Natural Fault Zones Research Articles

Development of fluid pathways and associated diagenetic variations in a natural CO2 leaking fault zone.

Abstract Permeability within fault zones can vary through time due to repeated deformation events and rock–fluid interactions. Understanding the history of fault zone alteration is critical when building hydrogeologic models and evaluating the risk of mechanical rock failure during subsurface storage. Newly acquired drill core recovered within the fault damage zone and fault core of the Little Grand Wash fault (LGWF) is combined with observations from outcrop, optical petrography, computerized tomography image analysis, and ultrasonic velocity measurements to characterize the rock types and preserved structural deformation features within this fault zone. These data are used to understand the history of mechanical rock failure and mineralization associated with this fluid-charged fault system. We identify multiple structural features and use their cross-cutting relationships to understand the history of deformation and their association with changes in fault zone permeability and rock mechanical properties. At the LGWF zone, structural deformation features vary temporally and are used to recognize a decoupling between fault slip and fluid flow. The formation of most open-mode veins occurred after shear failure within the LGWF zone. Early-developed shear bands are cut by carbonate veins, that are in turn cut by shear fractures, followed by a second phase of vein formation and ultimately by folding in the fault core. This sequence of formation reflects changes in mechanical rock properties due to subsurface rock–fluid interactions and indicates that the alteration of rock matrix by secondary carbonate cement results in increased unconfined compressive strength, decreased permeability, and increased ultrasonic velocity. In this fault zone, and possibly other fault zones, the changes in rock properties associated with deformation can be detected through their geophysical response.

Multiple Seismic Slip‐Rate Pulses and Mechanical and Textural Evolution of Calcite‐Bearing Fault Gouges

AbstractNatural fault zones are complex, spatially heterogeneous systems. Rock deformation experimental studies simplify the complexity of natural fault zones either as a surface discontinuity between intact rocks (bare‐rock surfaces) or as a few mm‐thick gouge layer. However, depending on the simplified fault type and its slip history, the response to applied deformation can vary. In this work, we conduct laboratory experiments for investigating the evolution of mechanical parameters of simulated faults made of calcite gouge subjected to multiple (four) identical seismic slip‐rate pulses. We observed that, as the number of applied slip‐rate pulses increased, (a) initial friction and steady‐state friction remained approximatively constant, (b) peak friction and normalized strength excess increased and, (c) the slip distances to achieve peak and steady‐state friction, Da and Dc, decreased. The greatest changes occurred between the first and the second slip‐rate pulse. From this pulse onward, the dissipated energy of the calcite gouge fault was similar to those obtained in bare‐rock surfaces experiments. Microstructural analysis showed that, strain is localized in up to two (recrystallized) principal slip zones (PSZ) with sub‐micrometric grain size, surrounded by low porosity sintered and non‐sintered comminuted gouge domains. We conclude that previous seismic slip episodes impact on both the structure and the strain localization processes within a fault, contributing to its shear fabric evolution. We highlight that the strain localization process identifies the PSZ, dissipating the least amount of energy within the entire experimental fault zone.

Rupture Jumping and Seismic Complexity in Models of Earthquake Cycles for Fault Stepovers with Off-Fault Plasticity

ABSTRACT Fault stepovers are prime examples of geometric complexity in natural fault zones that may affect seismic hazard by determining whether an earthquake rupture continues propagating or abruptly stops. However, the long-term pattern of seismicity near-fault stepovers and underlying mechanisms of rupture jumping in the context of earthquake cycles are rarely studied. Leveraging a hybrid numerical scheme combining the finite element and the spectral boundary integral methods, FEBE, we carry out fully dynamic simulations of sequences of earthquakes and aseismic slip for both compressive and tensile stepovers with off-fault plasticity. We consider a rate-and-state friction law for the fault friction and pressure-sensitive Drucker–Prager plasticity for the off-fault bulk response. We observe that the accumulation of plastic deformation, an indication of off-fault damage, is significantly different in the two cases, with more plastic deformation projected in the overlapping region for the tensile stepover. The seismic pattern for a tensile stepover is more complex than for a compressive stepover, and incorporating plasticity also increases complexity, relative to the elastic case. A tensile stepover with off-fault plasticity shows rupture segmentation, temporal clustering, and frequent rupture jumping from one fault to another. These results shed light on possible mechanisms of rupture jumping in fault stepovers as well as the long-term evolution of the fault zone.

