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Abstract\nKEY POINTSAncient faults dissect low-grade Mesoproterozoic sedimentary rocks of the 1673–1455 Ma Edmund Basin and the 1455–1075 Ma Collier Basin in the Capricorn Orogen, Western Australia, and reveal a protracted history of basin formation and subsequent deformation. K–Ar dating of illite from fine-grained fault gouge separates (ideally <0.1 µm) provide robust dates of the most recent fault movement. From the Godfrey–Mount Vernon Fault, a sample of siltstone from the Kiangi Creek Formation (1590–1514 Ma) in the Edmund Basin, which was collected adjacent to faulting, provided a whole-rock date of 1506 ± 30.2 Ma. This is interpreted as a mixed age, encompassing modification of the depositional age by subsequent faulting during the extensional opening of the Edmund Basin. A fault gouge sample dated at 1141.4 ± 58.6 Ma, also from the Godfrey–Mount Vernon Fault, is consistent with the tectono-thermal 1321–1171 Ma Mutherbukin Tectonic Event identified in underlying high-grade Gascoyne Province basement rocks. Fault gouge sample dates of 885.5 ± 45.9 Ma from the Godfrey–Mount Vernon Fault and 782.2 ± 40.1 Ma from the Quartzite Well Fault define the younger newly identified crustal shortening during the Kuparr Tectonic Event. Concomitant tectonic activity in eastern Australia and South China has been considered as a possible precursor of the breakup of Rodina. In southern Africa, the Zambezi Supracrustal Sequence has been dated at 880–820 Ma and the Munali Intrusive Complex at 862–857 Ma also identified as rift-related.K–Ar dating of illite 1M polymorph in fine clay fractions (<0.1 μm) of fault gouge reveals the most recent fault movement in low temperature shallow crustal Proterozoic sedimentary basins.K–Ar dating on listric normal faults confirms timing of the opening of the Edmund Basin, in the Capricorn Orogen, Western Australia, during the extensional 1689–1619 Ma Mangaroon Orogeny.Edmund Basin inversion and folding during the 1321–1171 Ma Mutherbukin Tectonic Event is confirmed from K–Ar dated reverse fault reactivation of splays faults of the Mount Vernon and Quartzite Well faults, which offset Edmund Basin strata.The newly defined 885–782 Ma transpressive Kuparr Tectonic Event is identified from K–Ar dating on the Mount Vernon and Quartzite Well faults. [ABSTRACT FROM AUTHOR]

The main goal of active fault investigation is obtaining evidence of the Quaternary fault activity through trenching. To accomplish this, electrical resistivity surveys are widely utilized to accurately detect faults buried beneath alluvium. In this study, 2D and 3D electrical resistivity surveys were conducted at the Miho site of the southern Yangsan Fault, the Quaternary active fault area already well-studied via trench investigations. The efficacy of electrical resistivity surveys for fault detection was examined by comparing the resistivity distributions against data in the trench logs. At the Miho site, three lines were installed for the 2D electrical resistivity survey and 22 lines were set within a 27 × 27 m square area for the 3D electrical resistivity survey. The length of each survey line was 27 m with an electrode spacing of 1 m. A dipole-dipole array was used to measure the potential difference between each measurement electrode using the same transmitted current and voltage. To derive the 2D and 3D resistivity distributions from the data acquired in the field, the inversion programs DC_2DPRO and DC_3DPRO were employed. Comparison of the 2D resistivity distribution with trench log data shows that the zone of dacitic welded tuff west of the Quaternary fault plane has relatively high resistivity, while the zone of foliate gouge and breccia derived from sedimentary rocks east of the Quaternary fault plane has a low resistivity of less than 40 Ω·m. From these results, it is evident that the method is effective, particularly when different rock types are distributed on either side of the fault boundary or when highly conductive materials, such as clay-rich fault gouges, are present within the fault zone. The resistivity distribution in the 3D survey area can be depicted using both block diagrams and depth-specific slices, facilitating a spatial understanding of the continuity of fault (or fracture) zones. Consequently, this study demonstrates that 3D surveys offer numerous advantages over 2D surveys by accurately capturing planar structures and enabling spatial interpretation based on 3D resistivity distribution. [ABSTRACT FROM AUTHOR]

Several borehole cores intersecting faults related to coseismic slip display high‐temperature features, including thermal decomposition of fault gouge. We present evidence that these features may be related to fluid drainage of the slip zone during seismic slip. We sheared water‐saturated kaolinite powders under both fluid drained and undrained conditions, expected for seismic slip at shallow crustal depths. Our results show typical dynamic weakening behavior regardless of conditions. Under fluid drained condition, restrengthening accompanied by the thermal decomposition of kaolinite occurs. In addition, thermal decomposition of kaolinite tends to be initiated at high normal stresses (>5 MPa) with short displacement (<5 m). We propose that thermal pressurization acts as a weakening mechanism but ceases because of fluid drainage, triggering kaolinite thermal decomposition. This finding explains seismic‐slip‐related clay anomalies at depth rather than at the surface, as observed in the borehole after the 1999 Mw 7.6 Chi‐Chi earthquake, Taiwan. Plain Language Summary: Seismic faulting at depth can drive thermochemical reactions within the slip zone, given the high slip velocity and large displacement. Several slip‐zone samples from deep drilling projects following catastrophic earthquakes have exhibited high‐temperature geological characteristics, which were not present in fault zone outcrops hosting surface ruptures. We sheared kaolinite (as an analogue of fault zone materials) under both fluid drained and undrained conditions, simulating conditions expected during seismic slip at borehole depths. Our results show that, regardless of the applied conditions, the materials tend to weaken dramatically during shearing. However, when fluids are allowed to drain from the slip zone, there is a subsequent strengthening accompanied by the thermal decomposition of kaolinite. We suggest that thermal pressurization operates as the weakening mechanism but is ceased due to fluid drainage, resulting in the thermal decomposition of kaolinite gouges. In addition, the thermal decomposition of kaolinite tends to be triggered at large normal stresses. Because kaolinite is a common component in both fault zones and subduction zones at shallow depths, our findings have potential implications for reported thermally driven reactions within slip zones as a potential seismic indicator at shallow crustal depths. Key Points: Thermal pressurization operates as the dynamic weakening mechanism during experimental seismic slip regardless of the ambient conditionsThermal pressurization ceases because of fluid drainage and is followed by kaolinite thermal decompositionThermal decomposition of kaolinite tends to be initiated at large normal stresses in the seismic slip range [ABSTRACT FROM AUTHOR]

