TS4.1

We present the results of the laboratory studies on fluid-initiated fracture in the samples of porous-fractured rocks that have been initially saturated with a pressure-injected fluid and then tested under increasing fluid pressure in saturated rocks. The tests were conducted at the Geophysical observatory “Borok” of Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences. The laboratory is equipped with electrohydraulic press INOVA-1000. The experiments were conducted on the rock samples with substantially different porosity. The tested samples were made of Buffalo sandstones, granites from the well drilled in the area of Koyna-Warna induced seismicity, and of granites from the well in the Voronezh crystalline massif. The permeability of granite samples was varied by their controlled artificial cracking by successive heating and cooling. A preliminarily dried sample was initially subjected to uniaxial loading in uniform compression (confining pressure). Loading was performed at a constant strain rate until the moment when the growth rate of acoustic emission (AE) activity began to accelerate which indicated that the stress level approaches ultimate strength. Since that, the loading rate was decreased by an order of magnitude, and water was infused into a sample from its top face. The bottom end of a sample was tightly sealed and impermeable to water. After this, the pore pressure in the sample that had got saturated with water to that moment was raised in steps whose amplitudes were varied. The obtained results of the laboratory studies show that the character and intensity of fluid initiation of fracture markedly differ under primary fluid injection into the dry porous-fractured samples and under the subsequent increases of the pore pressure in the saturated samples. The time delay of acoustic response relative to fluid initiation and the amplitude of the response proved to be larger in the case of water injection into dry samples than in the case of raising the pore pressure in saturated samples. The theoretical analysis of fluid propagation in a pore space of an air-filled sample in the model with piston-type air displacement has shown that in the case of water injection into a dry sample, the fluid pressure front propagates more slowly than in the saturated sample.

Investigation of the acoustic activity and GR b-value responses to the cyclic variations of the pore pressure in the fluid saturated rocks was studied in addition. The changes of b-value were found both for increasing and decreasing of the pore pressure. Obtained laboratory results are similar to results from the investigations of the seasonal variations of the induced seismicity in the area of Koyna-Warna water reservoirs.

The work was supported partly by the mega-grant program of the Russian Federation Ministry of Science and Education under the project no. 14.W03.31.0033 and partly by the Interdisciplinary Scientific and Educational School of Moscow University «Fundamental and Applied Space Research».

How to cite: Kartseva, T., Smirnov, V., Ponomarev, A., Patonin, A., Isaeva, A., Shikhova, N., Potanina, M., and Stroganova, S.: Fluid Initiation of Fracture in Dry and Water Saturated Rocks, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-899, https://doi.org/10.5194/egusphere-egu21-899, 2021.

We present the results of the laboratory studies of the activization of acoustic emission in fluid-saturated and uniaxial stressed sandstone and granite samples under the electrical current action. The experiments were carried out at the Geophysical observatory “Borok” of Schmidt Institute of Physics of the Earth (Russian Academy of Sciences) using servocontrolled press INOVA-1000 under strain control.

We recorded acoustic emission (AE), axial load, axial and radial strain of the sample and controlled the electric current flowing through the sample. The electrodes for creating an electric potential difference were mounted at the ends of the cylindrical samples. The experiments were carried out both in the presence and in the absence of a galvanic contact of the electrodes with the sample. We examined dry cores and partially saturated cores with an aqueous NaCl solution of various concentrations.

A significant increase in acoustic activity (more than several times) was found during periods of current action, as well as a decrease in activity after termination of electric action. Radial strain increases during periods of electric current flow, which indicates an increase in the sample volume. We did not find acoustic emission initiation on dry samples and on fluid-containing samples in the absence of galvanic contact of the electrodes with the samples.

The increase in the AE activity depends mainly on the electrical power and the duration of the exposure interval. The product of these parameters gives the amount of Joule heat. This indicates that the mechanism of AE initiation by electric current is of a thermal nature. Acoustic activation increases with an increase in the heat generated by the electric current passing through the sample. This makes it possible to relate the initiation of fracturing by thermal expansion of the fluid in the sample cracks and an increase in pore pressure. Found increasing of the radial deformation during the heating intervals supports this idea. Thus, the discovered phenomenon can be considered as a consequence of an unconventional way of increasing pore pressure in rocks saturated with a conducting fluid.

The effect of increasing the acoustic emission activity under electric current action is observed both in mechanically stressed samples and in free, unloaded samples.

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

How to cite: Ponomarev, A., Smirnov, V., Patonin, A., and Kartseva, T.: Initiation of Acoustic Emission in Fluid-Saturated Rock Samples under Electric Current Action, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-952, https://doi.org/10.5194/egusphere-egu21-952, 2021.

Observational and numerical studies have shown that the asymmetric geometry of thrust faults with respect to the Earth’s surface leads to a complex dynamic behavior of updip ruptures. Here, we use an experimental technique that combines ultrahigh-speed photography and digital image correlation to characterize the dynamics of transitioned supershear laboratory thrust earthquakes near the free surface with coherent full-field maps of dynamic displacements, velocities, and stresses associated with the ruptures at intervals of one microsecond. The experimental measurements visualize how the free surface breaks the symmetry in the velocity field with a larger velocity magnitude at the hanging-wall and significant rotations into a nearly vertical motion of the hanging wall and footwall motion at a dip angle much shallower than that of the fault. As indicated by the evolving stress maps, these rotations lead to significant normal stress reductions, with a temporal complete release in experiments that were conducted under small initial compressive load. The method enables us to measure the evolving on-fault friction in real time and to unravel the history dependent nature of friction on slip, slip rate as well as fast variations in normal stress. We show that the shear frictional resistance exhibits a significant lag in response to normal stress variations and identify a predictive frictional formulation that captures this effect. Our findings provide guidance to theoretical earthquake source mechanics models by furnishing the necessary on-fault physics needed for the numerical simulation of the rupture process.

How to cite: Tal, Y., Runino, V., Rosakis, A., and Lapusta, N.: Thrust-fault dynamics and frictional resistance response inferred though laboratory earthquakes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1056, https://doi.org/10.5194/egusphere-egu21-1056, 2021.

In this communication, we present a novel numerical framework which consists in a direct coupling between a discrete micromechanical modelling of rock damaging processes and a continuous modelling of elastic deformation and acoustic waves. It includes a polygon-based conforming Discrete Element Method (DEM) with a cohesive zone model (CZM, [1]) for the discrete part and a meshfree formulation for the continuum part. This framework is applied to the numerical reproduction of sawcut triaxial tests performed in the lab on marble samples under seismogenic conditions [2]. Realistic boundary conditions (in terms of the elasticity of the loading system, of the absorption of the elastic waves and of the fluid pressure applied on the lateral boundaries) are introduced. Constitutive laws (in the continuum part) and micromechanical parameters (in the discrete part) are calibrated by performing independant simulations based on experimental results found in the literature [3].

Upon loading, this model provides information on the system behavior that nicely complement the experimental data, such as (i) the progressive damaging of the contacting surfaces, leading to the emission of granular matter in the interface, to the formation of a gouge layer, and to a modification of the interface rheology, (ii) the space and time distribution and statistics and the detailed kinematics of the slip events related to the interface evolution, and (iii) the acoustic wave emission and propagation in the medium associated with such events.

The model shows that, depending on the experimental conditions (confining pressure, loading rate, surface roughness, etc.), and without relying to any prior choice of slip- or rate-dependent friction laws, a large number of sliding regimes can emerge from this system. This includes large stress drops, regular stick-slip, or stable sliding. This model thus provides an unprecedented view of both local and global phenomena at stake during lab earthquakes, at sampling rates in both space and time which remain out of reach for experimental instrumentation.

[1]. Mollon, G. (2015). “A numerical framework for discrete modelling of friction and wear using Voronoi polyhedrons”, Tribology International, 90, 343-355
[2]. Aubry, J. (2019). “Séismes au laboratoire: friction, plasticité et bilan énergétique”, PhD Thesis, Ecole Normale Supérieure.
[3]. Fredrich, J. T.; Evans, B. & Wong, T.-F., (1989). “Micromechanics of the brittle to plastic transition in Carrara marble”, Journal of Geophysical Research: Solid Earth,

How to cite: Mollon, G., Aubry, J., and Schubnel, A.: A micromechanically calibrated numerical model reproducing earthquake cycle in the lab, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1114, https://doi.org/10.5194/egusphere-egu21-1114, 2021.