Precursory Signals to Injection Induced Fault Reactivation in the Laboratory Using Active Ultrasonic Monitoring Methods

AbstractInduced earthquakes are still highly unpredictable, and often caused by variations in pore fluid pressure. Monitoring and understanding the mechanisms of fluid‐induced fault slip is essential for seismic risk mitigation and seismicity forecasting. Fluid‐induced slip experiments were performed on critically stressed faulted sandstone samples, and the evolution of the actively sent ultrasonic waves throughout the experiment was measured. Two different fault types were used: smooth saw‐cut fault samples at a 35° angle, and a rough fault created by in situ faulting of the samples. Variations in the seismic slip velocity and friction along the fault plane were identified by the coda of the ultrasonic waves. Additionally, ultrasonic amplitudes show precursory signals to laboratory fault reactivation. Our results show that small and local variations in stress before fault failure can be inferred using coda wave interferometry for time‐lapse monitoring, as coda waves are more sensitive to small perturbations in a medium than direct waves. Hence, these signals can be used as precursors to laboratory fault slip and to give insight into reactivation mechanisms. Our results show that time‐lapse monitoring of coda waves can be used to monitor local stress changes associated with fault reactivation in this laboratory setting of fluid‐induced fault reactivation. This is a critical first step toward a method for continuous monitoring of natural fault zones, contributing to seismic risk mitigation of induced and natural earthquakes.

A Diffuse Interface Method for Earthquake Rupture Dynamics Based on a Phase‐Field Model

AbstractIn traditional modeling approaches, earthquakes are often depicted as displacement discontinuities across zero‐thickness surfaces embedded within a linear elastodynamic continuum. This simplification, however, overlooks the intricate nature of natural fault zones and may fail to capture key physical phenomena integral to fault processes. Here, we propose a diffuse interface description for dynamic earthquake rupture modeling to address these limitations and gain deeper insight into fault zones' multifaceted volumetric failure patterns, mechanics, and seismicity. Our model leverages a steady‐state phase‐field, implying time‐independent fault zone geometry, which is defined by the contours of a signed distance function relative to a virtual fault plane. Our approach extends the classical stress glut method, adept at approximating fault‐jump conditions through inelastic alterations to stress components. We remove the sharp discontinuities typically introduced by the stress glut approach via our spatially smooth, mesh‐independent fault representation while maintaining the method's inherent logical simplicity within the well‐established spectral element method framework. We verify our approach using 2D numerical experiments in an open‐source spectral element implementation, examining both a kinematically driven Kostrov‐like crack and spontaneous dynamic rupture in diffuse fault zones. The capabilities of our methodology are showcased through mesh‐independent planar and curved fault zone geometries. Moreover, we highlight that our phase‐field‐based diffuse rupture dynamics models contain fundamental variations within the fault zone. Dynamic stresses intertwined with a volumetrically applied friction law give rise to oblique plastic shear and fault reactivation, markedly impacting rupture front dynamics and seismic wave radiation. Our results encourage future applications of phase‐field‐based earthquake modeling.

The effect of strain on the crystallinity of carbonaceous matter: Application of Raman spectroscopy to deformation experiments

Raman spectroscopy, applied to the carbonaceous matter (CM) present in fault zones, is a potentially powerful tool to aid in the reconstruction of slip conditions, provided the respective effect of heating and strain can be disentangled. For this purpose, we have carried out static heating and deformation experiments on low-grade sediments containing disseminated CM particles and tracked the evolution of CM crystalline ordering (crystallinity) using Raman spectroscopy. Heating at high pressure (200 MPa), at varying temperatures (500 to 800 °C) and durations (3 to 1080 h) resulted in an increased crystallinity, which was observed by an increase in the intensity ratio of the Disordered Band over the Graphite Band (Intensity Ratio R1 parameter) of the Raman spectra. The experimental relationship between R1, T and duration was compiled into a law of evolution, which was then used to design the deformation experiments. Experiments were performed in a Paterson rig at 150 MPa or in a Griggs-type rig at 1 GPa. We observed two types of deformation microstructures: (1) purely brittle ones, such as breccia along slip planes and (2) ductile ones, such as foliated layers, resulting from grain reorientation and comminution along with dissolution precipitation. In all deformation microstructures, an increase in R1 is visible and interpreted to be the result of higher crystallinity of carbonaceous particles. Such increase is negligible in breccia, but is significant, up to 40%, in ductile deformation zones. The slow strain rates prevailing in ductile deformation prevented any local temperature increase, therefore the increase in crystallinity of the CM can be solely attributed to deformation. Thus, in natural fault zone, the increased crystallinity of CM reported in the literature and interpreted to be the result of frictional heating during earthquakes might rather be the result of strain localization without transient temperature increase.

