However, extrapolating these small-scale laboratory studies to large-scale geophysical observations remains a significant challenge. This is where numerical simulations become essential, serving as a bridge between scales and enhancing our understanding of fault mechanics. Together, laboratory experiments, numerical simulations, and geophysical observations are complementary and necessary to understand fault mechanisms across different scales.
In this session, we aim to convene multidisciplinary contributions that address multiple aspects of earthquake mechanics, combining laboratory, geophysical, and numerical observations, including:
(i) the interaction between the fault zone and the surrounding damage zone;
(ii) the thermo-hydro-mechanical processes associated with all the different stages of the seismic cycle;
(iii) bridging the gap between the different scales of fault deformation mechanisms.
We particularly encourage contributions with novel observations and innovative methodologies for studying earthquake faulting. Contributions from early career scientists are highly welcome.
How to cite: Barras, F. and Brantut, N.: Bridging inner fault shear localisation and the propagation of earthquake rupture, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15062, https://doi.org/10.5194/egusphere-egu26-15062, 2026.
Fault slip stability is governed by the competition between the elastic energy stored in the surrounding medium and the rate of frictional weakening of the fault, which depends on its constitutive frictional properties. Laboratory shear experiments validate this framework under simplified conditions and homogeneous boundary conditions. In contrast, natural faults are heterogeneous across multiple scales, with variations in stress, frictional properties, fault-zone structure, and elastic properties as documented in laboratory, numerical, and field studies. In gouge-filled faults, heterogeneities are dynamically coupled: stress concentrations promote shear localization, which modifies gouge fabric that controls the frictional properties, ultimately dictating fault stability and rupture dynamics. Resolving how spatially heterogeneous stress and evolving fault-zone structure interact to control rupture nucleation and propagation therefore requires experimental approaches that move beyond homogeneous assumptions.
Here we present results from large-scale biaxial shear experiments on a quartz gouge–filled fault (75 cm × 8 cm) using two forcing-block materials—Nylon 6 and PMMA—with different elastic stiffnesses. The fault was densely instrumented to investigate how stress heterogeneity and evolving shear fabric control slip behavior under different nominal normal stresses from 3 to 10 MPa and a constant loading rate of 10 µm/s. Far-field stress and displacement were monitored using load cells and LVDTs (1 kHz), while forcing blocks deformation was measured using Digital Image Correlation (DIC, 2–30 Hz). During laboratory earthquakes, local fault slip and volumetric deformation were recorded using eddy-current displacement sensors (1.25 MHz) and high-speed DIC (10 kHz). Emitted acoustic waves were recorded at 3.125 MHz using an array of 33 piezoelectric sensors calibrated through ball-drop tests, active-ultrasonic survey, laser vibrometry, and spectral-element waveform modeling.
The experiments produce a broad spectrum of slip behaviors, from stable creep, slow ruptures to fast, dynamic events. Transitions from slow to fast slip are promoted by increasing normal stress and decreasing elastic stiffness. Co-seismic slip, peak slip velocity, and high-frequency acoustic energy increase systematically with cumulative fault slip, increasing normal stress, and decreasing loading stiffness. Direct measurements of slip enable estimation of the critical nucleation length, which decreases with increasing cumulative slip and normal stress, in agreement with theoretical predictions. Finite-element modeling shows that the experimental geometry induces heterogeneous stress distributions promoting the development of heterogeneous shear fabrics and spatially variable frictional responses. When shear fabric is well developed, normal stress is low, and the nucleation lengths are correspondingly large, stress heterogeneities have little impact on slip dynamics, which is dominated by system spanning events with regular, periodic seismic cycles. Conversely, at higher normal stress—conditions associated with smaller nucleation lengths—and/or poorly developed shear fabric, stress heterogeneity drives complex slip behavior, including partial and full ruptures and rupture cascades characterized by strongly spatially variable stress drops. These results highlight how the coupled evolution of stress, shear fabric, and frictional heterogeneities controls slip dynamics in gouge-filled faults.
How to cite: Mastella, G., Volpe, G., Van den Ende, M., De Solda, M., Corbi, F., Funciciello, F., Marone, C., and Marco, S.: Heterogeneous Stress–Driven Shear Localization Governing Fault Slip in Decimeter-Scale Quartz Gouge under Variable Surrounding Stiffness, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14558, https://doi.org/10.5194/egusphere-egu26-14558, 2026.
How large earthquakes rupture across mechanically distinct fault systems remains a fundamental unresolved problem in seismology. New Zealand straddles the Australia–Pacific plate boundary, where relative plate motion is partitioned between deep Hikurangi subduction and widespread dextral strike-slip faulting in the upper plate, such as the Marlborough Fault System. This tectonic configuration favours complex, multi-fault earthquakes, as illustrated by the 2016 Mw 7.8 Kaikōura earthquake and the 2013 Mw 6.5 and Mw 6.6 Cook Strait sequence in central New Zealand. Investigating the source physics of such complex ruptures and their relationship to plate-boundary architecture will provide essential constraints on fault interaction processes and seismic hazard in large-scale convergent margins.
On the evening of 23 January 1855, a major earthquake struck central New Zealand with intense ground shaking across much of the North and South Island. Empirical ground intensity was estimated up to ten, and local tsunami waves reached nearly 10 m in the Cook Strait (Grapes and Downes, 1997; Clark et al., 2019). This event is remarkable for having generated significant uplift - maximum uplift of 6.4 m measured in the southern Wellington coast (McSaveney et al., 2006) - and extreme dextral slip, measured as ~18.7 m at Pigeon Bush along the Wairarapa fault (Rodgers and Little, 2006). Seismological evidence suggests an offshore hypocentre at ~25 km depth and a magnitude exceeding 8.0; however, in the absence of instrumental observations, it remains unclear whether significant slip also occurred on the Hikurangi subduction interface (Beavan & Darby, 2005). Resolving the role of the deeper Hikurangi megathrust, particularly its potential synchronous activation with upper-plate faults, is therefore crucial for understanding the rupture mechanics of this event and for improving seismic hazard assessments in densely populated plate-boundary regions.
In this study, we investigate the source complexity and associated ground motions of multi-fault earthquakes using physics-based dynamic rupture simulations of the 2016 Kaikōura and 1855 Wairarapa earthquakes in central New Zealand. We construct dynamic source models accounting for updated geological and seismological constraints, including regional tectonic stress fields (Townend et al., 2012), national fault networks (Seebeck et al., 2024), nonlinear rheology, and three-dimensional subsurface structures. These key geophysical constraints are essential in reproducing the instrumental observations in the case of the 2016 Kaikōura earthquake (e.g. Ulrich et al. 2019). Rupture magnitude and ground shaking of historical earthquakes are validated against geological measurements, landslide inventories, and tsunami run-up. Beyond observation-driven scenarios, we systematically explore the sensitivity of rupture dynamics and ground motions to variations in tectonic conditions in historical earthquakes. These simulations will provide physical constraints on rupture kinematics and fault interactions, offering insights into improving near-source ground-motion models and regional seismic hazard assessments.
