Abstract The brittle–ductile transition, typically located near the base of the seismogenic zone, controls strain localization and the evolution of physical properties, with direct implications for fluid transport and fault mechanics. However, quantitative constraints on strain partitioning and porosity evolution across this transition remain limited, particularly in low‐porosity carbonate rocks. We conducted triaxial compression experiments on dry Carrara marble at confining pressures of 5 and 100 MPa using two complementary techniques: (a) acoustic emission (AE) monitoring coupled with P‐wave velocity measurement, and (b) in situ X‐ray microtomography combined with digital volume correlation (DVC). AE activity remains low at all pressures, with source mechanisms becoming increasingly compaction‐dominated at the highest confinement. P‐wave velocities fall markedly during deformation by up to 60% at 5–10 MPa, ∼50% at 40 MPa, and ∼40% at 100 MPa, reflecting pervasive microcrack development. DVC reveals that the proportion of dilative volumetric strain decreases with confinement from nearly 90% at 5 MPa to 50% at 70–90 MPa, yet dilation remains a major component and is comparable to compaction even at the highest pressures. X‐ray–derived bulk porosity increases from <1% to ≈1.5% at low confinement prior to failure, and to 10% at higher confinements, indicating substantial porosity generation during distributed dilatant cataclastic flow. Together, these results provide the first quantitative link between P‐wave velocity reduction, volumetric strain partitioning, and transient porosity generation across the brittle to semi‐ductile transition, revealing how the interplay of dilation and compaction governs both elastic properties and strain localization in low‐porosity crystalline carbonate.

When a porous rock is subjected to overall compressive loading, either increasing pore pressure or decreasing confining pressure could result in rock failure. The stress path and the applied pressure change rate may affect the initiation and propagation of fractures within brittle materials. Understanding the physical mechanisms leading to failure is crucial for underground engineering applications and geo-energy exploration and storage. We conducted triaxial compression experiments on porous Bentheim sandstone samples at different stress paths and pressure change rates. First, at a constant confining pressure of 35 MPa and pore pressure of 5 MPa, intact cylindrical samples were axially loaded up to about 85% of the peak strength. Subsequently, the axial piston position was fixed, and then either the pore pressure was increased or the confining pressure was decreased at two different rates (0.5 MPa/min or 2 MPa/min), leading to final catastrophic failure. The mechanical results revealed that samples subjected to higher rates of decreasing effective confining pressure exhibited larger stress drop rates, higher slip rates, higher total breakdown work, higher rates of acoustic emissions (AEs) before failure, and higher post-failure AE decay rates. In contrast, the applied stress path did not significantly affect rock failure characteristics. Comparison of located AE events with post-mortem microstructures of deformed samples shows a good agreement. The AE source type determined from the P-wave first-motion polarity shows that shear failure dominated the fracture process when approaching failure. Gutenberg-Richter b-values revealed a significant decrease before failure in all tests. Our results indicate that, in contrast to the stress path, the rate of effective stress change strongly affects fracturing behavior and AE rate changes.

Abstract In many locations, earthquakes in the upper crust occur in carbonate rocks, whose deformation affects aftershock sequences and postseismic relaxation. We investigate the stress relaxation process and strain evolution in Jinping dolomite marble (porosity <0.2%) subjected to multi‐stage stress ramp‐relaxation cycles under confining pressures of 2–70 MPa. Distributed Optical Fiber Sensing, a real‐time, high‐resolution technique capable of detecting micrometer‐scale strain changes, is employed to quantify the effects of confining pressure, differential stress, and relaxation time on the spatiotemporal evolution of strain. During stress relaxation, the high sensitivity of the inelastic strain rate to differential stress indicates that brittle creep, driven by time‐dependent microcracking and frictional sliding, is the primary mechanism governing differential stress reduction in Jinping dolomite marble. Differential stress and axial inelastic strain rates during relaxation follow an Omori‐like power‐law decay, with an apparent exponent increasing from 0.6 to 1 as strain becomes increasingly localized when approaching system‐size failure. The deviation of the observed from the theoretical , predicted by mean‐field and velocity‐strengthening models, arises from internal stress redistribution and the heterogeneous evolution of damage within the sample‐device system. Confining pressure and relaxation onset stress affect strain heterogeneity and localization, modulating the spatial evolution of high‐strain events and the observed relaxation behavior. These results suggest that inelastic strain accumulation via brittle creep may constitute an important component of postseismic deformation in the upper crust and highlight the linkage between spatial strain localization and temporal relaxation processes.