Mapping Buried Fault Zones in a Granitic Pluton Using Aeromagnetic Data

Globally, the largest geothermal resources are linked to the crystalline basement, which could be exploited using Enhanced Geothermal Systems (EGS). The planned underground laboratory GeoLaB aims to facilitate the large-scale application of EGS technologies by performing in-situ high-flowrate experiments on fault zones. The Tromm Granite at the northwestern margin of the Upper Rhine Graben was preselected as one of two potential sites for GeoLaB. To improve the understanding of the natural fault network in the target area, a drone-based aeromagnetic survey was conducted in March 2022, covering an area of about 13.3 km2 in the center of the pluton. A selection of eight filters to enhance the signal was tested against each other using a synthetic model. Based on this comparison, the tilt derivative appears to be the most reliable method for mapping and characterizing natural fault zones. The filtered aeromagnetic dataset reveals a network of linear anomalies interpreted as altered fault zones. The relative abundance of magnetic lineaments correlates broadly with slip tendency in the current stress field. Thus, the aeromagnetic may provide insights into the reactivation potential and deformation history of fault zones, which significantly influence the hydraulic properties. However, additional geophysical surveys or exploration wells are needed to validate the results.

Time-dependent frictional properties of granular materials used in analogue modelling: implications for mimicking fault healing during reactivation and inversion

Abstract. Analogue models are often used to model long-term geological processes such as mountain building or basin inversion. Most of these models use granular materials such as sand or glass beads to simulate the brittle behaviour of the crust. In granular material, deformation is localised in shear bands, which act as an analogue to natural fault zones and detachments. Shear bands, also known as faults, are permanent anomalies in the granular package and are often reactivated during a test run. This is due to their lower strength compared to the undeformed material. When the fault movement stops, time-dependent healing immediately begins to increase the strength of the fault. Faults that have been inactive for a long time therefore have a higher strength than younger faults. This time-dependent healing, also called time consolidation, can therefore affect the structure of an analogue model as the strength of the fault changes over time. Time consolidation is a well-known mechanism in granular mechanics, but it is poorly described for analogue materials and on the timescales of typical analogue models. In this study, we estimate the healing rate of different analogue materials and evaluate the impact on the reactivation potential of analogue faults. We find that healing rates are generally less than 3 % per 10-fold increase in holding time, which is comparable to natural fault zones. We qualitatively compare the frictional properties of the materials with grain characteristics and find a weak correlation of healing rates with sphericity and friction with an average quality score. Accordingly, in models where there are predefined faults or reactivation is forced by blocks, the stability range of the fault angles that can be reactivated can decrease by up to 7∘ over the duration of 12 h. The stress required to reactivate an existing fault can double in the same time, which can favour the development of new faults. In a basin inversion scenario, normal faults cannot be inverted because of the strong misorientation, so time consolidation plays little additional role for such models.

Seismic Fault Slip Behavior Predicted From Internal Microphysical Processes

AbstractEarthquake simulation and hazard prediction are strongly hampered by insufficient physical knowledge of the constitutive behavior of faults. Laboratory studies of carbonate fault friction suggest that seismogenic rupture on faults in carbonate terrains can be explained by a transition from high friction at low initial sliding velocities (V) to low friction at seismic slip velocities, that is, by rapid dynamic weakening. One proposed explanation for this weakening, invokes frictional heating resulting in deformation by grain boundary sliding accommodated by solid‐state diffusion (sometimes referred to as “viscous” or “superplastic” flow). We recently added this dynamic weakening mechanism to a microphysically based model addressing the (rate‐and‐state) frictional behavior of granular gouges undergoing low V shear characteristic of rupture nucleation and arrest. In the present study, we applied the full model to simulate seismic slip in laboratory carbonate faults. Assuming that slip localizes in a principal shear band within the fault (gouge) zone, and accounting for grain size evolution with velocity and temperature, the model reproduces the frictional, thermal and (micro‐)structural evolution observed during seismic slip experiments. In particular, it predicts spatial and temporal evolutions of grain size, porosity, and dominant deformation mechanisms, within and outside the assumed shear band, consistent with trends identified in the laboratory and natural fault zones.