Optically stimulated luminescence (OSL) dating utilises the detection of trapped charge in minerals, and has an ultralow closure temperature. There is the potential for direct dating of fault movement using fault gouges, because frictional heating caused by large earthquakes can reduce the OSL signal intensity of minerals within gouges. In this study, we conducted quartz OSL dating on four fault gouge and breccia samples from a surface outcrop of the Atotsugawa Fault, one of the most active dextral strike-slip faults in central Japan, where the last large earthquake occurred in AD1858, with an estimated magnitude of 7. The natural OSL signal intensity of fine-grained quartz was clearly below the signal saturation level, with the fraction of saturation (n/N) between 0.30 ± 0.02 and 0.39 ± 0.03, indicating there was signal resetting by past earthquakes. However, the apparent OSL ages ranged from 22 ± 1 to 72 ± 4 ka, two orders of magnitude older than the age of the last earthquake. To explain the significant age overestimation, we measured the thermal stability of the OSL signal, and used a thermal model with punctuated episodic losses to constrain the average shear heating temperature experienced during an individual faulting event. For an independently known recurrence interval of 2.5 ka and a presumed shear heating duration of 1 s, the observed n/N and the measured thermal stability of the OSL signals correspond to a resetting temperature of ~ 300 °C during a single earthquake event. [ABSTRACT FROM AUTHOR]

Under critical conditions where experimental fault slip exhibits self‐sustained oscillation, effects of normal stress oscillation (NSO) on fault strength and stability remain poorly understood, as do potential effects of NSO on natural and induced seismicity. In this study, we employed double direct‐shear testing to investigate the frictional behavior of a synthetic, slightly velocity‐weakening (SVW) fault gouge (characterized by self‐sustained oscillation under quasi‐static shear loading), when subjected to NSO at different amplitudes (5%–20% of 5 MPa) and frequencies (0.001–1 Hz). During the experiment, fault displacement and gouge layer thickness were measured. Transmitted ultrasonic waves were also employed to probe grain contact states within the gouge layer. Our results show that fault weakening and unstable slip can be triggered at NSO frequencies ranging from 0.03 to 0.1 Hz and amplitudes exceeding 5%. Interestingly, an amplified shear stress drop and weakening effect were observed when the NSO frequency fell in 0.05–0.1 Hz. Analysis of transmitted ultrasonic waves in tests on the SVW gouge revealed fault dilation, accompanied by unstable slip and weakening. By extending an existing microphysical model (the "Chen‐Niemeijer‐Spiers [CNS]" model), to account for elastic effects of NSO on gouge microstructure and grain contact state, the mechanical and wave data obtained in our experiments on the SVW gouge was reproduced, suggesting an approach for modeling fault instability under upper crustal (SVW) conditions where normal stress is perturbed by subsurface operations, such as periodic gas storage stimulation of reservoir formations. Plain Language Summary: To mitigate induced fault slip and seismicity associated with subsurface industrial activities that create oscillating stress, it is crucial to comprehend the effects of oscillation on fault friction. We used an experimental fault to mimic a potentially unstable fault at shallow depth and tested the effects of normal stress oscillation (NSO) on its shear strength. Various amplitudes and frequencies of NSO were investigated. An active ultrasonic source was employed to probe the microstructure of the gouge layer. Our results suggest that unstable slip can be easily triggered by applying NSO. Fault weakening and shear stress fluctuations are amplified within a limited range of oscillation frequency. Increasing amplitude causes greater fault weakening and stress fluctuations. The transmitted ultrasonic waves are sensitive to the change in mechanical properties of fault zone, reflecting that fault dilation is the main mechanism associated with fault weakening and instability. We present a microphysical model that reproduces and explains the mechanical and ultrasonic results, offering a potential route to extrapolate laboratory data to field conditions in the future. Key Points: Fault weakening and unstable events can be readily triggered by normal stress oscillations when the gouge is slightly velocity‐weakeningFault resonance occurs at oscillation frequencies in the range 0.05–0.1 HzAn extended microphysical model was built to quantify the mechanical behavior and tested using active ultrasonic technology [ABSTRACT FROM AUTHOR]

Laboratory acoustic emissions (AEs) serve as small-scale analogues to earthquakes, offering fundamental insights into seismic processes. To ensure accurate physical interpretations of AEs, rigorous calibration of the acoustic system is essential. In this paper, we present an empirical calibration technique that quantifies sensor response, instrumentation effects, and path characteristics into a single entity termed instrument apparatus response. Using a controlled seismic source with different steel balls, we retrieve the instrument apparatus response in the frequency domain under typical experimental conditions for various piezoelectric sensors (PZTs) arranged to simulate a three-component seismic station. Removing these responses from the raw AE spectra allows us to obtain calibrated AE source spectra, which are then effectively used to constrain the seismic AE source parameters. We apply this calibration method to acoustic emissions (AEs) generated during unstable stick-slip behavior of a quartz gouge in double direct shear experiments. The calibrated AEs range in magnitude from −7.1 to −6.4 and exhibit stress drops between 0.075 MPa and 4.29 MPa, consistent with earthquake scaling relation. This result highlights the strong similarities between AEs generated from frictional gouge experiments and natural earthquakes. Through this acoustic emission calibration, we gain physical insights into the seismic sources of laboratory AEs, enhancing our understanding of seismic rupture processes in fault gouge experiments. [ABSTRACT FROM AUTHOR]

Hazards created by the landslide damming of rivers have become common in tectonically active mountainous areas. However, it remains unclear how tectonic effects may influence the formation and characteristics of landslide dams. The purpose of this paper is to explore how tectonic effects impact the drivers, geomorphic features, and activity characteristics of landslide dams along a fault zone. We investigated 83 landslide dams clustered along a fault zone in the Bailong River Basin. Most of the dams are located in areas of high tectonic stress, resulting from the rapid river incision and destruction of slope structure caused by intense tectonic activities in these areas. Statistical analysis, InSAR monitoring, and field investigation revealed that different tectonic effects were associated with significant differences in the geomorphic features, activity characteristics, and controlling factors of the landslide dams. Thus, we identified three distinct patterns of landslide dams in tectonically active mountainous areas: (1) Topography-driven landslide dams are caused by rapid rock uplift and river incision. Here, the steep terrain enhances the development of small landslides, the narrowness of the channels favors river damming, and the residual deposits on the hillslope remain active. (2) Tectonic activities promote the development of structural planes in the rock mass and reduce its strength, ultimately forming structural plane-controlled landslide dams. Although their volumes are not very large, the strong erosion resistance of rockslides can cause river damming and maintain the stability of deposits. (3) Fractured rock mass-controlled landslide dams are composed of broken rock and fault gouge. The extremely low strength of these materials allows them to form very large landslides that can easily dam the river, and maintain a slow-moving state. Through a geomorphological and geological model, our study offers new insights and enhances the understanding of the formation and characteristics of landslide dams induced by tectonic activity in mountainous regions. [ABSTRACT FROM AUTHOR]