The issues concerning the relationship between two self-similarity parameters—the Gutenberg– Richter b- and Omori p-values—in the aftershock sequences are explored. In the laboratory experiments, under fracture initiation in the rock by sharp jumps in the axial stress, a correlation between the p- and b-values is revealed in the fracture relaxation regimes similar to aftershocks. The correlation observed in the experiments on water-saturated sandstone samples with the preliminarily formed faults is negative and clearly pronounced. The correlation in the case of dry samples of migmatite and concrete proved to be positive, but its statistical significance is lower than for the wet samples. The analysis of the literature data on detecting the connection between parameters p and b in the natural aftershock sequences shows that the reported results are heterogeneous. Some authors conclude that these parameters are connected and that both positive and negative correlation is noted between them. Other authors present evidence suggesting the absence of any correlation. Our study of the natural aftershocks based on the data of regional earthquake catalogs has shown that the statistical estimates of the Gutenberg–Richter and Omori parameters are fairly sensitive to the quality and homogeneity of the input data. The key factors affecting the estimation quality of these parameters are established, and the procedure for selecting the aftershock catalogs for subject analysis is developed. The results of statistical estimating the Gutenberg–Richter and Omori parameters in the aftershock processes in the regions with different types of the tectonic regimes—subduction zones and regions of shear transform faults—have shown that that the correlation of these parameters in the subduction zones can be positive and negative either. In the zones of the transform faults, the connection between these parameters is not detected. Our study generalizes C.H. Scholtz’s idea that the Omori law can be explained by the superimposition of the relaxation processes having different relaxation times. According to the generalized model, the different sign of the correlation between the self-similarity parameters in the aftershock processes correspond to the different relaxation mechanisms with different types of the dependence of the relaxation time on the “size” of the relaxator. It is currently unclear which particular mechanisms are implemented in the aftershock processes. The relationship between the Omori and Gutenberg–Richter parameters revealed by our laboratory experiments and field studies (positive correlation, negative correlation, or lack of correlation) may indicate the implementation of different relaxation mechanisms in some or other particular conditions.

The work was supported partly by the mega-grant program of the Russian Federation Ministry of Science and Education under the project no. 14.W03.31.0033 and partly by the Interdisciplinary Scientific and Educational School of Moscow University «Fundamental and Applied Space Research».

How to cite: Smirnov, V., Kartseva, T., Ponomarev, A., Patonin, A., and Potanina, M.: On the Relationship between the Omori and Gutenberg–Richter Parameters in Aftershock Sequences, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1288, https://doi.org/10.5194/egusphere-egu21-1288, 2021.

The Italian Central Apennines are one of the most seismically active areas in the Mediterranean (e.g., L’Aquila 2009, Mw 6.3 earthquake). The mainshocks and the aftershocks of these earthquake sequences propagate and often nucleate in fault zones cutting km-thick limestones and dolostones formations. An impressive feature of these faults is the presence, at their footwall, of few meters to hundreds of meters thick damage zones. However, the mechanism of formation of these damage zones and their role during (1) individual seismic ruptures (e.g., rupture arrest), (2) seismic sequences (e.g., aftershock evolution) and (3) seismic cycle (e.g., long term fault zone healing) are unknown. This limitation is also due to the lack of knowledge regarding the distribution, along strike and with depth, of damage with wall rock lithology, geometrical characteristics (fault length, inherited structures, etc.) and kinematic properties (cumulative displacement, strain rate, etc.) of the associated main faults.

Previous high-resolution field structural surveys were performed on the Vado di Corno Fault Zone, a segment of the ca. 20 km long Campo Imperatore normal fault system, which accommodated ~ 1500 m of vertical displacement (Fondriest et al., 2020). The damage zone was up to 400 m thick and dominated by intensely fractured (1-2 cm spaced joints) dolomitized limestones with the thickest volumes at fault oversteps and where the fault cuts through an older thrust zone. Here we describe two minor faults located in the same area (Central Apennines), but with shorter length along strike. They both strike NNW-SSE and accommodated a vertical displacement of ~300 m.

The Subequana Valley Fault is about 9 km long and consists of multiple segments disposed in an en-echelon array. The fault juxtaposes pelagic limestones at the footwall and quaternary deposits at the hanging wall. The damage zone is < 25 m  thick  and comprises fractured (1-2 cm spaced joints) limestones beds with decreasing fracture intensity moving away from the master fault. However, the damage zone thickness increases up to ∼100 m in proximity of subsidiary faults striking NNE-SSW. The latter could be reactivated inherited structures.

The Monte Capo di Serre Fault is about 8 km long and characterized by a sharp ultra-polished master fault surface which cuts locally dolomitized Jurassic platform limestones. The damage zone is up to 120 m thick and cut by 10-20 cm spaced joints, but it reaches an higher fracture intensity where is cut by subsidiary, possibly inherited, faults striking NNE-SSW.

Based on these preliminary observations, faults with similar displacement show comparable damage zone thicknesses. The most relevant damage zone thickness variations are related to geometrical complexities rather than changes in lithology (platform vs pelagic carbonates).  In particular, the largest values of damage zone thickness and fracture intensity occur at fault overstep or are associated to inherited structures. The latter, by acting as strong or weak barriers (sensu Das and Aki, 1977) during the propagation of seismic ruptures, have a key role in the formation of damage zones and the growth of normal faults.

How to cite: Chinello, M., Fondriest, M., and Di Toro, G.: Structural characterization of fault damage zones in carbonates (Central Apennines, Italy), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1525, https://doi.org/10.5194/egusphere-egu21-1525, 2021.

Earthquake ruptures arrest due to either encountering a barrier with high fracture energy or entering unfavorable stress conditions. Our large-scale laboratory earthquake experiments use heterogeneity in initial stress to confine the rupture within a 3-m long saw-cut granite fault. All earthquake processes, i.e., initiation, propagation, and arrest, were spontaneous and contained within the simulated fault. We proposed an analytical crack model to fit our experimental measurements and to better constrain the features in the spatial distribution of both slip and stress changes. Similar to natural earthquakes, laboratory measurements show coseismic slip that gradually tapers near the rupture tips. Measured stress changes show roughly constant stress drop in the center of the ruptured region, a maximum stress increase near the rupture tips, and a smooth transition in between, in a region we describe as the earthquake arrest zone. In our experiments, the earthquake arrest zone is more than one order of magnitude wider than the cohesive zone described by fracture mechanics. We propose that the transition in stress changes and the corresponding linear taper observed in the slip distribution are the result of rupture termination conditions primarily controlled by the initial stress distribution and are not related to the fault strength evolution. We also performed dynamic rupture simulations that confirm how arrest conditions can affect slip distribution and static stress changes, especially near the tip of an arrested rupture. If applicable to larger natural earthquakes, this distinction between the earthquake arrest zone resulted from heterogeneous initial stress and a cohesive zone that depends primarily on strength evolution has important implications for how seismic observations of earthquake fracture energy should be interpreted.

How to cite: Ke, C.-Y., McLaskey, G., and Kammer, D.: Spatial Distribution of Slip and Stress Changes in Contained Laboratory-Generated Earthquakes with Heterogeneous Initial Stress, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1918, https://doi.org/10.5194/egusphere-egu21-1918, 2021.

The nucleation and evolution of major crustal-scale seismogenic faults in the crystalline basement as well as the process of strain localization represent a long-standing, but poorly understood, issue in structural geology and fault mechanics. Here, we addressed the spatio-temporal evolution of the Bolfin Fault Zone (BFZ), a >40-km-long exhumed seismogenic splay fault of the 1000-km-long strike-slip Atacama Fault System. The BFZ has a sinuous fault trace across the Mesozoic magmatic arc of the Coastal Cordillera (Northern Chile). Seismic faulting occurred at 5-7 km depth and ≤ 270 °C in a fluid-rich environment as recorded by extensive propylitic alteration and epidote-chlorite veining. The ancient (125-118 Ma) seismicity is attested by the widespread occurrence of pseudotachylytes both in the fault core and in the damage zone. Field geological surveys indicate nucleation of the BFZ on precursory geometrical anisotropies represented by magmatic foliation of plutons (northern and central segments) and andesitic dyke swarms (southern segment) within the heterogeneous crystalline basement. Faulting exploited the segments of precursory anisotropies that were favorably oriented with respect to the long-term stress field associated with the oblique ancient subduction. The large-scale sinuous geometry of the BFZ may result from linkage of these anisotropy-pinned segments during fault growth. This evolution may provide a model to explain the complex fault pattern of the crustal-scale Atacama Fault System.