Carbonate reservoirs host faults that can be reactivated in the manner of aseismic creep or large earthquakes. In this study, we experimentally investigate how fault geometry and confining pressure control slip stability and microseismicity in low-porosity (∼1%) carbonates. Displacement-controlled triaxial shear tests were performed at varying confining pressures of 30–90 MPa on carbonate samples cut by smooth (saw-cut) and rough (fractured) faults, while acoustic emission (AE) activity and ultrasonic P-wave transmission were simultaneously monitored. Carbonate samples display pressure-sensitive frictional strength, and strain partitioning is found to be influenced by fault geometry. Rough-fault samples accumulate significant inelastic matrix deformation, characterized by axial strain at the onset of fault reactivation rxn > 0.35% and a relative contribution of fault-zone slip to total shortening Rslip < 70%, and generate clustered AEs. In contrast, saw-cut faults localize deformation almost entirely on the fault plane, with rxn < 0.16% and Rslip > 86%, and predominantly aseismic sliding. As the confining pressure increases from 30 MPa to 90 MPa, we observe a mechanical transition from stick-slip events to stable sliding with a shear-enhanced compaction deformation mechanism, as supported by a gradual increase in the compaction source type of associated AEs. The temporal evolution of ultrasonic P-wave velocities and P-wave amplitudes can act as a proxy for tracking evolving damage and fault coupling. These results link laboratory fault mechanics with field observations of earthquakes in carbonate, highlighting the combined role of pressure and fault geometry in governing strain partitioning, fault stability and energy dissipation.

Multi-stage uniaxial and triaxial stress relaxation tests were performed on Weschnitz granodiorite, Beishan granite, and Jinping dolomite marble to investigate the deformation evolution before system-size failure, and to study stress relaxation responses. Optical fiber sensing was used to measure distributed strain for full-field strain reconstruction across the sample surface. Strain heterogeneities due to imperfect boundary conditions are detected before and during the linear elastic deformation phase in all samples. The initial strain heterogeneity in the elastic phase is found to control the subsequent inelastic strain localization in granodiorite and granite samples. Macroscopic brittle splitting or faulting in granitic samples eventually occurs within or at the boundaries of the strain localization zones. In contrast, dolomite marble has a more homogeneous strain distribution, with increased differential stress promoting strain delocalization. The reduced axial strain rates during stress relaxation promote time-dependent deformation mechanisms, leading to different spatial distributions of strains. Stress relaxation does not significantly change the degree of strain localization in granite, but it promotes strain delocalization in marble after the onset of dilatancy. In multi-stage tests, inelastic strain accumulates mainly during stress relaxation in granite samples and during stress ramping in marble samples. The different strain distributions and relaxation responses between rock samples result from different deformation mechanisms: localized strain in granite results from clustered microcracking, whereas distributed strain in dolomite marble is driven by both microcracking and low-temperature plasticity (e.g., dislocation glide). These results suggest that lithological differences may result in different precursor signals before system-scale failure and postseismic bulk relaxation responses.