Altered minerals introduced by water-rock reactions during geothermal energy recovery may play a role in the frictional properties of fault gouges, perhaps inducing earthquakes. Based on the geological conditions of potential geothermal reservoirs in northeastern Shenzhen, this paper considers the critical pore pressure (about 50 MPa) and critical effective normal stress (about 20 MPa), which may lead to reactivation of the Henggang-Luohu fault zone due to fluid injection into a potential geothermal reservoir at a buried depth of about 2.5 km with the temperature of 150 °C. Under such hydrothermal conditions (50 MPa fluid pressure and 150 °C), thermodynamic calculations suggest that the reaction of Yanshanian granular granite and water, at equilibrium, can produce silica, kaolinite, and chlorite at a mass ratio of 16:7:2. Here we report 19 direct shear experiments performed on simulated fault gouges consisting of the binary mixture of silica + granite, kaolinite + granite, and chlorite + granite, and the multivariate mixture of silica + kaolinite + chlorite + granite, to investigate the effects of altered minerals on frictional properties of simulated granite fault gouges. Velocity stepping experiments were conducted on the wet samples with 10% water content at an initial normal stress of 20 MPa under drained conditions at room temperature. On this basis and together with acoustic emission observation and microstructural analysis, we discussed the likely mechanism responsible for the observed behavior. The results show that, for the presence of a single altered mineral, silica has little effect on the friction coefficient, though it contributes to velocity weakening and even stick-slip events. Whereas, clay minerals can significantly decrease the friction coefficient, though they have a contribution to velocity strengthening. For the presence of a mixture of silica + kaolinite + chlorite, clay minerals play a dominant role in controlling frictional properties of the simulated fault gouges: altered minerals can also significantly decrease the friction coefficient and contribute to velocity strengthening. Development of R1 shear planes was observed using microstructural analysis for the simulated granite fault gouges consisting of 0% or 33% of the altered minerals, while the R1 shear plane cannot be observed anymore in the sample consisting of 67% of the altered minerals. The number of acoustic emission events decreases significantly when the content of the altered minerals is 33%~67%. This may indicate a transition in the deformation mechanism from brittle to semi-plastic. Our study suggests that if geothermal energy recovery is carried out in northeastern Shenzhen, the fluid injection pressure should be controlled to avoid the slip of the fault. [ABSTRACT FROM AUTHOR]

The slip behavior of crustal faults is known to be controlled by the composition of the fault gouge, but the exact mechanisms, especially considering the role of water‐rock interactions, are still under investigation. Here, we use a geochemical approach measuring the cation exchange capacity (CEC) of several phyllosilicate minerals and non‐clays, using CEC as a proxy for the ability to bind water to the mineral surfaces and/or develop electrostatic forces between particles. Laboratory shearing experiments show that low CEC materials (<3 mEq/100 g) tend to exhibit high friction, low cohesion, and velocity‐weakening frictional behavior. Phyllosilicate minerals exhibit CEC values up to 78 mEq/100 g and correspondingly lower friction coefficients, higher cohesion, and a tendency for velocity‐strengthening friction. Zeolite behavior is atypical, exhibiting a high CEC value typical of phyllosilicates but the strength and frictional characteristics of a non‐clay with low CEC. This suggests that the structure of the mineral is important for non‐phyllosilicates. For phyllosilicates, our results can be explained by water bound to mineral surfaces, creating bridges of hydrogen or van der Waals bonds between particles. The enhanced particle bonding for high CEC materials is consistent with high cohesion under zero effective stress conditions, and lowered friction by trapping bound water between the mineral surfaces under normal load. Bound water may explain the tendency for velocity‐strengthening friction in high CEC materials by hindering a Dieterich‐type time‐dependent frictional strengthening mechanism. Plain Language Summary: How faults in the Earth move and their potential for earthquakes is controlled by the types of minerals in the fault, but exactly how is still being studied. Here, we measured the cation exchange capacity (CEC) of several different minerals, including various platy‐shaped minerals, where CEC is used to measure a very small electrical charge on the mineral surfaces and how strongly the minerals attract and hold water. Laboratory shearing experiments show that low CEC materials tend to be strong, and become weaker when sheared faster. Platy minerals exhibit large CEC values and lower strength, and tend to become stronger when sheared faster. We suggest that mineral surface chemistry controls fault movement because high CEC materials have a stronger electrical charge, meaning they can hold large amounts of water on their surfaces. This water becomes trapped and reduces fault strength but can also hold the particles together depending on the stresses on the fault. The mineral zeolite does not fit the pattern shown by the other materials, having a high CEC seen in platy minerals, but the strength characteristics of a non‐platy mineral with low CEC. This suggests that grain shape is also important along with mineral surface charge. Key Points: For a wide range of mineral types, large cation exchange capacity (CEC) correlates with low friction, high cohesion, and velocity‐strengthening frictionThe mechanism for the observed patterns may be strong bond bridges between particles formed by water in high CEC materialsZeolite is an exception and is strong and velocity‐weakening despite high CEC, indicating that grain shape is also important [ABSTRACT FROM AUTHOR]