How to cite: Masoch, S., Gomila, R., Fondriest, M., Jensen, E., Mitchell, T., Pennacchioni, G., Cembrano, J., and Di Toro, G.: Structural evolution of a crustal-scale seismogenic fault in a magmatic arc: The Bolfin Fault Zone (Atacama Fault System), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3093, https://doi.org/10.5194/egusphere-egu21-3093, 2021.

Earthquakes happen with frictional sliding, by releasing all the stresses accumulated in the pre-stressed surrounding medium. The geological third body (i.e. fault gouge), coming from the wear of previous slips, acts on friction stability and plays a key role in this sudden energy release. A large part of slip mechanisms is influenced, if not controlled, by fault gouge characteristics and environment. We aim to link third body properties (geological, mechanical, physical…) to its rheological behavior by testing numerically different types of dense geological third body (% of porosity, % of cohesion, grains shapes…) with distinct contact laws. Different granular samples are generated to simulate a mature fault gouge with mineral cementation between particles. The gouge is then inserted between two rock walls to realize direct shear experiments with Discrete Element Modelling in the software MELODY2D (Mollon, 2016). A dry contact model is considered to investigate mechanisms without fluid (displacement-driven and under constant confining pressure). Researches are based here on a millimeter-scale portion of gouge, considering that the output values could be used in another model at larger scale.

The peak strength can be sharp, short, and intense for dense and highly cohesive cases (angular particles, 15% initial porosity) and relatively low for ultra-dense samples (polygonal particles, 0% initial porosity). The observed regimes also correspond to an evolution of the amount of ductility within the sample. A very dense or highly cohesive sample behaves as a brittle material, whereas a typical cohesionless and porous geological layer tends to behave as a ductile material. The evolution of gouge characteristics truly influences the shape and formation time of Riedel shear bands. A change in contact laws between particles (%cohesion, friction) modifies the entire kinematics of Riedel bands formation. Indeed, with cohesion between particles, Riedel bands are directly linked to the importance of the dilation phase, depending itself on the initial porosity present within the sample (Casas et al., 2020). Then, increasing friction not only changes the principal orientation or Riedel bands but makes them more numerous within the gouge. It also leads to a more sudden post-peak weakening, which is prone to switch the fault behavior from a ductile aseismic response to a brittle seismic slip, depending on the stiffness of the surrounding medium. Global stiffness of the gouge also has an important role to play on Riedel bands formation, and it can be defined as a combination of multiple parameters such as initial porosity, shape and size of particles, numerical stiffness, gouge thickness… The local Breakdown energy, or energy needed to weaken the fault, is also calculated to be connected to Riedel bands formation.

How to cite: Casas, N., Mollon, G., and Daouadji, A.: Rheology and kinematics of dense granular fault gouges with DEM: shear bands formation and evolution, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3215, https://doi.org/10.5194/egusphere-egu21-3215, 2021.

The Rawil depression north of the Rhone Simplon fault zone (southwestern Swiss Alps) was host of the Mw = 5.8 Sion earthquake in 1946 (Fäh et al., 2011). It is nowadays one of the seismically most active regions in Switzerland and seismicity forms a cluster, which is elongated approximately in WSW-ENE direction over 40-50 km. In November 2019, a remarkable earthquake sequence occurred within the center of this cluster north of the village of Anzère, with more than 300 earthquakes up to ML = 3.3 recorded by the Swiss Seismological Service within 20 days.

Detecting associated full-scale 3D fault patterns solely based on earthquake hypocenters is challenging because of commonly too limited spatial resolution and insufficient number of seismic events. Within the framework of SeismoTeCH, we aim to improve these limitations by a combination of high-precision hypocenter relocation techniques, reconstruction of subsurface fault patterns and correlative links between surface and subsurface data. Assuming that a fault is seismically active multiple times and that the seismic stress-release is initiated at different locations along the fault, we can calculate 3D fault plane orientations from the hypocenter locations. Together with the 17 focal mechanisms derived for the Anzère sequence, we are able to gain geometrical and kinematic information of the seismic faults in 3D. Our analysis reveals a seismically active transpressional step-over structure within a dextral strike-slip fault zone. With remote sensing and field observations, we detect exhumed faults with similar orientations and kinematics that presumably represent step-over structures, interconnecting previously known strike-slip fault zones.

Although seismic activity occurs at depths between 3-5 km, we conclude that the observed surface fault systems in the Rawil depression can be correlated in terms of fault patterns with those assumed at depth. The linkage of the recent seismicity with structural observations of exhumed, potentially paleo-seismic faults in combination with recent hypocenter relocation techniques therefore have great potential to provide further insights into fault linkage and earthquake rupturing processes.

References

Fäh, D., Giardini, D., Kästli, P., Deichmann, N., Gisler, M., Schwarz-Zanetti, G., Alvarez-Rubio, S., Sellami, S., Edwards, B., Allmann, B., Bethmann, F., Wössner, J., Gassner-Stamm, G., Fritsche, S., Eberhard, D., 2011. ECOS-09 Earthquake Catalogue of Switzerland Release 2011. Report and Database. Public catalogue, 17.4.2011. Swiss Seismological Service ETH Zürich, Report SED/RISK/R/001/20110417.

How to cite: Truttmann, S., Diehl, T., and Herwegh, M.: Bridging the Gap between Seismicity and Exhumed Faults: Insights from a Seismically Active Strike-Slip Fault Zone in the Rawil Depression (Northern Valais, Switzerland), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3324, https://doi.org/10.5194/egusphere-egu21-3324, 2021.

The energy dissipated during the friction weakening process at the front of an earthquake rupture, which is known as the fracture energy, is a key earthquake property. It directly affects the nucleation, propagation and arrest of earthquake ruptures, and, is therefore related to important questions, including the maximum possible size of earthquakes at a given fault section. However, estimating the fracture energy in the field is a difficult task and current approaches remain limited. In this work, we present near-fault strain measurements of large-scale laboratory earthquakes on a granite fault. The strain measurements present high-frequency fluctuations while the fault is sliding. These strain fluctuations are indicative of rupture fronts that propagate across the entire fault and occasionally reflect at the boundaries. Here, we will characterize these strain fluctuations by applying fracture-mechanics theory. We will demonstrate that the shape and time scales of the strain fluctuations are well described by the proposed analytical solution. We will further show that by fitting the amplitude of the theory to the experimental measurement, we can estimate the local fracture energy. We apply this process to determine the fracture energy for secondary rupture fronts, which appear within the sliding rupture area. The results are consistent with fracture energy estimates from laboratory-earthquake arrest experiments, but are orders of magnitude lower than reported values from small-scale rotary shear friction experiments. We will discuss the implications and potential of these observations.

How to cite: Kammer, D. and McLaskey, G.: Energy dissipation at the rupture front of laboratory earthquakes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3975, https://doi.org/10.5194/egusphere-egu21-3975, 2021.

Several earthquake source parameters cannot be estimated from the analysis of seismic waves, instead, they may be derived from field surveys and experimental studies. Among these parameters, the fault strength evolution (t_f (t) in MPa) and the frictional power dissipation ( Q'= t_f (t) V(t) in MW m^-2, with V being the slip rate) during seismic slip control the moment release rate, the temperature increase in the slip zone and therefore the activation of coseismic fault dynamic weakening mechanisms. Frictional melts (preserved as pseudotachylytes) along the slip zone can be the result of relatively high Q'. In fact, shear heating is proportional to Q': the higher Q', the higher the heat production rate and, consequently, the faster the temperature increase in the slip zone and the steeper the temperature gradient in the boundary rocks (Nielsen et al., 2010). [PR1] The tonalite rocks used in this study come from the Gole Larghe Fault zone (Southern Alps, Italy), and they are made of minerals with different individual melting temperatures. The presence of a steep temperature gradient (high Q') with closely-spaced isotherms at the boundary walls, will cause the minerals to melt uniformly near the sliding surface (i.e. independently of their melting points), resulting in a relatively smooth pseudotachylyte-wall rock boundary. On the other hand, a gentle temperature gradient (low Q') with widely-spaced isotherms will mainly melt those minerals with low melting points, generating higher micro-roughness.

