore pressure varies linearly over the length of the sample, as expected from Darcy’s law and a constant permeability. However, with increasing pore pressure gradient (up to 60 MPa), the linearity is lost, as the permeability can no longer be assumed constant along the sample.

To interpret our results, we solved the diffusion equation, assuming that permeability varies exponentially with effective pressure. For steady state flow conditions, our observations of the pore pressure distribution in the samples are consistent with the theoretical predictions. In particular, we show that the shape of the pore-pressure distribution at steady-sate does not depend on permeability itself, but rather on the permeability pressure sensitivity factor: the larger the latter, the more non-linear the pore pressure in the samples.

How to cite: lin, G., Chapman, S., Fortin, J., and Schubnel, A.: Fluid diffusion and pore-pressure distribution in microcracked rocks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3630, https://doi.org/10.5194/egusphere-egu23-3630, 2023.

Past metallurgical sites and deposits account for a significant proportion of potentially contaminated sites in the European Union (EU):  about 100,000 have been identified only in the North West regions of the EU. While recent wastes from sites still in operation are commonly recovered, this is not the case for old aggregated materials with a high content of ferrous (and other) metals, white and black slag, etc., which are considered to be sources of pollution and are costly to manage or dispose of. These sites could be considered as opportunities to recover large volumes of resources (metals, materials and land) using urban mining techniques if they were better characterized.

The induced polarization (IP) method is a geophysical method known to be sensitive to the presence of various metallic particles disseminated in the soil layers. If qualitative interpretation of the measured IP parameters in the field (i.e. resistivity and chargeability) are widespread, quantitative interpretation in terms of concentrations of different metallic particles is yet to be developed.

The example of the Pompey field site (FR), investigated as part of the NWE-REGENERATIS project (https://www.nweurope.eu/projects/project-search/nwe-regeneratis-regeneration-of-past-metallurgical-sites-and-deposits-through-innovative-circularity-for-raw-materials/), is used in this study to present the interest in using time domain IP (TDIP) field measurements to characterize metallurgical past deposits. Several paths are explored to convert resistivity and chargeability TDIP tomographies into quantitative interpretation of metallic element concentrations: (1) extraction of frequency data from TDIP field measurements; and (2) upscaling of lab results through numerical simulations.

Regarding (1), TDIP measurement were made with different time windows (different frequencies), giving us access to spectral IP (SIP) processing and interpretation at 5 frequencies. These new frequency interpretations of the TDIP can be compared to lab measurements and facilitate the upscaling of the found petrophysical relationships.

Regarding (2), in order to interpret the TDIP results in terms of concentration of metallic particles, known petrophysical relationships and geochemical measurements obtained at the lab scale need to be interpreted at the field scale. We propose to use a Bayesian framework for inferring field-scale metallic particles concentrations, taking into account heterogeneity and anisotropy within the inversion schemes. This work is ongoing.

For both (1) and (2), it is crucial to find the best petrophysical relationships linking the IP parameters to concentration and size of metallic particles. Wong (1979) developed a physics-based electrochemical model that is still used today. We further investigate the Wong model to explore the role of the background porous medium itself in determining the IP signature of disseminated metallic particles and discuss the sensitivity of the model to estimate metallic grains concentration.

All these different research paths lead to a better understanding of metallic particles IP signature at a small scale, as well as discussions on how to use these findings to better characterize and reevaluate past metallurgical sites and deposits.

This study was funded by the North West Europe (NWE) Interreg project called NWE-REGENERATIS that aims at the regeneration of past metallurgic sites and deposits through innovative circularity for raw materials, and by Schlumberger-Doll Research Center (USA, MA).

How to cite: Kessouri, P., Ryckebusch, C., Fernandez-Visentini, A., and Slater, L. D.: IP signature of metallic particles: lessons learnt from field and laboratory experiments, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4452, https://doi.org/10.5194/egusphere-egu23-4452, 2023.

Electrical resistivity tomography and electromagnetic inverse modelling are particularly useful to explore orogenic systems because the most important conductive components of rock masses are economically-significant minerals (semi-metals like graphite, and semi-conducting minerals like sulphides), as well as certain clays and permeating saline fluids. Despite the efficiency of electrical measurements, anisotropic properties of the crust, which affect almost all acquired data, may lead to serious misinterpretation of the subsurface geology if they are ignored during data analysis. Understanding the geological causes of electrical anisotropy and heterogeneity, and considering their influence in field-scale electrical measurements, can provide crucial information on the crustal architecture, pore fluid network, as well as revealing the internal structure of fault zones, and increasing the accuracy of location of critical mineral deposits. To this end, we aim to quantify the electrical properties of mid- to lower-crustal metamorphic and magmatic lithologies based on their micro- to macrostructures, conductive components and fluid contents as measured by laboratory methods. Our research also contributes to, and advances, the likely outcomes of the ICDP-supported project DIVE (Drilling the Ivrea-Verbano ZonE). DIVE is currently exploring the hidden portions of the continental lower crust and crust-to-mantle transition zone of the Ivrea-Verbano Zone (Western Alps, Italy) in two boreholes at the sites of Megolo (DT-1a) and Ornavasso (DT-1b), separated by  7 km distance in Val d’Ossola. The first DIVE borehole, DT-1b, was completed in December 2022, reaching a depth of 578.5 metres, and rock cores of metapelite, gneiss, amphibolite, migmatite, and pegmatite were recovered. Some drillcores contained a range of potentially conductive lithologies, including sulphide- and graphite-bearing metapelites. In this research we are measuring electrical conductivity on a representative benchmark suite of bedrock outcrop samples from the region around the DIVE boreholes at elevated pressure and temperature. We are currently characterising the microstructural arrangement and distribution of conductive phases within these samples by electron beam methods. To properly understand electrical property measurements of the natural samples we determine the contributions of each key conductive phase (graphite and sulphides). The bulk resistivity of a mixture of quartz+10% graphite, which was synthesized in a solid–medium piston-cylinder apparatus, at temperature of 22.5 °C and pressure of 0.5 GPa, was found to be 1 Ω.m. No change in bulk resistivity was observed with increasing temperature up to 1000 °C. We will present the results of additional tests to be undertaken between January and April 2023 at this conference. Our data will be employed in interpretation of wireline electrical logs and borehole-to-surface electrical surveys from DT-1a and DT-1b.

How to cite: Mansouri, H., Toy, V., Klimm, K., Bagdassarov, N., Pistone, M., Greenwood, A., and Hetényi, G.: Quantification of electrical properties of deep crustal rocks based on their mineral modal proportion, fabric, and pressure-temperature conditions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5997, https://doi.org/10.5194/egusphere-egu23-5997, 2023.

Clays are sedimentary minerals that are ubiquitous in the Earth’s continental crust. They have remarkable adsorption, catalytic and containment properties due to their high surface charge and very large specific surface area. However, their microstructural and electrochemical properties are not completely understood. In this study, we have developed a new petrophysical model to interpret laboratory spectral induced polarization measurements on kaolinite, illite and montmorillonite muds when salinity increases (from around 0.01 mol L^-1 to 1 mol L^-1 NaCl initially). Our model considers electrical conduction in the bulk and diffuse layer waters as well as polarization of the Stern layers of illite aggregates and Stern layers and interlayer spaces of Na-montmorillonite aggregates with different shapes and sizes. Maxwell-Wagner polarization was considered as well. By fitting predicted to measured SIP spectra, we found that the basal surface of clays controls Stern layer polarization and that the interlayer space of Na-montmorillonite may polarize in the mHz to kHz frequency range. Our study is a step forward to better understand the high surface conductivity response of clays inferred from resistivity and induced polarization measurements.

How to cite: Leroy, P., Maineult, A., Mendieta, A., and Jougnot, D.: A petrophysical model for the spectral induced polarization of clays, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6640, https://doi.org/10.5194/egusphere-egu23-6640, 2023.

Since the great paradigmatic revolution initiated by Mandelbrot, we know that fractals are ubiquitous in nature. From coastlines to plant growth, fractal mathematics help us to describe and quantify many of nature’s properties. In the same way, the fractal theory can be applied to porous and fractured media. In recent decades, numerous research studies have shown that fractal theory provides a solid framework to describe the properties of geological media. Based on advanced physical knowledge at the microscale, it is possible to use fractal patterns to describe transport properties in porous and fractured media. Fractal laws can be applied to describe the size distribution of pores and fractures, fracture widths, and pore irregularities, but also to relate these pore sizes to pore tortuosities. In this contribution, we review the significant advances that have been made in the field of petrophysics by applying fractal mathematics to describe fundamental petrophysical properties such as porosity, permeability, electrical conductivity, thermal conductivity, and electrokinetic and electroosmotic coupling coefficients. These new petrophysical models are based on the upscaling procedure applied to different fractal objects such as the Sierpinski carpet, Koch curves, Pigeon holes, and Menger sponge, among others. Among the interesting results obtained by means of fractal-based petrophysics, one can derive transport properties of saturated or partially saturated media, above and below freezing temperature, and considering hysteretic behavior and reactive media dissolution/precipitation processes. Integrating these fractal-based petrophysical relationships into the laboratory or field-scale, numerical simulations are now opening a wide range of potential avenues for progress in near-surface and reservoir geophysics.

How to cite: Jougnot, D., Guarracino, L., Soldi, M., Rembert, F., Luo, H., Solazzi, S., and Thanh, L. D.: Predicting transport properties in porous and fractured media, how fractal-based models can help petrophysicists?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6657, https://doi.org/10.5194/egusphere-egu23-6657, 2023.

Low-field nuclear magnetic resonance (NMR) is a minimally-invasive geophysical method often used to characterize pore spaces, water content, and fluid transport and distribution in geologic materials. NMR measurements are based on the magnetization and relaxation behavior of the spin magnetic moment of hydrogen atoms in external magnetic fields. These measurements can be taken in the field, such as from a borehole or the surface of the Earth, or in the laboratory using a bench-top apparatus. Numerical simulations of NMR signals are great tools to better understand the relaxation behavior of pore water under different scenarios, explore the effect of changes in the composition or geochemical characteristics of the geologic material, verify experimental findings, and improve the interpretation of field measurements. They can also be used to examine situations where traditional interpretation of NMR signals fails, such as in complex, heterogeneous geometries with pore coupling effects. In a pore coupled system, significant magnetization exchange between pores of different sizes occurs during the measurement time, which makes it difficult to independently characterize the pore environments. Using numerical simulations, it is possible to explore the factors that control pore coupling, such as surface relaxivity, pore-network connectivity and other pore-network characteristics, can be explored independently in a controlled setting. In this work, we introduce common numerical modelling approaches used for simulating NMR responses in geologic materials, along with their limitations and traditional workflows. We present two specific examples: a Random Walk (RW) simulation to test the effect of different pore-network connectivity features on pore coupling in a simplified pore geometry, and a Finite Element Method (FEM) simulation approach to visualize the distribution of magnetization density within a single pore. NMR is a promising hydrogeophysics tool gaining popularity and finding new applications for near-surface exploration. A better understanding of the NMR signals in diverse and complex scenarios is essential for the adequate design of experiments and field campaigns and for the correct interpretation of NMR measurements at different scales. The use of numerical modelling strategies can help improve this understanding, leading to more accurate and reliable measurements and interpretations.

How to cite: Soto Bravo, F., Zhang, C., and Jia, L.: Reviewing numerical simulation methods of nuclear magnetic resonance signals in porous media., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6941, https://doi.org/10.5194/egusphere-egu23-6941, 2023.

Interactions between mineral phases and fluids in the subsurface inevitably lead to mineral precipitation reactions and dissolution. While these processes are the major drive behind many geochemical changes in aquifer systems, their detection, monitoring and characterization is difficult. Geoelectrical methods provide potential to investigate precipitation and dissolution reactions in rocks non-invasively. However, with the measurement of the DC electrical conductivity alone, changes in pore water salinity, mineralogy, or pore space characteristics can hardly be differentiated. The ambiguity in the identification of these processes can be reduced by also measuring the spectral induced polarization (SIP) response, i.e., the frequency-dependent complex electrical conductivity, given the sensitivity of especially the imaginary component to textural and chemical characteristics. In order to assess the capability of this approach, we conducted multiple laboratory experiments on quartz-rich sandstone samples in which different precipitation scenarios were provoked under controlled conditions while monitored with SIP. The experimental setup consists of two reactant solutions in contact with both sides of the sample, leading to a reaction within the sample as diffusion from each side into the rock goes on. We used reactant solutions of NaHCO₃and CaCl₂in varying molality, the mixing of which in the sample’s pore space results in CaCO₃formation. By varying samples and solutions, three different components contributing to the complex conductivity response during the ongoing precipitation could be identified. The onset of the chemical reaction is clearly visible in the temporal evolution of imaginary conductivity at relatively low frequencies. The observed temporal peak can be associated with changes in the pH value due to the infiltration of the reactant at earlier times and the reduction in pH with calcite precipitation. This explanation is supported by additional experiments performed on a similar sample, where pH was altered by infiltration of NaHCO₃only. A second spectral high-frequency peak shows up at later stages of the experiments, suggesting that here the main changes of the pore surfaces in response to the precipitation are occurring. This phenomenon could not be recreated by using the infiltration of a pure electrolyte solution or the infiltration of NaHCO₃. The last component in the complex conductivity response is the continuous increase of the real component due to the increasing salinity of the pore water, which also could be reproduced in comparative measurements. Our results show the potential of complex conductivity measurements for precipitation monitoring in rocks, including improved textural and chemical characterization. Given the applicability of complex conductivity imaging at the field scale, the method thus holds promise for monitoring tasks in the context of, for example, carbon capture and storage, enhanced geothermal energy, soil stabilization, and capture of dissolved contaminants, which are of increasing societal relevance.

How to cite: Mansfeld, A. M. and Kemna, A.: Impact of chemical subprocesses during calcite precipitation in sandstones on the measured SIP response and their identification, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7902, https://doi.org/10.5194/egusphere-egu23-7902, 2023.

Oil saturation is important for shale reservior to identify favorable sections and mapping the geological sweet spot. Current oil saturation evaluation methods, including experiments and empirical formulas, are not suitable for shale reservoir because of the complex mineral, fluid components and pore structure characteristics. To establish the shale oil saturation calculation model, X-ray diffraction, one-dimension and two-dimension nuclear magnetic resonance (NMR), and oil-water two-phase displacement experiments were employed on shale samples collected in the upper sub-member of the fourth member of the Eocene Shahejie Formation in the Dongying sag, Jiyang Depression, Bohai Bay Basin. After data analysis, the reason for whether oil is produced in the displacement experiments were explained, distribution characteristics of different shale components in the NMR T1-T2 map were analyzed, and a new shale oil saturation calculation method was proposed using NMR T2 sensitive parameters that reflected the changes of NMR T2 spectrum morphological characteristics with different oil saturation calibrated by NMR T1-T2 map at different displacement stage. The results indicated that the pore structure of shale samples is complex and show strong heterogeneity according to the NMR T2 spectrum, and the distribution of shale pore size is the main factor determining whether there is oil in the volumetric cylinder in the displacement experiment under the premise of the slight difference of wettability. NMR T1-T2 map is an effective way to identify different components (kerogen and solid bitumen, adsorbed oil, free oil, structural and adsorbed water, free water) of shale samples, and usually, kerogen and solid bitumen distributed in the top left of the T1-T2 map with T1>10 ms, T2<0.1 ms. Based on this, T2 threshold for free oil and adsorbed oil are 2 and 0.2 ms, and the corresponding threshold of pore radius are 40 and 4 nm according to the NMR theory. As NMR T2 spectrum sensitive parameters, geometric mean and interval porosity corresponding to the first peak are positively and negatively correlated with oil saturation respectively. With understanding this, oil saturation calculation method is established using the above two parameters and the Root Mean Square Error (RESM) between the measure oil saturation and the calculated results is 5.78%, which reflecting the accuracy and validity of the method. In general, this method allows the shale oil saturation to be accurately calculated and provides a parameter basis for the determination of favorable sections and evaluation of resource of shale oil reservoir. Moreover, it also offers a new idea for the oil saturation predication by NMR logging.

How to cite: Zhang, S., Cai, J., Yan, J., Zhu, X., and Wang, M.: Oil saturation quantitative evaluation in lacustrine shale: A novel insight from NMR T1-T2 experiments, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8629, https://doi.org/10.5194/egusphere-egu23-8629, 2023.

The seismic characterization of fractured geological formations is of importance for a wide range of applications throughout the Earth, environmental and engineering sciences, such as, for example, hydrocarbon exploration and production, CO­­₂ sequestration, monitoring of enhanced geothermal reservoirs, nuclear waste storage, and tunneling operations. Seismic methods are indirect in nature, and, hence, comprehensive modelling techniques are required to translate corresponding observations into rock physical properties. In this regard, numerous works have employed the theoretical framework of poroelasticity in order to explore the seismic response of particularly complex and elusive parameters of fluid-saturated fracture networks, such as their fracture density and interconnectivity. This is motivated by the fact that poroelasticity allows to account for fluid pressure diffusion effects between connected fractures as well as between fractures and their embedding background. Fluid pressure diffusion prevails when zones of contrasting compliance are traversed by a seismic wave, as this results in pressure gradients, which induce oscillatory fluid flow and, consequently, energy dissipation. This form of energy dissipation has a significant impact on seismic velocity dispersion, attenuation, and anisotropic characteristics, which are key seismic observables. While a wide range of approximations are employed to represent fracture properties in order to compute the seismic response of formations, they do tend to inherently ignore the complex interrelationships between the lengths, compliances, apertures, and permeabilities of fractures remains, as of yet, unaccounted for. In this work, we seek to alleviate this in combination with a poroelastic modelling approach to explore how length-dependent fracture scaling characteristics affect the effective seismic properties of fractured rocks. We start by revisiting canonical models with two orthogonally intersecting fractures of different lengths to analyze the interactions occurring when fractures are affected by a seismic wavefield. We then proceed to explore how scaling relations affect these results. Finally, we consider fracture networks with realistic stochastic length distributions, for which we compare the effective seismic response with and without the proposed length-dependent scaling of the fracture characteristics. Our results demonstrate that the scaling of fracture properties does indeed have a significant effect on the seismic response, as it dramatically reduces the contribution of smaller fractures to fluid pressure diffusion between connected fractures, which, in turn, affects the overall seismic characteristics of the formation.