The alternating stress caused by periodic high-pressure injection and extraction in gas storage can potentially induce fault slippage, compromising the sealing integrity of faults within these storages sites. Understanding the mechanical behavior of faults under alternating stress is crucial for ensuring the long-term stability and safety of gas storage operations. To explore the impact of fault dip angle and fault gouge strength on fault slip characteristics, fault samples were prepared with uniaxial compressive strengths of 20.1, 30.2, 42.4, and 51.4 MPa at two distinct dip angles. Triaxial compression experiments were conducted under alternating stress conditions corresponding to operational pressures at a specific gas storage site in China. The results indicate that faults with dip angles of 30° and 45° tend to fail at their weakest points. The increasing strength of fault gouges shifts failure mechanisms from interfacial failure between gouges and the surrounding rock towards internal gouge failure, often accompanied by shear failure across sections, resulting in characteristic "X"-shaped conjugate shear failures. The decrease in the ratio of bedrock strength to fault gouge strength elucidates the observed phenomena of an initial reduction followed by increased fault deformation. Transition points for faults with 30° and 45° dips occur around the strength ratios of 1.7/1 and 1.2/1, respectively. Fault damage exhibits a negative correlation with fault gouge strength and a positive correlation with fault dip angle. Samples with a higher-strength fault gouge at a 30° dip angle generally incur less damage compared to those with a lower-strength fault gouge at a 45° dip angle. Moreover, higher maximum static friction coefficients denote greater fault resistance to slipping, with 30° faults consistently demonstrating superior resistance compared to 45° faults. Additionally, a higher-strength fault gouge consistently enhances slip resistance under identical dip angles. [ABSTRACT FROM AUTHOR]

Fault zones are usually filled with fault gouge and accompanied by fault water. The coupled effect of fault gouge and water significantly impacts the slip behavior of the fault, which may weaken the fault structure and further induce rupture propagation and earthquakes. In this study, we carried out a laboratory experiment to investigate the fluid-induced slip behavior of fault filled with gouge. The friction evolution characteristic associated with fluid pressure and effective stress was investigated during the slip process. In addition, the role transformation process of the gouge on the slip behavior of fault was revealed. The experimental result indicates that the friction on the filled fault surface is significantly affected by fault gouge. The rupture of the gouge promotes fault slip and the fluid pressure plays a vital role in the initiation of fault slip. The fault gouge enhances the shearing strength of the fault and acts as a barrier before the initial slip under fluid injection. Nevertheless, the fault gouge would accelerate the fault slip and transform into lubricant after the initial slip.

Abstract Optically stimulated luminescence (OSL) dating utilises the detection of trapped charge in minerals, and has an ultralow closure temperature. There is the potential for direct dating of fault movement using fault gouges, because frictional heating caused by large earthquakes can reduce the OSL signal intensity of minerals within gouges. In this study, we conducted quartz OSL dating on four fault gouge and breccia samples from a surface outcrop of the Atotsugawa Fault, one of the most active dextral strike-slip faults in central Japan, where the last large earthquake occurred in AD1858, with an estimated magnitude of 7. The natural OSL signal intensity of fine-grained quartz was clearly below the signal saturation level, with the fraction of saturation (n/N) between 0.30 ± 0.02 and 0.39 ± 0.03, indicating there was signal resetting by past earthquakes. However, the apparent OSL ages ranged from 22 ± 1 to 72 ± 4 ka, two orders of magnitude older than the age of the last earthquake. To explain the significant age overestimation, we measured the thermal stability of the OSL signal, and used a thermal model with punctuated episodic losses to constrain the average shear heating temperature experienced during an individual faulting event. For an independently known recurrence interval of 2.5 ka and a presumed shear heating duration of 1 s, the observed n/N and the measured thermal stability of the OSL signals correspond to a resetting temperature of ~ 300 °C during a single earthquake event. Graphical Abstract

The fault gouge, a crushed fault rock resulting from seismic slip, exhibits variations in crystallinity and particle size contingent on the magnitude of deformation. Therefore, the fabrication of fault gouge analogues with carefully controlled properties such as crystallinity, grain size, and mineral composition is necessary to systematically study their role in a range of fault slips. While the single mineral fault gouge analogues have been reproduced using a milling method, the fabrication methods for multi-mineralic fault gouge analogues with varying particle size and crystallinity are rarely reported, yet. In this study, we propose two methodologies for fabricating multi-mineralic fault gouges controlling the degree of amorphization and the particle size using a high-energy ball mill: mixing-grinding (MG) and grinding-mixing (GM) types. The MG type was designed to simulate the fault gouge that the same grinding energy is applied to all the constituent minerals, while the GM type can be controlled the degree of deformation of each mineral according to the research objective. To investigate the effects of these methodologies on the reduction in crystallinity and particle size, we compared the characteristics of MG type_6h and GM type_6h samples, both ground for 6 h. Consequently, despite undergoing the same grinding duration, the GM type_6h sample exhibited more significant reduction in crystallinity and more heterogeneous particle size. Additionally, we fabricated the GM type_Xc50 sample, where the crystallinity of all constituent minerals was reduced to < 50%. For this, each mineral was ground for an optimized duration that reduced its crystallinity to < 50%, after which the samples were mixed. Consequently, the GM type_Xc50 sample demonstrated the greatest reduction in crystallinity and the most uniform particle size distribution. To fabricate the generation process of natural fault gouge, it is appropriate to use the MG type to apply the same energy to constituent mineral, resulting the deformation reflecting its hardness. The GM type is recommended for use in fabricating the characteristics of the fault gouge that has undergone intense deformation at the slip surface, which allows for controlling the uniform crystallinity and particle size reduction of each constituent mineral. Our results suggest that the diverse nature of naturally occurring fault gouges can be fabricated in laboratory settings by adjusting the grinding conditions. This study offers effective methods for fabricating fault gouge analogue for frictional experiments for fault slip and potential applications in simulating geochemical reactions in fault zones. [ABSTRACT FROM AUTHOR]

Carbonaceous materials in seismic fault zones may considerably influence seismic fault slip; however, the formation mechanism of carbonaceous materials remains unclear. In this study, we proposed a novel hypothesis for the formation of carbonaceous materials in fault gouge. Thus, we conducted a CO2 hydrogenation experiment in a high-temperature reactor at a co-seismic temperature, with fault gouge formed during the Wenchuan earthquake as the catalyst. Our experimental results demonstrate that carbonaceous materials in fault zones are formed on the fault gouge during the chemical reaction process, suggesting that the carbonaceous materials are possibly generated from the catalytic hydrogenation of CO2, followed by thermal cracking of its products. The results of this study provide a theoretical basis for understanding fault behavior and earthquake physics.