To consider these different scenarios, we collected samples of natural pseudotachylytes belonging to ‘wavy’ faults, together with samples of injection veins (tensile cracks with Q' ->  0). A ‘wavy’ fault presents shear cracks from compressional (high Q'), neutral, and extensional (low Q') domains along strike. We performed a series of experiments using a rotary shear apparatus (i.e., SHIVA, Di Toro et al., 2010) to produce artificial pseudotachylytes at increasing slip rates and normal stresses corresponding to values of increasing Q', ranging from 5 to 25 MW m^-2. The micro-roughness is then measured from optical and scanning electron microscope images obtained both from natural and artificial samples for comparison. We found that in the experimental samples, the micro-roughness is inversely proportional to Q', as predicted by the theoretical model. Natural samples show similar trends with the higher micro-roughness present in the injection veins where  Q' ->  0. This study demonstrates the robustness of the relation between and fault micro-roughness in both natural and experimental samples. However, further investigations are required to calibrate this methodology to estimate quantitatively the frictional power dissipated during natural earthquakes.

How to cite: Lazari, F., Castagna, A., Nielsen, S., Griffith, A. W., Resor, P., Gomila, R., and Di Toro, G.: Estimate of earthquake power dissipation from exhumed ancient faults (Gole Larghe fault zone, Italy)., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4114, https://doi.org/10.5194/egusphere-egu21-4114, 2021.

Failure and fault slip in crystalline rocks is associated with dilation. When pore fluids are present and drainage is insufficient, dilation leads to pore pressure drops, which in turn lead to strengthening of the material. We conducted laboratory rock fracture experiments with direct in-situ fluid pressure measurements which demonstrate that dynamic rupture propagation and fault slip can be stabilised (i.e., become quasi-static) by such a dilatancy strengthening effect. We also observe that, for the same effective pressures but lower pore fluid pressures, the stabilisation process may be arrested when the pore fluid pressure approaches zero and vaporises, resulting in dynamic shear failure. In case of a stable rupture, we witness continued after slip after the main failure event that is the result of pore pressure recharge of the fault zone. All our observations are quantitatively explained by a simple spring-slider model combining slip-weakening behaviour, slip-induced dilation, and pore fluid diffusion. Using our data in an inverse problem, we estimate the key parameters controlling rupture stabilisation, fault dilation rate and fault zone storage. These estimates are used to make predictions for the pore pressure drop associated with faulting, and where in the crust we may expect dilatancy stabilisation or vaporisation during earthquakes. For intact rock and well consolidated faults, we expect strong dilatancy strengthening between 4 and 6 km depth regardless of ambient pore pressure, and at greater depths when the ambient pore pressure approaches lithostatic pressure. In the uppermost part of the crust (<4 km), we predict vaporisation of pore fluids that eliminates dilatancy strengthening. The depth estimates where dilatant stabilisation is most likely coincide with geothermal energy reservoirs in crystalline rock (typically between 2 and 5 km depth) and in regions  where slow slip events are observed (pore pressure that approaches lithostatic pressure). 

How to cite: Aben, F. and Brantut, N.: Dilatancy stabilises shear failure in rock, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5626, https://doi.org/10.5194/egusphere-egu21-5626, 2021.

Faulting in seismically active regions commonly involves the deformation of unconsolidated to poorly lithified sediments at shallow to near-surface depths. When compared to classic crustal strength profiles that predict a velocity-strengthening behaviour for the first few km of depth, the propagation of seismic rupture to the surface appears counterintuitive. Rock deformation experiments have shown an inverse relationship between normal stress and displacement needed to the onset of dynamic weakening during seismic slip, meaning that for a seismic rupture to be able to propagate towards the surface, displacements should be large enough to counter the progressive decrease of normal and confining stresses.

In this contribution, we document the occurrence of mirror-like faults that formed within 20-30 m-thick, unconsolidated colluvium fan deposits at the hanging wall of the active Vado di Corno Fault Zone (VCFZ) in the Central Apennines, Italy. The deposits lie in direct contact with the master normal-fault surface, are Late Pleistocene to Holocene in age, and consist of angular carbonate clasts with grain size ranging ~0.1-10 mm derived from the dismantling of the adjacent VCFZ footwall. Field observations of cross cutting relationships and marker layer displacements suggest a maximum formation depth of the faults of c. 20-30 m and slip accommodated along single faults on the order of few cm. Faults are organised in three sets: subvertical, N-S and NE-SW trending faults, and WNW-ESE striking faults, synthetic and antithetic to the VCFZ master fault surface (N195/55°). Faults are commonly lineated with a dip-slip to slightly oblique kinematic.

Detailed microstructural analysis of the mirror faults shows extreme strain localization on a 2-5 µm thick principal slip zone composed of calcite nanograins ranging 10s-100s nm in size with amorphous material and phyllosilicates occurring along grain boundaries and within intragranular porosity. Locally, aggregates of nanograins coalesce and transition to µm-sized polygonal, larger grains. Calcite nanograins are mostly equant, with straight grain boundaries, 120° dihedral angles, and negligible porosity. These microstructures strongly resemble high temperature recrystallization structures documented along seismic faults exhumed from >5 km of depth, where stresses are significantly larger. In our case, field constraints show that deformation occurred in very confining stress conditions and with limited displacement.

Collectively, our observations provide new documentation on the conditions for the formation of mirror faults and new insights into the mechanics of faulting and strain accommodation in the shallowest part of the crust (< 1 km).

How to cite: Demurtas, M., Plümper, O., Ohl, M., Balsamo, F., and Pizzati, M.: Mirror fault formation and coseismic slip at surface conditions: an example of faulting in unconsolidated deposits in the Central Apennines (Italy), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5846, https://doi.org/10.5194/egusphere-egu21-5846, 2021.

The delayed aftershocks 2018 Mw 6.2 on April 10 and Mw 5.8 on Sept 1 and 2019 Mw 6.7 on January 20, Mw 6.4 on June 14, and Mw 6.2 on November 4, associated with the Mw 8.3 2015 Illapel Earthquake occurred in the ​​central Chile. The seismic source of this earthquake has been studied with the GPS, InSAR and tide gauge network. Although there are several studies performed to characterize the robust aftershocks and the variations in the field of deformation induced by the megathrust, but there are still aspects to be elucidated of the relationship between the transfer of stresses from the interface between plates towards delayed aftershocks with the crustal structures with seismogenic potential. Therefore, the principal objective of this study is to understand how the stress transfer induced by the 2015 Illapel earthquake of the heterogeneous rupture mechanism to intermediate-deep or crustal earthquakes. For this, coulomb stress changes from  finite fault model of the Illapel earthquake and with the biggest aftershocks in year 2015 are used. These cumulative stress pattern provides substantial evidences for the delayed aftershocks in this region. The subducting Challenger Fault Zone and Juan Fernandez Ridge heterogeneity are existing feature, which releases the accumulated coulomb stress changes and provide delayed aftershocks.  Therefore along with stress induced by a large earthquake such as Mw 8.3 from Illapel 2015 along with biggest aftershocks, have a direct mechanism that may activate the  delayed aftershocks. Our study suggests  the activation of crustal faults in this research as a risk assessment factor for the evaluating in the seismic context of the region and useful for another subduction zone.

How to cite: Lema, F. and Shrivastava, M.: The delayed aftershocks of the Illapel Earthquake Mw 8.3, 2015, Chile, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6978, https://doi.org/10.5194/egusphere-egu21-6978, 2021.

Understanding the evolution of fault strength over multiple interseismic periods is crucial to quantifying seismic hazard. According to Coulomb’s failure criterion, restrengthening, or healing, may result from an increase in friction and/or in cohesion. Classic sliding experiments on rocks and fault gouges are not able to resolve the contribution of cohesion to the healing of frictional interfaces. Here, we present a zero nominal normal stress friction experiment capable of large displacements that exhibits similar complexity as the deforming lithosphere (intermittent, aperiodic deformation; Gutenberg-Richter-type scaling of event sizes). This Couette-type apparatus is designed to shear millimeter-thick layers of columnar ice, grown in-situ in a meter scale circular water tank. When the system is driven at low sliding velocities, the ice plate fractures and sliding occurs along a complex, non-prescribed frictional interface. Water beneath the ice can percolate through the sliding interface and freeze, increasing its strength. A torque gauge and an array of acoustic emission transducers are used to measure the shear strength of the frictional interface and to monitor acoustic activity. Previous work, using constant values of sliding velocity, showed that deformation occurs via a combination of slow and fast slip events, and that the “seismic” part consists of two populations of acoustic emission (AE) events: standalone and correlated, with different Gutenberg-Richter b-values. The asymmetric shape of the shear stress (torque) fluctuations was attributed to cohesion-dominated strength recovery. We are currently using a new, high speed sampling system to investigate the relationship between the stress fluctuations and the concurrent AE activity in constant as well as variable sliding velocity experiments. This work aims to evaluate the effect of time-dependent processes on systems that deform intermittently.