Rocks exhibit a brittle failure mode, leading to system-size failure through cataclastic faulting processes involving microfracture coalescence and frictional sliding, resulting in localized deformation. In contrast, the ductile failure mode can be described as a distributed deformation at the macroscopic scale, although there may be significant grain-scale heterogeneities. The transition between these two modes is an important research area because it is assumed to occur at the base of the seismogenic zone where large earthquakes may nucleate. Understanding strain evolution and partitioning between brittle and ductile failure modes may shed light on the preparation process for large earthquakes. To investigate the transition from brittle to ductile deformation, we performed two series of experiments on Carrara marble core samples: conventional triaxial experiments with acoustic emission recording at GFZ Potsdam, and dynamic in situ 4D X-ray imaging experiments on beamline BM18 at the European Synchrotron Radiation Facility. Carrara marble is used as a rock model because this transition can be achieved at room temperature. We performed the experiments at room temperature and confining pressures between 5 and 100 MPa. For the synchrotron experiments, we segmented the images and implemented digital volume correlation (DVC) analyses between tomogram acquisitions to quantify the evolution of volumetric and shear strain components during the transition from the brittle to ductile regime. The results show that the transition is controlled by the dynamics of microfractures, even in the ductile regime. Below 40 MPa of confining pressure, deformation localizes along faults, particularly at 5 and 10 MPa. At 40 MPa, tomograms reveal the formation of a localized shear zone and macroscopically distributed deformation, resembling a semi-brittle regime. The DVC reveals the spatial extent of the strain directed into faults. A limited number of acoustic emissions recorded at this confining pressure revealed the prevalence of aseismic activity during deformation. Above 40 MPa, deformation shifts to a non-localized pattern at the core sample scale, involving the opening of microfractures, possibly due to the cataclastic flow mechanism accommodating this regime.

Geothermal energy plays a vital role in decarbonizing electricity and heat supply. Effective utilization of geothermal resources hinges on identifying or generating permeable reservoir zones and understanding how effective pressure variations affect fluid circulation and reservoir properties by poroelastic deformation. Hydrothermal alteration can modify the petrophysical properties of geothermal reservoir rocks, which may increase or decrease its productivity. Understanding these alteration effects is essential to predict and optimize long‐term sustainable geothermal operations. Here, we investigate the impact of hydrothermal alteration on poroelastic and hydraulic properties of diverse lithologies in a series of deformation tests performed at several confining (0–80 MPa) and pore pressure (10–30 MPa) levels. Experimental results of hydrothermally altered dikes and phyllites obtained from the Blue Mountain geothermal field (Nevada, USA) are compared to thermally cracked La Peyratte granite (France) and correlated with petrophysical properties, mineral composition, and microstructures. Argillic alteration of dikes increases porosity and storage capacity but lowers thermal conductivity and increases pore compressibility. Conversely, silicate precipitation in phyllites increases stiffness and thermal conductivity but also reduces porosity and permeability. Experimentally determined effective pressure coefficients range from 0.1 to 0.9, differ for permeability and volumetric strain and decrease with increasing effective pressure. The presence of compliant microcracks and crack‐like pores significantly increases the stress sensitivity of La Peyratte granite and silicified phyllites. This study demonstrates how thermal and chemical alteration impacts poromechanical and petrophysical characteristics of geothermal targets, which ultimately govern reservoir stability and subsidence, induced seismicity as well as fluid and heat extraction efficiency during geothermal operations.

Rock-magnetic properties are widely used to resolve the composition, concentration, size, shape, and alignment of iron oxides in rock samples. Although anisotropy of magnetic susceptibility is measured routinely, acquisition of remanent magnetization as well as hysteresis loops are usually measured in one direction only. This may lead to biased interpretation of the results. In our study, observation of strong directional dependence of remanence acquisition curves and hysteresis loops in experimentally deformed samples of hematite ore is reported. While the original undeformed hematite ore sample exhibited magnetic properties practically independent of the direction, the specimens which experienced deformation in torsion showed significant anisotropy. For example, the anisotropy of saturation magnetization reached several tens of percent. Our results suggest that experimental deformation resulted in neoformation of magnetite nanoparticles, as well as shift of the original hematite coercivity spectra toward lower values. These findings have significant impact on the remanent and induced magnetization measurement protocols and interpretation of the data. Anisotropy of these parameters should be checked at least in cases when hematite is assumed to be present in the samples, or the samples underwent creep deformation.