How to cite: Quiroga, G., Solazzi, S., Barbosa, N., Rubino, J. G., Favino, M., and Holliger, K.: Effective seismic properties of fractured rocks: the role played by fracture scaling characteristics, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8860, https://doi.org/10.5194/egusphere-egu23-8860, 2023.

Quantifying in-situ stress is crucial for predicting drilling-induced tensile fractures, wellbore failures, proper well placement, hydro-fracture treatment optimization and sand production. A comprehensive mechanical earth model incorporating pore pressure, stress state, and rock mechanical properties enable us to study the cause of failure observed in the well. The study is focused on a petroleum reservoir in Western Offshore India. In this study, an attempt is made to estimate in-situ stresses present in the field. Well-log data calibrated with available direct pressure measurements viz. Modular Dynamic Test (MDT) and Leak Off Test (LOT) data are used to predict the pore pressure and minimum horizontal stress. Vertical stress is estimated by extrapolating the density log; for minimum and maximum horizontal stress, the poroelastic approach is adopted. Key rock strength parameters were estimated using standard correlations and regional studies. Wellbore stability analysis was carried out, and the results were calibrated with the actual mud weight used. Natural fractures present in the reservoir are sensitive to stress distribution which in turn is sensitive to changes in pore pressure distribution. Many exploratory and development wells have been drilled in the area, but very few have recorded DSI (Dipole Shear Sonic Image) and FMI (Formation Micro-Scanner Image) logs. With the available log data, the study was carried out to quantify the rock mechanical parameters and the stress magnitudes of the field. The study aims to model the study area's geomechanical aspect for better prediction of drilling-induced challenges, thereby reducing NPT and optimizing drainage.

How to cite: Pradhan, S. P. and Sundli, K. C.: Mechanical Earth Modelling for Petroleum Reservoir in Western Offshore India: Tensile Failure Study, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11037, https://doi.org/10.5194/egusphere-egu23-11037, 2023.

Seismic wave velocities (both P and S waves) measurements have been carried out for carbonate rock samples collected from Lesser Himalayan deposits exposed along Alaknanda valley between Rudraprayag to Helang village in Uttarakhand, India. This study has been carried out to evaluate the effect of the petrophysical and the mechanical properties of rocks on seismic wave velocities. On the core samples, petrophysical and mechanical measurements were performed where porosity, density, water absorption, and seismic wave velocities were first determined, followed by measuring the uniaxial compressive strength (UCS), and Brazilian tensile strength (BTS). Thin sections were prepared to measure the petrographic parameters (textural properties and mineralogical composition). This study focuses mainly on grain size and mineral composition. Petrographic investigation and X-ray diffraction (XRD) analysis were done to identify their mineralogy. Both petrographic and XRD analysis revealed that the main constituting minerals are dolomite, and in minor amounts, calcite, quartz, and opaque minerals are present. Interrelationships between seismic wave velocities and porosity, density, mineral constituents, grain size, uniaxial compressive strength, and Brazilian tensile strength were obtained using regression analysis. It has been concluded that there are significant positive correlations between compressional wave velocity and uniaxial compressive strength (r² = 0.82), Brazilian tensile strength (r² = 0.67). Similarly, strong to moderate correlations were found between shear wave velocity and uniaxial compressive strength (r² = 0.73), Brazilian tensile strength (r² = 0.68). Weak to moderate negative correlations were found between seismic wave velocities and porosity. Moderate positive correlations have been found between seismic wave velocities and dry density. There is moderate negative correlation has been found between uniaxial compressive strength and grain size. Furthermore, it has also been concluded that the influence of grain size on rock strength was more important than mineral content.

How to cite: Sahu, A., Singh, S., Kumar Samadhiya, N., and Joshi, A.: Correlating Seismic Wave Velocities with the Physicomechanical Properties of Carbonate Rocks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16816, https://doi.org/10.5194/egusphere-egu23-16816, 2023.

A workflow is presented to estimate the size of a representative elementary volume and 3-D hydraulic conductivity tensor based on the discrete fracture network (DFN) fluid flow analysis through the case study performed for a granitic rock mass near the low and intermediate level radioactive waste disposal site in southeastern Korea. Intensity and size of joints were calibrated using the first invariant of fracture tensor for the 2-D DFN of the study area. Effective hydraulic apertures were obtained by analyzing the results of field packer tests. The representative elementary volume of the 2-D DFN was determined to be 20m square by investigating the variations in the directional hydraulic conductivity for blocks of different sizes. The directional hydraulic conductivities calculated from the 2-D DFN exhibited strong anisotropy related to hydraulic behaviors of the study area. The 3-D hydraulic conductivity tensor for the fractured rock mass of the study area was estimated from the directional block conductivities of the 2-D DFN blocks generated for various directions in 3-D. The orientations of the principal components of the 3-D hydraulic conductivity tensor were found to be identical to those of the delineated joint sets in the study area.

How to cite: Um, J.-G. and Bae, J.: Estimation of 3-D hydraulic conductivity tensor for a granitic rock mass near the low and intermediate level radioactive waste disposal site in Korea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1696, https://doi.org/10.5194/egusphere-egu23-1696, 2023.

Characterizing fluid transport and pore pressure diffusion is key for understanding and monitoring many natural (e.g., seismically active zones and volcanic systems) and engineered environments (e.g., enhanced geothermal reservoirs and CO₂ underground storage). Borehole hydraulic testing allows to infer relevant properties of the probed sub-surface volume, such as, for example, its transmissivity and diffusivity, for assessing the governing flow regime as well as for detecting the presence of hydraulic boundaries. Periodic hydraulic tests (PHT) achieve these objectives using a time-harmonic fluid injection procedure while measuring the fluid pressure response in monitoring boreholes. The relevant information on the pressure diffusion process occurring in the probed formation is retrieved from the phase shifts and amplitude ratios between the injected flow rate and the interval pressure. In general, the interpretation of PHT data relies on the assumption that the pressure diffusion process is uncoupled from the solid deformation of the probed rock volume. We present a poroelastic numerical approach to investigate the role played by hydromechanical coupling (HMC) effects during PHT and to assess whether and to what extent additional mechanical information can be extracted from these tests. We focus on (i) the influence of the borehole wall deformation on the wellbore storage coefficient S_w, which quantifies the difference between the injected flow rates and those actually entering the porous formation; and on (ii) the HMC effects associated with the presence of fractures in the formation. Following the commonly taken approach, we also interpret the synthetic data from the numerical poroelastic approach using the uncoupled diffusion solution. For different rock physical properties, we demonstrate that, in homogeneous formations, the uncoupled diffusion solution reproduces the poroelastic results. In this scenario, neglecting the effect of the deformation of the borehole wall on S_w upon injection can lead to an underestimation of both the transmissivity and diffusivity, which becomes worse for shorter oscillation periods. We also show that the effective values of S_w depend on the shear modulus of the formation and do not change with the oscillatory period. Based on this evidence, we present a methodology to obtain the effective S_w along with the hydraulic properties using observations at various oscillatory periods. Next, we consider formations containing hydraulically open and compliant fractures intersecting the borehole perpendicularly. Here, a single uncoupled diffusion model is not able to fully describe the poroelastic response of the medium at different periods. Furthermore, the presence of fractures significantly affects the effective value of S_w: it increases with respect to the one associated with the intact homogeneous rock, and the HMC effects associated with the compressibility contrast in the formation result in a period dependence of S_w. The characteristic period of the latter is primarily related to the diffusivity and size of the fractures. This result is particularly relevant for the planning and interpretation of monitoring experiments, in which the mechanical properties of the formation are expected to evolve, such as, for example, hydraulic stimulation procedures, seismic and/or volcanic regions, and injection of wastewater and CO₂ for subsurface storage.

How to cite: Barbosa, N., Müller, T., Favino, M., and Holliger, K.: Poroelastic modeling of borehole-based periodic hydraulic tests in non-fractured and fractured porous rocks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4952, https://doi.org/10.5194/egusphere-egu23-4952, 2023.

Measurements of permeability at high-pressure and high-temperature are critical to model and understand the behaviour and evolution of geothermal systems. To perform such measurements and provide constraints on the permeability of crustal rocks, we designed and tested a new apparatus.

Our high-pressure, high-temperature permeameter consists of three independent parts: the permeant gas circuit, the confining fluid circuit, and the heating element. For each measurement, a cylindrical sample is placed between the up- and downstream platens, into an annular Viton jacket which is secured within the pressure vessel. A confining pressure can be applied to the sample by filling the void space between the vessel and jacket through the inlet with kerosene. The confining pressure can be increased up to 50 MPa using a high-pressure hand pump. The temperature of the system can then be increased from room-temperature to up to 150 °C using a heating mantle wrapped around the pressure vessel and connected to a control box. After the confining pressure and temperature have been applied to the system, the permeability measurement is performed by flowing nitrogen (the permeant gas) through the sample while monitoring the pressure differential between the upstream pressure transducer and atmospheric pressure downstream of the sample at different volumetric flow rates (the steady-state method), measured using the downstream flowmeter.

Using this new experimental apparatus, the permeability of Lanhélin granite (from France) samples were measured. Cylindrical samples were prepared and thermally stressed (heated to 700 °C) to ensure that their permeabilities lie in the range that can be measured in our set-up (> 10^-18 m²). Permeability measurements were then performed under confining pressures of 2, 5, 10, 15, 20, 30, 40, and 50 MPa at room temperature, 50, and 100 °C. Our results provide the evolution of the permeability of microfractured granite in various pressure and temperature conditions, which will serve to inform numerical modelling designed to explore the influence of in-situ conditions on fluid flow within a fractured geothermal reservoir.

How to cite: Carbillet, L., Heap, M., and Baud, P.: The evolution of permeability with pressure and temperature in microfractured granite, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5623, https://doi.org/10.5194/egusphere-egu23-5623, 2023.

The study of fault damage zones is key to the understanding of fault-related fluid flow in the upper crust with many applications, including groundwater and hydrocarbon exploration, and underground storage of CO₂ and H. Many studies reveal that a relationship exists between fault damage zone width and net fault displacement. Despite this positive relationship, several factors such as the tectonic setting, the depth of deformation, the deformation mechanisms, and the evolving mechanical properties of fault rocks affect damage zone characteristics (e.g., width, asymmetry, fracture attitude, deformation intensity). Furthermore, recent studies show that the overall along-strike fault geometry may play a pivotal role in controlling damage zone characteristics. In particular, areas such as tip regions, linkage sectors, relay ramps and step-overs can be characterised by fault damage zone parameters markedly different from sectors away from these structural complexities. In this contribution, we present new structural data of fault damage zone parameters acquired along the 8 km long extensional Kornos-Aghios Ioannis Fault (KAIF) on Lemnos Island, North Aegean Sea, Greece. The KAIF deforms lower Miocene effusive and hypabyssal magmatic rocks and middle Eocene to lower Miocene turbidites. Deformed rock volumes along the KAIF are locally strongly altered by hydrothermal fluids (e.g., hydrothermal silicification). We provide a detailed characterization of the KAIF in terms of 2D fault geometry (mapped at 1:1000 scale) and kinematics and we present a characterization of fault damage zone parameters, including frequency and attitude of subsidiary fault-related fractures, in different fault sectors. The acquired data allowed us to define the boundaries of fault damage zones in the different sectors and to discuss the differences in terms of fracture attributes in linking- and tip-damage zones compared to damage zones away from these structural complexities. Our results show that fault damage zones in linkage and tip sectors are wider and that fault-related fractures are more clustered around several subsidiary faults with centimetre- to metre-offsets. Also, secondary fractures in linkage and tip sectors are less systematically oriented, thus increasing fracture network connectivity and, consequently, facilitating fluid mobility in structurally complex fault sectors.

How to cite: Berio, L. R., Balsamo, F., Pizzati, M., Storti, F., Curzi, M., and Viola, G.: Along-strike fault geometry controls damage zone parameters: the case of the Kornos-Aghios Ioannis Extensional Fault (Lemnos Island, NE Greece), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5675, https://doi.org/10.5194/egusphere-egu23-5675, 2023.

Fluid flow within low permeable reservoirs such as carbonates is primarily controlled by faults, fractures and other structural networks which can be defined through properties such as intensity, connectivity and aperture. These properties can vary not only among individual fractures but also between scales which can influence uncertainties within permeability calculations and fluid flow simulations. Therefore, understanding the interactions and variations within these networks is fundamental to deriving properties such as permeability and characterising fluid flow through naturally fractured reservoirs.

Determining fracture network properties of reservoirs can be undertaken using several methods across different scales from both surface and subsurface sources. However, where subsurface data is limited (e.g., within the geothermal reservoirs of Northern Bavaria), outcrop analogues become vital for obtaining the important information required for characterising fracture networks. Outcrops such as quarry sections can be imaged and scanned using both 2D and 3D photogrammetry techniques, from which fault and fracture networks can be detected and analysed. Previous work has presented a method to upscale fracture networks to 2D permeability tensors from outcrop sections through independently assigning properties to individual fractures within the networks. However, upscaling the networks to larger scales can lead to uncertainties due to variations within the modelled fracture networks. It its therefore important to understand the how the permeability tensor varies between scales and dimensions to reduce upscaling uncertainties.

Using examples from multiple outcrops within the Franconian Basin, Germany, we present an improved workflow to derive the tensors between dimensions and an investigation of the relationships among fracture networks at different scales. This will show the effect on permeability within geothermal reservoirs in the region and how to reduce the uncertainty in upscaling outcrops to subsurface reservoir scale.

How to cite: Smith, R., Prabhakaran, R., and Koehn, D.: Investigating scale variations in outcrop derived permeability tensors and the effect on geothermal fluid in upscaling naturally fractured reservoirs, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6756, https://doi.org/10.5194/egusphere-egu23-6756, 2023.

The investigation of hydro-mechanical properties in rock formations is of utmost importance for several geological and engineering applications, e.g. for carbon-dioxide or hydrogen underground storage, exploitation of groundwater, geothermal or oil and gas reservoirs, hydrothermal ore deposits, and the mechanics of earthquakes. In particular, modeling fluid flow into networks of discontinuities (i.e. faults and fractures) is a key task in all these studies. Due to the high complexity of such processes, involving a significant number of feedbacks and occurring at different spatial and time scales, the achievement of a satisfying representation of the physical problem remains a challenge.

In the last decades, a variety of modeling approaches have been proposed in literature, accounting for different orders of complexity and using several computational methods. Analytical solutions are commonly based on simplistic assumptions about the process, allowing for simple fracture geometries and/or implying incompressible Newtonian fluids; as well as about the medium, considered elastic and permeable (or impermeable). Nevertheless, these solutions are still widely employed, as they provide significantly quick, first-order solutions compared to more sophisticated approaches.

On the other hand numerical models, typically accounting for a higher number of parameters and concurrent effects, are expected to return more realistic solutions. Those based on Finite Element Methods (FEMs) or similarly discretized domains, for example, permit to model the fractured rock mass with information that can be inferred from geological surveys and geophysical techniques.

As anticipated, important limitations in the use of more advanced modeling approaches could be the computing time and model size or resolution, not always allowing for cost-efficient solutions. In this study, we aim at a comprehensive review and benchmarking of the main classes of existing methods, comparing their results obtained for an identical dataset. In this way, we are able to highlight the advantages and disadvantages of each technique, defining the differences in accuracy and the ranges of applicability of these methods. 

The outcomes of our work are intended as a cross-benchmarking among available models, as well as a starting point for the future development of novel improved techniques in the field of fluid flows dynamics in networks of discontinuities.

How to cite: De Paolo, E., Bistacchi, A., Casiraghi, S., and La Valle, F.: Comparative analysis of analytical and numerical solutions for hydraulic properties upscaling in fractured media, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6830, https://doi.org/10.5194/egusphere-egu23-6830, 2023.

The Hengill region is one of the largest areas of high geothermal gradient and subsurface heat flow in Iceland, and hosts two of its largest geothermal power plants: Hellisheiði and Nesjavellir, which have a combined capacity of 423 MWe and 560 MWth. The Hengill region is located in a unique tectonic setting, characterized by the convergence of three plate boundary segments: the oblique-spreading Reykjanes Peninsula (RP), the orthogonal-spreading rift of the Western Volcanic Zone (WVZ) and the transform, South Iceland Seismic Zone (SISZ). Unlike most tectonic triple junctions, which occur on the ocean floor, the Hengill Triple Junction (HTJ) is exposed above sea level, thus providing a unique opportunity to study the interplay between three plate boundary segments and the local deformation processes occurring at their convergence site. Additionally, the injection of fluids due to on-going geothermal operations enhances the natural tendency of the region for seismic activity, and results in a significant level of induced seismic hazard. Because slip on pre-existing faults is triggered when the applied shear stress surpasses the frictional strength of the fault, regions that are naturally subjected to higher shear stresses are more prone to fault re-activation due to fluid re-injection. Therefore, a spatial variation in tectonic stresses may result in varying induced seismicity potential within a region.