Experimental Plastic Reactivation of Pseudotachylyte‐Filled Shear Zones

AbstractPseudotachylytes are fine‐grained fault rocks that solidify from melt that is produced in fault zones during earthquakes. Exposed sections of natural fault zones reveal evidence of postseismic plastic deformation (i.e., reactivation) of pseudotachylyte, which suggests these rocks may contribute to aseismic slip behavior in regions of repeated seismicity. To measure the plastic flow behavior of pseudotachylyte, we performed high‐temperature deformation experiments on pseudotachylyte from the Gole Larghe Fault Zone, Italy. Plastic reactivation of pseudotachylyte occurs at temperatures above 700°C for strain rates accessible during laboratory experiments. Extrapolation of experimental results to natural conditions demonstrates that pseudotachylyte deforms via diffusion creep at crustal conditions and is much weaker than host rocks in seismically active regions. Importantly, the presence of plastically deforming pseudotachylyte may influence the thickness of the seismogenic layer in some fault zones that experience repeated seismicity.

Frictional properties and microstructural evolution of dry and wet calcite–dolomite gouges

Abstract. Calcite and dolomite are the two most common minerals in carbonate-bearing faults and shear zones. Motivated by observations of exhumed seismogenic faults in the Italian Central Apennines, we used a rotary-shear apparatus to investigate the frictional and microstructural evolution of ca. 3 mm thick gouge layers consisting of 50 wt % calcite and 50 wt % dolomite. The gouges were sheared at a range of slip rates (30 µm s−1–1 m s−1), displacements (0.05–0.4 m), and a normal load of 17.5 MPa under both room-humidity and water-dampened conditions. The frictional behaviour and microstructural evolution of the gouges were strongly influenced by the presence of water. At room humidity, slip strengthening was observed up to slip rates of 0.01 m s−1, which was associated with gouge dilation and the development of a 500–900 µm wide slip zone cut by Y-, R-, and R1-shear bands. Above a slip rate of 0.1 m s−1, dynamic weakening accompanied the development of a localised < 100 µm thick principal slip zone preserving microstructural evidence for calcite recrystallisation and dolomite decarbonation, while the bulk gouges developed a well-defined foliation consisting of organised domains of heavily fractured calcite and dolomite. In water-dampened conditions, evidence of gouge fluidisation within a fine-grained principal slip zone was observed at a range of slip rates from 30 µm s−1 to 0.1 m s−1, suggesting that caution is needed when relating fluidisation textures to seismic slip in natural fault zones. Dynamic weakening in water-dampened conditions was observed at 1 m s−1, where the principal slip zone was characterised by patches of recrystallised calcite. However, local fragmentation and reworking of recrystallised calcite suggests a cyclic process involving formation and destruction of a heterogeneous slip zone. Our microstructural data show that development of well-defined gouge foliation under the tested experimental conditions is limited to high velocities (>0.1 m s−1) and room humidity, supporting the notion that some foliated gouges and cataclasites may form during seismic slip in natural carbonate-bearing faults.

The Mechanics of Creep, Slow Slip Events, and Earthquakes in Mixed Brittle‐Ductile Fault Zones

AbstractGeological observations show that fault zone composition varies and often accommodates a mixture of brittle and ductile deformation. There is growing evidence that the nature of this mixture may play an important role in determining whether the fault creeps steadily or slides in slow slip events (SSEs) and/or fast earthquakes. Using numerical experiments of slip events in a fault zone of finite thickness, we explore how the ratio of brittle to ductile material and the absolute friction change resulting from a variation in slip velocity, affect energy partitioning and slip behavior in brittle‐ductile mixtures. We treat brittle material as Mohr‐Coulomb elastoplastic and ductile material as Maxwell viscoelastic. We simulate velocity‐weakening () behavior in the brittle part of the mixture and velocity‐strengthening () behavior in the ductile part using a rate‐and‐state formulation dependent on plastic strain accumulation. We show that: (1) mixtures can exhibit multiple slip behaviors including earthquakes and slow slip, (2) highly brittle mixtures do not tend to generate SSEs while weakly brittle mixtures can generate slow slip over a wider range of compositions, (3) structural features formed during simulated creep, SSEs, and earthquakes share notable similarities with structures observed in natural fault zones. We find that slip‐synchronous strengthening in the ductile portion of the mixture controls whether a rupture propagates as SSEs of yearlong durations. Shorter duration SSEs occur when the length of the plastic shear segments formed during slip is similar to the characteristic weakening distance for an earthquake.