How to cite: Li, D., Howell, A., Coffey, G., Clark, K., Litchfield, N., Langridge, R., Caballero Leyva, E., Benites, R., Williams, C., Bora, S., and Gerstenberger, M.: Filling a missing piece of the 1855 Wairarapa earthquake: Rupture characteristics and implications for regional seismic hazard, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6574, https://doi.org/10.5194/egusphere-egu26-6574, 2026.
The largest earthquakes on planet Earth occurred along the frictional interface between subducting and overriding plates (i.e., the megathrust) at convergent margins. Some of these destructive events have occurred over the last 20 years, such as the 2004 Mw 9.0 Sumatra‐Andaman, 2010 Mw 8.8 Maule, and 2011 Mw 9.0 Tōhoku‐Oki earthquakes. These large events are among the most devastating expressions of Earth's dynamics and, along with tsunamis, they represent a major hazard to society. Therefore, it is crucial to understand to which extent it is possible to predict the final size of a large rupture from the early stage of its propagation. We studied analog earthquakes in an apparatus - Foamquake (Mastella et al., 2022) - in which we used foam rubber to reproduce the upper plate and a 1 cm thick layer of granular materials to reproduce the subduction channel. The set-up is made of an elastic foam wedge with a dimension of 145 × 90 × 20 cm3 (i.e., the overriding plate analog) that overlies a planar, 10° dipping, rigid plate. Along the plate, a basal conveyor belt is driven with constant velocity (0.01 cm/s), reproducing a steady, trench-orthogonal subduction. To constrain the dynamics of analog earthquakes, we used a network of 11 Micro-Electro-Mechanical Systems (MEMS) accelerometers, distributed on the model surface, and measured the evolution of the trench orthogonal component of acceleration at 1 kHz. Additionally, we also used a top-view high-resolution camera (100 Hz), that allows us to derive surface displacements via Particle Image Velocimetry (PIV), that enables characterization of the final static rupture properties, while MEMS monitoring resolves the temporal evolution of spatiotemporal slip. We report 21 models with different frictional configurations of the analog megathrust, including asperities and barriers of varying dimensions, to produce thousands of events with different magnitudes. MEMS monitoring allows for characterization of the Source Time Function (STF) of each event. Preliminary analysis of the STFs indicates a weak correlation (i.e., R2<0.2) between the moment accumulated over different time windows during the early stages of rupture propagation and the final size of individual events. These results contribute, from an experimental perspective, to the ongoing debate on the stochastic versus deterministic nature of earthquake rupture growth.
How to cite: Pardo, S., Corbi, F., Guastamacchia, S., Mastella, G., Tinti, E., and Funiciello, F.: Can Earthquake magnitude be predicted at rupture onset? insights from scaled seismotectonic models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12207, https://doi.org/10.5194/egusphere-egu26-12207, 2026.
The critical nucleation length (Lc) is a fundamental parameter that characterizes the earthquake nucleation process and is theoretically proportional to the fracture energy and the inverse square of shear stress drop (Andrews, 1976, JGR). However, quantitative verification of this relationship has been limited due to difficulties in controlling the nucleation location and in achieving spatially dense measurements capable of resolving the nucleation extent and associated stress drop on the fault.
In this study, we conducted large-scale rock-friction experiments using the biaxial friction apparatus to investigate the scaling characteristics of the critical nucleation length Lc. The experimental configuration consists of vertically stacked Indian metagabbro blocks, comprising an upper block (L6.0 m × W0.5 m × H0.75 m ) and a lower block (L7.5 m × W0.5 m × H0.75 m ), forming a simulated fault with a nominal contact area of 6.0 m × 0.5 m. Strain gauges were installed at a distance of 15 mm from the fault surface with a spacing of 130 mm, and continuous measurements were recorded at a sampling rate of 1 MHz. Normal loading was applied using six independently controlled jacks, enabling controlled rupture nucleation confined within the fault and high-resolution measurements of local shear-stress time history.
To establish a robust measurement criterion for Lc based on rupture velocity evolution, we performed 2D dynamic rupture simulations using spectral boundary integral equation software UGUCA (Kammer et al., 2021) with a linear slip-weakening law. We compared the theoretical Lc predicted from the frictional parameters and prescribed initial stress with Lc inferred from rupture-velocity-based criteria. By varying the critical slip distance Dc, we simulated nucleation processes with different Lc values. The results show that the preslip extent at which the rupture velocity reaches approximately 0.06Vs (where Vs is shear-wave velocity) is in good agreement with the theoretically predicted Lc, supporting the use of this criterion for quantifying and discussing the scaling of Lc.
Applying this criterion to the laboratory experiments, the estimated Lc values range from 0.4 m to 4.0 m, spanning nearly one order of magnitude. The average local shear stress drop within the estimated nucleation region was evaluated as the difference between the initial and residual shear stresses measured before and after the main shock. We observed that Lc clearly scales with the inverse of the shear stress drop, rather than the inverse square, which persists under different normal stress conditions. This scaling is consistent with the observation that the initial shear stress is close to the peak strength in the nucleation region, under the assumption that Dc remains nearly constant among events (approximately 1 μm in this study). These findings provide insight into the quantitative dependence of Lc on the shear stress drop and place important constraints on our understanding of earthquake nucleation processes.
How to cite: Matsumoto, Y., Okubo, K., Yamashita, F., and Fukuyama, E.: Earthquake Source Processes inferred from a 6-meter-long laboratory fault (1) Quantitative Evaluation of Critical Nucleation Length under Variable Stress Drops, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9293, https://doi.org/10.5194/egusphere-egu26-9293, 2026.
The seismic potential of a fault is controlled by its ability to regain strength between earthquakes (fault healing). Variations in healing rate among different rock types can cause local locking and elastic strain energy accumulation, potentially leading to earthquake nucleation. The 2016 Mw 6.5 Norcia mainshock nucleated in the Triassic Evaporites, the seismogenic layer of central Italy, composed of dolostones and anhydrites. Despite its relevance, the mechanical behavior of anhydrite remains relatively less constrained compared to other common crustal lithologies. Only a limited number of studies have systematically investigated the frictional properties of anhydrite, suggesting that its mechanical behavior is strongly sensitive to boundary conditions.
We conducted room humidity (RH) and water-saturated (WS) Slide-Hold-Slide (SHS) friction experiments on anhydrite gouge to assess and isolate the role of water on its healing properties. To inform mechanical data with the microphysical evolution of the fault, we used piezoelectric (PZT) sensors in transmission mode, which record ultrasonic S-wave (UW) propagation through the sample. Finally, mechanical and ultrasonic measurements were complemented by microstructural analyses of the post-mortem sample.