The local interplay of the three converging tectonic regimes, and their effect on the stress fields within the triple junction region, has been examined through a combination of regional structural mapping and a numerical model of the plate boundary interactions using the Boundary Element Method (BEM). Large scale structural mapping and analytical models of oblique rifting were used to estimate the degree of rift obliquity for individual fissure swarms. Our results reveal that the transition from the highly oblique rift system of the RP towards the spreading-orthogonal rift of the WVZ is smooth, and manifests as a rotation of the trend of fissures and eruptive ridges, and the strike of normal faults formed in response to the local stress field developed in the HTJ. These results were then correlated with those from the BEM model, which allows us to predict the orientation, relative magnitude, and distribution of stresses within the study area. Finally, the shear stress distribution determined from the BEM model was plotted against the location of both natural and induced seismic events detected in the region over a time span of 26 months, by the COSEISMIQ project (Grigoli et al., 2022). Our results show that seismic events cluster in either at the triple junction or SW of it, within the highly stressed regions of the RP. Furthermore, the seismicity transitions from scattered to non-existent towards the north of the region, where shear stresses also diffuse. The good correlation between the high shear stress regions predicted by the model and distribution of seismicity suggests that this approach may provide a valuable and cost-effective tool for seismic hazard prediction within regions with complex tectonic settings.

How to cite: Stanton-Yonge, A., Mitchell, T., Meredith, P., Gunnarsdóttir, S., Ósk Snæbjörnsdóttir, S., and Hjorleifsdottir, V.: Structural patterns and states of stress at the Hengill Triple Junction, SW Iceland: implications for fluid-injection induced seismic hazard, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7330, https://doi.org/10.5194/egusphere-egu23-7330, 2023.

Rising demand for energy and green energy has led to increasing subsurface activities, such as geothermal energy sites. These increasing human activities in the subsurface have caused substantial induced earthquakes in more densely populated areas, increasing the risks of operating safely. Well-known examples of induced seismicity, due to geothermal sites, are the M5.4 earthquake in Pohang (South Korea) or the M3.4 earthquake in Basel (Switzerland).  

Monitoring and forecasting earthquakes have been a topic of interest for years. Predictions are often made by production scenarios, probabilistic models, or average earthquake size distribution (b-value). Only a few studies focus on predicting fluid-induced seismicity by using seismic monitoring methods. Pore fluid changes play an important role in the reactivation of the fault strength and stability. Variations in pore pressure can cause a drop in the stresses along the fault plane and cause fault instability and movement resulting in induced seismicity.  Monitoring and predicting the stress changes along the fault planes can therefore be essential in forecasting induced seismicity and mitigation, potentially reducing the risks of operating (in denser populated areas). However, monitoring the degree of these changes remains challenging. Most studies using seismic methods to monitor induced seismicity on a field scale or laboratory scale focus on either passive monitoring or active monitoring. This study combines the two methods and shows how they complement each other in monitoring and mitigation of fault reactivation in the laboratory. We have performed pore fluid injection experiments on faulted sandstones to reactivate the faults while monitoring both actively (active seismic) and passively (acoustic emission).

These results show that both acoustic monitoring techniques can be used to detect the different fault reactivation stages: linear strain build-up, early creep (pre-slip), stress drop (main slip), and continuous sliding phase. However, using active monitoring the early creep phase is detected slightly earlier than using passive monitoring. Combining the methods shows that the stress changes along the fault can be detected with more detail in more accuracy. As a result, the combination of passive and active techniques may be useful for monitoring faulted or critically stressed reservoirs that experience pore pressure changes.

How to cite: Veltmeijer, A., Naderloo, M., and Barnhoorn, A.: Seismic monitoring of laboratory fault reactivation by pore fluid injection, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7596, https://doi.org/10.5194/egusphere-egu23-7596, 2023.

Hydrochemical stimulation by acidification of geothermal wells is a standard procedure to remove drilling mud and to improve hydraulic contact between borehole and reservoir. Several successive stimulations using hydrochloric acid were monitored by both online measurement and conventional analysis. The results show recovery curves with distinct two-step exponential temporal decrease of the chloride concentration. The initial decrease is fast, representing water and acid flow along pathways which are very well connected to the borehole. After the fluid from these flow paths has been recovered, the concentration decreases at a lower rate. This can be attributed to water flowing in less well connected flow paths. With additional stimulations the chloride concentration curve approaches a mono-exponential decrease. This indicates that the flow paths within the reach of the stimulation get more homogeneous.

A numerical model of flow and solute transport in the borehole and the surrounding geothermal reservoir was developed in order to simulate the observed chloride-recovery behaviour in the course of a number of successive hydrochemical stimulations. The finite-element model was adapted to match the observed hydraulic and hydrochemical data range.

Simulation hereby allows to separate time-dependent single contributions from the different flow paths to the total recovery concentration. Based on this information, indications of structural change due to the successive acidification steps can be derived from the chloride-recovery curves of each step. Furthermore, for typical settings the minimum time and volume of solution can be estimated which is required to achieve a significant structural signal

The derived structural information can be useful to predict the long-term behaviour of a geothermal injection well which during operation is exposed to a mild but constant chemical stimulation by the injected cold and, with respect to chloride in the rock matrix, undersaturated water. 

How to cite: Bartels, J., Schätzl, P., and Baumann, T.: Hydrochemical Stimulation in Fractured Carbonate Rocks - Monitoring and Simulation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8016, https://doi.org/10.5194/egusphere-egu23-8016, 2023.

Hydraulic shearing of natural fractures or fault zones is a key mechanism for enhancing permeability in engineered geothermal systems (EGS) in order to extract geothermal energy from crystalline basement rocks. Shear reactivation is achieved by hydraulic stimulation in an injection borehole, involving a complex hydro-seismo-mechanical response of fractured crystalline rock. A major challenge is to predict which fractures are reactivated at which reactivation pressures, in order to efficiently design the injection protocols and create a large fracture network for sufficient fluid circulation and heat exchange.

The Bedretto Underground Laboratory for Geosciences and Geoenergies (BedrettoLab) in Switzerland serves as an in situ test-bed where meso-scale hydraulic stimulation experiments are conducted to better bridge the knowledge gap between laboratory scale experiments and complex reservoir scale processes (Ma et al. 2022). The BedrettoLab is located in a 100 m long enlarged section of the Bedretto tunnel (Ticino, Switzerland), with an overburden of more than 1000 m of granite. Several characterization, monitoring, and two stimulation boreholes were drilled. One of the stimulation boreholes (referred to as ST1) is 400 m long, 45°-dipping, and was equipped with a multi-packer system that partitions the borehole into 15 intervals. Before conducting two multi-stage hydraulic stimulation phases in borehole ST1, the rock volume was characterized with various geophysical logging tools, hydraulic tests, and mini-frac tests for stress measurements (Bröker and Ma 2022, Ma et al. 2022).

Along the stimulation borehole, we mapped multiple clusters of sub-parallel pre-existing open fractures and fault zones that are preferentially oriented for reactivation in the estimated stress field. In this work, we compare our preceding probabilistic slip tendency and reactivation pressure estimates with the results from hydraulic stimulation experiments. The interval pressure and flowrate data from the stimulations reveal a reactivation of the natural fractures associated with an increase in injectivity. A comparison of the expected stress field around the stimulation interval with the observed reactivation pressure indicates that the fractures were likely reactivated by hydraulic shearing. The observed reactivation pressures are in the range of the preceding estimates, but a precise estimation is challenging due to the large number of input parameters, i.e. stress magnitudes and orientation, fracture orientation, pore pressure, coefficient of friction, and their uncertainties.

References:

Bröker, K., & Ma, X. (2022). Estimating the Least Principal Stress in a Granitic Rock Mass: Systematic Mini-Frac Tests and Elaborated Pressure Transient Analysis. Rock Mechanics and Rock Engineering. https://doi.org/10.1007/s00603-021-02743-1

Ma, X., Hertrich, M., Amann, F., Bröker, K., Gholizadeh Doonechaly, N., Gischig, V., Hochreutener, R., Kästli, P., Krietsch, H., Marti, M., Nägeli, B., Nejati, M., Obermann, A., Plenkers, K., Rinaldi, A. P., Shakas, A., Villiger, L., Wenning, Q., Zappone, A., … Giardini, D. (2022). Multi-disciplinary characterizations of the BedrettoLab -- a new underground geoscience research facility. Solid Earth, 13(2), 301–322. https://doi.org/10.5194/se-13-301-2022

How to cite: Bröker, K., Ma, X., Stadler, D., Doonechaly, N. G., Hertrich, M., and Giardini, D. and the Bedretto Team: Slip tendency and reactivation pressure prediction of natural fractures at the Bedretto Underground Laboratory, Switzerland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8221, https://doi.org/10.5194/egusphere-egu23-8221, 2023.

Changes in stress regimes impact the geometry of fracture networks and affect the porosity and permeability of carbonate reservoirs. This is, predominantly, because of the complexity of the deformation phases, the poor understanding of the mechanical and diagenetic mechanisms that affect apertures, and the difficulty in precisely characterizing aperture distributions in the subsurface. Utilizing outcrop data analysis and displacement-based linear elastic finite element modelling, we study the effect of stress regime change on fracture network permeability. The model is based on fracture networks, specifically fracture sub-structures.

The Latemar, which is primarily affected by subsidence deformation and Alpine compression, is used as an outcrop analogue for isolated (Mesozoic) carbonate formations with fracture-dominated permeability. We apply a novel strategy involving two compressive boundary loading conditions constrained by the study area's NW-SE and N-S stress directions. Stress-dependent heterogeneous apertures and effective permeability were computed by: (i) using the local stress state within the fracture sub-structure and (ii) running a single-phase flow analysis considering the fracture apertures in each fracture sub-structure.

Our results show that the impact of the modelled far-field stresses at: (i) subsidence deformation (first stage loading) from the NW-SE, and (ii) Alpine deformation (second stage loading) from the N-S, increased the overall fracture aperture and permeability. In each case, increasing permeability is associated with open fractures parallel to the orientation of the loading stages and with fracture densities. The anisotropy of permeability is affected by shear dilation and is increased by the density and connectedness of the fracture network. The two far-field stresses simultaneously acting within the selected fracture sub-structure at a different magnitude and orientation do not necessarily cancel out each other in the mechanical deformation modelling. These stresses effect the overall aperture and permeability distributions. These effects, which may be ignored in simpler stress-dependent permeability, can result in significant inaccuracies in permeability estimation, especially in the subsurface carbonate reservoirs.

How to cite: Igbokwe, O. A., Timothy, J., Kumar, A., Yan, X., Mueller, M., Verdecchia, A., Meschke, G., and Immenhauser, A.: Effect of stress regime change on fractured carbonate’s permeability: A case of Latemar carbonate buildup (The Dolomites, Northern Italy) , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9317, https://doi.org/10.5194/egusphere-egu23-9317, 2023.

The characterization and modelling of fractured reservoirs of geofluids are becoming increasingly important in the ongoing energy transition and climate crisis. Fractured reservoirs are fundamental for critical applications such as CO₂ sequestration, H₂ and natural gas storage, exploitation of geothermal fluids and hydrothermal ore deposits, and management and safeguard of groundwater resources (deep aquifers are considered more resilient in drought scenarios). In addition, the characterization of fracture networks is relevant in earthquake mechanics, slope stability and engineering geology.

Characterization of natural fracture systems and fracture networks is often based on characterization of outcrop analogues, with measurement of large structural datasets with a combination of field and remote sensing techniques (e.g. Digital Outcrop Models - DOMs), leading to statistical and topological analysis. Numerous studies provide significant amounts of data from a broad variety of methodologies and protocols used in the field. These methodologies aim at characterizing fracture systems by a large number of parameters. Individual “fracture” sets are characterized by genetic features (e.g. joint vs. stylolite), relative chronology, spatial distribution (regular, random, clustered...), density and intensity (e.g. P₂₀ and P₂₁), and by statistical distributions of spacing, orientation, length, height, and aperture (the latter being a dynamical property that varies with fluid pressure and confining stress). Fracture networks composed by several sets are also characterized by topology and connectivity (characterized for instance in terms of fracture terminations or with graphs).

Here we propose a thorough review, supported by rich case studies, of quantitative methods for fracture network characterization and analysis on DOMs. This review aims at determining the most relevant and efficient methods for field and remote-sensing measurement, and best-practice statistical analysis techniques, in order to accurately characterize outcrop analogues that can be used to model fractured reservoirs.

How to cite: Bistacchi, A., Mayolle, S., Casiraghi, S., De Paolo, E., Martinelli, M., Agliardi, F., and La Valle, F.: Quantitative structural analysis of fracture networks in outcrop analogues of fractured reservoirs: a review of measurement methodologies and statistical techniques, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9661, https://doi.org/10.5194/egusphere-egu23-9661, 2023.

Faults and fractures have a crucial role in controlling the permeability in carbonate reservoirs, as they can act as a conduit or barrier for fluid flow. Reservoir-scale outcrop analogue studies provide a useful tool to investigate their spatial distribution and connectivity and to establish the relationships between small-scale structures with larger structures that can be identified in the subsurface.

In this contribution, we describe the preliminary results of a structural study carried out in the Matera's High, South Italy, as an analogue for porous carbonate structures that could be used as CO2 storage fields. Matera High is located on the western side of the Murge region, at the boundary between the Apulian foreland and the foredeep of the southern Apennines thrust belt. It consists of an asymmetrical horst structure involving the Cretaceous carbonates of the Apulian platform (Calcare di Altamura). The Calcare di Altamura is unconformably overlain by Plio-Pleistocene shallow-marine coarse-grained carbonates (Calcarenite di Gravina). The Calcare di Altamura is moderately tilted and is characterised by NW-SE striking normal faults with a throw variable from centimetres to tens of meters. The Cretaceous sequence is also characterised by widespread joints, whose intensity increases approaching faults. The Plio-Pleistocene carbonate succession has very few faults. It is dominated by deformation bands organized into 3 main sets dipping at high angles and striking N-S, NW-SE, and NE-SW. This geological setting allows us to conduct a detailed structural study on an area of about 80 km2, investigating how deformation structures affect the secondary porosity in tight limestone and porous calcarenites. The study was conducted at multiple scales in the field and laboratory and includes (1) geological mapping and structural measurements of faults, fractures and deformation bands; (2) use of linear scan-lines to characterise the deformation bands density across faults; (3) use of photogrammetric techniques to obtain Virtual Outcrop Models (VOMs); (4) development of 3D model based on statistical and topological analysis obtained from scan lines and scan areas in the field and VOMs, (5) petrophysical logging (uniaxial strength, in situ permeability, gamma ray) to highlight the factors that control the formation of the deformation bands, (6) image analysis of blue-resin impregnated thin section and optical cathodoluminescence images, and (7) He-density and Hg-intrusion porosimetry to quantify host rock and deformation bands porosity and pore size distribution.

The preliminary results suggest that the combination of fieldwork, VOMs and laboratory measurements allow the characterization of the deformation bands with more confidence to obtain conceptual and quantitative models about its effects on the fluid flow which can be used for reservoirs characterization for CO2 sequestration.

How to cite: Freda, G., Mittempergher, S., Balsamo, F., Di Cuia, R., and Ricciato, A.: Deformation bands characterization in porous carbonates: a case study from the Matera High (Southern Italy), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9673, https://doi.org/10.5194/egusphere-egu23-9673, 2023.

Groundwater accounts for around 25% of the world’s fresh water supply. Due to the increasing anthropogenic pressure on shallow aquifers as well as climate change that is impacting global groundwater recharge, there is an increasing need to access deeper groundwater resources, which are frequently hosted in fractured-rock formations. The migration of groundwater (and other types of fluids, in general) in fractured rocks allows the contact between fluids in geochemical disequilibrium with the host rocks (i.e., large geochemical gradients) promoting water-rock reactions inside the fractures. These reactions may influence the permeability and porosity, as well as they may lead to fracture sealing. So, a thorough understanding of the coupled hydro-chemical processing that occur in fractured media is important for applications such as the sustainable exploitation of the afore-mentioned reserves, the protection and remediations of aquifers used for drinking water production or the safety analyses of deep geological repositories for spent nuclear fuel, energy storage, nuclear waste disposal sites, etc.. In fractured rocks, groundwater flows in specific pathways and interacts with the host rock which may lead to the change in the hydro-geochemical conditions. The prediction of these interactions become critical for a proper management of the different applications. Therefore, the understanding and modelling of fluid-fracture interaction is of high scientific and commercial interest.

Using the software dfnWorks, it is possible to model the fluid transport using a non-reactive Lagrangian method (particle tracking). In this contribution, we intend to implement geochemical reactions in dfnWorks to quantify the impact of these reactions in the fracture network. In fact, flow of water through Discrete Fracture Networks leads to interaction between water and the minerals occurring in the fracture plane and thus alters the underlying groundwater flow patterns. Thus, using these DFN-based reactive transport simulations, we aim at predicting the effect that chemical reactions have on flow and channeling. Besides presenting a proof-of-concept set of calculations, we will also present preliminary results of a real-case application, where fracture filling is produced as a result of a chemical imbalance.