Enhanced geothermal systems (EGSs) developed by hydraulic stimulation are promising for exploiting petrothermal heat by improving fluid pathways in low-permeable geothermal reservoir rocks. However, fluid injection into the subsurface can potentially cause large seismic events by reactivating pre-existing faults, which is a significant barrier to EGSs. The management of injection-induced seismicity is, therefore, essential for the success of EGSs. During the hydraulic stimulation of an EGS, fluid can be injected into a fault zone or into the rock matrix containing pre-existing faults adjacent to the injection well. The differences in hydromechanical responses between fluid injection into and adjacent to a fault have not been investigated in detail. Here, we performed triaxial fluid injection experiments involving injecting fluid directly and indirectly into a fault in granite rock samples to analyse the distinct hydromechanical responses and estimate the injection-induced seismicity in both cases. Our results suggest that in addition to directly injecting fluid into a critically stressed fault, injecting into nearly intact granite adjacent to the fault could also cause injection-induced seismic hazards owing to the high fluid pressure required to create new fractures in the granite matrix. It is, therefore, important to carefully identify pre-existing faults within tight reservoirs to avoid injecting fluid adjacent to them. Additionally, once prior unknown faults are delineated during hydraulic stimulation, appropriate shut-in strategies should be implemented immediately to mitigate seismic risks. This article is part of the theme issue 'Induced seismicity in coupled subsurface systems'.

Little is known about the impact of pressure (P) and temperature (T) on faulting behavior and the transition to fault locking under high P-T conditions. Using a Paterson gas-medium apparatus, triaxial compression experiments were conducted on Carrara marble (CM) samples containing a saw-cut interface at similar to 40 degrees to the vertical axis at a constant axial strain rate of similar to 1 x 10(-5) s(-1), P = 30-150 MPa and T = 20-600 degrees C. Depending on the P-T conditions, we observed the complete spectrum of deformation behavior, including macroscopic (shear) failure, stable sliding, unstable stick-slip, and bulk deformation with locked faults. Macroscopic failure and stable sliding were limited to P < 100 MPa and T = 20 degrees C. In contrast, at P >= 100 MPa or T >= 500 degrees C, faults were locked, and samples with bulk deformation experienced strain hardening at strains <= 8.8%. At T = 100-400 degrees C and P <= 100 MPa, we observed unstable stick-slip behavior, where both fault reactivation stress and subsequent stress drop increased with increasing pressure and temperature, associated with increasing matrix deformation and less fault slip. Microstructures indicate a mixture of microcracking, twinning and dislocation activity (e.g., kinking and undulatory extinction) that depends on P-T conditions and peak stress. The transition from slip to lock-up with increasing pressure and temperature is induced by an enhanced contribution of crystal plastic deformation. Our results show that fault reactivation and stability in CM are significantly influenced by P-T conditions, probably limiting the nucleation of earthquakes to a depth of a few kilometers in calcite-dominated faults. Plain Language Summary The nucleation depth of natural earthquakes is often limited to a certain depth range that depends on lithology and environmental conditions (e.g. T and P). Here, we performed an experimental study on Carrara marble with saw-cut and polished faults in a triaxial deformation apparatus at pressures up to 150 MPa and temperatures up to 600 degrees C to investigate the conditions under which the fault is reactivated or already locked. Due to an increasing amount of plastic deformation with increasing temperature and pressure, the deformation of the pre-faulted rock partitions increasingly into enhanced matrix deformation and less fault slip, associated with higher reactivation stress and larger stress drops. Extrapolating our laboratory results to natural calcite-dominated faults suggests that earthquakes may occur at depths of a few kilometers. At greater depths, faults are likely locked, and calcite rocks dominantly deform in the ductile regime.