How to cite: Korkolis, E., Gimbert, F., Weiss, J., and Renard, F.: Irregular stick-slip and the role of cohesion in an ice friction experiment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7436, https://doi.org/10.5194/egusphere-egu21-7436, 2021.

As repeated slip events occur on a fault, energy is partly dissipated through rock fracturing and frictional processes in the fault zone and partly radiated to the surface as seismic energy. Numerous field studies have shown that the core of intraplate faults becomes wider on average with increasing total displacement (and hence slip events). In this study we compile data on the fault core thickness, total displacement and internal structure (e.g., fault core composition, host rock juxtaposition, slip direction, fault type, and/or the number of fault core strands) of plate boundary faults to compare to intraplate faults (within the interior of tectonic plates). Fault core thickness data show that plate boundary faults are anomalously narrow by comparison to intraplate faults of the same displacement and that they remain narrow regardless of how much total displacement they have experienced or the local structure of the fault. By examining the scaling relations between seismic moment, average displacement and surface rupture length for plate boundary and intraplate fault ruptures, we find that for a given value of displacement in an individual earthquake, plate boundary fault earthquakes typically have a greater seismic moment (and hence earthquake magnitude) than intraplate events. We infer that narrow plate boundary faults do not process intact rock as much during seismic events as intraplate faults. Thus, plate boundary faults dissipate less energy than intraplate faults during earthquakes meaning that for a given value of average displacement, more energy is radiated to the surface manifested as higher magnitude earthquakes. By contrast, intraplate faults dissipate more energy and get wider as fault slip increases, generating complex zones of damage in the surrounding rock and propagating through linkage with neighboring structures. The more complex the fault geometry, the more energy has to be consumed at depth during an earthquake and the less energy reaches the surface.

How to cite: Shipton, Z., McKay, L., Lunn, R., Pytharouli, S., and Roberts, J.: Do Intraplate and Plate Boundary Fault Systems Evolve in a Similar Way with Repeated Slip Events?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7733, https://doi.org/10.5194/egusphere-egu21-7733, 2021.

This paper focuses on the study of the temporal evolution of seismicity and related fluid migration following major earthquake sequences occurred in the central Apennines and Eastern California Shear Zone over the last two decades: The 1997 Colfiorito sequence, the 2009 L’Aquila sequence, the 2016 Amatrice-Norcia sequence and the 2019 Ridgecrest sequence. The availability of different high-quality seismic catalogs offers the opportunity to evaluate in detail the temporal evolution of the earthquake's size distribution (or b value) and estimate the effect of the fluid flow process in triggering seismicity. For all seismic sequences, the b value time series show a gradual decrease from a few months to one year before mainshocks. The gradual decrease in the b value is interpreted in terms of coupled fluid-stress intensity as a gradual increase in earthquake activity due essentially to the short-term to intermediate-term pore-fluid fluctuations. Based on laboratory experiments results, the observed short–term fluctuation of b value is presented here as an accelerating cracks growth due essentially to the fluid flow instability.  Despite that the occurrence of seismic precursors could have been predictable in areas with high dense seismic networks, the different b value time series show a difficulty to establish a correspondence between the duration of the foreshock activity and the magnitude of the next largest expected earthquake. This may explain that the spatial and temporal evolution of fluid migration controls the size of the ruptures.

How to cite: Jugurtha, K.: Role of fluid on earthquake occurrence: Example of the 2019 Ridgecrest and the 1997-2016 Central Apennines sequences, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8081, https://doi.org/10.5194/egusphere-egu21-8081, 2021.

Imaging and characterizing transform fault sections that are capable to produce large earthquakes is crucial for evaluating seismic hazard and subsequent risk for nearby population centers. The Marmara Fault near the megacity of Istanbul is one of the best defined seismic gaps in the world and its complexity is captured by seismological, geodetic and geological data. A local dense seismic array (MONGAN) provides a high resolution data set allowing to image the Ganos fault separating two different geological units in the western Marmara region. First results of the waveform analysis from this array present systematic early-phase arrivals at the seismic stations located on the northern block of the Ganos fault which comprises geological units including older and more compact materials than that of the southern block. This difference in the arrival times causes the earthquake epicenters to shift further north than the real locations. In this preliminary results, the early-arrivals will be evaluated according to source azimuths and distances, and possible earth models and wave paths will be discussed. The results have implications for rupture directivity during future earthquakes as input for hazard and risk models for the Marmara region.

How to cite: Yalcinkaya, E., Bohnhoff, M., Martinez-Garzon, P., Görgün, E., Pınar, A., and Alp, H.: Velocity changes across the Ganos segment of the North Anatolian Fault Zone in NW Turkey from systematic variations in body wave arrival times, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8152, https://doi.org/10.5194/egusphere-egu21-8152, 2021.

One of the most alarming recent findings in geo-science is the dramatic rise in the rate of human-induced earthquakes in the past decade. This is due to the fluid injection or extraction in deep reservoirs for hydrocarbon production, wastewater and CO₂ storage and exploitation of geothermal resources which result in the reactivation of nearby faults. These reservoirs are often located 2-3 km depth (i.e. 30 MPa), and are hosted in or covered by sedimentary carbonate layers. As carbonate undergoes a brittle-ductile transition with increasing confining pressure from values of around 20 MPa, ductile deformation can play an important role on the nucleation and propagation of earthquakes on carbonate faults. Here, we investigate the role of increasing ductile behaviour on fault frictional parameters. The research is performed through the new biaxial apparatus installed at EPFL, the HighSTEPS (High Strain TEmperature Pressure Speed) apparatus, able to measure frictional parameters in a wide range of shearing velocities (10^-6 m/s – 0.2 m/s) and under unique boundary conditions representative of the Earth’s crust, i.e., normal stress up to 100 MPa, confining pressure up to 100 MPa, pore fluid pressure up to 100 MPa and temperature up to 120°. The induced stress state in bare surface samples was previously studied by a comparison between results of FEM numerical analyses and experimental ones. Under shear loading conditions, the principal stress σ1 is oriented at about 25° to the vertical axis, and the confining pressure corresponds to the principal stress σ2. Tests are performed under different values of applied confining pressure (1 - 60 MPa) and normal stress (1.5 – 90 MPa) on the faults, keeping constant the ratio between σn/σ3 around ~ 3, to mimic faults at different depth. We present experimental results mapping carbonate fault mechanical behaviour from low shearing velocity 10^-6 m/s to high shearing velocity 10^-1 m/s. Moreover, experimental results are modelled with rate-and-state friction laws (RSFLs) to define rate and state parameters related to the critical conditions for fault stability and its dependence on the presence of ductile deformation. These results shed new light on the nucleation and propagation of earthquake within the brittle-ductile transition in carbonate bearing rocks.

How to cite: Figura, F., Giorgetti, C., Lebihain, M., and Violay, M.: Frictional behaviour of carbonate bearing faults at brittle-ductile transition, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8174, https://doi.org/10.5194/egusphere-egu21-8174, 2021.

Many natural fault cores comprise volumes of extremely fine, low permeability, clay-bearing fault rocks. Should these fault rocks undergo transient volume changes in response to changes in fault slip velocity, the subsequent pore pressure transients would produce significant fault weakening or strengthening, strongly affecting earthquake nucleation and possibly leading to episodic slow slip events. Dilatancy at slow slip velocity has previously been measured in quartz-rich gouges but little is known about gouge containing clay. In this work, the mechanical behaviour of synthetic quartz-kaolinite fault gouges and their volume response to velocity step changes were investigated in a suite of triaxial deformation experiments at effective normal stresses of 60MPa, 25MPa and 10MPa. Kaolinite content was varied from 0 to 100wt% and slip velocity was varied between 0.3 and 3 microns/s.