How to cite: Harb, I., Grandia, F., Trinchero, P., and Hyman, J.: Modelling fluid flow and water-rock interaction in fractured crust using a Discrete Fracture Network approach, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9694, https://doi.org/10.5194/egusphere-egu23-9694, 2023.

Inferences have to be made about likely structures and their effects on fluid flow in a geothermal reservoir at the pre-drilling stage. This is the case for the potential geothermal reservoir in Variscan metasedimentary rocks that are expected to occur in the subsurface at Göttingen. Simple mechanical modelling, using reasonable ranges of values for rock properties, stresses and fluid pressures, is used here to predict the range of possible structures that are likely to exist in the sub-surface and that may be generated during thermal and hydraulic stimulation. Mohr diagrams are a useful way for predicting and illustrating how rocks can go from a stable stress state (i.e., no fracturing occurs) to an unstable stress state (i.e., fracturing occurs). This transition can occur if there are changes in: (1) the failure envelope; (2) the stresses; and/or (3) fluid pressure. Mohr diagrams are used to show under what fluid pressures and tectonic stresses different types and orientations of fractures are likely to be reactivated or generated. The approach enables the effects of parameters to be modelled individually, and for the types and orientations of fractures to be considered. This modelling is useful for helping geoscientists consider, model and predict the ranges of mechanical properties of rock, stresses, fluid pressures and the resultant fractures that are likely to occur in the sub-surface.

The Mesozoic rocks of the Somerset coast, UK, are used to illustrate how Mohr diagrams can help understand the history of fracturing. Such understanding is useful for predicting which fractures are likely to occur in the subsurface, which is important for predicting reservoir behaviour.

How to cite: Peacock, D., Leiss, B., and Sanderson, D.: Use of Mohr diagrams to predict fracturing in rock, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10311, https://doi.org/10.5194/egusphere-egu23-10311, 2023.

Pore fluid pressure in the geological formation at depth varies spatially and temporarily. An increase in pore fluid pressure leads to a reduction in effective normal stress and thus affects the rock strength and deformation mode. Extremely high pore fluid pressure induces very low normal stress conditions, where an extension or extension-shear hybrid fractures are formed. To better quantify the stress states and fluid pressure during fracture formation, it is crucial to document mechanical strength and the transition from tensile to shear fracture at low effective stress with elevated pore fluid pressure. However, all previous experimental studies were conducted under dry conditions. Here, we investigate the effects of pore fluid pressure on tensile and hybrid fractures in Berea sandstone by conducting triaxial extension deformation experiments under pore-fluid-pressure controlled conditions at effective maximum principal stress (σ₁' = σ₁ - P_p, where σ₁ is total maximum principal stress and Pp is pore fluid pressure) ranging from 10 to 130 MPa. Fracture strength, inelastic strain, strain at failure, fracture angle to σ₁', and the amount of comminution increase with σ₁'. The transition of extension to shear fracture occurs at σ₁' = ~ 30 MPa, based on the fracture angle and the degree of comminution. All the saturated or pore fluid pressure-controlled test specimens exhibit lower fracture strength than dry samples, and the difference is distinct when the minimum principal stress is tensile (i.e., σ₃' < 0). This implies that pore fluid pressure more effectively assists the breakage of the bonds and opening of the microcracks in the extension fracture regime. A series of triaxial extension experiments at σ₁' = 20 and 50 MPa with various combinations of σ₁ and Pp indicate that the fracture angle to σ₁' is independent of σ₁ and Pp in the extension fracture regime at σ₁' = 20 MPa, and that fracture angle increases with σ₁ and Pp in the extension-shear hybrid fracture regime at σ₁' = 50 MPa. This implies that the estimation of in-situ stress and pore fluid pressure from natural or human-induced deformation at low effective pressure (such as joints, veins, and drilling-induced tensile fractures) requires careful consideration of the mode of fractures formed.

How to cite: Kitajima, H., Ruplinger, C., and Tilley, C.: Experimental investigations on effects of pore fluid pressure on extension and extension-shear mixed-mode fracture in Berea sandstone, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10433, https://doi.org/10.5194/egusphere-egu23-10433, 2023.

                                        Hydraulic fracturing is a useful stimulation technique to create cracks in unconventional reservoirs and enhance the effective fluid transmissivity to recover gas from natural gas reservoirs or heat from geothermal reservoirs. However, due to fracturing the overall strength and load bearing capacity of the reservoir is compromised. This may be a serious concern if the reservoir is below a dam or any other massive structures. In such case significant settlement can take place as the result of mixed mode fracturing inside the reservoir which might already have natural fractures. Phase field method based on the formulation of mixed-mode fracturing has been adopted in the present work for modeling fracture propagation in a saturated porous medium when subjected to fluid pressure and increasing overburden load. A numerical method has been developed using Finite element method (FEM) for solving the displacement and damage field, and Finite volume method (FVM) for solving flow field due to its flux conservative nature which is automatically satisfied for each FVM cell. Our FEM code alone has been first validated for modelling mixed mode fracturing considering a single fracture as a notch against the published experimental and numerical results for elastic medium subjected to compressive load. In this method, the notch does not have any material and computational mesh is refined around the notch as commonly done by others. We have later developed an alternative method where the pre-existing crack is modelled as a fully damaged zone. In this method, a structured and uniform grid can be used to obtain same fracturing pattern and load-deflection curve. The alternative method has a few advantages such as it can be easily applied for reservoir with many cracks without any grid refinement around the pre-existing cracks, and it can be easily coupled with FVM code for modeling fluid flow. Our numerical modelling code is capable to simulate fracturing along with the branching and merging effects. We have simulated load-bearing capacity of a fractured reservoir subjected to increasing overburden load. The reservoir is considered to consists of many randomly oriented but poorly connected natural fractures. The load-bearing capacity and load-deflection curves are compared for reservoirs with and without hydraulic fracturing. The simulations have been performed for different set of natural fractures to understand the effect of fracture density and other fracture network properties on the load-deflection curves.

How to cite: Kar, S. and Chaudhuri, A.: Phase field method to model mixed-mode fracturing in fluid saturated porous reservoir, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10795, https://doi.org/10.5194/egusphere-egu23-10795, 2023.

 Deep geothermal reservoirs are often naturally or hydraulically fractured media which consist of a rock matrix and a network of randomly oriented discrete fractures of different sizes. In the present study, a three-dimensional model of coupled flow, heat transfer and deformation of fractured geothermal reservoir is developed. Because of high thermal expansibility and high mobility, supercritical C0₂ is under consideration as an alternative fluid to water/brine for extracting energy from geothermal reservoir. However, for simulation of CO₂ based EGS, two phase flow model should be included and this makes the coupled model far more nonlinear and complex. FEHM which is one of the most robust code developed at LANL is capable to simulate the multi-physics problem of geosciences. We have found that the computational cost for CO₂ based EGS a few times higher than that of water based EGS. Due to temperature drawdown and pressure difference between injection and production wells thermo-poro-elastic stresses are induced within the reservoir. This can influence larger shear dislocation, and normal opening /closing of fracture causing changes in the fracture aperture and permeability during the extraction of the geothermal energy from a reservoir. The correlation of the variation of fracture permeability with the variation of the local stress tensor has been taken into account in this study. To study the permeability alteration on geothermal energy extraction for water and CO₂  based EGS different fracture networks, different values of injection temperature and injection pressure are considered. Three-dimensional fracture networks of randomly oriented rectangular fractures with different sizes, and dip angles are created using ADFNE Matlab code. A structured computational mesh is created for simulating the multiphase flow, heat transfer and geomechanics. The nodes belonging to the fractures are assigned appropriate permeability, thermal conductivity and mechanical properties depending on the fracture aperture. The values of these properties are subjected to alteration based on local stress values.

How to cite: Adhikary, S. S. and Chaudhuri, A.: Thermo-poro-elastic stress induced aperture alteration of fractured geothermal reservoir and its effect on geothermal energy production, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10797, https://doi.org/10.5194/egusphere-egu23-10797, 2023.

During fault evolution different rock types are fractured and sheared within the fault core, producing fault gouges with heterogeneous mineralogical composition. Mineral composition exerts a primary control on fault frictional properties and hence on fault slip behaviour. Understanding the conditions that lead to seismic or aseismic fault slip is of great interest to earthquake hazard assessment both for natural and induced seismicity. Although the effect of single mineral phases is probably the most documented factor in laboratory tests, no clear link has been established to understand how systematic variation of different mineral phases in gouge mixtures influences the macroscopic frictional behaviour.

Here we present an experimental study designed to probe the control of mineral composition on fault friction and stability responses. We selected three representative mineral phases, commonly found in fault zones, that are known to have severely different frictional properties: muscovite (phyllosilicate), quartz (granular silicate) and calcite (granular carbonate). Thirty double direct shear experiments were performed using a biaxial rock deformation apparatus (BRAVA) on powders (with grain sizes < 125 µm) of pure minerals and their mixtures at normal stress of 50 and 100 MPa, at room temperature and water saturation conditions. After an initial sliding of 10 mm at 10 µm/s to develop a steady state shear fabric, slide‐hold‐slide sequences (30-1000 s) and velocity steps (0.3-300 µm/s) were employed to evaluate static healing and frictional stability, respectively.

Our experimental data indicate that the mineralogical composition of fault gouges significantly affects the frictional strength, healing, and stability with a non-trivial pattern. Increasing phyllosilicate (muscovite) content results in a decrease of the frictional strength, from 0.62 for pure calcite and 0.56 for pure quartz down to 0.33 for pure muscovite powders. This effect is more marked in calcite-rich mixtures rather than quartz-rich ones, possibly due to favourable conditions for fluid-assisted pressure-solution at grain contacts. Calcite-muscovite interaction also favours a reduction of frictional healing and a more marked velocity-strengthening behaviour (promoting stable sliding and fault creep) in comparison to quartz-muscovite mixtures.

How to cite: Ruggieri, R., Pozzi, G., and Collettini, C.: Mineralogical control on fault friction and stability: a systematic study on quartz, calcite and muscovite ternary mixtures., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12483, https://doi.org/10.5194/egusphere-egu23-12483, 2023.

A number of six in situ hydraulic fracturing experiments were carried out at the Äspö Hard Rock Laboratory (Sweden) in 2017 in a depth of 410 m. Here we present electric self-potential monitoring during the conventional and the step-wise cyclic injection experiments HF2 and HF3. Electric self-potential data were acquired through a two-sensor array, each including nine measuring probes and one base probe, that were installed at the 410 m and 280 m levels. The experimental borehole F1 is drilled in the direction of S_hmin, perpendicular to the expected fracture plane. The self-potential sensors are installed sub-parallel to S_hmin at level 410 at a distance of 50-75 m to the borehole F1 and sub-perpendicular to S_hmin at level 280 m at a distance of 150-200 m to F1. The self-potential data were measured with a sampling rate of 1 Hz. Here, we propose a 1-D modelling of the streaming potential that approximates the measured self-potential data. These streaming potential gradients ∆V are estimated from the simulated pressure signals and the coupling coefficient.

How to cite: Haaf, N., Guarracino, L., Jougnot, D., and Schill, E.: Electric self-potential monitoring of hydraulic fracturing experiments in the Äspö Hard Rock Laboratory, Sweden., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13820, https://doi.org/10.5194/egusphere-egu23-13820, 2023.

Many reservoir rocks of productive geothermal energy resources display low porosity and matrix permeability. Therefore, to enhance fluid flow, fault zones and natural fracture networks are increasingly targeted for geothermal energy exploitation that are hydraulically connected to geothermal wells by stimulating the reservoir units. To this end, fluid is injected into the reservoir, which is generally believed to reduce effective stress and induce minor slip along stressed faults. Fluid injection can also lead to induced microseismicity and remotely-triggered earthquakes at great distances from the target reservoirs. The Blue Mountain geothermal field produces the largest seismic activity during maintenance shutdowns of injection and production requiring additional mechanisms such as poroelastic stress effects. In order to improve seismic hazard assessment and the understanding of induced seismicity around injection wells, we explore the coupling between matrix permeability, fault zone hydrology and mechanical behavior.

One main goal of this work is to provide insight into the scale-dependence of permeability by comparing laboratory results with previous permeability measurements using tidal responses in three different idle wells in Blue Mountain. We present a series of laboratory experiments performed on rock samples collected from the DB2 well at the Blue Mountain geothermal site in Humboldt County, Nevada, USA. The geothermal field benefits from the intersection of two W- and NW-directed normal faults resulting in high permeability of the geothermal reservoir production zone controlled by a brittle damage zone. Samples were obtained from two different lithologies, both of Triassic age, that constitute the reservoir. The first set of samples are low-porosity, (0.4 vol%) quartz-dominated (~50 – 60 wt%) phyllites, which exhibit zones with pronounced fracturing and elevated porosity (3.8 vol%). The second set of samples are felsic intrusive rocks with moderate to high porosity (7 – 15 vol%) due to strong hydrothermal alteration and clay mineral formation. Samples from both lithologies were selected from different sections of the damage zone showing varying degrees of faulting, from intact to highly brecciated, containing mineralized veins. We determine flow and poroelastic properties of cylindrical samples with a length of 2 cm and a diameter of 5 cm, subjected to stepwise cyclic variation of pore (<40 MPa) and confining pressure (<45 MPa). At each pressure step, we measure volumetric strain changes to derive the bulk modulus and effective stress coefficient, and use steady-state or pore pressure oscillation to determine permeability.

In addition to the tidal response and laboratory results, we developed a high-resolution seismicity catalog based on more than three years of continuous waveforms records from 2016 to 2019. We performed template-matching and differential travel-time inversions and use the resulting seismic events with magnitudes between 0.7 – 2.7 to search for seismicity migration patterns associated with discrete injection events. Integration of field and laboratory results can improve the characterization of the permeability structure of the fault zone at Blue Mountain and help to understand the mechanisms that trigger seismic events during production shutdown as well as the role of poroelastic stress release.

How to cite: Schuster, V., Rybacki, E., Schleicher, A. M., Cladohous, T. T., Koirala, R., and Göbel, T. H. W.: Permeability and Compressibility Evolution of Fractured and Intact Reservoir Rocks from the Blue Mountain Geothermal Field, Nevada, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13949, https://doi.org/10.5194/egusphere-egu23-13949, 2023.

Biot coefficient and Skempton coefficient are key descriptors of the coupled hydro-mechanical (HM) behavior of fluid-saturated porous materials. Biot coefficient defines a relationship between an applied load, fluid pressure and the stress that effectively acts on the solid skeleton. Skempton coefficient defines the temporary pore pressure variation caused by the application of a load in undrained conditions. The product of the two coefficients establishes the impact of an applied load on the solid skeleton, and thus the material deformation, under undrained conditions. The two coefficients are generally estimated through analytical expressions valid for isotropic homogeneous materials, or they are experimentally estimated at the laboratory sample-scale.

In this work, we define a framework for the evaluation of equivalent Biot coefficient and Skempton coefficient at the scale of a fractured rock mass. We derive theoretical expressions that estimate the two equivalent coefficients from the properties of both the porous intact rock and the discrete fracture network (DFN), including fractures with different orientation, size, and mechanical properties. These formal expressions are validated against results from fully coupled hydro-mechanical simulations on systems with explicit representation of deformable fractures and rock blocks. We show that the coefficients largely vary with the fracture orientation and density, which implies that disregarding the presence of fractures may incur an incorrect evaluation of the HM response. We also discuss the variability of the coefficients under different settings of DFN properties, including realistic scaling conditions of size-dependent and stress-dependent fracture properties.

How to cite: De Simone, S., Darcel, C., Kasani, H. A., Mas Ivars, D., and Davy, P.: Equivalent Biot and Skempton coefficients for fractured rocks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14955, https://doi.org/10.5194/egusphere-egu23-14955, 2023.

Subsurface engineering, such as geothermal energy extraction, requires knowledge of the rupture of geomaterials. Of particular importance is the time- and rate-dependence of material strength, which impacts fracture architecture and thus hydraulic conductivity and system permeability. Cyclic soft stimulation (CSS) techniques have been developed to maximise the efficiency of resource extraction whilst minimising large amplitude, fluid-injection induced seismicity. Here, we explore the benefits of cyclic stimulation experimentally, utilising novel “pulsed pumping” hydraulic fracture tests in which fluid pressure is cycled within the central borehole of a suite of 20x20cm cylinders of dense granite. The response is monitored at high-resolution by fibre-optic circumferential strain measurements, fluid pressure data and acoustic emission recording. Using cyclic high-low pressure square waves, we found that breakdown pressure was reduced by up to 15% compared to the monotonic case in which pressure was increased by applying a constant flow rate. Whilst peak pressure had the primary control on the number of cycles to failure, increasing the minimum pressure in the borehole (thus increasing mean pressure) further reduced breakdown pressure, suggesting that even small pressure fluctuations during hydraulic stimulation may reduce the largest stress drops, and hence the magnitude of induced seismic events. Strain measurements detected accelerating precursory deformation a few cycles prior to failure, hinting at the opportunity for responsive stimulation practices where activity can be monitored in real-time. These novel large-scale, high-resolution experiments were complemented by indirect tensile measurements at a range of strain rates, and by cyclic fatigue Brazilian disc testing at a range of peak loads and cycle amplitudes. These results further highlight the increasing contribution of time-dependent deformation during slower and cyclic loading, resulting in lower peak loads and reducing large magnitude fracturing events. The generated S-N curves demonstrate that weakening by cyclic hydraulic pressurisation mimics relationships defined by conventional fatigue testing of geomaterials. Such experimental constraints will be of great benefit to the development of cyclic stimulation practices for subsurface engineering.