Fault normal stress (σn) changes dynamically during earthquakes. However, the impact of these changes on fault strength is poorly understood. We explore the effects of rapidly varying σn by conducting rotary‐shear experiments on simulated fault gouges at 1 μm/s, under well‐drained, hydrothermal conditions. Our results show both elastic and anelastic (time‐dependent but recoverable) changes in gouge layer thickness in response to step changes and sinusoidal oscillations in σn. In particular, we observe dilation associated with marked weakening during ongoing σn‐oscillations at frequencies >0.1 Hz. Moreover, recovery of shear stress after such oscillations is accompanied by transient (anelastic) compaction. We propose a microphysically based friction model that explains most of the observations made, including the effects of temperature and step versus sinusoidal perturbation modes. Our results highlight that σn‐oscillations above a specific frequency threshold, controlled by the loading regime and frictional properties of the fault, may enhance seismic hazards. Plain Language Summary: Faults in the crust sometimes experience rapid stress changes, caused by nearby or remote earthquakes, by seasonal impoundment and discharge of reservoirs, by hydrocarbon or geothermal energy production, or by reservoir stimulation. The impact of these stress changes on the earthquake potential of faults is poorly understood. This study explores such effects through laboratory experiments on simulated faults under upper crustal PT conditions, perturbing the normal stress on the fault in various ways. Our results show that the shear stress supported by the fault, and the fault thickness, respond instantly to normal stress changes, followed by a transient evolution. In particular, we observed dilation (fault‐normal expansion) associated with marked weakening during fast oscillation. We propose a micromechanical model that can qualitatively explain the general experimental observations. Our results indicate that varying the normal stress on a fault at frequencies above a specific threshold may enhance seismic hazard. Key Points: We observed dynamic weakening and dilation of shearing gouges subjected to rapid oscillations in normal stressFault compaction/dilation explains the shear stress evolution in response to different modes of normal stress perturbationWe propose a micromechanical model for gouge friction under time‐variable normal stress conditions [ABSTRACT FROM AUTHOR]

Performing stimulation experiments at approximately 1 km depth in the Bedretto Underground Laboratory for Geosciences and Geoenergies necessitates identifying and characterizing the target fault zone for on-fault monitoring of induced fault slip and seismicity, which presents a challenge when attempting to understand seismogenic processes. We discuss the multidisciplinary approach for selecting the target fault zone for experiments planned within the Fault Activation and Earthquake Ruptures (FEAR) project, for which the aim is to induce the fault slip and seismicity for an earthquake magnitude of up to 1.0 while enhancing the monitoring and control of fluid-injection experiments. Structural geological mapping, remote sensing, exploration drilling and borehole logging, ground-penetration radar, and laboratory investigations were employed to identify and characterize the target fault – a ductile–brittle shear zone several meters wide with an intensely fractured volume spanning over 100 m. Its orientation in the in situ stress field favors reactivation in normal to strike-slip regimes. Laboratory tests showed slight velocity strengthening of the fault gouge. The fault's architecture, typical for crystalline environments, poses challenges for fluid flow, necessitating detailed hydraulic and stress characterization before each of the FEAR experiments. This multidisciplinary approach was crucial for managing rock volume heterogeneity and understanding implications for the dense monitoring network. Successfully identifying the fault sets the stage for seismic activation experiments commencing in spring 2024. [ABSTRACT FROM AUTHOR]

The Anninghe Fault (ANHF) is a major left‐lateral strike‐slip fault in southwestern China and one of the main seismogenic fault zones with a history of strong earthquakes. To understand the frictional properties of natural granitic gouges from the principal slip zone, we conducted hydrothermal friction experiments using both saw‐cut and ring shear methods. These experiments were performed at temperatures (T) of 25–600°C, pore pressures (Pf) of zero (dry), 30 and 100 MPa, sliding velocities (V) of 0.01–100 μm/s and effective normal stresses (σneff ${\sigma }_{\mathrm{n}}^{\text{eff}}$) of 68, 100, and 200 MPa. The (apparent) friction coefficient is low (μ < 0.5) at high T (600°C), high Pf (100 MPa) and low V (<1 μm/s); but high (μ > 0.6) under all other T, Pf and V conditions. Under high Pf, the velocity dependence of friction, (a‐b), displays three regimes with increasing temperature, from positive below ∼100°C to negative at 100–300°C (at V = 1–3 μm/s) or else 100–450°C (at V = 30–100 μm/s), becoming positive again above 300–450°C. At low Pf, the negative (a‐b) expands to the range ∼300–600°C. Microstructural observations and microphysical interpretation imply that the frictional weakening and transitions in (a‐b) are related to competition between dilatant granular flow and deformation of the fine‐grained gouge by intergranular pressure solution accompanied by healing phenomena (leading to cavitation‐creep‐like behavior). Our results provide a possible explanation for the distribution of earthquakes at different depths in the continental crust, in particular for the depth range of the seismogenic zone between 4 and 24 km along the ANHF. Plain Language Summary: The left‐lateral Anninghe Fault (ANHF), bounding the Tibetan Plateau in China, accommodates part of the distributed convergence between the Indian and Eurasian plates. It has accommodated numerous historical earthquakes during the last few centuries. However, the mechanisms controlling the frictional behavior of natural granitic fault rocks (gouges) collected from ANHF are poorly understood. Laboratory friction experiments were performed at a wide range of temperatures (T), velocities (V) and shear displacements (X), employing varying pore fluid pressure (Pf). The gouges showed potentially unstable shearing behavior at 100–450°C (corresponding to 4–18 km depth), extending up to 600°C (24 km depth) at low Pf (≤30 MPa) and low V (0.04 μm/s). Low frictional strength obtained in large displacement experiments at 600°C indicates a transition from brittle to ductile and seismic‐aseismic (velocity‐strengthening) behavior. Based on microstructural analysis and a microphysical model, we infer that the observed behavior is due to competition between brittle/frictional flow of the granular gouge and creep involving stress‐driven dissolution of the finely crushed constituent grains. Our results offer a possible explanation for the distribution of earthquakes at different depths between 4 and 24 km along the ANHF. Key Points: New experiments confirm a low friction coefficient (μ = 0.37) for granitic gouge at high temperature hydrothermal conditionsThe temperature range characterized by velocity‐weakening narrows with decreasing velocity and with increasing pore fluid pressurePressure solution is a key deformation mechanism controlling the velocity dependence of friction in fine‐grained granite gouge [ABSTRACT FROM AUTHOR]