Probing source mechanisms of natural and induced earthquakes is a powerful tool to unveil associated rupture kinematics. The source processes of failure and slip instability driven by stress loading are affected by fault geometry, but the source ruptures of injection-induced seismicity in relation to fault structures and local stress states remain poorly understood. We have conducted a series of fault reactivation and slip experiments on sandstone samples containing faults with different surface roughness (smooth saw-cut fault and fractured rough fault). We impose progressive fluid injection to induce fault slip, and simultaneously monitor the associated acoustic emission (AE) activity. Using high-resolution AE recordings, we perform full moment tensor inversion of all located AE sources, and investigate the changes of AE source characteristics associated with induced fault slip and their relation to fault roughness. For the complex and rough fault, we observe significant non-double-couple components of AE sources and a high degree of focal mechanism heterogeneity. The temporal changes of AE mechanisms associated with injection-induced fault slip on the smooth fault reveal increasing proportions of double-couple components and decreasing variability of AE focal mechanisms when approaching the onset of slip events. The observed inconsistency between the nodal planes of AE sources and the macroscopic fault plane orientation is attributed to the development of secondary fracture networks surrounding the principal slip surface. We analyze changes in the magnitude-frequency characteristics and source mechanisms of AEs with faultnormal distance, showing that for the smooth (mature) fault, Gutenberg-Richter b-value of on-fault seismicity is lower and focal mechanisms are less heterogeneous, compared to off-fault seismicity. Our results emphasize the important role of roughness-related changes in local fault geometry and associated stress heterogeneity for source mechanisms and rupture kinematics of injection-induced seismicity.

Unconsolidated, undrained triaxial deformation tests were performed on sandy facies Opalinus Clay at 50 MPa confining pressure to characterize the effect of water and microfabric orientation on the deformation behavior, mechanical properties, and P-wave velocity evolution. Dry and wet (≈ 8 and > 95% initial water saturation, respectively) samples with 12.6 ± 0.4 vol% porosity were deformed parallel and perpendicular to the bedding direction at a constant strain rate of 5 × 10 –6 s −1 . Dry samples revealed semi-brittle behavior and exhibited strain localization at failure, while deformation was more ductile at saturated conditions, promoting stable, slow faulting. Peak strength, Young’s modulus, and number of cumulative acoustic emissions decreased significantly for wet samples compared to dry samples; the opposite was observed for Poisson’s ratio. P-wave velocity anisotropy was significantly altered by differential stress, primarily due to the interplay between pore and fracture closure and stress-induced microcrack formation. For samples that were deformed perpendicular to bedding, we observed a reduction and reversal of P-wave velocity anisotropy with increasing differential stress, whereas anisotropy of parallel samples increased. The results suggest that water saturation reduces the pressure at the brittle-ductile transition and that the elastic properties and anisotropy of sandy facies Opalinus Clay can be significantly altered in an anisotropic stress field, e.g., adjacent to fault zones or tunnel excavations. Changes in elastic anisotropy are primarily controlled by the orientation between the pre-existing microfabric and the maximum principal stress direction, stress magnitude, and the degree of water saturation.

Surface roughness ubiquitously prevails in natural faults across various length scales. Despite extensive studies highlighting the important role of fault geometry in the dynamics of tectonic earthquakes, whether and how fault roughness affects fluid-induced seismicity remains elusive. Here, we investigate the effects of fault geometry and stress heterogeneity on fluid-induced fault slip and associated seismicity characteristics using laboratory experiments and numerical modeling. We perform fluid injection experiments on quartz-rich sandstone samples containing either a smooth or a rough fault. We find that geometrical roughness slows down injection-induced fault slip and reduces macroscopic slip velocities and fault slip-weakening rates. Stress heterogeneity and roughness control hypocenter distribution, frequency-magnitude characteristics, and source mechanisms of injection-induced acoustic emissions (AEs) (analogous to natural seismicity). In contrast to smooth faults where injection-induced AEs are uniformly distributed, slip on rough faults produces spatially localized AEs with pronounced non-double-couple source mechanisms. We demonstrate that these clustered AEs occur around highly stressed asperities where induced local slip rates are higher, accompanied by lower Gutenberg-Richter b-values. Our findings suggest that real-time monitoring of induced microseismicity during fluid injection may allow identifying progressive localization of seismic activity and improve forecasting of runaway events