Faulting Processes Unveiled by Magnetic Properties of Fault Rocks

AbstractAs iron‐bearing minerals—ferrimagnetic minerals in particular—are sensitive to stress, temperature, and presence of fluids in fault zones, their magnetic properties provide valuable insights into physical and chemical processes affecting fault rocks. Here, we review the advances made in magnetic studies of fault rocks in the past three decades. We provide a synthesis of the mechanisms that account for the magnetic changes in fault rocks and insights gained from magnetic research. We also integrate nonmagnetic approaches in the evaluation of the magnetic properties of fault rocks. Magnetic analysis unveils microscopic processes operating in the fault zones such as frictional heating, energy dissipation, and fluid percolation that are otherwise difficult to constrain. This makes magnetic properties suited as a “strain indicator,” a “geothermometer,” and a “fluid tracer” in fault zones. However, a full understanding of faulting‐induced magnetic changes has not been accomplished yet. Future research should focus on detailed magnetic property analysis of fault zones including magnetic microscanning and magnetic fabric analysis. To calibrate the observations on natural fault zones, laboratory experiments should be carried out that enable to extract the exact physicochemical conditions that led to a certain magnetic signature. Potential avenues could include (1) magnetic investigations on natural and synthetic fault rocks after friction experiments, (2) laboratory simulation of fault fluid percolation, (3) paleomagnetic analysis of postkinematic remanence components associated with faulting processes, and (4) synergy of interdisciplinary approaches in mineral‐magnetic studies. This would help to place our understanding of the microphysics of faulting on a much stronger footing.

Optically Stimulated Luminescence Signal Resetting of Quartz Gouge During Subseismic to Seismic Frictional Sliding: A Case Study Using Granite‐Derived Quartz

AbstractAttempts to determine the age of faulting recorded by fault rocks have been made using K–Ar, fission‐track, electron spin resonance, and luminescence dating methods. Here we report the unambiguous reduction (resetting) of optically stimulated luminescence (OSL) signals of quartz gouge after friction experiments and estimate the seismological and geological conditions required for OSL signal resetting in natural fault zones. In the experiments, we used quartz sand with a particle size of <150 μm and equivalent dose of 31.5 ± 16.6 Gy. Friction experiments were conducted with a rotary‐shear, high‐velocity friction apparatus at slip rates (V) of 200 μm s−1 to 1.3 m s−1, normal stress of 1.0 MPa, and displacement of 10 m. In the experiments conducted under dry conditions, the OSL signal starts to decrease from V = 0.25 m s−1 and becomes near zero at V ≥ 0.65 m s−1. OSL signal resetting is also observed in the experiment sheared at 1.3 m s−1 under water‐added conditions. At V = 0.25 and 0.40 m s−1, partial resetting occurs, which is characterized by coexistence of particles with and without an OSL signal. OSL signal intensity shows a strong correlation with applied power density and frictional heat during high‐velocity friction, and the signal exponentially decreases with increasing power density and temperature. The power density required for partial and complete OSL signal resetting is ~0.17 and 0.6 MW m−2, respectively. Assuming a co‐seismic fault slip rate of 0.6 m s−1, the depths required for partial and complete resetting are expected to be ≥11 and ≥42 m.