Upon a 10-fold velocity increase or decrease, gouges of all kaolinite-quartz contents displayed measurable volume change transients. The results show the volume change transients are independent of effective normal stress but are sensitive to gouge kaolinite content. Peak dilation values did not occur in the pure quartz gouges, but rather in gouges containing 10wt% to 20wt% kaolinite. Above a kaolinite content of 10wt% to 20wt%, both dilation and compaction decreased with increasing gouge kaolinite content. At 25MPa effective normal stress, the normalised volume changes decreased from 0.1% to 0.06% at 10wt% to 100wt% kaolinite.  The gouge mechanical behaviour shows that increasing the gouge kaolinite content decreases the gouge frictional strength and promotes more stable sliding, rather than earthquake slip. Increasing the effective normal stress slightly decreases the frictional strength, enhances the chance of earthquake nucleation, and has no discernible effect on the magnitude of the pore volume changes during slip velocity changes.

Low permeabilities of clay-rich fault gouges, coupled with the observed volume change transients, could produce pore pressure fluctuations up to 10MPa in response to fault slip. This assumes no fluid escape from an isolated fault core. Where the permeability is finite, any pore pressure changes will be mediated by fluid influx into the gouge. Volume change transients could therefore be a significant factor in determining whether fault slip leads to earthquake nucleation or a dampened response, possibly resulting in episodic slow slip in low permeability fault rock volumes.

How to cite: Ashman, I. and Faulkner, D.: Dilation and compaction accompanying changes in slip velocity in clay-bearing fault gouges, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8679, https://doi.org/10.5194/egusphere-egu21-8679, 2021.

The understanding of earthquake physics is hindered by the poor knowledge of fault strength and temperature evolution during seismic slip. Experiments reproducing seismic velocity (~1 m/s) allow us to measure both the evolution of fault strength and the associated temperature increase due to frictional heating. However, temperature measurements were performed with techniques having insufficient spatial and temporal resolution. Here we conduct high velocity friction experiments on Carrara marble rock samples sheared at 20 MPa normal stress, velocity of 0.3 and 6 m/s, and 20 m of total displacement. We measure the temperature evolution of the fault surface at the acquisition rate of 1 kHz and over a spatial resolution of ~40 µmwith optical fibers conveying the infrared radiation to a two-color pyrometer. Temperatures up to 1250 °C and low coseismic fault shear strength are compatible with the activation of grain size dependent viscous creep.

How to cite: Di Toro, G., Aretusini, S., Núñez-Cascajero, A., Spagnuolo, E., Tapetado, A., and Vasquez Garcia, M. C.: Fast and localized temperature measurements during simulated earthquakes in carbonate rocks, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9047, https://doi.org/10.5194/egusphere-egu21-9047, 2021.

This discussion assumes that there are ionospheric anomalies in total electron count (TEC) as precursors to major earthquakes. Very careful work by Thomas et al. (2017) and others remove TEC anomalies when correlated with natural events such as geomagnetic or solar activity. Without these data, correlation between ionospheric disturbances and large earthquakes (M ≥ 7.0) occurs infrequently (~20% of events) and is within the standard error resulting from the small sample size. There are two possibilities: (1) either the mechanism of volatile (including radon) release that occurs in some regions precursory to major seismic events is unrelated to ionospheric disturbances; or (2) the occurrence of these volatiles is related first to geomagnetic and solar activity. The first hypothesis is easily falsified. In addition to careful statistical analysis by Thomas et al. and others, the mechanism for travel through the lower atmosphere of matter arising on the ground as a stable electric signal is not physically plausible. The second hypothesis awaits falsification, as the correlation fits the data. If natural events such as geomagnetic and solar activity are a trigger for large earthquakes, a plausible mechanism ought to be explored. In considering the effects of ionospheric disturbances on ground-based phenomena, geomagnetically induced currents (GIC) are a reasonable model. GIC occur generally at high latitudes and are responsible for the electrocorrosion of bridges and other metal infrastructure. Fluids laden with dissolved ions occur in faults and are a potential conduit for GIC. Electromagnetic fields induced by ionospheric anomalies may be present at depth. Can these types of fields weaken earth materials? One reason dilatancy diffusion models fell out of favor is scale. The microcracks observed are too small to hold the volume of volatiles required to account for observed changes to groundwater. If instead the presence of electric and magnetic fields aid in the liberation of volatiles and dissolution of certain minerals in rock, seismic events may occur. Andrén et al. (2016), for example, note decreasing groundwater (Si and Na) ion concentrations (ratio 2:1) as well as a small decrease in Ca and an increase in K ion concentrations during a period leading up to two consecutive M > 5 earthquakes in Hafralækur, Iceland. They took well cuttings for petrographic analysis: The observed groundwater changes are consistent with contemporary replacement of labradorite with analcime and the precipitation of zeolite minerals before and during the seismic activity, respectively, when the cuttings were taken. These observations fit the data well. In some cases, solar and geomagnetic activity cause ionospheric anomalies. These then induce electromagnetic currents in faults. The resulting fields aid in the dissolution of certain minerals and release volatiles, which are then precursory to seismic events. Groundwater changes before and after such events are related to the dissolution and subsequent precipitation of minerals in the rock. This rock weakening hypothesis fits the data, and is a simple explanation for how correlations between ionospheric disturbances caused by solar or geomagnetic events and large seismic events may arise.

How to cite: Helman, D.: Ionospheric Induction of Earthquakes: Fitting the Data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9614, https://doi.org/10.5194/egusphere-egu21-9614, 2021.

The Ganos Fault (GF) is the westernmost onshore segment of the North Anatolian Fault Zone (NAFZ) and was last activated in the Mw7.4 Ganos/Mürefte earthquake in 1912. The GF is a first order linear and a right lateral strike-slip fault with a locking depth of 8-17 km. A 40-station seismic array has been deployed between 2017 and 2020 along the GF to study the fault zone characteristics at depth. Fault Zone Head Waves (FZHW) are an important diagnostic signal to detect velocity contrast across fault and thus identify them as interfaces. A fault consisting of a sharp material contrast between different lithologies is expected to generate FZHW. They spend a large portion of their propagation paths refracting along the bimaterial interface. The head waves propagate with the velocity and motion polarity of the faster block, and are radiated from the fault to the slower velocity block where they are characterized by an emergent waveform with opposite motion polarity to that of the direct body waves. The FZHW are the first arriving phases at locations on the slower block with normal distance to the fault less than a critical distance. The high station coverage across the fault will allow us to observe micro-earthquake activity and FZHW close to the seismically active region of the GF throughout the entire seismogenic depth down to approximately 20 km thereby enhancing the resolution of seismological observations in that area. Preliminary results from MONGAN array allow to identify FZHWs at several stations in waveforms originating from events in the western Marmara Sea. We focus on the interpretation of a distinct first phase (FZHW) contained in the waveform coda that is well separated from the direct P wave. FZHWs are visible in many waveforms and have a specific time delay before the direct P wave arrivals at each station. Based on a polarization analysis of records at MONGAN stations, this first phase is interpreted as a FZHW at an interface near the study area. Its particle motion is consistent with FZHW and the direct P wave produced by the bimaterial interface. This is an indication of a bimaterial interface along the GF where the northern block is faster than the southern block.

How to cite: Gorgun, B., Yalcinkaya, E., Gorgun, E., Bohnhoff, M., and Alp, H.: Imaging the rupture zone of the 1912 Ganos earthquake using fault zone head waves from a local seismic network, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9764, https://doi.org/10.5194/egusphere-egu21-9764, 2021.

Current conceptual fault models define a seismogenic zone, where earthquakes nucleate, characterised by velocity-weakening fault rocks in a dominantly frictional regime. The base of the seismogenic zone is commonly inferred to coincide with a thermally controlled onset of velocity-strengthening slip or distributed viscous deformation. The top of the seismogenic zone may be determined by low-temperature diagenetic processes and the state of consolidation and alteration. Overall, the seismogenic zone is therefore described as bounded by transitions in frictional and rheological properties. These properties are relatively well-determined for monomineralic systems and simple, planar geometries; but, many exceptions, including deep earthquakes, slow slip, and shallow creep, imply processes involving compositional, structural, or environmental heterogeneities. We explore how such heterogeneities may alter the extent of the seismogenic zone.