&lt;p&gt;Many reservoir rocks of productive geothermal energy resources display low porosity and matrix permeability. Therefore, to enhance fluid flow, fault zones and natural fracture networks are increasingly targeted for geothermal energy exploitation that are hydraulically connected to geothermal wells by stimulating the reservoir units. To this end, fluid is injected into the reservoir, which is generally believed to reduce effective stress and induce minor slip along stressed faults. Fluid injection can also lead to induced microseismicity and remotely-triggered earthquakes at great distances from the target reservoirs. The Blue Mountain geothermal field produces the largest seismic activity during maintenance shutdowns of injection and production requiring additional mechanisms such as poroelastic stress effects. In order to improve seismic hazard assessment and the understanding of induced seismicity around injection wells, we explore the coupling between matrix permeability, fault zone hydrology and mechanical behavior.&lt;/p&gt;&lt;p&gt;One main goal of this work is to provide insight into the scale-dependence of permeability by comparing laboratory results with previous permeability measurements using tidal responses in three different idle wells in Blue Mountain. We present a series of laboratory experiments performed on rock samples collected from the DB2 well at the Blue Mountain geothermal site in Humboldt County, Nevada, USA. The geothermal field benefits from the intersection of two W- and NW-directed normal faults resulting in high permeability of the geothermal reservoir production zone controlled by a brittle damage zone. Samples were obtained from two different lithologies, both of Triassic age, that constitute the reservoir. The first set of samples are low-porosity, (0.4 vol%) quartz-dominated (~50 &amp;#8211; 60 wt%) phyllites, which exhibit zones with pronounced fracturing and elevated porosity (3.8 vol%). The second set of samples are felsic intrusive rocks with moderate to high porosity (7 &amp;#8211; 15 vol%) due to strong hydrothermal alteration and clay mineral formation. Samples from both lithologies were selected from different sections of the damage zone showing varying degrees of faulting, from intact to highly brecciated, containing mineralized veins. We determine flow and poroelastic properties of cylindrical samples with a length of 2 cm and a diameter of 5 cm, subjected to stepwise cyclic variation of pore (&lt;40 MPa) and confining pressure (&lt;45 MPa). At each pressure step, we measure volumetric strain changes to derive the bulk modulus and effective stress coefficient, and use steady-state or pore pressure oscillation to determine permeability.&lt;/p&gt;&lt;p&gt;In addition to the tidal response and laboratory results, we developed a high-resolution seismicity catalog based on more than three years of continuous waveforms records from 2016 to 2019. We performed template-matching and differential travel-time inversions and use the resulting seismic events with magnitudes between 0.7 &amp;#8211; 2.7 to search for seismicity migration patterns associated with discrete injection events. Integration of field and laboratory results can improve the characterization of the permeability structure of the fault zone at Blue Mountain and help to understand the mechanisms that trigger seismic events during production shutdown as well as the role of poroelastic stress release.&lt;/p&gt;

This repository contains the digitalized data from the following original publications and also intermediate results for our manuscript. 1) Gleason, G. C., & Tullis, J. (1995). A flow law for dislocation creep of quartz aggregates determined with the molten salt cell. Tectonophysics, 247(1-4), 1–23. 2) Zhou, Y., Zhang, H., Yao, W., Dang, J., & He, C. (2017). An experimental study on creep of partially molten granulite under high temperature and wet conditions. Journal of Asian Earth Sciences, 139, 15–29.

The reactivation and stability of faults depend on a number of parameters, like rock composition, (effective) normal pressure, fault roughness, and loading rate. However, not much in known about the impact of temperature on faulting behavior. Using a Paterson-type gas deformation apparatus, triaxial compression experiments were conducted on dry Carrara marble samples containing a saw-cut oriented at about 40° to the axial stress direction. The tests were performed at constant axial strain rate of 1x10-5 s-1, confining pressures, P, between 30 and 150 MPa, and temperatures, T, in the range of 20 to 600°C. Under these conditions, intact Carrara marble deforms mainly in the semi-brittle regime and brittle, localized, deformation associated with strain weakening occurs only at room temperature and P < 100 MPa. At similar temperature, saw cut samples show formation of a new fracture zone inclined at 30° to the loading direction at P = 30 MPa and fault reactivation with stable sliding on the preexisting fault at P = 50 MPa. At higher temperatures up to 400°C and pressures < 100 MPa, we observed a mixture of matrix deformation and unstable (stick-slip) sliding on the fault. The peak stress at the onset of fault reactivation increased with P and T, resulting in higher associated peak strain. Also, the peak stress drop increased with increasing peak stress. At high T (>400°C) and P (>100 MPa) the fault remained locked and samples revealed ductile matrix creep with strain hardening, where the strength is almost similar to the strength of intact sample deformed under similar conditions. Microstructural observations reveal intense microcracking at the lowest P-T conditions. Samples exhibiting stick-slip behavior show a thin, discontinuous gouge layer and high twin density in specimens with late (high stress) fault reactivation. Bulk creeping samples reveal less damage and the fault appears to be partially sealed. Electron backscatter diffraction measurements suggest a slightly increasing crystallographic preferred orientation in the direct neighborhood to the fault compared the matrix under most conditions. Our results indicate that the fault reactivation stress of marbles increases with both, pressure and temperature, limited by the frictional strength. Above the brittle-ductile transition, the strength is limited by the bulk flow strength, which depends on total strain due to strain hardening, eventually leading to failure at high strain.

