EMRP1.2 | The Mechanics of Earthquake Faulting: a multiscale approach
The Mechanics of Earthquake Faulting: a multiscale approach
Co-organized by SM4/TS2
Convener: Stefano Aretusini | Co-conveners: Matteo Demurtas, Michele Fondriest, Gina-Maria Geffers, Francois Passelegue, Berit Schwichtenberg
Orals
| Mon, 24 Apr, 08:30–12:25 (CEST)
 
Room K1
Posters on site
| Attendance Tue, 25 Apr, 10:45–12:30 (CEST)
 
Hall X3
Orals |
Mon, 08:30
Tue, 10:45
Earthquake mechanics is controlled by a spectrum of processes covering a wide range of length scales, from tens of kilometres down to few nanometres. The geometry of the fault/fracture network and its physical properties control the global stress distribution and the propagation/arrest of the seismic rupture. At the same time, earthquake rupture nucleation, rupture and arrest are governed by fracture propagation and frictional processes occurring within extremely localized sub-planar slipping zones. The co-seismic rheology of the slipping zones themselves depends on deformation mechanisms and dissipative processes active at the scale of the grain or asperity. The study of such complex multiscale systems requires an interdisciplinary approach spanning from structural geology to seismology, geophysics, petrology, rupture modelling and experimental rock deformation. In this session we aim to convene contributions dealing with different aspects of earthquake mechanics at various depths and scales such as:

- the thermo-hydro-mechanical processes associated with co-seismic fault weakening based on rock deformation experiments, numerical simulations and microstructural studies of fault rocks;
- the study of natural and experimental fault rocks to investigate the nucleation mechanisms of intermediate and deep earthquakes in comparison to their shallow counterparts;
- the elastic, frictional and transport properties of fault rocks from the field (geophysical and hydrogeological data) to the laboratory scale (petrophysical and rock deformation studies);
- the internal architecture of seismogenic fault zones from field structural survey and geophysical investigations;
- the modeling of earthquake ruptures, off-fault dynamic stress fields and long-term mechanical evolution of realistic fault networks;
- the earthquake source energy budget and partitioning between fracture, friction and elastic wave radiation from seismological, theoretical and field observations.
- the interplay between fault geometry and earthquake rupture characteristics from seismological, geodetic, remote sensed or field observations;

We particularly welcome novel observations or innovative approaches to the study of earthquake faulting. Contributions from early career scientists are solicited.

Orals: Mon, 24 Apr | Room K1

Chairpersons: Stefano Aretusini, Michele Fondriest, Berit Schwichtenberg
08:30–08:35
08:35–08:45
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EGU23-10016
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ECS
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solicited
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On-site presentation
Carolina Giorgetti, Marie Violay, and Cristiano Collettini

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.

08:45–08:55
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EGU23-7933
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On-site presentation
Alexis Cartwright-Taylor, Maria-Daphne Mangriotis, Ian G. Main, Ian B. Butler, Florian Fusseis, Martin Ling, Edward Andò, Andrew Curtis, Andrew F. Bell, Alyssa Crippen, Roberto E. Rizzo, Sina Marti, Derek D. V. Leung, and Oxana V. Magdysyuk

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.

08:55–09:05
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EGU23-8778
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On-site presentation
Francois Renard, Benoit Cordonnier, Mai-Linh Doan, Michele Fondriest, Bratislav Lukic, and Erina Prastyani

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.

09:05–09:15
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EGU23-8634
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ECS
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On-site presentation
Sofia Michail, Paul Antony Selvadurai, Sara Beth Leach Cebry, Antonio Felipe Salazar Vásquez, Patrick Bianchi, Markus Rast, Claudio Madonna, and Stefan Wiemer

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= 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 (Pc = 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= 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= 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.

09:15–09:25
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EGU23-1330
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ECS
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Virtual presentation
Tatiana Kartseva and Nikolai Shapiro

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.

09:25–09:35
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EGU23-3982
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ECS
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On-site presentation
Navid Kheirdast, Michelle Almakari, Carlos Villafuerte, Marion Y. Thomas, and Harsha S. Bhat

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.

09:35–09:45
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EGU23-6517
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On-site presentation
Guilhem Mollon, Jérôme Aubry, and Alexandre Schubnel

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.

09:45–09:55
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EGU23-16290
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On-site presentation
Harsha Bhat, Michelle Almakari, Navid Kheirdast, Carlos Villafuerte, and Marion Thomas

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.

09:55–10:05
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EGU23-6974
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ECS
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On-site presentation
Roxane Ferry, Marion Thomas, and Louise Jeandet

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.

10:05–10:15
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EGU23-7849
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ECS
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On-site presentation
Carlos Villafuerte, Kurama Okubo, Esteban Rougier, Raul Madariaga, and Harsha S. Bhat

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.