Our results show that fault healing follows a log-linear dependence on hold time under both RH and WS conditions, but with markedly different magnitudes. Water-saturated experiments exhibit a healing rate nearly three times larger (β ~ 0.024) than RH experiments (β ~ 0.009). Ultrasonic measurements reveal a systematic log-linear increase in S-wave velocity during hold periods. This growth is significantly more pronounced in WS samples, where S-wave velocity increases by more than ~2.8% per decade of hold time, compared to ~1% in RH conditions. Microstructural observations indicate that RH samples deform through distributed cataclastic processes accommodated by multiple R-shear bands, whereas WS samples exhibit extreme strain localization along B-shear zones characterized by intense grain-size reduction and the development of a compressive foliation, consistent with semi-brittle deformation.
These results demonstrate that water fundamentally alters the healing efficiency and deformation style of anhydrite faults. Moreover, they show that ultrasonic wave measurements provide a powerful, independent tool to track fault restrengthening during simulated interseismic periods. The observed increase in S-wave velocity can be directly linked to an increase in the shear modulus of the gouge, which appears to be greater in the presence of water, probably due to fluid-assisted healing processes. Together, the high healing rates and the mechanical stiffening of the microstructure inferred from S-wave velocity measurements suggest that anhydrite gouge may be capable of efficiently accumulating elastic strain energy during interseismic periods. Our findings suggest that fluid-assisted healing in anhydrite-bearing fault zones may play a critical role in controlling fault stability and seismic behavior in natural settings.
How to cite: Mauro, M., Guglielmi, G., Trippetta, F., and Scuderi, M.: Fluid-assisted frictional healing revealed by ultrasonic waves, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12872, https://doi.org/10.5194/egusphere-egu26-12872, 2026.
Understanding how fluid injection perturbs stressed faults and triggers induced seismicity has become an urgent challenge in geophysics and hazard mitigation. Observations from subsurface fluid injection associated with geoenergy exploitation show that variations in injection pressure and rate and injected volume can strongly modulate seismicity rates and magnitudes. Yet, comparable injection operations may result in stable creep, slow slip, or dynamic rupture, highlighting persistent gaps in our understanding of the physical processes governing fluid-driven fault reactivation.
Here, we investigate fluid-induced fault reactivation through decimetric-scale laboratory experiments on granite samples containing a 45° precut fault. Experiments are conducted in a biaxial apparatus under critically stressed conditions at 3 MPa normal stress, with independent control of normal and shear stresses. Fluids are injected directly into the fault surface while fault slip is measured using fibre-optic sensors (mini-SIMFIP) installed across the fault. Seismic activity is monitored through passive acoustic emission recordings, and repeated active ultrasonic surveys are performed throughout the experiments to track wave velocity changes and map fluid diffusion along the fault.
By systematically varying the injection rate, we observe a clear transition from aseismic creep to slow slip and dynamic rupture. In all cases, fault slip nucleates at the injection point and subsequently propagates within the pressurized region of the fault. High injection rates generate localized overpressure near the injection point, triggering abrupt and seismic fault reactivation. During high-rate injection, we observe a pronounced drop in P-wave velocity, indicating strong mechanical perturbation of the fault zone, followed by a progressive velocity increase as fluids diffuse along the fault. In contrast, low injection rates lead to stable, aseismic slip confined to the pressurized zone, while intermediate rates produce a progressive reactivation sequence in which slip initiates aseismically, evolves into slow slip, and eventually transitions to dynamic rupture as the pressurized region expands.
Our results show that injection rate governs fault slip behavior by controlling where slip nucleates and whether it remains confined to, or propagates beyond, the pressurized zone and accelerates dynamically.
How to cite: Pignalberi, F., Osten, J., Selvadurai, P., Spagnuolo, E., Jalali, M., and Amann, F.: Illuminating Fluid-Induced Fault Reactivation: Laboratory Insights into Injection-Rate Control on Slip Evolution and Seismic Nucleation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2082, https://doi.org/10.5194/egusphere-egu26-2082, 2026.
Cyclic fluid injection can promote repeated fault reactivation and transient permeability changes, a behavior often discussed within the fault-valve concept where faults alternate between acting as hydraulic barriers and conduits. Such cycles are relevant to both natural hydrothermal systems and industrial activities that modify pore pressure in the subsurface.
The complex evolution of fault permeability and strength during and after fault reactivation calls for a more complete description of the underlying physical processes. Current state-of-the-art fault reactivation models generally represent these weakening and strengthening mechanisms at the macroscopic scale using phenomenological laws, such as the widely used rate-and-state framework. Although these formulations have proven successful in reproducing some observed fault behaviors, they rely on empirically determined parameters and still leave part of the relevant physics insufficiently described.
Here we investigate these processes using a discrete element method (DEM) approach coupled with a pore-scale finite volume (PFV) scheme. Similarly to the framework proposed by Nguyen et al. (2021), the DEM models the granular gouge, while PFV simulates pore-pressure evolution and fluid flow through the evolving pore geometry, with full two-way coupling between solid deformation and fluid pressure, thus relating the macroscopic response of the system to the micromechanical phenomena at work.
Using this coupled approach, we simulate both monotonic and cyclic injection protocols designed to represent fault-valve cycles. We quantify how permeability evolves before, during, and after reactivation, and we explore the influence of key controlling factors: (i) initial permeability, (ii) initial stress state prior to injection, and (iii) confining stress. We also estimate seismic moments associated with individual reactivation events where the recovered moments remain bounded by the injected-volume constraint M0,max =GΔV (McGarr, 2014). Overall, by adding grain-scale observations to trends reported in laboratory and in situ studies, this work helps interpret permeability transients and their implications for triggered seismicity, in order to provide more realistic models of fluid-induced fault reactivation.
References
Nguyen, H. N. G., Scholtès, L., Guglielmi, Y., Donzé, F. V., Ouraga, Z., & Souley, M. (2021). Micromechanics of sheared granular layers activated by fluid pressurization. Geophysical Research Letters. https://doi.org/10.1029/2021GL093222
McGarr, A. (2014). Maximum magnitude earthquakes induced by fluid injection. Journal of Geophysical Research: Solid Earth, 119, 1008–1019. https://doi.org/10.1002/2013JB010597
How to cite: Larkem, O., Scholtès, L., and Golfier, F.: A Multi-Scale Approach to Fault-Valve Systems and Their Evolution, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6001, https://doi.org/10.5194/egusphere-egu26-6001, 2026.