How to cite: Kendrick, J. E., Mouli-Castillo, J., Lamur, A., Fraser-Harris, A., Lightbody, A., Chandler, M., Edlmann, K., McDermott, C., and Shipton, Z.: Constraining the impact of cyclic hydraulic stimulation on granites, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14982, https://doi.org/10.5194/egusphere-egu23-14982, 2023.

The geothermal potential in the deep subsurface of North Rhine-Westphalia (NRW) has been scarcely explored. Due to the increasing demand for zero-carbon heating applications, there has been a renewed phase of seismic exploration in the region to investigate potential geothermal aquifers. The prime exploration target is the Lower Carboniferous Ruhr Basin and the Devonian substratum of the Variscan foreland, as they host deep carbonate aquifers. Interpretation of the recently-acquired 70-km-long 2D seismic profiles, together with the 2D legacy seismic data in the northern part of the Ruhr basin, the Münsterland region, reveals two carbonate units; the Dinantian platform facies and the Givetian massive facies. These are located at depths that range from c. 4500 m to 6000 m. Considering their great burial depth, the permeability of the carbonate rocks is considered to be primarily facilitated by fault zones with dense fracture swarms. Therefore, understanding the complex deformation history of this fossil foreland basin is crucial to evaluate its geothermal potential. We here reveal the timing and geometric evolution of the fault zones. Using a multiattribute seismic analysis, we delineate three major types of faults; (1) SW–NE-⁠trending, syn-fold thrusts, (2) WNW–ESE-striking normal faults, and (3) E–W and N–S strike-slip faults. Interestingly, we do not observe in the available data flexure-induced faults that are typical for foreland basins and would be expected to strike parallel to the SW–NE-oriented Variscan Orogen. To constrain the relative timing of fault activity, we mapped seven well-constrained and age-calibrated stratigraphic horizons within the Carboniferous molasse sequence and the Cretaceous cover. The preliminary results confirm the observations of previous researchers that the thrust faults formed after the deposition of the Late Carboniferous strata, as evidenced by their concordant folding. Thrusts are crosscut by the normal faults, suggesting that the latter formed at a subsequent stage. Most of the strike-slip faults cut through the Carboniferous–Cretaceous unconformity, with some culminating in the Cretaceous cover as positive flower structures. Notably, one flower structure is co-linear with a thrust fault in the Carboniferous, suggesting that there is some degree of kinematic linkage between the two structural levels. Possibly, some of the optimally-oriented thrusts were reactivated and grew upward as strike-slip faults during Late Cretaceous transpression. Such multiphase evolution of fault zones may enhance permeability structure, since each reactivation event potentially contributes to the widening of the deformation zones, thereby increasing the density of interconnected fractures. In this study, we demonstrate how the integration of new seismic data provides valuable insights into the structural evolution of the Ruhr Basin and its geothermal potential. A 3D seismic acquisition campaign is planned in the investigated region. Using its results, we intend to conduct a high-resolution fault throw analysis to further constrain the kinematic development of the deformation structures.

How to cite: Shipilin, V. and Dölling, M.: Structural evolution in the northern Ruhr basin: A case study of urban geothermal exploration in the Münsterland Region, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15424, https://doi.org/10.5194/egusphere-egu23-15424, 2023.

Previous hydrocarbon exploration in the Ordos, Tarim, and Sichuan basins of China has indicated that strike-slip faults play an important role in controlling reservoir distribution. High hydrocarbon production within strike-slip fault zones in these basins indicates that the fault zones not only act as conduits or seals for hydrocarbon migration, but also provide space for hydrocarbon accumulation. The productivity of different wells, however, can vary within one strike-slip fault zone, suggesting that variability in fault zone architecture controls hydrocarbon enrichment. To date, very few studies have explored fault zone architecture in the southern Ordos Basin, inhibiting oil exploration and development. We explored faults in the Jinghe Oilfield in the southern Ordos Basin by integrating outcrops, wellbore cores, well logs, and 3D seismic data. We carried out fault segmentation, qualitative characterization of fault zone architecture, and quantitative characterization of the boundary between the damage zone and wall rock. The results showed that fault zone architecture is complicated by fault segmentation, architectural configuration, and damage zone asymmetry. Strike-slip faults can be divided into transtensional, strike-slip, and transpressional segments along the fault strike, with transtensional and strike-slip segments dominant in the Jinghe Oilfield. Each segment is further complicated by different configurations of gouge, breccia, and fracture zones along the fault dip. Compared with the strike-slip segments, transtensional and transpressional segments showed more complexity, with the fracture density and damage zone width of the hanging wall being greater than that of the footwall. Transtensional segments with braided and horsetail structures showed more complexity owing to the presence of multiple fault cores and damage zones around the main fault and its subsidiary faults. Quantitative analysis showed that the fault zone width was the greatest for transtensional segments, intermediate for transpressional segments, and the lowest for strike-slip segments. We determined a positive linear relationship between the relative widths of the fault core and fault zone. The cavities in breccia zones and fractures in damage zones provide conduits and storage space for hydrocarbon migration and accumulation. We conclude that damage zones in transtensional segments, particularly in the hanging wall, are primary potential targets for petroleum exploration and development.

How to cite: Meng, Y., Chen, H., Luo, Y., Zhao, Y., Tang, D., and He, F.: Architecture of intraplate strike-slip fault zones in theYanchang Formation, Southern Ordos Basin, China: Characterizationand implications for their control on hydrocarbon enrichment, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17164, https://doi.org/10.5194/egusphere-egu23-17164, 2023.

Subsurface storage of CO₂ and retrievable green energy (i.e. compressed air and hydrogen) in depleted gas reservoirs, deep saline aquifers and salt caverns, are considered as key transitions within the energy sector to enable many countries to meet their net zero carbon emissions targets. One of the main challenges in underground storage is the presence of potential leakage pathways (such as caprock fractures/faults) which pose a threat to economic feasibility and to the environment. Seeking an efficient technique to remediate leakage pathways is crucial to guarantee sealing efficacy. Microbially induced carbonate precipitation (MICP) is regarded as a promising bio-grouting technique for leakage remediation due to its advantages such as the low water-like viscosity and micron-size microbes, which enables permeation far from the injection point and excellent penetration into fine aperture fractures.

MICP utilizes ureolytic bacteria, commonly Sporosarcina pasteurii (S. pasteurii) to produce highly active urease enzyme, which can catalyze urea hydrolysis and result in the production of carbonate ions. Carbonate precipitation occurs if a calcium source is supplied, which can effectively remediate leakage by filling void space and improve strength by bonding solid grains.

We propose a micro-continuum model which accounts for the coupled hydro-bio-chemical MICP processes in fractured porous rock. In this micro-continuum model, the flow is controlled by the Darcy-Brinkman-Stokes equation over a number of control volumes (i.e. voxels based on X-ray CT scan). The kinetic parameters of urea hydrolysis and calcium carbonate precipitation are calibrated based on batch experiments. The bacteria deposition parameters, which feature the bacteria deposition in fractured rock, are calibrated based on experimentally measured bacteria breakthrough curves in a 1-dimensional column filled with fracture gouge materials. The proposed micro-continuum model can well represent the decreases in porosity and permeability of an artificially-cut anhydrite fracture filled with anhydrite gouges under MICP treatment cycles. Our modelling results also suggest that when the CaCl₂ concentration is equal to or higher than the urea concentration in the injected cementing solution, the rate of microbially induced carbonate precipitation is predominantly limited by the kinetics of urea hydrolysis rather than the kinetics of calcium carbonate precipitation. The proposed micro-continuum model serves as a useful tool for evaluating MICP treatment strategies and may be upscaled to predict and optimise field-scale leakage remediation.

How to cite: Sang, G., Lunn, R., El Mountassir, G., and Minto, J.: Micro-Continuum Modelling of Coupled Hydro-Bio-Chemical MICP Processes in Fractured Rock, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17201, https://doi.org/10.5194/egusphere-egu23-17201, 2023.

Planetary magnetic field production mechanism may require consideration of fermi electrons. While avoiding the Boussinesq approximation and considering a presence of Fermi electrons in a planetary core, a new hypothesis how the planetary magnetic field may operate is proposed. The overall topology concerns both the core’s north hemisphere (NH) and south hemisphere (SH) that produce its own magnetic polarities due to a sense of the Earth’s rotation (Coriolis effect). Magnetism can be generated due to the electric current form the core’s fermi electrons that follow the more conducting spiraling plumes from the convection heat exchange. NH produces magnetic flux directed toward the north (reversed polarity) while SH produces magnetic flux directed to the south (normal polarity). When NH is more buoyant than SH, the overall dipolar reversed polarity is produced. When SH of the core is more buoyant, the overall normal magnetic polarity is produced. Overall planetary magnetic field is then generated from a core’s heat exchange competition between its NH and SH. For this hypothesis supports is found from the theoretical arguments, from the topology of finite element modeling, and from the evidence of a historical magnetic reversal record. Calculations considering the presence of Fermi electrons in the core allow for heat gradient generated magnetic flux estimate between 0.1 mT and 3 mT inside the liquid core. Finite element modeling topology of simulated magnetic dipoles near inner/outer core boundary (IOB) oriented only northward in NH and southward in SH supported that todays’ surface magnetic field observations are consistent with the outer core fields between 0.1 mT and 3 mT. Individual treatment of normal and reversed polarity durations supported that a predominance of magnetic polarity durations relates to the existing temperature models near the core/mantle boundary (CMB) that have a consistent effect on the heating exchange within the core.

How to cite: Kletetschka, G.: Origin of the Earth’s magnetic field from the Fermi electrons, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2458, https://doi.org/10.5194/egusphere-egu23-2458, 2023.

Differential rotation of Earth’s inner core relative to the mantle above plays an important role in core dynamics and the core-mantle coupling. The rotation has been inferred from temporal changes of repeating seismic waves traversing the inner core. In our recent study¹, we report remarkable observations that all the paths previously with significant temporal changes have now exhibited little changes over the recent decade. The consistent global pattern suggests strongly that the inner core rotates as a whole and the rotation has paused in the recent decade with a net torque of ~10¹⁶ Nm. Furthermore, the recent pattern seems associated with a gradual turning-back as a part of a long-period (about seven decades) oscillation with another turning point in the early 1970s. The multidecadal periodicity coincides with changes in several other geophysical observations, including the global mean temperature², the global mean sea level rise³, and especially the length of day (LOD) and magnetic field variations⁴, pointing to a common resonating system of the Earth. Our observation provides important constraints to geodynamo models and the mantle-inner core gravitational coupling and offers key evidence for dynamic interactions between the Earth’s layers from the deepest interior to the surface.

References:

1. Yang, Y., & Song, X. (2023). Multidecadal variation of the Earth’s inner-core rotation. Nature Geoscience (in press). https://doi.org/10.1038/s41561-022-01112-z

2. Zotov, L., Bizouard, C., & Shum, C. K. (2016). A possible interrelation between Earth rotation and climatic variability at decadal time-scale. Geodesy and Geodynamics, 7(3), 216–222. https://doi.org/10.1016/j.geog.2016.05.005

3. Ding, H., Jin, T., Li, J., & Jiang, W. (2021). The contribution of a newly unraveled 64 years common oscillation on the estimate of present-day global mean sea level rise. Journal of Geophysical Research: Solid Earth, 126(8). https://doi.org/10.1029/2021JB022147

4. Roberts, P. H., Yu, Z. J., & Russell, C. T. (2007). On the 60-year signal from the core. Geophysical and Astrophysical Fluid Dynamics, 101(1), 11–35. https://doi.org/10.1080/03091920601083820

How to cite: Yang, Y. and Song, X.: Multidecadal variation of the inner core rotation and implications for global dynamics, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2826, https://doi.org/10.5194/egusphere-egu23-2826, 2023.

Core surface flow inversion using physics-informed neural networks

Jinfeng Li (1) and Yufeng Lin (1)

(1) Department of Earth and Space Sciences, Southern University of Science and Technology, Shenzhen 518055, China.

Physics-informed neural networks (PINNs) have recently been widely used to solve PDEs or ODEs. An attractive feature of this method is that it can calculate the derivatives without truncation errors by the automatic differentiation method (Lu et al., 2021). Another advantage is that it can solve the inverse problem with slightly modified code for solving the forward problem (Raissi et al., 2020). In this study, we use the PINN to inverse the core surface flow from the geomagnetic observations. We start from the radial component of the induction equation under the frozen-flux approximation (Robert and Scott, 1965) and tangentially geostrophic flows assumption (Hills, 1979). Instead of using the large-scale approximation, which assumes the flows that generate the observed secular variation (SV) are large-scale, we model the flow field in the physic space and construct the unobserved magnetic field based on the power spectrum of numerical dynamo simulations. We examine the nonuniqueness of the inversion results by pre-setting the different initial parameters of the neural network. Our tests show that the uncertainty of large-scale flow field is small and the inversion scheme is robust.

We retrieve the core surface flow field between 2000 and 2020 using the core magnetic field model CHAOS-7 (Finlay et al., 2020). We then perform the dynamic mode decomposition method (DMD) (Schmid, 2010) of the retrieved core flow. This method decomposes the flow field and SV into several eigenmodes with time evolution. The consistency time evolution between the flow and the SV modes indicates the inversion algorithm is stable. Moreover, we calculate the secular acceleration (SA) of the magnetic field for each dynamic modes and find the mode with 8 years period can match the jerk events occurred in the equatorial region.

Reference

• C. Finlay, C. Kloss, N. Olsen et al. 2020, The CHAOS-7 geomagnetic field model and observed changes in the South Atlantic Anomaly, Earth Planets Space, 72, 156.
• G. Hills, 1979, Convection in the Earth’s Mantle Due to Viscous Shear at the Core-Mantle Interface and Due to Large-Scale Buoyancy. PhD Thesis, New Mexico State University, Las Cruces.
• Lu, X. Meng, Z. Mao and G. E. Karniadakis, 2021, DeepXDE: A Deep Learning Library for Solving Differential Equations, SIAM Review, 63, pp. 208-228.
• Raissi, A. Yazdani and G. E. Karniadakis, 2020, Hidden fluid mechanics: Learning velocity and pressure fields from flow visualizations, Science, 367, pp. 1026-1030.
• H. Robert and S. Scott, 1965, On analysis of the secular variation. 1: A hydromagnetic constraint: Theory, Journal of Geomagnetism and Geoelectricity, 17, pp. 137-151.
• J. Schmid, 2010, Dynamic mode decomposition of numerical and experimental data, J. Fluid Mech, 65, pp. 5-28.

How to cite: Li, J.: Core surface flow inversion using physics-informed neural networks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3710, https://doi.org/10.5194/egusphere-egu23-3710, 2023.

Models and paleointensity data continue to consistently point to the Ediacaran Period as the most likely time for the onset of inner core nucleation (ICN). The geodynamo models of Driscoll (2016) and Driscoll and Davies (2022) predict a weak field state, where core kinetic energy exceeds magnetic energy, prior to ICN. The paleomagnetic record of the Ediacaran Period shows a hyper-reversal frequency and unusually high secular variation. But the most telling characteristic of the Ediacaran magnetic field that suggests the dynamo approached the weak field state is its time-averaged ultralow paleointensity, more than 10 times weaker than today (Bono et al., 2019). The field subsequently regained strength in the early Cambrian (Zhou et al., 2022), consistent with Ediacaran ICN. Here, we investigate the possibility that the magnetic field may have ceased completely for some part of the Ediacaran Period. We report new field strength values from whole rocks that are less than 1-2 microTesla. These values are amongst the lowest terrestrial fields ever recorded, heightening the possibility of environmental effects due to the weakened magnetosphere that may have in turn influenced biotic evolution. But even these ultralow field values may overestimate the true ambient field strength because of subsequent thermal viscous magnetic overprints carried by nonideal magnetic carriers in whole rocks. We will discuss our efforts to use single crystal paleointensity methods to isolate ideal magnetic carriers to resolve this question.

How to cite: Zhou, T., Tarduno, J., Kodama, K., Cottrell, R., and Bono, R.: Did the dynamo cease during the Ediacaran Period prior to inner core nucleation?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7723, https://doi.org/10.5194/egusphere-egu23-7723, 2023.

Earth's rotation period varies over many time scales ranging from diurnal to several milennia, in addition to its secular increase due to tidal friction. These variations in the rotation period imply an exchange of angular momentum between the mantle and other fluid layers of the Earth, such as the atmosphere, oceans, and the fluid outer core. In order to disentangle the role of the outer core, a good understanding of its low frequency eigenmodes is necessary. We attempt to build a relatively simple model of the Earth's fluid core including gravitational, viscous, and magnetic coupling with the mantle and the solid inner core. Our goal is to assess whether observed length-of-day variations can be partially attributed to outer core eigenmodes, and if that is the case, to explore the implications related to the outer core-mantle and outer-inner core coupling mechanisms.

How to cite: Triana, S., Rekier, J., Gerick, F., and Dehant, V.: Low frequency eigenmodes of the Earth's fluid core, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8209, https://doi.org/10.5194/egusphere-egu23-8209, 2023.