In crustal faults dominated by granitoid gouges, the frictional‐viscous transition marks a significant change in strength constraining the lower depth limit of the seismogenic zone. Dissolution‐precipitation creep (DPC) may play an important role in initiating this transition, especially within polymineralic materials. Yet, it remains unclear to what extent DPC contributes to the weakening of granitoid gouge materials at the transition. Here we conducted sliding experiments on wet granitoid gouges to large displacement (15 mm), at an effective normal stress and pore fluid pressure of 100 MPa, at temperatures of 20–650°C, and at sliding velocities of 0.1–100 μm/s, which are relevant for earthquake nucleation. Gouge shear strengths were generally ∼75 MPa even at temperatures up to 650°C and at velocities >1 μm/s. At velocities ≤1 μm/s, strengths decreased at temperatures ≥450°C, reaching a minimum of 37 MPa at the highest temperature and lowest velocity condition. Microstructural observations showed that, as the gouges weakened, the strain localized into thin, dense, and ultrafine‐grained (≤1 μm) principal slip zones, where nanopores were located along grain contacts and contained minute biotite‐quartz‐feldspar precipitates. The stress sensitivity exponent n decreased from a large number at 20°C to ∼2.2 at 650°C at the lowest velocities. These findings suggest that high temperature, slow velocity and small grain sizes promote DPC‐accommodated granular flow over cataclastic frictional granular flow, leading to the observed weakening and strain localization. Field observations together with extrapolation suggest that DPC‐induced weakening occurs at depths of 7–20 km depending on geothermal gradient. Plain Language Summary: Below the Earth's surface, rocks usually undergo a change in strength and deformation behavior as they reach deeper crustal levels, transitioning from strong and brittle behavior in the upper crust, to weak, and viscous near the middle crust. This change, known as the frictional‐viscous transition, defines the limit in depth where major earthquakes can occur. Some evidence from nature and laboratory experiments suggests that deformation by dissolution‐precipitation creep (DPC or "pressure solution") may play an important role in triggering this transition. However, these assumptions lack adequate verification and comprehensive understanding. Here, we conducted laboratory tests across a wide range of temperatures and deformation rates in order to reconstruct the frictional‐viscous transition in granitoid rocks, one of the most common rock types in the upper to middle continental crust. We found that granitoid gouges started to weaken and became viscous when being deformed at higher temperatures and lower deformation rates. As the rock became weaker, deformation localized in a distinct layer called the principal slip zone. This is very fine‐grained and contains tiny pores filled with many newly precipitated minerals, which proves active DPC. In nature, activation of DPC could trigger the frictional‐viscous transition at shallower crustal depths than otherwise expected. Key Points: The frictional‐viscous transition in granitoid gouges is promoted by increasing temperature and decreasing velocityThe transition involves forming a dense, ultrafine‐grained principal slip zone where dissolution‐precipitation creep operatesThis temperature‐, rate‐, and grain size‐sensitive creep mechanism greatly weakens the crust at depth >7 km depending on geothermal gradient [ABSTRACT FROM AUTHOR]

Natural 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. Plain Language Summary: Earthquakes are caused by the propagation of seismic ruptures and sliding of rocks along geological structures called faults. Within the fault, seismic ruptures propagate in mm‐cm thick slip zones that cut cm‐ to meters‐thick fault cores. Both slip zones and fault cores typically exhibit microstructural assemblages different from those of nearby rocks. Laboratory experiments are used to investigate the processes that result in the decrease of fault strength with slip and slip‐rate and govern seismic rupture propagation. However, experiments are performed on simplified fault cores consisting of either a surface discontinuity between intact rocks (i.e., slip zone = "bare‐rock surfaces") or a mm‐thick layer of powdered rocks (i.e., slip zone in a "fault gouge"). In this work, we investigate how slip zones form and evolve in an experimental fault (gouge) core depending on seismic slip history. We apply repeated pulses of seismic slip and show that (a) in the first pulse a slip zone with similar microstructure and friction properties to bare‐rock is formed, (b) in subsequent pulses these slip zones are abandoned and new slip zones are formed in the fault core. Notably, these structural changes, occur to minimize the mechanical energy dissipated in the FC. Key Points: Friction experiments applying up to four consecutive seismic slip‐rate pulses (1 m/s), in fault gouge and bare‐rock surfacesThe greatest changes in dissipated energy occurred between the first and the second slip‐rate pulseStrain localization in the principal slip zones minimizes the energy dissipated within the whole experimental fault zone [ABSTRACT FROM AUTHOR]

The standard rate‐and‐state friction (RSF) has extensively captured frictional behaviors, but it fails to explain the velocity dependence of frictional stability transition and widespread slow‐slip events (SSEs) in experiments and nature adequately. An alternative microphysical Chen‐Niemeijer‐Spiers (CNS) model can well describe the velocity dependence of frictional behaviors of granular gouges. Using the original CNS model, standard RSF parameters can be quantified microphysically. However, some micro‐parameters are not easy to estimate quantitatively, making it difficult to extrapolate to natural and experimental conditions. Here, we simplify the microphysically‐derived RSF parameters including direct effect a, evolution effect b, and critical slip distance Dc, as well as equivalent values (aeq, beq, and Deq). The simplified friction parameters directly illustrate their velocity dependence, namely the essentially constant a, aeq, and Dc, negatively velocity‐dependent b and beq, as well as varying Deq for different laws. They are roughly consistent with experimental results in various fault gouges. A modified CNS model is further derived from the original CNS model, establishing a direct link between the standard RSF and CNS models. The modified CNS model exhibits virtually identical frictional behaviors to the original CNS, but differs from the standard RSF at large velocity perturbations. Moreover, the linearized stability analysis indicates that the critical stiffness for the modified CNS model is velocity‐dependent. Compared with the standard RSF, the modified CNS model not only explains the velocity dependence of frictional stability transition, but also exhibits a more gradual transition for SSEs with a broader range of stiffness ratios. Plain Language Summary: Geological observations have illustrated a growing body of slow fault slip events that radiate little or no seismic energy, but the underlying mechanisms are not fully understood. This study examines two widely‐used friction models, namely, an empirical friction model and a microphysically‐based model. Both of them have been employed to capture experimental and natural observations, but the microphysical model shows significant velocity dependence of friction and is able to interpret friction parameters of the empirical friction model microphysically. We further examine the microphysically‐derived friction parameters, omit some micro‐parameters that contribute negligibly, directly describe their velocity dependence, and derive a simple friction model. The modified friction model is roughly consistent with the microphysically‐based model. More importantly, the modified model can not only describe the velocity dependence of frictional stability transition, but also explain the widespread slow fault slip events in experiments and nature. Key Points: Microphysically‐derived rate‐and‐state friction parameters are simplifiedA modified Chen, Niemeijer, and Spiers (CNS) model is derived from the original microphysical modelThe modified CNS model captures a more gradual transition for slow‐slip events [ABSTRACT FROM AUTHOR]