Coffee break
Chairpersons: Stefano Aretusini, Berit Schwichtenberg, Gina-Maria Geffers
10:45–10:55
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EGU23-2911
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solicited
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On-site presentation
Andre R. Niemeijer, Evangelos Korkolis, Tanmaya Mishra, Rens Elbertsen, Beunen Jop, and Ivan Pires de Vasconselos

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.

10:55–11:05
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EGU23-531
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ECS
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On-site presentation
Simone Masoch, Michele Fondriest, Rodrigo Gomila, Giorgio Pennacchioni, José Cembrano, and Giulio Di Toro

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.

11:05–11:15
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EGU23-9616
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ECS
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On-site presentation
Wei Feng, Lu Yao, Rodrigo Gomila, Shengli Ma, and Giulio Di Toro

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 th and, (2) the frictional healing rate βμ/log th >0. This increase in fault frictional strength with th 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 (σeff =50MPa), and temperature (T) ranging from 25 to 400 ℃  under dry or pore fluid (deionized H2O) pressure (Pf=30 MPa) conditions. In the SHS sequence, the imposed slip velocity was V=10 μm/s, and hold time th 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 th, consistent with previous experiments. Moreover, the Δμ and β values in the 2nd SHS sequence are slightly higher than those in the 1st 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.

11:15–11:25
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EGU23-16777
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ECS
|
On-site presentation
Raphael Affinito, Derek Elsworth, and Chris Marone

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 σn’, with Ppnratios 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.

11:25–11:35
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EGU23-14607
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On-site presentation
Alexandros Stathas and Ioannis Stefanou

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.

11:35–11:45
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EGU23-11776
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On-site presentation
|
Vito Rubino, Ares Rosakis, and Nadia Lapusta

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.

11:45–11:55
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EGU23-16320
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On-site presentation
Fabian Barras, Kjetil Thøgersen, Einat Aharonov, and François Renard

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.

11:55–12:05
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EGU23-10314
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ECS
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On-site presentation
Gaohua Zhu and Hongfeng Yang

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.

12:05–12:15
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EGU23-5033
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On-site presentation
Vincent Roche, Mirko van der Baan, and John Walsh

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.

12:15–12:25
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EGU23-7142
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ECS
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On-site presentation
Suli Yao, Hongfeng Yang, and Ziyue Tang

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.

Posters on site: Tue, 25 Apr, 10:45–12:30 | Hall X3

Chairpersons: Stefano Aretusini, Berit Schwichtenberg, Francois Passelegue
X3.1
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EGU23-7167
Men-Andrin Meier, Federica Lanza, and Patricia Martinez-Garzon

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.

X3.2
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EGU23-431
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ECS
|
|
Rossella Fonzetti, Luisa Valoroso, Pasquale De Gori, and Claudio Chiarabba

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.

X3.3
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EGU23-15563
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ECS
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Piercarlo Giacomel, Daniel Faulkner, Valère Lambert, and Michael Allen

In the framework of empirically-derived rate- and state- friction (RSF) laws, friction constitutive parameters a, b, and Dc  (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, Dc2,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 Dc1 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 Dc2 may significantly differ if Dc2 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 Dc2.

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 Dc2 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.

X3.4
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EGU23-6468
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ECS
|
Mattia Pizzati, Nicolò Lieta, Anita Torabi, Luca Aldega, Fabrizio Storti, and Fabrizio Balsamo

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.

X3.5
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EGU23-7327
Janis Aleksans, Conrad Childs, and Martin Schöpfer

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.

X3.6
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EGU23-7594
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ECS
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Nathalie Casas, Carolina Giorgetti, Cristiano Collettini, and Marco Maria Scuderi

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.

X3.7
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EGU23-8163
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ECS
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Job Arts, André Niemeijer, Martyn Drury, Ernst Willingshofer, and Liviu Matenco

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.

X3.8
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EGU23-8389
Wen-Jie Wu, Li-Wei Kuo, Chia-Wei Kuo, Wei-Hsin Wu, and Hwo‐Shuenn Sheu

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.

X3.9
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EGU23-2537
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Highlight
Giulio Di Toro, Alessio Chiesurin, Elena Spagnuolo, Rodrigo Gomila, and Sukanta Roy

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 11th, 1967 (ML6.3, 177 casualties), the largest human-induced earthquake so far. Major earthquakes (ML>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 ML>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.

X3.10
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EGU23-9600
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ECS
Rodrigo Gomila, Wei Feng, and Giulio Di Toro

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+thold /tcutoff)) – 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 2nd and 3rd with 5000s, and geochemically contrasted against a non-sheared sample. The fluid (deionized water) saturated gouges were kept under an effective normal stress (σneff) of 10 MPa, a fixed temperature T of 300°C and a constant pore fluid pressure Pf 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 (1st and 2nd experiments) and from 3s to 300000s (3rd), 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 1st, 2nd and 3rd 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.

X3.11
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EGU23-10779
Sylvain Barbot

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.

X3.12
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EGU23-14284
Chiara Cornelio, Stefano Aretusini, Elena Spagnuolo, Giulio Di Toro, and Massimo Cocco

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.