Impact of reservoir heterogeneity on fluid injection-induced fault slip: insights from numerical simulation

In order to meet the growing energy demand worldwide, the technology of hydraulic fracturing for shale gas extraction has been rapidly developed and applied in recent years. However, man-made fluid injection has also caused several problems of high societal concern such as induced earthquakes. The focus of this paper is on the process of induced fault activation in shale reservoirs caused by hydraulic stimulation activities. We constructed a two-dimensional numerical model representing the horizontal cross-section of a faulted reservoir and simulated important coupled hydro-mechanical processes during fluid injection. We parameterized our model with the site investigation data of an actual shale gas project including in-situ stress, rock properties and injection protocol, etc. We represented the natural fault zone as a geological composite involving a fault core and a damage zone consisting of a set of subsidiary fractures parallel to the fault strike. We coupled the solid deformation and fault displacement with Darcy-type fluid flow based on poroelasticity principles and hydro-mechanical constitutive relationships. The heterogeneous nature of the permeability of the shale formation is mimicked by a random field model governed by a log-normal probability density function and directional correlation lengths, such that we can explore the impact of reservoir heterogeneity on the fault slip behaviour. In addition, we also studied the effects of fault length, dip angle and injection point-to-fault distance on the fault slip. Specifically, we analysed the link between fault slip and fluid flow field, permitting the capture of fluid movement in the heterogeneous faulted reservoir, the temporal and spatial evolution of preferential flow channels and their consequences on induced fault slip. The results of this research have important implications for understanding the triggering processes and mechanisms of fault slip during shale gas exploitation as well as many other injection-related engineering activities.

Friction experiments under in-situ stress reveal unexpected velocity-weakening in Nankai accretionary prism samples

The Nankai Trough hosts diverse fault slip modes, ranging from slow slip events to megathrust earthquake ruptures. We performed laboratory friction experiments on samples collected by the Integrated Ocean Drilling Program offshore Kii Peninsula, Japan. This study systematically investigates the effect of effective normal stress on frictional strength and the velocity-dependence of friction for natural fault zone and wall rock samples collected from depths of 270 to ∼450 meters below seafloor (mbsf), and over a range of shearing velocity spanning from 0.01–30 μm/s. In addition, cohesive strength was determined before and after each velocity step experiment while the sample was unloaded. We tested both powdered and intact specimens at estimated in-situ effective stresses, as well as at higher stresses representing deeper portions of the megasplay fault (Sites C0004, C0010) and the frontal thrust zone (Sites C0007, C0006). The apparent coefficient of sliding friction μs varies between 0.22–0.53 and correlates inversely with clay content. Direct measurements of cohesive strength show that on average 11% of the residual shear strength, and up to ∼30% for some specimens, can be attributed to cohesion. Friction coefficient slightly decreases as a function of increasing effective normal stress, attributed to a decreasing proportion of cohesive strength. The lowest μs values are observed for samples from the frontal thrust zone Site C0007 and the megasplay fault Site C0010. All samples show a combination of velocity-strengthening and velocity-weakening behavior, but intact samples tested under in-situ effective normal stress and low shearing velocities (<∼1 μm/s) exhibit consistently large velocity-weakening frictional behavior. The observed velocity-weakening behavior is related to the induration state of the material, which affects the real area of contact along shear surfaces. This is supported by direct measurements of cohesive strength, showing that higher cohesion values in intact samples correspond with a more pronounced evolutionary effect in velocity step tests that, in turn, map to lower values of the rate-dependent friction parameter a−b. We propose that fault zones in the Nankai subduction zone are likely to be velocity-weakening, and thus potentially able to host the nucleation of unstable slip, from seismogenic depths to the seafloor. We also find that testing disaggregated fault zone samples and employing effective stresses exceeding those in-situ lead to overestimation of a−b, emphasizing the importance of testing intact samples under in-situ conditions in laboratory friction studies.

Modelling fluid flow in complex natural fault zones: Implications for natural and human-induced earthquake nucleation