Determining the shear‐velocity dependence of dry granular friction can provide insight into the controlling variables in a dry granular friction law. Some laboratories believe that the quality of this study is at the forefront of the discipline for the following reasons. Results suggest that granular friction is greatly affected by shear‐velocity (v), but shear experiments over the large range of naturally occurring shear‐velocities are lacking. Herein we examined the shear velocity dependence of dry friction for three granular materials, quartz sand, glass beads and fluorspar, across nine orders of magnitude of shear velocity (10−8–2 m/s). Within this range, granular friction exhibited four regimes, following a broad approximate "m" shape including two velocity‐strengthening and two velocity‐weakening regimes. We discuss the possible physical mechanisms of each regime. This shear velocity dependence appeared to be universal for all particle types, shapes, sizes, and for all normal stresses over the tested range. We also found that ultra‐high frequency vibration as grain surfaces were scoured by micro‐chips were formed by spalling at high shear velocities, creating ∼20 μm diameter impact pits on particle surfaces. This study provides laboratory laws of a friction‐velocity (μ‐v) model for granular materials. Plain Language Summary: Numerous phenomena of earth science can be simplified to shearing granular systems, such as landslide gouge and rock avalanches. It is believed that friction varies with shear velocity, and this is no exception for dry granular materials. Studying the law of friction variation over as large a range of velocity changes as possible aids in establishing a full life‐cycle hypothesis for landslides from initiation, through movement to cessation. We sheared granular materials across nine orders of magnitude, from 10−8 to 2 m/s and obtained an "M" shape law for the variation of friction with velocity. By analyzing the acoustic frequency of shear experiments at different velocities, we discovered that the acoustic frequency increased with shear velocity. By comparing the microscopic morphology of the particle surface before and after high‐velocity shearing, we hypothesized that the impact pits may be left by particle collisions during high‐velocity shearing. We proposed that the role of ultra‐high frequency vibration on high‐velocity shear weakening should be considered, which provides a basis for accurately revealing the physical mechanism of granular friction variation with velocity. Key Points: This paper examines the shear velocity dependence of friction for dry granular materials, across nine orders of magnitude in shear velocity (10−8–2 m/s)An "M‐shaped" law for the variation of friction with velocity is obtained including two velocity‐strengthening and two velocity‐weakening lawsThere is microscopic evidence that ultra‐high frequency vibration may cause high velocity shear weakening [ABSTRACT FROM AUTHOR]

A long-term opening problem in geophysics and engineering is the generation mechanism of rock pulverization (e.g., fault gouge) within the shear zone when subjected to triaxial stress environment and dynamic loads. A novel triaxial Hopkinson bar is developed to apply quasi-static triaxial confinement and dynamic loads onto the testing rocks and to measure the energy release from the shearing zone. The results elucidate shear strain-rate threshold (i.e., over 125 s−1) for granite at the triaxial prestress of (10, 10, 10) MPa is required for fine fragments/gouge generation. The effects of shear and normal pre-stresses on dynamic shear strength and surface energy for rock pulverization are investigated, which will help explain fine fragments formed during the high-rate loading events. It is suggested that, except for the quasi-static cumulative slip attrition and multiple dynamic events, rock pulverization or fault gouge could be generated at relatively shallow depths (low geo-stress) in a single dynamic earthquake processes. Highlights: Triaxial static-dynamic coupled shear tests of rock were conducted using a novel triaxial Hopkinson bar (Tri-HB). The effects of strain rate and confinement on dynamic shear behaviors of rock. Abundant fault gouge can be formed at stress deformation conditions associated with a fast shear crack propagating (i.e., shear strain rate over 125 s−1). [ABSTRACT FROM AUTHOR]

As a particularly common mineral in granites, the presence of feldspar and feldspar‐chlorite gouges at hydrothermal conditions has important implications in fault strength and reactivation. We present laboratory observations of frictional strength and stability of feldspar (K‐feldspar and albite) and feldspar‐chlorite gouges under conditions representative of deep geothermal reservoirs to evaluate the impact on fault stability. Velocity‐stepping experiments are performed at a confining stress of 95 MPa, pore pressures of 35–90 MPa, and temperatures of 120–400°C representative of in situ conditions for such reservoirs. Our experiment results indicate that the feldspar gouge exhibits strong friction (μ ∼ 0.71) at all experimental temperatures (∼120–400°C) but when T > 120°C, the frictional response transitions from velocity‐strengthening to slightly velocity‐weakening. At constant confining pressure and temperature, increasing the pore pressure increases the friction coefficient (∼0.70–0.85) and the gouge remains slightly velocity weakening. The presence of alteration‐sourced chlorite leads to a transition from velocity weakening to velocity strengthening in the mixed gouge at experimental temperatures and pore pressures. As a ubiquitous mineral in reservoir rocks, feldspar is shown to potentially contribute to unstable sliding over ranges in temperature and pressure typical in deep hydrothermal reservoirs. These findings emphasize that feldspar minerals may increase the potential for injection‐induced seismicity on pre‐existing faults if devoid of chloritization. Plain Language Summary: Granites are an important host for deep geothermal reservoirs where the potential hazard of injection‐triggered seismicity offsets the advantages of low‐carbon energy. Feldspars in these granites are susceptible to water‐rock interactions and produce chlorite under appropriate hydrothermal conditions. Previous studies have shown that chlorite‐rich gouges, as well as its coexistence with other minerals (not yet including feldspar), may generate aseismic sliding during fluid‐injection into such reservoirs. We conduct laboratory measurements of frictional properties of simulated feldspar and feldspar‐chlorite mixed gouges to determine the likelihood of generating earthquakes. We show that high temperatures and pore pressures may favor unstable sliding on feldspar gouges–and thus the potential to generate earthquakes. However, chlorite as an alteration product stabilizes faults and reduces the potential for earthquakes. We emphasize the importance of feldspar and its chloritization in modulating fault slip and the possible risk of triggered earthquakes in anthropogenic hydrothermal systems. Key Points: Pure‐feldspar gouge changes from velocity‐strengthening to velocity‐weakening with increasing temperaturesAt elevated pore pressures, pure‐feldspar gouge remains slightly velocity‐weakeningChlorite formed under feldspar chloritization reduces frictional strength but minimizes fault instability at hydrothermal conditions [ABSTRACT FROM AUTHOR]