Transient creep of crustal rocks is important to explain time‐dependent geological processes such as postseismic deformation following a large continental earthquake. While the steady‐state creep flow law parameters of quartz and feldspar, major minerals in the upper and lower crust, are well known, the physical mechanism behind transient creep and the corresponding flow law parameters are poorly understood. We quantify the flow law parameters for both quartz and granulite (mixture of plagioclase and pyroxene) under wet conditions with a nonlinear Burgers model using a Markov chain Monte Carlo (MCMC) method. Modeling results yield an activation energy of 70 ± 20 kJ/mol and a stress exponent of 2.0 ± 0.1 for transient creep of quartz aggregates. For granulite/feldspar, we find activation energies of 280 ± 30 and 220 ± 20 kJ/mol and stress exponents of 1.0 ± 0.2 and 0.9 ± 0.1 under mid (1050–1100°C) and high (1125–1150°C), temperature conditions, respectively. The stress exponents and activation energies of transient creep are consistently smaller than those of steady‐state creep for both quartz and granulite/feldspar. Combined with results for transient creep of olivine that were previously obtained (Masuti & Barbot, 2021, https://doi.org/10.1186/s40623-021-01543-9 ), we suggest that the activation energies and stress exponents of transient creep are smaller than those of steady‐state creep for volumetrically important silicate minerals of the crust and upper mantle. Extrapolation of the estimated flow law parameters of granulite/feldspar to natural conditions suggests that transient creep may dominate during the postseismic period and lasts longer than previously thought.

&lt;p&gt;Transient creep of the lower crustal minerals such as feldspar is important to explain postseismic deformation following a large continental earthquake. However, transient creep of feldspar is poorly understood and the flow law parameters are unknown so far. Therefore, we performed constant strain rate deformation experiments on synthetic fine-grained anorthite aggregates under wet conditions using a Paterson-type gas deformation apparatus. We conducted tests at temperatures from 1000 &amp;#186;C to 1200 &amp;#186;C and confining pressure of 400 MPa. Typical strain rates in our experiments were 1x10&lt;sup&gt;-4&lt;/sup&gt; s&lt;sup&gt;-1&lt;/sup&gt;, 2.5x10&lt;sup&gt;-4&lt;/sup&gt; s&lt;sup&gt;-1&lt;/sup&gt;, 5x10&lt;sup&gt;-4&lt;/sup&gt; s&lt;sup&gt;-1&lt;/sup&gt;, and 7.5x10&lt;sup&gt;-4&lt;/sup&gt; s&lt;sup&gt;-1&lt;/sup&gt;, including some strain rate stepping experiments. In general, the transient creep accounted for 6-8% of the total strain (~10-15%), which is high compared to 2-3 % transient deformation observed in previous experiments on anorthite, quartz, and olivine aggregates. Inspection of the microstructures of deformed samples using transmission electron microscopy reveal dislocation activity and antiphase domain boundaries. Analysis of steady-state creep data indicates that the samples were deformed at the boundary between diffusion and dislocation creep with a power law stress exponent of ~1.4 and an activation energy of 272 kJ/mol. Because a constitutive equation for transient creep of feldspar is not well established, we estimated transient creep flow law parameters using inter-granular and intra-granular models. In the intergranular model for a polycrystalline aggregate, where grains are randomly oriented, &amp;#160;it is assumed that low strain (i.e., transient creep) is accommodated by individual grains with soft/easy slip orientation and high strain (steady-state creep) is accommodated by grains with hard/strong slip orientation. In contrast, in the intra-granular model, both transient creep and steady-state deformation are dominated by intragranular processes, such as long-range elastic interactions of dislocations. In the intragranular approach, we find that the full stress vs. strain curve (i.e., including transient and steady-state creep) can be modelled using a stress exponent of ~1.5 and an activation energy of ~200 kJ/mol. Applying the intergranular model, we get a stress exponent of ~3 and an activation energy of ~130 kJ/mol for transient creep of anorthite aggregates. Extrapolated to natural strain rates, these two approaches will have different implications in modelling postseismic deformation.&lt;/p&gt;