Pore fluid overpressures in active fault systems can drive fluid flow and cause fault weakening and seismicity. In return, deformation accommodated by different modes of failure (e.g. brittle vs. ductile) also affects fault zone permeability and, hence, fluid flow and pore fluid pressure distribution. Current numerical simulation techniques model how fluid flow controls fault reactivation and associated seismicity. However, the control exerted by pore fluid pressure on the transition from slow aseismic fault sliding to fast seismic sliding, during the earthquake nucleation phase, is still poorly understood. Here, we model overpressured, supercritical CO2 fluid flow in natural faults, where non-linear, complex feedback between fluid flow, fluid pressure and fault deformation controls the length of the nucleation phase of an earthquake and the duration of the interseismic period. The model setup is an analogue for recent seismic source events in the Northern Apennines of Italy (e.g. Mw 6.0 1997-98 Colfiorito and Mw 6.5 2016 Norcia earthquakes). Our modelling results of Darcy fluid flow show that the duration of the nucleation phase can be reduced by orders of magnitude, when realistic models of fault zone architecture and pore pressure- and deformation-dependent permeability are considered. In particular, earthquake nucleation phase duration can drop from more than 10 years to a few days/minutes, while the seismic moment can decrease by a factor of 6. Notably, the moment of aseismic slip (M0=109Nm) obtained during the nucleation phase modelled in our study is of the same order as the detection limit of local strain measurements using strain meters. These findings have significant implications for earthquake early warning systems, as the duration and moment of the nucleation phase will affect the likelihood of timely precursory signal detection. Interestingly, aseismic slip has been measured up to a few months before some recent large earthquakes, although in a different tectonic context than the model developed here, rekindling interest in the nucleation phase of earthquakes. In addition, our results have important implications for short and long term earthquake forecasting, as crustal fluid migration during the interseismic period may control fault strength and earthquake recurrence intervals.

Experimental investigations on dating the last earthquake event using OSL signals of quartz from fault gouges

Obtaining a reliable age of the latest seismic slip event along an active fault is important for seismic hazard assessment. Here, we observe changes in the optically stimulated luminescence (OSL) signal of quartz crystals due to frictional heating in artificial fault gouges (comprising a mixture of quartz grains and Ca-bentonite powder). The fault gouge was deformed using a high-velocity rotary-shear apparatus at room temperature and room humidity. At a seismic slip rate of 1.31 m·s−1, intense slip localization occurred along a very thin layer (~300 μm thick) within the simulated fault zones (1 mm thick). The estimated temperature of the slip-localized layer (SLL) increased by ~475 °C from frictional heating. The quartz OSL signals of the gouges were fully reset, most noticeably for the SLL. In contrast, there was rare slip-localization at subseismic slip rates (0.06–0.001 m·s−1), for which the estimated temperature rise in the SLL was ~120 °C; hence, the quartz OSL signal was not reset under this condition. The results suggest that quartz OSL dating can be used to constrain the age of the latest seismic event in natural quartz-bearing fault zones where a SLL occurs.

Simulation of slip transients and earthquakes in finite thickness shear zones with a plastic formulation

We perform numerical experiments of damped quasi-dynamic fault slip that include a rate-and-state behavior at steady state to simulate earthquakes and a plastic rheology to model permanent strain. The model shear zone has a finite width which represents a natural fault zone. Here we reproduce fast and slow events that follow theoretical and observational scaling relationships for earthquakes and slow slip events (SSEs). We show that the transition between fast and slow slip occurs when the friction drop in the shear zone is equal to a critical value, Δμc. With lower friction drops, SSEs use nearly all of mechanical work to accumulate inelastic strain, while with higher friction drops fast slips use some of the mechanical work to slip frictionally. Our new formulation replaces the state evolution of rate and state by the stress evolution concurrent with accumulation of permanent damage in and around a fault zone.

Strain localization in dry sheared granular materials: A compactivity-based approach.

Shear banding is widely observed in natural fault zones as well as in laboratory experiments on granular materials. Understanding the dynamics of strain localization under different loading conditions is essential for quantifying strength evolution of fault gouge and energy partitioning during earthquakes and characterizing rheological transitions and fault zone structure changes. To that end, we develop a physics-based continuum model for strain localization in sheared granular materials. The grain-scale dynamics is described by the shear transformation zone (STZ) theory, a nonequilibrium statistical thermodynamic framework for viscoplastic deformation in amorphous materials. Using a finite strain computational framework, we investigate the initiation and growth of complex shear bands under a variety of loading conditions and identify implications for strength evolution and the ductile to brittle transition. Our numerical results show similar localization patterns to field and laboratory observations and suggest that shear zones show more ductile response at higher confining pressures, lower dilatancy, and loose initial conditions. Lower pressures, higher dilatancy, and dense initial conditions favor a brittle response and larger strength drops. These findings shed light on a range of mechanisms for strength evolution in dry sheared granular materials and provide a critical input to physics-based multiscale models of fault zone instabilities.