The thermal conductivity values through Earth’s core and planetary cores have important implications for the thermal evolution and magnetism of these bodies. For the outer cores of small terrestrial planetary bodies, this study constrains the thermal conductivity of liquid Fe-8wt%S-4.5wt%Si at pressures 2-5 GPa. Thermal conductivity was estimated using the Wiedemann-Franz Law from electrical resistivity measurements of a small Fe alloy sample at high pressures and high temperatures in a 1000-ton cubic anvil press. The powder samples were prepared by mixing powders of three compositions: Fe, FeS, and Fe-9wt%Si. Electron microprobe analysis and micro X-ray diffraction verified the elemental composition and crystallographic structure of the sample material both before and after pressurization.

Resistivity-temperature plots of the Fe-8wt%S-4.5wt%Si data display trends common to Fe mixed with significant amounts of Si: a general rise in resistivity to a peak, a drop in resistivity through the melt, and a leveling of resistivity through the liquid state. Two reversals in slope occur between 800 K and 1000 K. At each integral pressure value between 2-5 GPa, an electrical resistivity in the range 300±100 μΩ·cm was found. Using the Sommerfeld value of the Lorenz number, thermal conductivities in the range 15±5 W/m/K were estimated. Comparative plots including resistivity data of Fe, Fe-4.5wt%Si, Fe-17wt%Si, and Fe-20wt%S are instructive to illuminate the relative effects of S and Si on the resistivity and thus the thermal conductivity and adiabatic heat flow of core mimetic Fe alloys. If the pressure at the top of the core is constrained using the assumptions of hydrostatic equilibrium and a bulk silicate mantle, then these thermal conductivity results may be applied to a number of known small terrestrial bodies, such as Io, in the case of a dominantly Fe-S-Si liquid outer core.

How to cite: Lenhart, E., Yong, W., and Secco, R.: Outer Core Heat Flux in Small Terrestrial Bodies from Electrical Resistivity Measurements of Liquid Fe-8S-4.5Si at High Pressure, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10229, https://doi.org/10.5194/egusphere-egu23-10229, 2023.

Recent satellite missions provide accurate measurement of the time derivative of the Gaussian coefficients from which the secular variation spectrum can be calculated. The ratio of the magnetic energy spectrum to the secular variation spectrum gives a typical scale τ for the temporal variation of the geomagnetic field as a function of the spherical harmonics degrees l. There is much interest in the scaling of τ with l: τ ~ l ^β. Numerical simulations and the frozen flux hypothesis suggest the simple relation τ ~ l ^-1 while observational studies give a diverse range of value for β. A question here is whether the frozen flux hypothesis is applicable. It is plausible that magnetic diffusion can be neglected inside the outer core. However, the situation in a boundary layer under the core-mantle boundary (CMB) is less clear. A related question is whether τ observed at the Earth's surface is relevant to what is happening in the interior of the outer core as the form of the magnetic field above the CMB is constrained by the boundary conditions at the CMB. Here we use a numerical dynamo model to investigate these questions. We extend the definition of τ to the inside of the outer core. We find that in our simulations the exponent β undergoes a sharp transition just beneath the CMB, magnetic diffusion plays a role in the scaling of τ above the CMB and the frozen flux hypothesis is not applicable here.

How to cite: Tsang, Y.-K.: Scaling of the geomagnetic secular variation time scales, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10439, https://doi.org/10.5194/egusphere-egu23-10439, 2023.

The energy dissipation in the fluid flow near the boundary separating the core and the mantle (i.e. the CMB) of a planet or moon with a fluid interior is a crucial parameter to understand its rotational dynamics. This boundary layer is typically very small compared to the core radius, and can become turbulent under certain conditions, which presents a challenge for global scale simulations of the flow in the fluid core. Here we construct a local Cartesian model to study the boundary layer of a precessing planet or moon. The solutions we derive in the laminar regime, i.e. where the Reynolds number Re is small and the non-linear term is neglected, are consistent with previous studies. This gives us confidence to push the model further into the turbulent regime. We solve numerically the governing equations, i.e., the Navier-Stokes equation and the continuity equation for an incompressible fluid in a rotating frame. We observe that, when the flow is turbulent, the boundary layer dissipation is increased, compared to its laminar counterpart, as expected. Moreover, we found that the velocity profile agrees with the law of the wall, a theory developed to study turbulent flow near a solid boundary. Based on our numerical results, we further construct a turbulence model using similarity theory. Last but not least, due to chemical interaction on the planetary core-mantle boundary, small-scale topography or surface roughness might exist. To investigate this topographic effect, we impose a sinusoidal topography in our local model. Preliminary results show further increase of the dissipation. Our results may provide valuable insight into the boundary layer dissipation near the CMB for both the Earth and the moon.

How to cite: Shih, S.-A., Triana, S. A., and Dehant, V.: Turbulent Dissipation in the Boundary Layer of Precession Driven Flow in a Sphere, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10448, https://doi.org/10.5194/egusphere-egu23-10448, 2023.

Modern observations show that the fast fluctuations in geomagnetic acceleration and fluid core surface flow motions always occur at the equatorial regions, which may arise from the rapidly hydromagnetic waves atop the Earth’s core. But, the exact origins of these waves are still unclear, though the so-called eMAC waves may provide a potential mechanism. Given that the physical expressions of describing the physical properties (e.g., equatorial confinement and latitudinal distribution, damping rate, eigen-period) and the perturbed magnetic fields of the eMAC waves have not been given before, this work carefully revisits the currently eMAC wave theory and firstly gives the systematically analytical expressions for these physical properties. Importantly, the perturbation analysis indicates that the eMAC wave model can own the high accuracy (i.e., the relative errors are less than 5%) to describe the low-latitude waves with latitude below 25 degrees, which can cover the regions where the equatorial waves mainly locate. In summary, this work provides an important complement for the currently eMAC wave theory. The results of this work are significant to understand the physical mechanism responsible for the origins of the inferred equatorial waves, their physical properties and the dynamics of the Earth’s equatorial regions.

How to cite: Duan, P.: Analytical model of the equatorial Magnetic-Archimedes-Coriolis waves propagating at Earth’s core surface and the potential implications, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12751, https://doi.org/10.5194/egusphere-egu23-12751, 2023.

We investigate the dipole–multipole field transition in rapidly rotating dynamos in the low-inertia regime relevant to planetary cores. Here, the Rossby number is small on the planetary core depth as well as on the length scale of core convection. Attention is focused on the dynamics of slow Magnetic-Archimedean-Coriolis (MAC), or magnetostrophic, waves generated in the energy-containing scales of the dynamo. The suppression of the slow MAC waves in a strongly driven dynamo is dynamically similar to the excitation of these waves in a moderately driven dynamo evolving from a small seed magnetic field. While the former regime causes polarity reversals, the latter regime produces the axial dipole field from a multipolar state. For either polarity transition, a Rayleigh number based on the mean wavenumber of the energy-containing scales bears the same linear relationship with the peak Elsasser number measured at the transition. This self-similarity can provide an estimate of the Rayleigh number that admits polarity reversals.

How to cite: Majumder, D., Sreenivasan, B., and Maurya, G.: The dipole–multipole transition in planetary dynamos, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15288, https://doi.org/10.5194/egusphere-egu23-15288, 2023.

Mercury possesses an internally generated global magnetic field which significantly differs from Earth’s magnetic field in geometry and strength. While being much weaker (1% of Earth’s surface field strength), Mercury’s magnetic field is strongly aligned to the rotation axis and the magnetic equator is offset towards north. These characteristics of the field have been a challenging task for dynamo modelling. Current dynamo models for Mercury suggest that a stably stratified layer below the core-mantle boundary is necessary to explain the the weak, axisymmetric and offset dipole magnetic field. Although, having different geophysical measurements by NASA’s MESSENGER mission the inner core size of the planet is barely constrained. While interior models from geodetic measurements suggests an inner core sizes which can occupy half of the total core, dynamo models which generate a Mercury-like magnetic field have mostly a rather small inner core of around 400 km. In this study we performed dynamo simulations with a stably stratified layer below the core-mantle boundary which are able to reproduce Mercury’s magnetic field characteristics and we vary the inner core size in these models systematically. First results of the study reveal, that only dynamo models with a small inner core well below 750 km radius are capable of reproducing a Mercury-like magnetic field, while models with a larger inner cores cannot reproduce the offset magnetic equator.

How to cite: Kolhey, P., Heyner, D., Wicht, J., Gastine, T., and Plaschke, F.: Constraints for Mercury’s Inner Core Size by Dynamo Modelling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15588, https://doi.org/10.5194/egusphere-egu23-15588, 2023.

Seismological observations of J-phases, the seismic waves traversing the Earth’s inner core (IC) as shear waves, are critical to understanding the inner core shear properties. That, in turn, will shed light on the solidification process and the evolution of the inner core and our planet. Most body-wave detections of the J waves have been controversial due to their small amplitudes, which involve energy conversion from P- to S- and vice versa at the inner core boundary. Recent advances in understanding the nature of the late coda correlation offer a new way to sample the deep Earth, including the shear properties of the Earth’s inner core. The correlation-based features provide the sensitivity of the periods between 15 and 50 s, placing it between the body waves and normal mode data. Therefore, the observations of late coda correlation are vital in refining the shear properties of the IC, such as velocity, anisotropy, and attenuation.

This study employs several uninvestigated J-wave correlation features detected in the global coda-correlation wavefield building on the study of Tkalčić and Pham (2018) that determined the shear wave speed reduction of 2.5% relative to PREM. The correlation features observed in the coda-correlation wavefield arise from similar seismic phases in which one contains a shear-wave leg in the IC. Improved data selection process and knowledge acquired from recent theoretical and observational developments in understanding the anatomy of coda correlation wavefield enable significant improvements in the data quality. We benchmark the waveforms of observed correlation features using numerical modeling, confirm the observations of J waves and inner core solidity and update its shear properties’ values, including shear-wave speed and Poisson’s ratio.

How to cite: Costa de Lima, T., Pham, T.-S., Ma, X., and Tkalčić, H.: New constraints on shear properties of the Earth’s inner core from the global correlation wavefield, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16047, https://doi.org/10.5194/egusphere-egu23-16047, 2023.

Earth’s core currently sustains a geodynamo through chemical convection in the liquid outer core. This power source originates from the growth of the solid inner core, where light elements are partitioned to the liquid at the lower most outer core. The inner core is expected to be ~1 Gyr old, meaning that for most of Earth history, the geodynamo required alternate power sources to produce a magnetic field. The paleomagnetic record shows that the field has been persistent for the last 3.5 Gyrs. Secular cooling is not capable of providing sufficient power for the geodynamo to remain active during this time if conductive heat transport is large. Recent experiments and calculations find that the thermal conductivity of the core is high, suggesting that the power available for geodynamo action would have been exhausted significantly before inner core growth began. Of the alternate power sources available to supplement secular cooling, precipitation of light elements is the most hopeful. We explore the solubility of silicon and other candidate light elements in iron-rich liquids of the core through ab initio calculations of partitioning. We apply these results to a thermodynamic model of partitioning, informed by experimental partitioning. When incorporated into thermal history models of the deep Earth, we find that the geodynamo can be sustained by silicon precipitation, provided that the oxygen concentration of the ancient core is less than 1.1 wt%. These results highlight the importance of the initial composition of the core and interaction between light elements on the available precipitative power in the core.

How to cite: Wilson, A., Pozzo, M., Alfè, D., Walker, A., Pommier, A., Greenwood, S., and Davies, C.: Precipitation of light elements from Earth’s liquid core: Can exsolution power the ancient geodynamo?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16247, https://doi.org/10.5194/egusphere-egu23-16247, 2023.

How to cite: Hao, S., Tong, P., and Chen, J.: Adjoint-State Surface Wave Tomography for Azimuthally Anisotropic Media: Eikonal Equation-Based Methods and Incorporation of Surface Topography, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2522, https://doi.org/10.5194/egusphere-egu23-2522, 2023.

The singularity points are very important for elastic waves propagation in low-symmetry anisotropic media (Stovas et al., 2021a). Being converted into the group velocity domain, they result in internal refraction cone with anomalous amplitudes and very complicated polarization fields. In elastic orthorhombic (ORT) media, there is always one singularity point in the essential symmetry plane, (0,1,2) singularity points in remaining non-essential symmetry planes and (0,1) points in-between the symmetry planes (Stovas et al., 2021b).

I analyze the conditions for existence of a singularity point in-between the symmetry planes.

In order to do that I fix the diagonal elements of the stiffness coefficient matrix, c_jj , j=1,6, and introduce new variables d₁₂ = c₁₂ +c_66, d₁₃ = c₁₃ +c₅₅ and d₂₃ = c₂₃ +c₄₄. I also assume that the symmetry plane 2-3 is the essential one by introducing the inequality c₅₅ < c₄₄ < c₆₆ . If c₆₆ < c₄₄ < c_55, the 2-3 plane is still essential one but the properties of non-essential planes will interchange. In case of other inequalities for “S wave” stiffness coefficients, the corresponding properties of singularity points can be obtained by a cyclic rotation of stiffness coefficients and symmetry planes (Stovas et al., 2023).

For selected essential symmetry plane (2-3), I propose to fix two variables d₁₂ and d_13, and set the variable d₂₃ as a free variable. By changing d₂₃ only, I can define the trajectory of a singularity point in-between the symmetry planes. This trajectory is given by a continuous line connecting the symmetry planes. Then I define the traces of this trajectory on symmetry planes (maximum two points for each plane) by a specific value of the variable d₂₃. These 6 values can be used for intervals of d₂₃ where the singularity point in-between the symmetry planes exists. Analysis shows that there are 7 zones in (d₁₂ , d₁₃) plane with different intervals of d₂₃, which guarantee the existence of singularity point in-between the symmetry planes. There are 3 intervals of d₂₃ in one zone, two intervals in two zones and one interval in three zones. There is no singularity point in-between the symmetry planes for any d₂₃ in remaining zone. These zones are separated by three straight lines that defined by d₁₂ = d_12(critical) , d₁₃ = d_13(critical)  and d₁₃ = α d_12, where α guarantees that trajectory of singularity point meets the essential symmetry plane.

 References

Stovas, A., Roganov, Yu., and V. Roganov, 2021a, Geometrical characteristics of P and S wave phase and group velocity surfaces in anisotropic media, Geophysical Prospecting, 68(1), 53-69.

Stovas, A., Roganov, Yu., and V. Roganov, 2021b, Wave characteristics in elliptical orthorhombic medium, Geophysics, 86(3), C89-C99.

Stovas, A., Roganov, Yu., and V. Roganov, 2023, On singularity points in elastic orthorhombic media, Geophysics, 88(1), C11-C32.

How to cite: Stovas, A.: On singularity point in-between the symmetry planes in elastic orthorhombic media, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2613, https://doi.org/10.5194/egusphere-egu23-2613, 2023.

Talc and chloritoid are common metamorphic minerals observed in mafic-ultramafic rocks and/or high-pressure metapelitic rocks comprising subducting slabs. Crystallographic preferred orientations (CPOs) of elastically anisotropic minerals have been known to be important for interpreting seismic anisotropy observed in subduction zones. However, studies on the CPOs of talc and chloritoid have been very limited. In this study, CPOs of talc and chloritoid in garnet-chloritoid-talc schist samples from ultrahigh-pressure Makbal Complex (Tianshan, Kazakhstan-Kyrgyzstan) which has been regarded as a part of subducting slab were measured using SEM/EBSD technique. CPO-induced seismic properties of both talc and chloritoid were analyzed and compared. The results showed that both talc and chloritoid displayed strong CPOs characterized by the [001] axes aligned subnormal to the foliation (see also Lee et al., 2021). CPO-induced seismic properties of polycrystalline talc and chloritoid were calculated and they showed that both P-wave anisotropy (AVp = 5 – 72 %) and high S-wave anisotropy (AVs = 10 – 24 %) of talc and chloritoid were much higher than those of garnet (AVp = 0.4 %, AVs = 0.9 – 1.0 %). In addition, the AVp of polycrystalline talc was much higher than that of polycrystalline chloritoid. Analysis of S-wave delay time and fast-polarization direction based on the modelling study of subduction zone geometry showed that the CPOs of talc and chloritoid induced a long delay time of 0.3 – 0.5 s and trench-parallel polarization direction for high dip-angle subduction, which is consistent with the observation of strong trench-parallel seismic anisotropy in subduction zones. Our results suggest that the strong CPOs of talc and chloritoid would influence trench-parallel seismic anisotropy induced by subducting slab in subduction zones. Lee et al., 2021, Seismic anisotropy in subduction zones: evaluating the role of chloritoid, Frontiers in Earth Science, 9, 1-16.

How to cite: Lee, J. and Jung, H.: Crystallographic preferred orientations of talc and chloritoid and implications for seismic anisotropy in subduction zones, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3136, https://doi.org/10.5194/egusphere-egu23-3136, 2023.

It has been suggested that the present-day locations of ultra-low velocity zones (ULVZs), which are thin features just above the core-mantle boundary (CMB), are influenced by mantle convection; however, apart from their preferential locations, there is little direct evidence for this connection. Observations of deep mantle anisotropy can be used to infer mantle dynamics but are not usually jointly analyzed with ULVZ structure. We newly detect and characterize a ULVZ beneath the Himalaya, located approximately at the edge between an (almost) isotropic and a large anisotropic region in the lowermost mantle. Using global wavefield simulations to model realistic mineral physics scenarios, we show that the seismic anisotropy is indicative of northeast-southwest flow directions. The southwestwards flow is likely induced by slab remnants at the CMB, and the ULVZ is located at the southwestern edge of the anisotropic province, which is indicative of slab-induced ULVZ displacement. 