Earthquakes represent the most visible manifestation of tectonic stress accumulation and release within the Earth’s lithosphere. Despite their fundamental importance, the absolute stress levels that drive earthquake rupture remain poorly constrained. In particular, fault shear strength is strongly influenced by fluid pressure at depth, yet its magnitude and variability, especially within the locked zones of megathrusts and across different tectonic regimes, are still not well understood. In this study, I estimate the shear strength of the lithosphere by quantifying the total energy budget of large earthquake ruptures. The total released energy is partitioned into radiated energy and energy dissipated during fault slip, commonly referred to as breakdown (or fracture) energy (G). Radiated energy can be robustly estimated from seismic waveforms or earthquake source time functions. In contrast, reliable estimates of fracture energy are more challenging. To address this, I employ a finite-width slip-pulse model for steady-state dynamic rupture propagation (Rice et al., 2005), combined with heterogeneous kinematic rupture models of large earthquakes. The analysis is based on 208 finite-fault rupture models from the NEIC database, spanning Mw ≥ 7 earthquakes between 1990 and 2025. The adoption of a self-healing pulse rupture framework is motivated by the widely observed property that earthquake rise times are approximately an order of magnitude shorter than total rupture durations, on average about one-seventh of the rupture time in my database.
My results indicate that fracture energy (G) is strongly controlled by the heterogeneous distribution of rupture velocity, rise time, and slip during earthquake rupture. Fracture energy is underestimated by approximately a factor of five when heterogeneity in the rupture process of large earthquakes is neglected, and by nearly an order of magnitude when estimates are based on a classical crack rupture model. Assuming that megathrust earthquakes undergo an almost complete strength drop during rupture, as observed for the 2011 Tohoku earthquake, our estimates represent lower bounds on fault shear strength across global subduction zones and the oceanic lithosphere. The results reveal pronounced fault weakening during megathrust ruptures, with a global average shear strength of approximately 6 MPa. Tsunami earthquakes correspond to the weakest faults, with shear strengths on the order of ~2 MPa, implying that fluid pressures are extremely elevated across most subduction interfaces worldwide. Using these shear strength estimates, I infer a global average pore-fluid pressure ratio (λ = Pf / σlith) of approximately 0.9 for subduction megathrusts. In contrast, the oceanic lithosphere at mid-ocean ridges, transform faults, and fracture zones is nearly an order of magnitude stronger, indicating fluid pressures close to hydrostatic conditions. These pronounced contrasts demonstrate that fluid pressure may play a first-order role in controlling the strength of the Earth’s lithosphere.
How to cite: Pulido, N.: Estimation of fault fracture energy and shear strength drop in large earthquakes: Implications for fluid pressure and tectonic regime, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16466, https://doi.org/10.5194/egusphere-egu26-16466, 2026.
Understanding where and how rupture terminates on a fault is crucial because it controls earthquake magnitude and associated damage. But the in situ stress state, which is one of the key parameters governing rupture dynamics, is not directly measurable on natural faults. Laboratory experiments therefore provide an essential approach for investigating rupture termination (e.g., Bayart et al., 2016, 2018; Ke et al., 2018). Previous studies showed that termination can often be interpreted using an energy balance at the rupture tip within linear elastic fracture mechanics (LEFM), while also suggesting that incorporating additional processes such as long-tailed weakening or rate-dependent friction may improve the description (e.g., Paglialunga et al., 2022; Brener and Bouchbinder, 2021). In order to deepen our understanding of rupture dynamics, including how they terminate, it should be efficient to conduct a systematic investigation that controls the stress state and resulting rupture behavior in the laboratory. From this point of view, we have started a large-scale rock friction experiment. In our experiments, two metagabbro specimens are stacked vertically within the experimental frame. The contacting nominal area is 6.0 m long by 0.5 m wide. Six hydraulic jacks apply normal load to the upper block, and a single hydraulic jack applies shear load to the lower block, which is supported on low-friction rollers. Strain gauge arrays along the fault measure local shear stress every 130 mm at a sampling rate of 1 MHz. In experiment GB01-051, we first imposed 5 mm of shear displacement under a macroscopic normal stress of 2.8 MPa, generating repeated stick-slip events that nucleated at either the leading or trailing edge. We then gradually reduced the normal load on one of the normal jacks on the trailing-edge side while maintaining the shear load. This procedure produced clear nucleation near the unloaded jack followed by a full rupture across the entire fault. After restoring the loads to near-critical conditions, we repeated the procedure at the leading-edge side to generate fault ruptures. In a subsequent trailing-edge attempt, however, rupture terminated approximately halfway along the fault, despite a similar macroscopic stress level. Local stress measurements indicate that previous ruptures reduced the shear stress on the leading-edge side, lowering the available energy release rate for propagation and promoting termination. These results demonstrate that rupture initiation and termination can be manipulated through the evolving stress heterogeneity. We also estimated a lower bound on fracture energy from the measured stress drop using LEFM. Accounting for uncertainty in the termination location, the inferred value ranges from 0.032 to 0.29 J/m², consistent with prior experiments on the same rock type (Xu et al., 2019). Ongoing work will further quantify how controlled stress heterogeneity governs rupture termination.
How to cite: Yamashita, F., Fukuyama, E., Okubo, K., and Matsumoto, Y.: Rupture termination controlled by tuned stress heterogeneity on a 6-m-long laboratory rock fault, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8994, https://doi.org/10.5194/egusphere-egu26-8994, 2026.
Fault zones play a fundamental role in controlling subsurface fluid circulation and mineralization. Their capacity to alternately behave as barriers or conduits—commonly conceptualized within the fault-valve model—results from complex interactions among multiphysical processes (thermal, hydraulic, mechanical, and chemical) acting across a wide range of spatial and temporal scales within the fault structure.
Focusing on the gouge scale, we investigate the hydromechanical behavior of faults using a three-dimensional pore-scale modeling framework that couples a Discrete Element Method (DEM) with a pore-scale finite volume (PFV) scheme. Building on the approach of Nguyen et al. (2021), the DEM is used to model the gouge material as a granular assembly, while the PFV method is used to model fluid flow within the evolving pore space.
Using this coupled approach, we simulate a sheared, fluid-saturated granular gouge subjected to hydromechanical loading through a controlled cyclic fluid injection protocol applied at constant shear stress. The DEM-PFV model captures emergent behaviors consistent with laboratory and in situ observations, and reveals pronounced cycle-to-cycle variability in both the onset of reactivation (Sarma et al., 2025) and the magnitude of slip under repeated pressurization and depressurization. In particular, some cycles produce large slip episodes whereas others exhibit comparatively small slip under similar loading conditions. To connect this macroscopic variability to micromechanics, we track the evolution of the force-chain population throughout the cycles using the characterization method proposed by Peters et al. (2005). The results provide grain-scale insights into how internal load-bearing structures reorganize across cycles and how these force-chain dynamics relate to the occurrence of large-slip events in fault-valving sequences.
References
Nguyen, H. N. G., Scholtès, L., Guglielmi, Y., Donzé, F. V., Ouraga, Z., & Souley, M. (2021). Micromechanics of sheared granular layers activated by fluid pressurization. Geophysical Research Letters. https://doi.org/10.1029/2021GL093222
Sarma, P., Aharonov, E., Toussaint, R., & Parez, S. (2025). Fault gouge failure induced by fluid injection: Hysteresis, delay and shear-strengthening. Journal of Geophysical Research: Solid Earth. https://doi.org/10.1029/2024JB030768
Peters, J., Muthuswamy, M., Wibowo, J., & Tordesillas, A. (2005). Characterization of force chains in granular material. Physical Review E. https://doi.org/10.1103/PhysRevE.72.041307
How to cite: Scholtès, L., Larkem, O., and Golfier, F.: Micromechanical Investigation of Fault-Valving Cycles Using the Discrete Element Method, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9616, https://doi.org/10.5194/egusphere-egu26-9616, 2026.