We consider mixed viscous-frictional deformation and suggest a simple rule of thumb to estimate the role of heterogeneities by a combination of the viscosity contrast within the fault, and the ratio between the bulk shear stress and the yield strength of the strongest fault zone component. In this model, slip behaviour can change dynamically in response to stress and strength variations with depth and time. We quantify the model numerically, and illustrate the idea with a few field-based examples: 1) earthquakes within the viscous regime, deeper than the thermally-controlled seismogenic zone, can be triggered by an increase in the ratio of shear stress to yield strength, either by increased fluid pressure or increased local stress; 2) there is commonly a depth range of transitional behaviour at the base of the seismogenic zone – the thickness of this zone increases markedly with increased viscosity contrast within the fault zone; and 3) fault zone weakening by phyllosilicate growth and foliation development increases viscosity ratio and decreases bulk shear stress, leading to efficient, stable, fault zone creep. These examples are not new interpretations or observations, but given the substantial complexity of heterogeneous fault zones, we suggest that a simplified, conceptual model based on basic strength and stress parameters is useful in describing and assessing the effect of heterogeneities on fault slip behaviour.         

How to cite: Fagereng, A. and Beall, A.: Effects of heterogeneity on frictional-viscous deformation and the depth-extent of the seismogenic zone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10039, https://doi.org/10.5194/egusphere-egu21-10039, 2021.

Knowledge of in situ stress fields is critical for a better understanding of deformation, faulting regime, and earthquake processes in seismically active margins such as the Hikurangi Subduction Margin (HSM), North Island, New Zealand. In this study, we utilize Leak-off Test (LOTs) data, borehole breakout widths measured from borehole image logs, and rock unconfined compressive strengths (UCS) derived from empirical P-wave velocity log relationships to estimate vertical (Sv), minimum (Shmin), and maximum horizontal stress magnitudes (SHmax) and interpret the likely faulting regime experienced in four boreholes (Kauhauroa-2, Kauhauroa-5, Titihaoa-1, and Tawatawa-1). Using the standard Anderson’s stress regime classification, relative stress magnitudes in Kauhauroa-5 at 1200-1700 m depth and Kauhauroa-2 at 1800-2100 m and  indicate that the stress state in the shallow crust of the central and northern part of HSM is predominantly strike-slip (SHmax≥Sv≥Shmin) and normal Sv≥SHmax> Shmin respectively. Moving to the offshore, southern HSM a dominant compressional stress regime (SHmax> Shmin >Sv), with some possible strike slip stress states are observed in Titihaoa-1 from 2240-2660 m and Tawatawa-1 from 750-1350 m. The observed normal/strike-slip stress state in Kauhauroa-2 and Kauhauroa-5 is consistent with the average SHmax orientation of 64° ± 18° (NE-SW) determined from borehole breakouts and dominantly NE–SW striking normal faults interpreted from seismic reflection data. The normal/ strike-slip regime in this area suggests that the stress regime here is probably influenced by the effect of the clockwise rotation of the HSM hangingwall associated with oblique Pacific-Australia plate convergence (ENE-WSW). Alternatively, these stress states could be the result of gravitational collapse due to rapid uplift of the subducting plate during the mid-Miocene. The compressional stress regime in the southern HSM in Titihaoa-1 and Tawatawa-1 is in agreement with the SHmax orientations of 148° ± 14° (NW-SE ) and 102° ± 16° (WNW-ESE) obtained from image logs and mapped NE–SW striking reverse faults in this region. This observation suggests that the tectonics here are strongly linked to the subduction of Hikurangi plateau under Australian Plate (NW-SE) or active frontal thrusts in the overriding plate. 

How to cite: Behboudi, E., McNamara, D., Lokmer, I., Wallace, L., and Manzocchi, T.: The state of stress in the shallow crust of the Hikurangi Subduction Margin hangingwall, New Zealand, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10217, https://doi.org/10.5194/egusphere-egu21-10217, 2021.

We used three-dimensional numerical simulations of the discrete element method (DEM) to investigate slip localization in sheared granular faults under seismic velocities. An aggregate of non-destructive spherical particles with assigned contact properties is subjected to direct shear with periodic boundary in horizontal directions. To investigate whether particle size distribution (PSD) influences slip accommodation, three distinct PSDs, namely Gaussian, log-normal, and power-law with fractal dimension D ranging from 0.8 to 2.6, are employed. In additional simulations, we impose a thin layer of particles with smaller grain size along the boundary as well as in the middle of the granular assemblages to simulate boundary and Y shears occurring in both natural and laboratory fault gouges. Transient microscopic properties, such as particle motion and contact forces, as well as macroscopic properties, such as friction, of the granular layer, are continuously monitored during numerical shearing. Results show that no visible slip localization is observed for all different PSDs based on the current particle motion analysis. On the other hand, we find that much more strain (i.e., displacement) is accommodated in the finer-grained layer even with a small contrast in grain size. Up to 90 % of the displacement is localized in a finer-grained layer when the contrast ratio of the grain size is 50 %. Since more frictional heat will be generated in the localized slip zone, the results provide crucial information on the heat generation and associated slip accommodation in sheared gouge zones. A possible mechanism of slip localization in the simulations is the transfer of the momentum across the granular system. We conclude that the occurrence of a weaker, fine-grained layer within a dense fault zone is likely to result in self-enhanced weakening of the fault planes.  Ongoing work includes (1) varying the thickness, grain size, and internal friction of the thinner layer; (2) applying triangulation methods to further analyze the microscale stress and strain tensor between particles; (3) changing the rolling friction of particles.

How to cite: Hung, C.-C., Niemeijer, A., Raoof, A., and Swijen, T.: Investigation of strain localization in sheared granular material using 3D numerical discrete element model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10506, https://doi.org/10.5194/egusphere-egu21-10506, 2021.

Potential energy stored during the inter-seismic period by tectonic loading around faults can be released through earthquakes as radiated energy, heat and rupture energy. The latter is of first importance, since it controls both the nucleation and the propagation of the seismic rupture. On one side, the rupture energy estimated for natural earthquakes (also called Breakdown work) ranges between 1 J/m² and tens of MJ/m² for the largest events, and shows a clear slip dependence. On the other side, recent experimental studies highlighted that at the scale of the laboratory, rupture energy is a material property (energy required to break the fault interface), limited by an upper bound value corresponding to the rupture energy of the intact material (1 to 10 kJ/m²), independently of the size of the event, i.e. of the seismic slip.

To reconcile these contradictory observations, we performed stick-slip experiments, as an analog for earthquakes, in a bi-axial shear configuration. We analyzed the fault weakening during frictional rupture by accessing to the on-fault (1 mm away) stress-slip curve through strain-gauge array. We first estimated rupture energy by comparing the experimental strain with the theoretical predictions from both Linear Elastic Fracture Mechanics (LEFM) and the Cohesive Zone Model (CZM). Secondly, we compared these values to the breakdown work obtained from the integration of the stress-slip curve. Our results showed that, at the scale of our experiments, fault weakening is divided into two stages; the first one, corresponding to an energy of few J/m², coherent with the estimated rupture energy (by LEFM and CZM), and a long-tailed weakening corresponding to a larger energy not observable at the rupture tip.

Using a theoretical analysis and numerical simulations, we demonstrated that only the first weakening stage controls the nucleation and the dynamics of the rupture tip. The breakdown work induced by the long-tailed weakening can enhance slip during rupture propagation and can allow the rupture to overcome stress heterogeneity along the fault. Additionally, we showed that at a large scale of observation the dynamics of the rupture tip can become controlled by the breakdown work induced by the long-tailed weakening, leading to a larger stress singularity at the rupture tip which becomes less sensitive to stress perturbations. We suggest that while the onset of frictional motions is related to fracture, natural earthquakes propagation is driven by frictional weakening with increasing slip, explaining the large values of estimated breakdown work for natural earthquakes, as well as the scale dependence in the dynamics of rupture.

How to cite: Paglialunga, F., Passelègue, F., Barras, F., Lebihain, M., Brantut, N., and Violay, M.: On the scale dependence in the dynamics of rupture, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10681, https://doi.org/10.5194/egusphere-egu21-10681, 2021.