How to cite: Wolf, J. and Long, M. D.: Slab-driven transport of ultra-low velocity material in the deep mantle, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3947, https://doi.org/10.5194/egusphere-egu23-3947, 2023.

Northeast (NE) China is located in the eastern Central Asian Orogenic Belt, and has a complex deformation history. The evolution of NE China has been controlled by the (Paleo-)Pacific Plate since the late Mesozoic and was affected by the closure of the Paleo-Asian and Mongol–Okhotsk oceans. Meanwhile, large strike-slip faults and extensive intraplate volcanisms characterize active tectonics in NE China. Different mechanisms have been proposed to interpret the origin of the intraplate volcanism, such as interactions between the lithosphere and the big mantle wedge, and the subduction-induced upwelling within the gap of the stagnant Pacific slab.

Seismic anisotropy describes the directional dependence of the seismic velocities. In NE China, seismic anisotropy not only reveals the past and present deformations in the lithosphere but also helps us clarify the possible intraplate volcanism. In this study, we apply the full-wave multi-scale anisotropy tomography method to investigate the seismic anisotropy in NE China. We measure the splitting intensities of SKS waves, which can be linearly inverted for the 3D variation of anisotropy. We employ broadband seismograms recorded at ~450 regional seismic stations (including ~250 temporary stations deployed for 2 years) of unprecedented density from teleseismic events of magnitudes greater than 5.5 occurring in 2009-2018. We obtain a total of 4249 splitting intensity measurements, and perform the multi-scale inversion using sensitivity kernels computed by normal-mode summation. The resulting 3D anisotropic model of the upper mantle in NE China shows a dominant NW-SE fast axis, which highlights a strong correlation between the intraplate volcanoes and upper-mantle seismic anisotropy, and indicates that NE China is still mainly controlled by the Pacific Plate.

How to cite: Suwen, J., Lin, Y., Zhao, L., and Chen, Q.-F.: Full-wave anisotropy tomography for the upper mantle of Northeast China using SKS splitting intensities, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4746, https://doi.org/10.5194/egusphere-egu23-4746, 2023.

Making use of the dense AlpArray and SwathD networks in the eastern Alps, a large dataset of Rayleigh phase-velocity measurements is extracted. This dataset is the basis for a 3D azimuthally anisotropic shear-velocity model of the Alpine crust. A 2-step inversion approach is followed: First, phase-velocity maps are created which are inverted for the shear velocity structure at depth in a second step. In both steps, a Bayesian (rjMcMC) approach is used to find the posterior distribution of anisotropic models. The model uncertainties are propagated from the phase-velocity maps to the depth inversion to make sure that the data is not overfitted. The final model shows a 2 layer anisotropy in the Alpine crust, the upper crustal layer is mostly orogen parallel and follows the major fault structures. The lower crustal to uppermost mantle layer shows orogen-perpendicular fast axis in the Alps and an anisotropy following the curvature of the Alps in the northern foreland. The importance of microfabric such as microcracks and oriented mineral grains is difficult to estimate from the presented model on the effective regional-scale anisotropy. But the results suggest that the azimuthal anisotropy may be largely controlled by macro-scale structures. The transition from upper to lower crustal anistotropy takes place at approx. 20 km depth which is unlikely to be due to the brittle-ductile transition. But it could indicate that upper and lower crust are only weakly coupled underneath the Alps.

How to cite: Kästle, E. D. and the AlpArray Working Group: Azimuthal Anisotropy in the Eastern Alpine Crust from Ambient Noise Tomography, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5235, https://doi.org/10.5194/egusphere-egu23-5235, 2023.

Crustal scale deformation along the fault zone and dipping Moho structures can be constrained by azimuthal seismic anisotropy. Reliable knowledge of the geometry of fault and its vertical extent in the crust and uppermost mantle that often controls observed seismic anisotropy parameters is of great importance for proper seismic hazard assessments in active tectonic settings. The North Anatolian Fault Zone (NAFZ) extending from Karlıova Triple Junction in the east to the Aegean Sea in the west poses actively deforming areas. To elucidate the crustal anisotropy along the NAFZ we will apply a novel receiver function method that simultaneously measures the Moho orientation and average bulk crustal anisotropy. It employs an algorithm in which transverse polarization component minimization (TPCM) applied on the Pms (Moho converted phase) is being integrated into a joint objective method (JOF). The method is advantageous as it restrains the random and coherent noise in the data. Prior to raw data-based anisotropic parameter estimations, we performed synthetic tests mainly considering two hypothetic models devised to be analogous to the NAFZ case. Model-1 assumes S-anisotropy in the crust oriented along N45°E with 4% of strength with flat Moho. The Model 2 involves the same anisotropic properties but with 25° of dipping Moho. Our synthetic tests show that this new approach is able to exactly resolve true model parameters assumed for Model 1 resulting in a 0.987 per cent of model accuracy. The resultant 0.225 s of time delay corresponds to ~4% of anisotropic strength considering a crustal thickness with 30 km and 4.8 km/s average isotropic S-wave velocity for the medium. Our results obtained for Model-2 still tend to converge the true model parameters but show slight discrepancies, in particular, for anisotropic parameters resolved with 0.3 s of time delay and N40°E oriented fast wave azimuth. The test for Model-2 achieves 27.5° as the dipping angle of Moho which is fairly close to its true model parameter. We observe relatively low model accuracy with 0.710 per cent in the case of Model-2. At the further stage of this work, we will utilize digital waveforms of teleseismic earthquakes recorded at the KOERI and AFAD permanent seismic station networks along the NAFZ.   

How to cite: Keleş, D., Eken, T., Wang, P., Huang, Z., and Taymaz, T.: Novel Application of Receiver Function Analyses Dependent on Splitting Measurement: Crustal Anisotropy along the NAFZ, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6183, https://doi.org/10.5194/egusphere-egu23-6183, 2023.

Seismic anisotropy measurements provide a lot of information on the deformation and structure of the Earth’s interior, in particular of the upper mantle. Conventional methods of measurement of anisotropy have their limitations, especially regarding depth resolution. Splitting intensity (SI) is a seismic observable, related to the amount of energy on the transverse component waveform and, to a first order, it is linearly related to the elastic perturbations of the medium through the 3-D sensitivity kernels, that can be therefore inverted, allowing a high-resolution image of the upper-mantle anisotropy. Starting from synthetic SKS waveforms, we first derived high-quality SKS splitting intensity measurements; then we used the splitting intensity data as input into tomographic inversion. This approach enables high‐resolution tomographic images of horizontal upper‐mantle anisotropy through recovering vertical and lateral changes in anisotropy and represents a propaedeutic step to the real cases of subduction settings. Additionally, this approach was able to detect regions of strong dipping anisotropy by allowing a 360° periodic dependence of the splitting vector. Single and thick layers of dipping angles between 30 and 60° are clearly represented with a high dt₂ value, while double layers or nearly vertical dips are more difficult to identify.

How to cite: Confal, J. M., Baccheschi, P., Pondrelli, S., VanderBeek, B. P., Karakostas, F., and Faccenda, M.: Testing the splitting intensity methodology to retrieve average, dipping, and depth dependent anisotropy from a complex subduction model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7321, https://doi.org/10.5194/egusphere-egu23-7321, 2023.

How to cite: Ward, J., Walker, A., Nowacki, A., Panton, J., and Davies, H.: The Memory of the Mantle: The Influence of a Time Varying Flow Field on Present Day Observations of Seismic Anisotropy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7527, https://doi.org/10.5194/egusphere-egu23-7527, 2023.

Despite the well known anisotropic structure of Earth’s upper mantle, the effect of seismic anisotropy on the construction of body wave shear velocity models remains largely ignored. Ignoring anisotropic heterogeneity can introduce significant model artefacts that may be misinterpreted as compositional and thermal heterogeneities. While effective anisotropic imaging strategies that improve model reconstruction have been developed for P-wave delay times, no such general framework exists for S-waves partly because, unlike P-waves, there is not a simple ray-based methodology for predicting S-wave travel-times through anisotropic media. Here, we apply a new methodology for the inversion of relative shear wave delay times and splitting intensity measurements for arbitrarily oriented hexagonally anisotropic model parameters using data collected across the western United States and Cascadia subduction system. We detail the data analysis procedure required for making measurements of shear wave observables suitable for anisotropic inversions (e.g. determination of incoming polarisation directions). We then present a preliminary anisotropic shear wave velocity model for Cascadia and compare the results to purely isotropic images. The imaged anisotropic heterogeneity is compared to the well-established patterns in shear wave splitting parameters observed in the study area.

How to cite: VanderBeek, B., Lo Bue, R., and Faccenda, M.: Imaging Upper Mantle Anisotropic Structure Using Teleseismic Shear Wave Delays and Splitting Intensity: Application to the Cascadia Subduction Zone, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7790, https://doi.org/10.5194/egusphere-egu23-7790, 2023.

Seismic anisotropy is a fundamental key to gain knowledge of the mantle dynamics and structure. The image of seismic anisotropy over the upper mantle can be obtained with several methods, including surface waves and SKS splitting measurements. Taken together, these anisotropic measurements contribute to extensively catch anisotropy at different depths, yielding insights into the structure and dynamics of the crust and upper mantle. Nevertheless, mantle images resulting from surface waves result in poor lateral resolution, while the nearly vertically propagating SKS waves, when interpreted in a ray-based framework, results in little or no depth resolution, not allowing to easily image the distribution of the anisotropy through depth. Though the anisotropic seismic nature of the upper mantle is well established by a wealth of observational research, most of common teleseismic body-wave tomography studies neglect P- and S-wave anisotropy, thus producing artefacts in tomographic models in terms of amplitude and localization of heterogeneities. To overcome this problem different tomographic methods have been implemented to invert SKS splitting observations for anisotropic structures, most of which based on finite-frequency sensitivity kernels that relate elastic model perturbations to splitting observations. In this study we adopted the tomographic method relying on the inversion of the splitting intensity, a measure of the amount of energy on the transverse component of the waveform. Since is linearly related to the elastic perturbations of the medium through the 3-D sensitivity kernels, SI can therefore be easily inverted, providing the basis for a better interpretation of shear wave splitting measurements. In this study, we first compute the splitting intensity (SI) and splitting parameters using teleseismic shear-wave recorded at 824 available permanent and temporary stations in Italy and surrounding regions. Then, the dataset of SI has been used as an input for the tomographic inversion. The results obtained show changes of the anisotropic properties with depth, especially for the strength of anisotropy. A progressive depth-increase in anisotropy intensity has been recovered over Italy, affecting the bulge of the Alps and Apennines chain and the southern Tyrrhenian subduction system. On the contrary, weaker anisotropy characterizes the transition zone from the Apenninic to Alps domain beneath the Po plain and the Adriatic domain. The anisotropic tomography models obtained in this study allowed us to recover for the first time a new 3D-imaging of seismic anisotropy of Italy down to the deeper layers, allowing to better understand the dynamic of asthenospheric mantle flow and its relation with subducting plate, as well as the rheology of the continental lithosphere.

How to cite: Baccheschi, P., Confal, J. M., Pondrelli, S., Faccenda, M., VanderBeek, B. P., and Huang, Z.: High-resolution imaging of the deep structure of Italy through SKS anisotropy tomography, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8129, https://doi.org/10.5194/egusphere-egu23-8129, 2023.

The Mediterranean region is an active plate margin characterized by the presence of both oceanic and continental lithosphere. Its tectonic history is marked by intense seismic and volcanic activity triggered by episodes of continental collision and slab rollback leading to the formation of mountain ranges and extensional basins. Our understanding of the structural heterogeneity and tectonic complexity of this region requires accurate imaging of the subsurface. Seismic anisotropy is a key parameter commonly used to study flow in the mantle and its relations with plate motions. In this study we present a three-dimensional anisotropic seismic tomography of the entire Mediterranean area performed using travel time from the new “Global Catalog of Calibrated Earthquake Locations” by Bergman et al. (2023). We present purely isotropic and anisotropic solutions. Compared to isotropic tomography, it is found that including the magnitude, azimuth, and, importantly, dip of seismic anisotropy in the inversions simplifies isotropic heterogeneity by reducing the magnitude of slow anomalies while yielding anisotropy patterns that are consistent with regional tectonics. The isotropic component of our preferred tomography model is dominated by numerous fast anomalies associated with retreating, stagnant, and detached slab segments. In contrast, relatively slower mantle structure is related to slab windows and the opening of back-arc basins. The anisotropic patterns reveal the deformation history of the area which has been characterized by intermittent phases of collision and tectonic relaxation. A diversity of dip angles is observed with near-horizontal and more steeply dipping fabrics found in different areas of the Entire Mediterranean, probably reflecting the entrainment effect of horizontal or vertical asthenospheric flows, respectively. We interpreted the high velocity zones of our best solution as subducting lithosphere and starting from this interpretation we built a 3D reconstruction of the main slabs found in the study region. To perform the tomography, we used the method proposed by Vanderbeek and Faccenda (2021) and already used by Rappisi et al. (2022) in a similar study on the Central Mediterranean area. This work returns the first anisotropic tomography of the entire Mediterranean and demonstrates the importance of seismic anisotropy to better constrain the upper mantle.

Bergman, E. A., Benz, H. M., Yeck, W. L., Karasözen, E., Engdahl, E. R., Ghods, A., ... & Earle, P. S. (2023). A Global Catalog of Calibrated Earthquake Locations. Seismological Society of America, 94(1), 485-495.

Rappisi, F., VanderBeek, B. P., Faccenda, M., Morelli, A., & Molinari, I. (2022). Slab Geometry and Upper Mantle Flow Patterns in the Central Mediterranean From 3D Anisotropic P‐Wave Tomography. Journal of Geophysical Research: Solid Earth, 127(5), e2021JB023488.

VanderBeek, B. P., & Faccenda, M. (2021). Imaging upper mantle anisotropy with teleseismic P-wave delays: insights from tomographic reconstructions of subduction simulations. Geophysical Journal International, 225(3), 2097-2119.

How to cite: Rappisi, F., VanderBeek, B. P., and Faccenda, M.: Seismic anisotropy tomography: new insight into upper mantle structure and dynamics beneath the Mediterranean region, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8301, https://doi.org/10.5194/egusphere-egu23-8301, 2023.

Most information on the P-wave seismic velocity of the lower continental is based on controlled-source wide-angle seismic experiments, for which the direction of wave propagation in the target region is largely horizontal. Following the common practice of interpreting such data in an isotropic framework, too high P-wave velocities would therefore be inferred in an anisotropic lower continental crust, for which the long axis of the anisotropy ellipsoid is roughly aligned with the horizontal direction. This, in turn, would result in a bias of the interpretation of the lower crustal bulk composition towards the mafic side and potentially also lead to incorrect estimations of crustal thickness. Anisotropy of this type can arise from the small-scale sub-horizontal alignment of anisotropic minerals and/or from large-scale sub-horizontal layering. To assess the likely importance of lower crustal anisotropy in general and the respective contributions of mineral fabric and layering in particular in Ivrea-type lower continental crust, we analyze a range of layered canonical models. The individual layers are parameterized based on published laboratory measurements of seismic velocities from pertinent rock samples. Our preliminary results indicate that anisotropy related to the mineralogical composition and fabric prevails over the corresponding effects of layering. For the considered canonical models, these effects are particularly prominent in the presence of metapelitic rocks, where a petrological interpretation of the inferred average horizontal P-wave velocity could indeed lead to a notable overestimation of the mafic component. Conversely, our initial results also indicate that in the absence of metapelitic rocks, the anisotropy-induced velocity bias may be sufficiently benign to allow for a reasonably reliable interpretation of the bulk composition of the lower continental crust based on the P-wave velocity inferred from wide-angle seismic data.

How to cite: Luo, Z., Müntener, O., Hetényi, G., and Holliger, K.: Effects of large-scale layering and small-scale mineral fabric on the seismic anisotropy of Ivrea-type lower continental crust, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8453, https://doi.org/10.5194/egusphere-egu23-8453, 2023.

 Characterized by persistent eruptive activity associated with a complex interaction between magma in its plumbing system and an articulated tectonic and geodynamic context, Mt. Etna (Sicily, Italy) is one of the most hazardous and monitored volcanoes in the world. Since the late 1990s, several seismic and tomographic experiments have been performed to obtain accurate images of the shallow-intermediate P-wave velocity structures of the volcano. Unfortunately, seismic tomography models, in particular those derived from body waves, typically relies on the approximation of seismic isotropy. This is a poor assumption considering that P-waves exhibit strong sensitivity to anisotropic fabrics and neglecting anisotropic heterogeneity can introduce significant velocity artefacts that may be misinterpreted as compositional and thermal heterogeneities (VanderBeek & Faccenda,2021; Lo Bue et al, 2022). Here, we discard the isotropic approximation and invert for P-wave isotropic (mean velocity) and anisotropic (magnitude of hexagonal anisotropy, azimuth and dip of the symmetry axis) parameters using the methodology proposed by VanderBeek & Faccenda (2021). We use active and passive seismic data collected by the TOMO-ETNA experiment (Ibanez et al. 2016a, b; Coltelli et al. 2016) between June and November 2014. We present 3D anisotropic P-wave tomography models of Etna volcano and compare them with purely isotropic images. Discriminating the anisotropic structures from the velocity artifacts allows to better recover the isotropic and anisotropic crustal structures and to improve our understanding on the major regional fault systems and on the processes that control magma and fluids ascent beneath the volcanic edifice.