Understanding the faulting dynamics of natural earthquakes is fundamentally limited by the scarcity of near-source observations and the incompleteness of knowledge about in situ conditions at depth. The Bedretto Underground Laboratory for Geosciences and Geoenergies (BedrettoLab) in Switzerland addresses these limitations through controlled hydraulic stimulation experiments that generate seismicity beneath more than 1 km of rock overburden, thereby bridging the scale gap between laboratory studies and observed natural earthquakes. A key advantage of the BedrettoLab is the ability to characterize in situ conditions prior to seismicity induction. This includes imaging the geometry of the target fault, estimating the local stress state and pore fluid pressure, and examining host and fault rock properties.
Seismicity induced by controlled hydraulic stimulation is recorded by a comprehensive suite of near-source instrumentation, including strong-motion seismometers, borehole geophones and accelerometers, high-frequency acoustic emission sensors, and fibre-optic cables enabling Distributed Acoustic Sensing (DAS) and Fibre Bragg Grating (FBG) measurements. Past experiments have successfully generated seismicity sequences with mainshock magnitudes between Mw −0.5 and 0.0. We construct dynamic rupture models for one such mainshock, constrained by the available near-source observations, to image slip distribution, rupture directivity, and rupture velocity at meter-scale resolution. We find that rupture directivity has a substantially stronger impact on spectral amplitudes than average stress drop. The inferred stress and friction drops are interpreted in terms of the maximum possible confining pressure, providing insights into dynamic weakening processes during earthquake rupture.
The next experiment aims to induce Mw 1.0 earthquakes along a selected fault zone. Using constraints from hydraulic fracture tests, fault geometry imaging, and injection protocols, we seek to forecast the potential rupture dynamics of the induced mainshock by generating a suite of dynamic rupture models representing plausible rupture scenarios, against which the observed mainshock dynamics can be evaluated. In particular, we assess how reliably pre-experiment slip tendency analyses translate into the actual rupture behavior under these controlled conditions. Ultimately, this research will advance our understanding of earthquake source physics and contribute to improved forecasting and mitigation of worst-case scenarios associated with hydraulic stimulation.
How to cite: Schliwa, N., Mosconi, F., Tinti, E., Lambiase, A., Tuinstra, K., Gabriel, A.-A., Rinaldi, A. P., Meier, M.-A., Cocco, M., and Giardini, D.: Imaging and forecasting rupture dynamics of induced microearthquakes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17445, https://doi.org/10.5194/egusphere-egu26-17445, 2026.
In fluid-rich faults, thermal pressurization (TP) is theoretically predicted to induce rapid fault weakening and facilitate large earthquakes, whereas dilatant strengthening (DS) can counteract this process through pore-space expansion. This study experimentally investigates the relative efficiency and condition of TP and DS using triaxial stick-slip tests with on-fault pore pressure (Pp) measurements. Dynamic stick-slip events were generated on four saturated, saw-cut, thermally cracked Westerly granite samples under varying effective confining (30–60 MPa) and pore pressures (25–45 MPa). Results reveal a systematic rupture transition from co-seismic Pp rise (TP-type) at low shear stress to Pp drop (DS-type) at higher shear stress, accompanied by a coupled fast–slow spectrum: fast TP → slow TP → slow DS → fast DS. Fast events reach slip velocities up to three orders of magnitude higher (0.5–10 mm s-¹) than slow ones (0.001–0.5 mm s-¹). The TP model under undrained, adiabatic conditions reproduces the measured Pp evolution, indicating a progressive shear-zone widening (0.05–0.5 mm, as overestimated values) that reduces TP efficiency and promotes slow events. For DS sequences, an increasing dilatancy coefficient is inferred, consistent with enhanced Pp drops. Breakdown energy shows no clear difference between TP and DS events, suggesting similar rupture energetics despite opposite pore-pressure evolution. Overall, this study provides the first direct experimental evidence of TP–DS transitions, demonstrating that TP governs early-stage weakening but diminishes as the shear zone widens, allowing DS to dominate. These results imply that in mature fault zones, after several seismic cycles, fault weakening may be mainly governed by co-seismic dilatancy, although strong, fast ruptures can still occur when the dilatant strengthening is not sufficient to stop the on-going rupture.
How to cite: Fan, C., Lin, G., Aubry, J., Deldicque, D., Giorgetti, C., S. Bhat, H., and Schubnel, A.: From thermal pressurization (TP) to dilatant strengthening (DS) during stick-slip ruptures on saturated saw-cut thermally cracked westerly granite, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9839, https://doi.org/10.5194/egusphere-egu26-9839, 2026.
Fault slip emerges from coupled frictional and hydromechanical processes, yet forecasting stress evolution remains challenging when fluid pressurization, drainage, and dilatancy modulate effective normal stress. We present laboratory double-direct shear experiments on quartz-rich natural fault gouge conducted under dry, 100% humidity, and constant fluid-pressure boundary conditions. Throughout quasi-periodic and irregular seismic cycles, we continuously acquire active-source ultrasonic waveforms transmitted across the gouge layer and derive cycle-resolved acoustic observables: P- and S-wave velocity changes and amplitude-based transmissivity metrics (band-limited RMS).
For our range of conditions, the acoustic properties exhibit robust, two-stage precursory evolution. During the early interseismic phase, acoustic transmissivity increases with contact stress, consistent with progressive asperity contact growth and rising contact stiffness (“healing”). This is followed by a late interseismic-to-preseismic transition characterized by gradual bulk velocity reduction, interpreted as distributed inelastic creep and microcrack growth within the gouge. For the 100% humidity condition, velocity changes track slip velocity, peaking as the fault locks and decreasing prior to dynamic slip. Under constant external fluid pressure, partial drainage and localized undrained behavior further modulate both elastic velocity and transmissivity through shear-induced porosity changes and associated pore-pressure transients. Localized slip regions can show transient acoustic velocity increases consistent with dilatancy hardening, while the bulk response trends toward overall velocity decrease as failure approaches.
We develop a mechanistic poromechanical framework that links the ultrasonic observables to evolving contact stiffness, porosity, and effective stress, providing a physical basis for interpreting travel-time and amplitude changes under fluid pressurization. As an additional validation, a lightweight sequence model trained on the acoustic observables can reconstruct cycle-scale shear-stress evolution and event timing, demonstrating that the acoustic measurements encode the state of the fault. These results highlight the role of fault zone elastic properties for detection of precursory processes prior to earthquake failure and illuminate the processes that occur during the preparatory stages of earthquake nucleation for fluid-saturated fault systems.