Rocks are heterogeneous materials that exhibit nonlinear elastic (anelastic) behavior in both the laboratory and Earth. In the laboratory, investigators have observed complex relationships between stress and strain that include hysteresis, finite relaxation times, and rate and stress path dependence.  These behaviors are linked to stress, porosity, permeability, material integrity and material failure, many of the characteristics we care about in the upper crust.  A limited number of studies in the Earth have confirmed that nonlinear elasticity can be measured in situ, but due to logistical challenges these investigations have not achieved the full potential of what can ultimately be learned from this type of investigation.  We adapted a ‘pump-probe’ type experiment common in laboratory studies, using solid earth tides as the low frequency pump and empirical Green’s function as the high frequency probe.  By probing the velocity at different points in the pump cycle, we constrain some important information about the stress-strain relationship.  Near the San Andreas Fault, we observe strongly nonlinear elastic behavior that increases with decreasing distance to the fault that characterizes the damage zone.  We also constrain important aspects of hysteretic behavior that are related to damage properties and possibly pore pressure.

How to cite: Delorey, A. and Johnson, P.: Probing the damage zone on the San Andreas Fault at Parkfield, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10926, https://doi.org/10.5194/egusphere-egu21-10926, 2021.

The architecture of fault damage zones combines various elements. Halos of intense fracturing forms around principal slip planes, possibly resulting from the shearing of slip surface rugosity or from dynamic stresses caused by earthquake ruptures. Splays forming off the tips and off the edges of a growing fault result in larger scale fracture networks and damage zones. Faults also grow by coalescence of en-echelon segments, such as Riedel fractures in a shear zone, and stress concentration at the steps results in linking damage zones. We show that these various elements of a shear-crack system can be recognized at seismogenic depth in earthquake sequences. Here we examine high-precision, absolute earthquake relocations for the Mw5.7 Magna UT, Mw6.4 Monte Cristo CA and Mw 5.8 Lone Pine CA earthquake sequences in 2020. We use iterative, source-specific, station corrections to loosely couple and improve event locations, and then waveform similarity between events as a measure for strongly coupling probabilistic event locations between multiplet events to greatly improve precision (see presentation EGU21-14608, and Lomax, 2020). The relocated seismicity shows mainly sparse clusters of seismicity, from which we infer multi-scale fault geometries. The uncertainty on earthquake locations (a few hundred meters) is typically larger than the width of halo damage zones observed in the field so that it is not possible to distinguish small aftershocks that could occur on a fracture within the halo or on a principal slip plane.

The relocated Magna seismicity shows a west-dipping, normal-faulting mainshock surface with an isolated, mainshock hypocenter at its base, surrounded up-dip in the hanging wall by a chevron of complex, clustered seismicity, likely related to secondary fault planes. This seismicity and a shallower up-dip cluster of aftershock seismicity correspond to clusters of background seismicity. The Lone Pine seismicity defines a main, east-dipping normal-faulting surface whose bottom edge connects to a steeper dipping splay, surrounded by a few clusters of background and reactivated seismicity. The space-time relation between background seismicity and multi-scale, foreshock-mainshock sequences are clearly imaged. The Monte Cristo Range seismicity (Lomax 2020) illuminates two, en-echelon primary faulting surfaces and surrounding, characteristic shear-crack features such as edge, wall, tip, and linking damage zones, showing that this sequence ruptured a complete shear crack system. In this example the width of the damage zone increases toward the earth surface.  Shallow damage zones align with areas of dense surface fracturing, subsidence and after-slip, showing the importance of damage zones for shaking intensity and earthquake hazard.

For all three sequences, some of the seismicity clusters delineate planar surfaces and concentrate along the edges of the suspected main slip patches. Other clusters of seismicity may result from larger scale damage associated with splay faults, en-echelon systems and linking zones, or with zones of background seismicity reactivated by stress changes from mainshock rupture. These types of seismicity and faulting structures may be more developed in the case of a complex rupture on an immature fault

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

How to cite: Henry, P., Lomax, A., and VIseur, S.: Imaging damage zones and fault growth processes with high-precision relocations of earthquake sequences., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13447, https://doi.org/10.5194/egusphere-egu21-13447, 2021.

Under high rates of coseismic slip, frictional melt may be generated at the shear zone potentially altering the dynamics and rendering classical rate-and-state friction laws ineffective. Pseudotachylytes (solidified frictional melt) created in laboratories and found in natural fault zones thus provide thermal and mechanical information critical to the study of dynamic shear zone processes, including thermal runaway, stress drop, and viscous braking. While extensive geochemical and mineralogical evidence has suggested the occurrence of disequilibrium melting during pseudotachylite generation, few studies have leveraged it to resolve the kinematics of co-seismic slip.

In this study, we optimize the kinematic parameters of the regularized Yoffe source function using the topographic relief of a pseudotachylyte/wall rock surface in combination with a one-dimensional fluid-mechanical-thermal finite element model. The model consists of solving a two-phase moving boundary problem with an internal heat source constrained by the slip kinematics of the Yoffe function in tandem with the Couette flow problem as an approximation to the shearing of the viscous melt. The topographic relief data come from a pseudotachylyte-bearing fault within the Gole Larghe fault zone, Italy measured using high-resolution X-ray tomography. On this fault surface, biotites are ~260 (±100) micron lower than the mean surface height as a result of preferential melting associated with a lower fusion temperature than quartz or feldspar. Using Monte Carlo sampling of the relief data distribution and Bayesian optimization, we optimize the kinematic parameters of the regularized Yoffe functions and resolve the statistics of shear stress evolution.

Our preliminary results show that the displacement-averaged shear stress in frictional melt ranges from 2 to 7 MPa with a mean value of 5.5 MPa. This is much smaller than estimates based on pseudotachylyte thickness and laboratory experiments, indicating a more complete stress drop than previously thought. The optimal Yoffe source functions have a mean total rise time of ~4 seconds, which is longer than that inferred from scaling laws. Simulations are ongoing and we look forward to interpreting the results in the context of source properties, source models, and energy partitioning for pseudotachylyte-bearing faults.

How to cite: Yang, D. and Resor, P.: Inverse Modeling of Earthquake Source Properties Constrained by Pseudotachylite Surface Roughness, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13889, https://doi.org/10.5194/egusphere-egu21-13889, 2021.

The onset of sliding between two rough surfaces held in frictional contact arises through the nucleation and propagation of rupture fronts, whose dynamics has been shown to obey the elastodynamics of a shear crack. By analogy with the fracture energy controlling the growth of brittle crack in intact material, a frictional rupture is governed by an associated rupture energy. In the context of earthquakes, this rupture energy is expected to control the nucleation and the transition from an accelerating slip patch or localized perturbation to a propagating seismic rupture. The microscopic origin of this rupture energy and its relation to the microcontacts topography remain however unsettled.

In this context, this study aims at bridging the macroscopic description of friction to the failure of contacting asperities and frictional wear prevailing at smaller scales. Recent studies demonstrated how the failure of two contacting asperities arises either by plastic deformation or brittle failure of their apices depending on whether their contact junction is respectively smaller or larger than a characteristic length scale. In this study, we investigate numerically how the different failure mechanisms of microcontact asperities impact the nucleation and propagation of frictional rupture fronts.

At a macroscopic level, we study the ability of an interface to withstand a progressively applied shearing, i.e. its frictional strength, while at the microscopic scale, we observe how the failure process develops across the microcontact junctions. We highlight how the microcontacts topography significantly impacts the nucleation and frictional strength, even when comparing interfaces with identical macroscopic properties and rupture energy. We present how the characteristic length governing microcontacts failure can be used to select which details of the surface roughness are homogenized along the tip of a nucleating slip front. Combining the approach proposed in this work with models solving normal contact between rough surfaces will open up new prospects to study the strength and rupture energy of frictional interfaces at the onset of sliding.

How to cite: Barras, F., Aghababaei, R., and Molinari, J.-F.: Bridging the failure of surface asperities to the macroscopic rupture energy during the onset of frictional sliding, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14445, https://doi.org/10.5194/egusphere-egu21-14445, 2021.

First results show no depth dependency in the shallowest 100 km but spatial variability with high stress drops focused to particular regions on the interface. We also find increased stress drop values in the crust when compared to events close or at the interface.

How to cite: Folesky, J., Hofman, R., and Kummerow, J.: Stress Drops from Trench to Depth in the Northern Chilean Subduction Zone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16514, https://doi.org/10.5194/egusphere-egu21-16514, 2021.