Coltelli, M., Cavallaro, D., Firetto Carlino, M., Cocchi, L., Muccini, F., D'Aessandro, A., ... & Rapisarda, S. (2016). The marine activities performed within the TOMO-ETNA experiment. Annals of Geophysics.

Ibáñez, J. M., Prudencio, J., Díaz-Moreno, A., Patanè, D., Puglisi, G., Lühr, B. G., ... & Mazauric, V. (2016a). The TOMO-ETNA experiment: an imaging active campaign at Mt. Etna volcano. Context, main objectives, working-plans and involved research projects. Annals of Geophysics, 59(4), S0426-S0426.

Ibáñez, J. M., Díaz-Moreno, A., Prudencio, J., Patené, D., Zuccarello, L., Cocina, O., ... & Abramenkov, S. (2016b). TOMO-ETNA experiment at Etna volcano: activities on land. Annals of Geophysics, 59(4).

Lo Bue, R., Rappisi, F., Vanderbeek, B. P., & Faccenda, M. (2022). Tomographic Image Interpretation and Central-Western Mediterranean-Like Upper Mantle Dynamics From Coupled Seismological and Geodynamic Modeling Approach. Frontiers in Earth Science, 10, 884100.

VanderBeek, B. P., & Faccenda, M. (2021). Imaging upper mantle anisotropy with teleseismic P-wave delays: insights from tomographic reconstructions of subduction simulations. Geophysical Journal International, 225(3), 2097-2119.

How to cite: Lo Bue, R., Faccenda, M., Cocina, O., Rappisi, F., and Vanderbeek, B. P.: Joint Active and Passive P-wave Tomography reveals Mt. Etna's Seismic Anisotropy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8563, https://doi.org/10.5194/egusphere-egu23-8563, 2023.

Olivine is a major phase in the upper mantle and its crystallographic preferred orientation (CPO) carries strong implications for the interpretation of seismic anisotropy and geodynamic models of the upper mantle. The computational model D-Rex (Kaminski et al., 2004) is often used to depict the evolution of olivine CPO under various flow patterns. In its tracing of the crystallographic orientation of olivine and orthopyroxene aggregate D-Rex includes the process of dynamic recrystallization, a fundamental process associated with deformation under dislocation creep. Dynamic recrystallization and deformation mechanism are incorporated in D-Rex via different parameters - the efficiency of nucleation of new grains, grain boundary mobility, and the threshold value below which the grains deform by grain boundary sliding (GBS) (i.e., deformation does not result in rotation or recrystallization). These parameters were benchmarked with experiments to fit the overall CPO evolution. While D-Rex is set to predict the CPO, an appraisal of other pivotal microstructural properties like grain size, dislocation density, and recrystallization fraction has been neglected. Here, we use the implementation of D-Rex within ASPECT to model the shearing of grain to first trace the microstructural properties and further test how they are affected by the dynamic recrystallization parameters. We synthesize the results of tens of runs under a range of parameter space and strains. We observe that for grain size distribution, as the nucleation and threshold value for GBS increase the spread of grain sizes is relatively low while increasing mobility causes a large spread of grain size associated with a small group of grains that dominate the overall CPO. At low strains, the intermediate-sized grains are the major contributor to the CPO while with increasing strain, a smaller fraction of large grains dominate the CPO. Further, we find that the biggest grains keep growing bigger, while the smallest grains oscillate around the GBS threshold. Our analysis highlights the gap between the natural evolution of olivine microstructure and the microstructural properties evolution in D-Rex. The use and implications of different suggested parameters will be discussed. 

  Kaminski, É., Ribe, N. M., & Browaeys, J. T. (2004). D-Rex, a program for calculation of seismic anisotropy due to crystal lattice preferred orientation in the convective upper mantle. Geophysical Journal International, 158(2), 744–752. https://doi.org/10.1111/j.1365-246X.2004.02308.x

How to cite: Vedavyas, S., Fraters, M., Billen, M., and Boneh, Y.: Appraisal of D-Rex parameterization in simulating olivine crystallographic preferred orientation (CPO) evolution using microstructural properties, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9453, https://doi.org/10.5194/egusphere-egu23-9453, 2023.

The implementation of stochastic methods in seismic tomography arises as a response to the limitations introduced by traditional non-linear optimization solvers. Since tomographic problems are generally ill-conditioned, additional constraints on the model are set in the misfit function, and the weight given to each minimization term has a level of arbitrariness; different solutions are obtained with different choices for the damping/smoothing factors. Non-linear optimization solvers are based on a perturbative approach which linearizes the forward modelling locally around a reference model, updated at each iteration until convergence. These methods need the evaluation of the derivatives of the predictions with respect to the parameters of the model, which is not always an easy task, and they generally do not provide the uncertainties associated with the solution model.
The Reversible-Jump Markov-Chain Monte Carlo is a stochastic method which performs a random walk in the model space sampling the posterior probability distribution associated with the model in the light of the observations. This method is a trans-dimensional Metropolis-Hastings where the number of parameters used to represent the continuous fields (as interpolation nodes) is treated as a parameter itself of the inversion, as the positions of the nodes. Using statistical estimators on the ensemble of models produced by the algorithm it is possible to extract a reference model, typically as an average of the ensemble. With this method no regularization is needed, and uncertainty can be estimated using the ensemble of models sampled. The limitations of non-linear optimization solvers are overcome at the cost of an increase in the computational time required.

The presented applications of this method involve seismic tomography in subduction zones, where the anisotropic component of the seismic velocity field is relevant, and the inversion of seismological data could provide an interesting insight into the dynamics of these regions. 

How to cite: Del Piccolo, G., VanderBeek, B., Faccenda, M., Morelli, A., and Byrnes, J.: Reversible-Jump, Markov-Chain Monte Carlo seismic tomographic inversion for anisotropic structure in subduction zones, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9927, https://doi.org/10.5194/egusphere-egu23-9927, 2023.

Seismic anisotropy is an observation that is believed to yield information on the flow pattern in the mantle. There are many studies of anisotropy in the upper mantle; however, the lower mantle is still underexplored, due to problems in seismic imaging and complexities of modelling of flow laws of
different minerals. In this study, we modelled the radially anisotropic behavior of two different geodynamic setups, one is the rising of a mantle plume from the core-mantle boundary to the surface, and another is subduction of a slab reaching the lowermost mantle. We use ASPECT for modelling large scale mantle flow and ECOMAN to simulate the development of lattice preferred orientation of mantle fabric. We use the slip system of Bridgmanite following the previous experimental study by Mainprice et al. (2008). We then couple the results from ASPECT to ECOMAN for modelling the radial anisotropy and maximum shear wave splitting direction. We show that in the part of the lowermost mantle surrounding the plume horizontally polarized shear waves (Vsh ) are faster than the vertically polarized ones (Vsv ) while the inside of the plume tail shows opposite signature. However, Vsh becomes greater than Vsv when the plume flattens out at the surface. We also find that the maximum splitting direction is horizontal outside the base of the plume and it becomes vertical inside the plume tail and again becomes horizontal at the surface. This result corroborates previous seismic observations (Wolf et al., 2019) of the Iceland plume at the core-mantle boundary. Moreover, our result for the slab setup reveals that as the slab reaches the lowermost mantle, Vsh becomes higher than Vsv and maximum splitting is horizontal at the base of the slab.

References
Wolf, J., Creasy, N., Pisconti, A., Long, M.D., Thomas, C., 2019. An investigation of seismic anisotropy in the lowermost mantle beneath Iceland. Geophys. J. Int. 219, S152–S166.

Mainprice, D., Tommasi, A., Ferré, D., Carrez, P., Cordier, P., 2008. Predicted glide system and crystal preferred orientations of polycrystalline silicate Mg-
perovskite at high-pressure: implicaitons for the seismic anisotropy in the lower mantle. Earth Planet. Sci. Lett. 271, 135–144.

How to cite: Roy, P. and Steinberger, B.: Modelling of seismic anisotropy in the lowermost mantle with rheologically constrained geodynamic setup, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10061, https://doi.org/10.5194/egusphere-egu23-10061, 2023.

Many researchers have used S-to-P (Sp) converted waves to detect the Moho discontinuity, the lithosphere-asthenosphere boundary (LAB) and mid-lithospheric discontinuities (MLD). The anisotropy of Earth’s lithosphere is typically constrained with shear-wave birefringence.  Both theory and reflectivity computations, however, argue for a substantial influence of anisotropy on the initial amplitude of the Sp converted wave.  The effects of compressional anisotropy on initial Sp amplitudes are stronger than the effects of shear anisotropy for anisotropy with a tilted axis of symmetry, a geometry that is often neglected in birefringence interpretations.  This Sp behavior is not typically studied, but it has the potential to test the hypothesis that the seismic lithosphere-asthenosphere boundary (LAB) is caused by a transition in anisotropic layering at the base of Earth’s tectonic plates.

We develop and apply multiple-taper correlation estimates for Sp receiver functions, applicable to either SV or SH incoming polarization, or for a linear combination of SV and SH. In the context of incoming SV-polarized body waves, e.g., SKS phases, algorithms from multiple-taper Ps RFs can be borrowed to apply moveout corrections before the Fourier transform to target a particular interface depth in the crust or mantle.  With synthetic seismograms, we find that SH “receiver functions” can be computed from incoming SV waves, promising a diagnostic detecting SKS birefringence and to estimate an average splitting signal from a station. The SV and SH waveforms can be “unsplit” in the frequency domain by the estimated average birefringence to reconstruct the S waves that impinge the lithosphere from the deep mantle.  We will report analyses with data from permanent stations of the Global Seismographic Network and the USGS ANSS.

How to cite: Park, J., Chen, X., and Levin, V.: Detecting Anisotropy from Back-Azimuth Amplitude Dependence of Sp Converted Waves, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10460, https://doi.org/10.5194/egusphere-egu23-10460, 2023.

Shear wave splitting (SWS) is a widely used technique to study the anisotropic properties of the Earth’s interior. The geological structure of the southeastern Korean Peninsula is represented as the Yangsan fault and Ulsan fault, which is controlled by the present-day compressional stress regime in the ENE-WSW direction. We analyzed shear wave splitting to understand the anisotropic features of the upper crust above the hypocentral depth in the southeastern Korean Peninsula using the local earthquake data from the Gyeongju Hi-density Broadband Seismic Network (GHBSN). The GHBSN is a dense array composed of 200 broadband stations, which covers an area of about 60×60 km² in the southeastern Korean Peninsula. We used the MFAST program (Savage et al., 2010) to measure the SWS parameters of fast polarization and delay time from shear waves of local earthquakes from January 2019 to December 2020. In addition, the TESSA program (Johnson et al., 2011) was employed to inspect the spatial variation in the anisotropy of the study region. To obtain reliable measurements of SWS parameters, rigorous constraints including quality control of the original waveforms were applied, and then, cycle-skipped measurements were manually removed. In final, we obtained the SWS measurements of 4260 records. Because the seismicity in the region is concentrated at the epicentral region of the 2016 Gyeongju earthquake sequence and the hanging wall of the Ulsan fault, raypaths are limited to a narrow azimuthal range. Both the raw and spatially averaged fast-polarization directions are dominant to be parallel either to major faults (structural anisotropy) or to the ENE-WSW (stress-induced anisotropy). Also, some stations and regions show bi- or multi-modal rose diagram of the SWS, representing that there is more than one factor of anisotropy to induce the SWS. The delay time of the SWS showed the right-skewed distribution. Tomographic result of the SWS delay time shows that, relatively high anisotropy is observed at the epicentral region of the 2016 Gyeongju earthquake sequence and the hanging wall of the Ulsan fault. It implies that microcracks at these regions are better developed compared to the remaining regions.

How to cite: Baek, J., Kang, T.-S., Heo, D., Ree, J.-H., Kim, K.-H., Rhie, J., and Kim, Y.: Upper crustal anisotropy in the southeastern Korean Peninsula from shear wave splitting of local earthquakes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10928, https://doi.org/10.5194/egusphere-egu23-10928, 2023.

3-component regional waveforms for ~14700 raypaths sampling India, Himalaya and Tibet, have been used for multi-taper polarization analysis of surface waves between periods of 10 and 120 s. Rayleigh (LR) and Love (LQ) wave energy arriving at the station within +/- 10 degree of the theoretical back azimuth have been used to compute 1D path average fundamental mode group velocity dispersion. Theoretical dispersion of fundamental and first two higher modes have been computed using each 1D path average velocity structure constructed from CRUST1.0 over IASP91 mantle model. These are compared with the observed dispersion dataset to identify and remove those periods with higher mode overlap with the observed fundamental mode picks. The shortlisted dispersion datasets consist of ~90% of the original dataset. To ascertain the lateral variation in the group velocity, the observed dispersion has been used to compute 2D tomography maps of LR and LQ group velocities at discrete periods between 10 and 120 s. From these maps, seven 2D profiles across northern India, Himalaya and Tibet have been extracted for modeling the radially anisotropic shear-wave velocity structure of the lithosphere. Haskell-Thompson (H-T) matrix method is used to calculate synthetic LQ dispersion. For the LR dispersion, the H-T method with reduced delta matrix has been used, considering Vph≠Vpv and Eta≠1. The inversion scheme uses genetic algorithms (GA) to search the model space parameterized using Vsh, Vph, Xi and thickness for 3 crustal layers and 2 mantle layers underlain by a mantle half-space. Synthetic tests have been performed using theoretical LR and LQ dispersion curves, computed from global models with 5% and 10% anisotropy, introduced in the mantle layers. The fit to the synthetic LR and LQ dispersion data and the model recovery using GA inversion is satisfactory for such tests. This inversion scheme is being applied to the observed LR and LQ dispersion data from the seven profiles and the results will be presented.

How to cite: Ghosh, M., Chakraborty, A., Dey, S., Kharjana, I., Sharma, S., Bhattacharya, S. N., and Mitra, S.: Radially anisotropic shear-wave velocity structure of northern India, Himalaya and Tibet, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11721, https://doi.org/10.5194/egusphere-egu23-11721, 2023.

We investigate the complex flow field around the Vrancea slab in Romania, a steeply sinking seismogenic lithospheric block that experienced lateral tear-off and possible rotation. The Vrancea slab is located beneath the South-East Carpathians and generates frequent seismicity despite its remote location from active collisional boundaries. We analyse core-refracted shear wave (SKS) splitting recorded by permanent broadband seismic stations from the Romanian Seismic Network for periods up to 10 years, and compare our results with seismic tomography of the upper mantle. We identify several stations with large backazimuthal variations of SKS fast axis polarization and delay times both in the slab hinge zone, the back-arc and the circumslab region, indicating complex mantle deformation patterns. To investigate the effect of a two-level rotated slab we invert SKS waveforms using cross-convolutional misfit combined with a neighbourhood search algorithm to model two layers of anisotropy. In the shallow mantle, anisotropy aligns with the upper slab strike and reorients along the strike of the lower slab at depths below the hinge zone. In the backarc trench-perpendicular anisotropy switches to trench-parallel, due to the recent retreat and roll-back of the slab. Our results have important implications for understanding SKS interference from subducted slab fragments and provide evidence of the recent retreat, break-off and rotation of two Vrancea slab levels sinking into the upper mantle.

How to cite: Petrescu, L., Mihai, A., and Borleanu, F.: Slab tear and rotation imaged with core-refracted shear wave anisotropy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12807, https://doi.org/10.5194/egusphere-egu23-12807, 2023.

The seismic imaging of radial anisotropy can be used as a proxy for the direction of mantle flow. Previous studies have imaged radial anisotropy throughout the mantle on a global scale and are starting to show some consistent features. However, the interpretation of existing models is hindered by the lack of uncertainties provided from the employed inversion method. To address this, we build a new global radially anisotropic model of the Earth’s upper mantle which consists of two main stages. Firstly, we build global Rayleigh and Love wave phase and group velocity maps using ~47 million measurements, including fundamental mode and up to 5^th overtone measurements, and compute their associated uncertainties. Weights according to similar paths and data uncertainties are employed in the inversions. We construct a total of 310 2D maps, at periods between T16-375 s, expanded in spherical harmonics up to degree lmax=60 and observe many relevant small-scale structures, such as e.g. the curvature of the Tibetan plateau at T~40s (fundamental mode). As expected, uncertainties are higher in regions of poor data coverage (e.g., southern hemisphere and oceans). Then, we invert for 1D profiles of radial anisotropy using two Monte Carlo based inversion methods: the Neighbourhood Algorithm (NA) and the Reversible-Jump Markov Chain Monte Carlo algorithm (RJMCMC). The NA has been widely used for seismic inversion, as it efficiently explores the parameter space. However, the advantage of the RJMCMC is that in addition to constraining e.g. radial anisotropy, it can also constrain e.g. layer thickness. We compare the 1D profiles from both methods, and their associated uncertainties, which will lead to a new global 3D model of radial anisotropy.

How to cite: Sturgeon, W., Ferreira, A. M. G., and Fox, M.: Towards constraining mantle flow through imaging of radial anisotropy, with full uncertainty quantification, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13670, https://doi.org/10.5194/egusphere-egu23-13670, 2023.