How to cite: Affinito, R., Yu, P., Elsworth, D., and Marone, C.: P- and S-wave precursors to lab earthquakes under variable drainage and fluid pressurization, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17533, https://doi.org/10.5194/egusphere-egu26-17533, 2026.
Understanding the mechanisms controlling fault slip requires dedicated experimental setups that bridge the gap between natural conditions and observable scales. These experiments may require the use of analogue materials adapted to the scale of the laboratory, which present interesting characteristics facilitating the observation of physical processes. Polystyrene, due to its low strength and elastic properties, and structure, offers an effective analog material to investigate mechanical fault processes at both large (metric) and small scales (cm). Its low elastic properties slows down deformation processes and enables in-situ small scale observations using X-ray microCT.
In this study, we performed uniaxial compression experiments on small polystyrene blocks with a pre-cut slip interface. Polystyrene of various initial densities were tested. Compression rates ranged from 0.1 to 1 mm/min to induce different slip modes, from slow slip to dynamic stick-slip events. Real-time 2D X-ray radiography was coupled with mechanical monitoring to capture the onset and evolution of slip along the interface. Additionally, 3D scans were acquired at various stages during compression, with the objective of evaluating the spatial distribution of deformation around the fault plane over time.
The aim of this study is to combine mechanical data and imaging in order to characterize internal density changes associated with deformation. Preliminary observations seem to highlight density contrasts in the bulk material around the fault plane, offering insight into potential precursory signs of slip.
This approach demonstrates the potential of X-ray microCT for high-resolution monitoring of analog fault models, with perspectives for quantifying strain localization and post-slip damage patterns. These results may contribute to the understanding of frictional behavior and rupture dynamics in scaled experiments.
How to cite: Walter, B. and Bonnelye, A.: In-situ X-ray imaging of stick-slip behavior in small-scale polystyrene fault analogs, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17231, https://doi.org/10.5194/egusphere-egu26-17231, 2026.
In laboratory studies, the tendency for earthquake nucleation on major plate-boundary fault zones is typically evaluated by measuring the change in shear strength upon controlled step changes in driving velocity. In such laboratory shear experiments simulating fault sliding, it is typical to neglect the cohesion and assume that all the measured shear strength is due to frictional resistance. Therefore, the results of such experiments are evaluated in terms of a friction coefficient calculated as the ratio of the shear strength to effective normal stress, where the “velocity-dependent friction” is quantified by the parameter a-b. However, previous work has shown that the sliding cohesion is not necessarily negligible, especially for water-saturated samples rich in clay-minerals. This comes from recent experiments have shown that, using a single-direct type shear geometry, the cohesion can be directly measured as a peak shear strength with zero applied normal load (zero effective normal stress). The technique can also be used for samples that have accumulated slip, thus providing a measure of the cohesion that exists during sliding or “sliding cohesion”.
Here, I use measurements of sliding cohesion to test the assumption that velocity-dependent fault strength is completely controlled by friction, and determine if cohesion plays a significant role. For these tests I use water-saturated powdered illite-rich Rochester shale, a material that consistently exhibits velocity-strengthening behavior. The velocity-dependent strength is first obtained with a series of standard 3-fold step increases in driving velocity in the range 0.1-100 μm/s under 10 MPa effective normal stress. The sliding cohesion is then measured in a series of experiments in which the samples were sheared for 5 mm under 10 MPa effective stress, the normal stress subsequently removed, and then sheared at each of the velocities used in the velocity-step test. From these tests, the velocity-dependent cohesion is calculated by substituting the cohesion for shear strength and calculating an equivalent “a-b” value that can be subtracted from the standard a-b value measured from the velocity step tests.
Preliminary results show that velocity-dependent cohesion is of the same order as a-b values, and accounts for up to about a third of the measured a-b values. The percentage of strength change related to cohesion rather than friction decreases as a function of increasing driving velocity. Although cohesion is a significant proportion of the velocity-dependent strength changes, removing the velocity-dependence of cohesion is insufficient to cause negative a-b values. However, this result can also be affected by the choice parameters used in the modeling technique that extracts the a-b values and must therefore be evaluated carefully. The magnitude of velocity-dependent cohesion suggests that it may represent a signification proportion of velocity-dependent sliding strength.
How to cite: Ikari, M.: Considering velocity-dependent cohesion in fault sliding stability, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14228, https://doi.org/10.5194/egusphere-egu26-14228, 2026.
In Kochi, JAMSTEC, Japan, we have developed rotary shear apparatuses to understand fault slip behavior of geologic materials at coseismic slip rates. However, coseismic fault behavior under hydrothermal conditions has remained challenging to reproduce in the laboratory. To address this issue, we developed a novel apparatus (HDR: hydrothermal rotary shear apparatus) in 2017, which is capable of high-velocity slip (~2 m/s) at temperatures of up to 600 ℃ and pore fluid pressures of up to 120 MPa. Using this apparatus, we have successfully carried out hydrothermal high-velocity friction experiments on various types of materials, including bare gabbro surfaces and gouges derived from quartz, gabbro, granite, olivine, calcite, and clay minerals under a wide range of pressure-temperature-velocity conditions. The experimental data obtained under hydrothermal conditions are sometimes markedly different from those typically observed in room-temperature experiments due to dynamic changes in fluid properties and chemical reactions in supercritical water. Further understanding of coupled interactions between fault slip, frictional heat, fluid properties, chemical reactions, etc. under hydrothermal conditions will be essential for constraining fault slip behavior in seismogenic-zone settings. In this presentation, we introduce some of our latest results obtained using HDR.
How to cite: Okuda, H., Tanikawa, W., Hamada, Y., Okazaki, K., Bedford, J., Hidaka, M., and Hirose, T.: Development of hydrothermal high-velocity rotary shear apparatus in Kochi, Japan: towards understanding fault slip behavior in the seismogenic zone environment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2718, https://doi.org/10.5194/egusphere-egu26-2718, 2026.