Postseismic relaxation after large earthquakes induces transient deformation of the solid Earth, particularly in the deeper part of the crust. The deformation of the upper and lower crust are mainly controlled by the rheological behavior of quartz and feldspar, respectively. The mechanical properties of quartz and feldspar at steady-state creep conditions are well constrained and flow law parameters are known from experimental calibrations. However, the physical mechanism underlying transient creep is poorly understood and the corresponding flow law parameters are unknown so far. Here, we constrain a constitutive framework that captures transient creep and steady state creep consistently using the mechanical data from laboratory experiments. The constitutive framework represents a Burgers assembly with a thermally activated nonlinear stress versus strain-rate relationship for the dashpots. Using the Markov chain Monte Carlo (MCMC) method, we uniquely determine the flow law parameters for both quartz and feldspar. We find an activation energy of 70±20 kJ/mol and a stress exponent of 2.0±0.1 for transient creep of quartz. For feldspar, the best-fit activation energies are 280±30 and 220±20 kJ/mol with stress exponents of 1.0±0.2 and 0.9±0.1 under mid- and high-temperature conditions, respectively. The stress exponents and activation energies of transient creep are consistently smaller than those of steady-state creep for both quartz and feldspar. The flow law parameters determined in this study could be used to quantify the contribution of transient creep in the postseismic deformation following a large continental earthquake.

The Opalinus Clay (OPA) formation is considered a suitable host rock candidate for nuclear waste storage. However, the sealing integrity and long-term safety of OPA are potentially compromised by pre-existing natural or artificially induced faults. Therefore, characterizing the mechanical behavior and microscale deformation mechanisms of faults and the surrounding rock is relevant for predicting repository damage evolution. In this study, we performed triaxial tests using saw-cut samples of the shaly and sandy facies of OPA to investigate the influence of pressure and mineral composition on the deformation behavior during fault reactivation. Dried samples were hydrostatically pre-compacted at 50 MPa and then deformed at constant strain rate, drained conditions and confining pressures ( p c ) of 5–35 MPa. Mechanical data from triaxial tests was complemented by local strain measurements to determine the relative contribution of bulk deformation and fault slip, as well as by acoustic emission (AE) monitoring, and elastic P-wave velocity measurements using ultrasonic transmissions. With increasing p c , we observe a transition from brittle deformation behavior with highly localized fault slip to semi-brittle behavior characterized by non-linear strain hardening with increasing delocalization of deformation. We find that brittle localization behavior is limited by p c at which fault strength exceeds matrix yield strength. AEs were only detected in tests performed on sandy facies samples, and activity decreased with increasing p c . Microstructural analysis of deformed samples revealed a positive correlation between increasing p c and gouge layer thickness. This goes along with a change from brittle fragmentation and frictional sliding to the development of shear zones with a higher contribution of cataclastic and granular flow. Friction coefficient at fault reactivation is only slightly higher for the sandy ( µ ~ 0.48) compared to the shaly facies ( µ ~ 0.4). Slide-hold-slide tests performed after ~ 6 mm axial shortening suggest stable creeping and long-term weakness of faults at the applied conditions. Our results demonstrate that the mode of fault reactivation highly depends on the present stress field and burial history.