Understanding dynamic weakening is of paramount importance because it involves thermally triggered physical-chemical processes that reduce the fault frictional resistance and facilitate earthquake propagation. Investigating the evolution of temperature (T), pore fluid pressure (Pf), and chemical reactions during slip therefore allows exploration of dynamic weakening mechanisms. However, direct in situ measurements of T and Pf within slip zones remain technologically challenging in laboratory high-velocity rotary shear experiments. Consequently, numerical simulations constrained by mechanical data are essential for inferring these critical parameters. We utilize a series of rotary-shear mechanical data on a combination of kaolinite and quartz. The experiments are conducted under undrained/drained conditions with thermocouples for temperature measurements. On the basis of these data, we develop a Thermo-Hydro-Mechanical-Chemical (THMC) modeling framework using COMSOL Multiphysics to estimate physical conditions within the Principal Slip Zone (PSZ) and to infer dynamic weakening mechanisms responsible for the observed frictional behavior. Under undrained conditions, the friction coefficient (µ) reaches a peak friction (µp) at ~0.28 and undergoes abrupt weakening, followed by a steady-state low-friction (µs) at ~0.1. This behavior corresponds to a measured T stabilizing at 320–360°C and a simulated Pf rapidly increasing and is maintained at ~2.4 MPa. It suggests that frictional heating induces pore water pressurization. Under drained conditions, µ reaches a µp ~0.3 at 0.7s and undergoes abrupt weakening during 1.2-1.7s, maintains µs ~0.17 between 2-4s and followed by a re-strengthening behavior at 4s. µ was accompanied by the changes of T and Pf. T increased to ~280°C at 1.2s, followed by a decrease to ~200°C. Meanwhile, the simulated Pf achieved the highest value (~1.9 MPa) at 1.2s and gradually decreased and reached a relatively lowest value (~0.3 MPa) at 4s due to the pore fluid drainage. Whether Pf is present is corresponding to the weakening and re-strengthening times. In addition, microstructural and mineralogical observations show thermal decomposition of kaolinite. Because thermal decomposition is a strong endothermic reaction, the temperature decreases during the later stages of the process due to the thermal decomposition of kaolinite. We suggest that thermal pressurization operates as the dynamic weakening mechanism during the initial slip stage, consistent with theoretical predictions and experimental documentation of thermal pressurization during rapid shear, as described by Rice (2006) and Ferri et al. (2010). Later, thermal pressurization ceases under drained conditions, resulting in complex frictional behavior. In general, our study provides essential insights into the dynamic weakening mechanism during experimental seismic slip. In addition, we suggest that drainage conditions may influence frictional behavior by affecting the generation and maintenance of pore fluid pressure during seismic slip.
How to cite: Huang, Y.-Q., Kuo, L.-W., Hung, C.-C., and Nguyen, T. T.: Thermo-hydro-mechanical-chemical modeling of synthetic wet gouges sheared at experimental seismic slip under fluid drainage conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3257, https://doi.org/10.5194/egusphere-egu26-3257, 2026.
While dynamic fluctuations in effective normal stress affecting fault zones are ubiquitous, arising from both complex natural phenomena like remote dynamic triggering of earthquakes and anthropogenic activities like industrial subsurface fluid injection, the precise influence of these perturbations and specifically their frequencies, on the macroscopic fault strength remains insufficiently characterized.The frequency of these pore-pressure variations is likely a key factor setting the timescale for the drained-to-undrained transition, thereby driving markedly different mechanical responses.
In this work, we present results from a coupled hydromechanical-discrete element model simulating a pre-stressed, fully saturated granular fault gouge subject to cyclic pore-pressure variations across three orders of magnitude in frequency. We observe that fault failure consistently occurs before the system reaches the traditional Mohr-Coulomb failure criterion. This early failure indicates that additional dynamic mechanisms, often neglected in effective stress analyses, play a dominant role in triggering instability. We investigate the driving forces responsible for this pre-Mohr-Coulomb failure and find they evolve distinctly with frequency. We evaluate three primary candidates driving this behavior: 1) seepage forces arising from the pore-pressure gradients, 2) contact weakening induced by granular agitation (vibration), and 3) inertial effects driven by acceleration from cyclic pore-pressurization. Our analysis isolates the contribution of each mechanism across the frequency spectrum, offering a new physical basis for understanding why dynamic pore-pressure perturbations can trigger slip earlier than what static friction laws predict.
How to cite: Sarma, P., Parez, S., Toussaint, R., and Aharonov, E.: Mechanisms of failure in a fluid-saturated fault gouge subject to cyclic pore-pressure oscillations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4224, https://doi.org/10.5194/egusphere-egu26-4224, 2026.
The Gutenberg-Richter law for the distribution of earthquake magnitude and the Omori law for the decay of aftershocks are two universal laws in seismicity. Although numerical models have been developed to reproduce these laws, they sometimes produce many more foreshocks and less aftershocks than observed. In this study, we simulate earthquake sequences on a 2D random fault network. The fault lengths follow a power-law distribution. Our simulations reproduce the Omori law, without producing many foreshocks. The event size distribution follows Gutenberg-Richter's law with the b-value expected from the fault length distribution, even though many earthquakes are multi-fault ruptures or partial ruptures. Ruptures sometimes propagate into other faults, though there are more partial ruptures than multi-fault ruptures. The frequency of partial and cascading ruptures increases with higher fault density or stronger velocity-weakening friction. Overall, this work illuminates how fault interaction controls the spatiotemporal pattern of seismicity.
How to cite: Ozawa, S.: Partial ruptures, cascading multi-fault ruptures, and aftershocks in 2D random fault network, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8601, https://doi.org/10.5194/egusphere-egu26-8601, 2026.
Fracture energy is a key scaling parameter governing the transition from quasi-static nucleation to dynamic rupture propagation, and it also controls the dynamic stress field around the rupture front. In principle, when fracture energy is uniform along a fault, a single intrinsic fracture energy should describe both quasi-static nucleation and the stress field during rupture propagation. However, this unifying role has not been fully validated experimentally, primarily because conventional laboratory faults are too small to capture the entire rupture process from nucleation to propagation toward limiting speed. Here, we compare the fracture energy evaluated from the shear stress changes associated with the dynamic rupture propagation (Γ) with that inferred from the independently observed critical nucleation length (GLc) on a 6-meter-long laboratory fault.
We conduct faulting experiments on a 6-meter-long laboratory fault and evaluate fracture energy by fitting a steady-state rupture model with a linear cohesive zone to local shear stress changes recorded 15 mm away from the fault. In the biaxial rock-friction apparatus, vertically stacked rock specimens form a simulated fault with dimensions of 6 m × 0.5 m. Six independently controlled jacks applying normal loading enable rupture nucleation at prescribed timing and location by unloading a selected jack while maintaining the shear stress near the peak frictional strength. Rupture velocity is constrained by cross-correlating shear stress histories between neighboring strain gauges spaced at 130 mm. Using the locally estimated rupture velocity, fracture energy Γ and cohesive zone size are determined by minimizing the residual between observed and modeled shear stress time histories.
Fracture energy inferred from critical nucleation length, GLc, is computed following the formulation of Palmer and Rice (1973) and Andrews (1976), using the critical nucleation length examined by Matsumoto et al. (2026, EGU) togather with the measured stress drops.
We analyze three nucleation-controlled events conducted under a macroscopic normal stress of 3 MPa. From local shear stress time histories associated with rupture velocities lower than 0.95 of the Rayleigh wave speed, we obtained an average fracture energy Γ of 0.04 ± 0.01 J/m². This value is consistent with GLc of 0.05 J/m², inferred from events with an average stress drop of 0.05 MPa. These results contribute to the quantitative interpretation of laboratory observations and to improved understanding of earthquake source processes on natural faults.
How to cite: Okubo, K., Yamashita, F., Matsumoto, Y., and Fukuyama, E.: Earthquake Source Processes inferred from a 6-meter-long laboratory fault (2): Fracture Energy Estimation from Dynamic Stress Fields, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14469, https://doi.org/10.5194/egusphere-egu26-14469, 2026.
Understanding earthquakes mechanisms still represents a challenge. The complexity of both the fault zone and the fault behaviour requires to make some simplifications and to downscale the studied system.
In our work, we aim at creating a down-scaled experimental fault model where the behaviour of the asperities and the shearing of the granular gouge are both considered. In order to do so, we borrow from the tribological approach the pin-on-disk experiment: the original experimental apparatus consists in a centimetric pin with a hemispherical extremity representing the fault asperity while a large flat rotating disk stands for the opposite surface of the experimental fault. Both parts are made in the same carbonate rock with controlled roughness. Co-seismic conditions (Velocity in the range [0.001 – 1] m/s, Normal stress in the range [4 – 400] MPa) are applied during the different experimental tests. A number of high-sampling-rate sensors are used to constrain the observation of the asperity-track contact during the simulated seismic events. Moreover, complete post-mortem analyses of the contact surfaces allow to quantify the mechanisms and to reconstruct friction scenarios in accordance with the time-series acquired during tests.
In a previous work, we determined the conditions most representative for a mature lab-fault. In the present study, we focus on the changes in wear and friction behaviours in the lab-fault linked to the slip velocity variations and the presence of granular gouge. Wear is mostly dependent on the slip velocity and the granular gouge layer thickness obtained at the mature conditions appears as an optimal thickness to limit wear. Here, velocity weakening is observed, with dramatic consequences on the microstructure of the contact surfaces. SEM and optical images show evidences of the combination of high stresses and heating on the first layer of minerals of the contact zone.
How to cite: Clerc, A., Mollon, G., Ferrieux, A., Lafarge, L., and Saulot, A.: Microstructural and Frictional Consequences of Slip Velocity Variations: Insights from a single-asperity lab-fault experiment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16867, https://doi.org/10.5194/egusphere-egu26-16867, 2026.
Earthquakes originate from frictional instabilities that nucleate and propagate along faults cutting through a multilayered crust. Geological observations show that fault zones are complex structures often composed of heterogeneous mineral assemblages with contrasting frictional rheologies, whose interaction strongly influences slip behavior during fault reactivation. Two major earthquakes (Mmax > 6) that struck central Italy in the past 30 years–the 1997 Colfiorito and 2016 Norcia events–nucleated within Triassic Evaporites (TE) consisting of anhydrites and dolostones. Previous laboratory experiments on stick-slipping faults in TE gouges highlighted the key role of shear zone fabric in controlling breakdown processes and slip dynamics. However, the individual contribution of each lithology to frictional failure mode remains unclear.
Here we present preliminary results from laboratory friction experiments on powdered TE samples aimed at disentangling the role of anhydrite and dolostone in controlling fault slip behavior and dynamics in TE faults. We conducted double-direct shear experiments on three gouge compositions: 100% anhydrite, 50:50 anhydrite-dolostone, and 100% dolostone. The experimental procedure comprises two main stages: (i) a fabric development phase, in which the gouge is sheared under 50 MPa normal stress at a load-point velocity of 10 μm/s; and (ii) a fault reactivation phase at boundary conditions designed to enhance frictional instabilities that are: normal stress reduced to 30 MPa, a low-stiffness element inserted in series with the shear loading axis, and re-shearing at 1 μm/s. We also monitored the evolution of the fault physical properties from fabric development to stick-slip, via an ultrasonic system that continuously transmits and receives acoustic waves through the experimental fault.
During the fabric development stage, all three fault gouges display stable sliding with a friction coefficient μ of ~ 0.6. In the fault reactivation stage, anhydrite faults exhibit slow (v < 20 μm/s), repetitive stick-slip with small stress drops (Δ𝛕 < 0.1 MPa), whereas dolostone faults accommodate shear through stable sliding. Interestingly, the 50:50 anhydrite-dolostone mixture does not exhibit intermediate behavior between the two end-members but instead develops larger (0.1 < Δ𝛕 < 0.4 MPa), yet generally slower (v < 10 μm/s), slip instabilities, indicating a nonlinear mechanical interaction between anhydrite and dolostone.
Post-experiment microstructural analyses reveal that single-component gouges deform via cataclastic flow and frictional sliding along boundary and Riedel shear bands. In contrast, the 50:50 mixture exhibits extremely localized boundary shear planes dominated by nanometric dolostone particles embedded within foliated anhydrite-dolostone S-C structures. These features suggest a significant contribution of ductile, distributed deformation to energy dissipation during slow frictional ruptures. Ongoing ultrasonic wave analyses aim to characterize the relationship between the evolution of the elastic properties of the fault gouge, its internal structure, and the resulting slip behavior.
Our results provide new insights into the complex interplay between different frictional rheologies within fault zones of the seismogenic layer of northern Apennines, and highlight the role of compositional heterogeneity in controlling fault slip dynamics and energy dissipation.
How to cite: Guglielmi, G., Mauro, M., Scuderi, M., Collettini, C., and Trippetta, F.: Complex interaction between brittle and ductile rheologies in slow laboratory earthquakes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18859, https://doi.org/10.5194/egusphere-egu26-18859, 2026.
The physics underlying earthquake precursory phenomena remains poorly constrained, particularly in crustal materials containing long preexisting fractures. We conduct axial compression experiments on pre-heated Fontainebleau sandstone with acoustic emission (AE) monitoring under confining and pore pressures of 30 and 5 MPa, respectively. Preliminary results show no systematic differences in precursory AE behavior between pre-heated and as-is samples. All samples exhibit b values between 0 and 1, with a wide range of overall b-value evolution toward failure, including both increasing and decreasing trends. Despite this variability, many samples show a local decrease in b value immediately before and during failure, preceded by a local increase near peak stress. These local b-value variations likely reflect distinct microfracturing processes in porous granular rocks, contrasting with the more monotonic b-value decrease commonly reported for crystalline rocks. Our results suggest that subtle differences in initial granular microstructure promote diverse precursory behavior under otherwise identical experimental conditions. Such variability may contribute to the range of precursory behavior observed in tectonic earthquakes, where many large events are not preceded by a decrease in b value. To investigate the role of aseismic deformation in fault nucleation, we are currently quantifying the evolution of microfracture distributions in deformed samples. In parallel, we are deforming samples containing long preexisting fractures by first pre-deforming them in the semibrittle regime at higher pressure–temperature conditions, followed by deformation to failure at lower pressure–temperature conditions. These experiments constrain the evolution of seismic and aseismic precursory signals toward large earthquakes in highly fractured crust.
How to cite: Kanaya, T.: Effect of long preexisting fractures on fault nucleation processes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22410, https://doi.org/10.5194/egusphere-egu26-22410, 2026.