The Periadriatic Fault system (PAF) ranks among the largest post-collisional structures of the European Alps. Recent geodetic data suggest that a fraction of the Adria–Europe convergence is still being accommodated in the Eastern Alps. However, the historical seismicity records along the easternmost segment of the PAF are ambiguous, and instrumental records indicate that seismotectonic deformation is mostly concentrated in the adjacent Southern Alps and Dinarides. Both electron spin resonance (ESR) and optically stimulated luminescence (OSL) dating methods can be used to date coseismic slip (with a combined range covering a few decades to a couple of million years) in slowly deforming fault zones, such as the PAF. Since the saturation doses of the quartz ESR signals are larger than quartz and feldspar OSL, ESR enables establishing a maximum age of the last resetting event of the system, while OSL allows constraining their minimum age when the signal is in saturation. We collected fault gouge samples from three localities along the easternmost segment of the PAF. For ESR, we measured the signals from the Al center in quartz comparing the results from the single aliquot additive dose (SAAD) and single aliquot regenerative (SAR) dose protocols. For OSL, we recorded the infrared stimulated luminescence signal at 50°C (IR50) and post-infrared infrared-stimulated luminescence signal at 225 °C (pIRIR225). Our dating results indicate that the studied segment of the PAF system accommodated seismotectonic deformation during the Quaternary, with a maximum age for the last resetting event of the system ranging from 1075 ± 48 to 552 ± 26 ka (ESR SAR) and minimum ages in the range from 196 ± 12 to 281 ± 16 ka (saturation of pIRIR225). The obtained ages suggest that the studied segment of the PAF could be considered at least as a potentially active fault. [ABSTRACT FROM AUTHOR]

Seismic faults are known to exhibit a high level of spatial and temporal complexity, and the causes and consequences of this complexity have been the topic of numerous research works in the past decade. In this paper, we investigate the origins and the structure of this complexity by considering a numerical model of laboratory earthquake experiment, where we introduce a fault with homogeneous mechanical properties but allow it to evolve spontaneously to its natural level of complexity. This is achieved by coupling the elastic deformability of the off‐fault medium (and therefore allowing for heterogeneous stress fields to develop) and the discrete degradation and gouge formation at the fault plane (and therefore allowing for structural heterogeneity to develop). Numerical results show the development of persistent stress, damage, and gouge thickness heterogeneities, with a much larger variability in space than in time. Strong positive correlations are found between these quantities, which suggest a positive feedback between local normal stress and damage rate, only mildly mitigated by the mobility of the granular gouge in the interface. For a wide range of confining stresses, after a sufficient number of seismic cycles, the fault reaches a state of established disorder with a constant roughness, a certain amount of periodicity at the millimetric scale, and a power law decay of the Power Spectral Density at smaller spatial scales. The typical height‐to‐wavelength ratio of geometrical asperities and the correlations between stress and damage profiles are in good agreement with previous field or lab estimates. Plain Language Summary: Earthquakes occur on faults in the Earth curst, and these faults are known to be highly complex, both from a geometrical and structural point of view. This complexity is now introduced artificially in many numerical and theoretical models of faults in order to derive meaningful knowledge related to earthquake mechanics. There is little knowledge, however, about the way this complexity develops. In this paper, we employ a numerical model to observe the spontaneous development of this complexity, in order to understand its origin, its structure, and its evolution. Key Points: A coupled discrete‐continuum model of laboratory earthquakes allows the spontaneous development of fault complexity along seismic cyclesFault heterogeneity is persistent in space and time, with a strong positive correlation between normal stress, damage, and gouge thicknessStress fields exhibit a millimetric periodicity and a self‐affinity at smaller scales, in quantitative agreement with previous estimates [ABSTRACT FROM AUTHOR]

Smøla island, situated within the mid-Norwegian passive margin, contains crystalline-basement-hosted intricate fracture and fault arrays formed during a polyphase brittle tectonic evolution. Its detailed study may strengthen correlation attempts between the well-exposed onshore domain and the inaccessible offshore domain, further the understanding of the passive margin evolution, and provide useful constraints on petrophysical properties of fractured basement blocks. A combination of geophysical and remote sensing lineament analysis, field mapping, high-resolution drill hole logging, 3D modelling, petrographic and microstructural studies, and fault gouge K–Ar geochronology made it possible to define five deformation episodes (D1 to D5). These episodes occurred between the post-Caledonian evolution of the regional-scale Møre–Trøndelag Fault Complex (MTFC) and the Late Cretaceous and younger crustal extension preceding the final stages of Greenland–Norway break-up. Each reconstructed deformation stage is associated with different structural features, fault and fracture geometries, and kinematic patterns. Synkinematic mineralisations evolved progressively from epidote–prehnite, sericite–chlorite–calcite, chlorite–hematite, hematite–zeolite–calcite, to quartz–calcite. K–Ar geochronology constrains brittle deformation to discrete localisation events spanning from the Carboniferous to the Late Cretaceous. Multiscalar geometrical modelling at scales of 100, 10, and 1 m helps constrain the extent and size of the deformation zones of each deformation episode, with D2 structures exhibiting the greatest strike continuity and D1 features the most localised. Overall, the approach highlighted here is of great utility for unravelling complex brittle tectonic histories within basement volumes. It is also a prerequisite to constrain the dynamic evolution of the petrophysical properties of basement blocks. [ABSTRACT FROM AUTHOR]

Studying the effect of mechanical perturbations on granular systems is crucial for understanding soil stability, avalanches, and earthquakes. We investigate a granular system as a laboratory proxy for fault gouge. When subjected to a slow shear, granular materials typically exhibit a stress overshoot before reaching a steady state. We find that short seismic pulses can reset a granular system flowing in steady state so that the stress overshoot is regenerated. This feature is shown to determine the stability of the granular system under different applied stresses in the wake of a perturbation pulse and the resulting dynamics when it fails. Using an analytical aging-rejuvenation model for describing the overshoot response, we show that our laboratory-derived theoretical framework can quantitatively explain data from two fault slip events triggered by seismic waves. [ABSTRACT FROM AUTHOR]

Constraining strain localization and the growth of shear fabrics within brittle fault zones at sub‐seismic slip rates is important for understanding fault strength and frictional stability. We conducted direct shear experiments on simulated sandstone‐derived fault gouges at an effective normal stress of 40 MPa, a pore pressure of 15 MPa, and a temperature of 100°C. Using a passive strain marker and X‐ray Computed Tomography, we analyzed the spatial distribution of deformation in gouges deformed in the strain‐hardening, subsequent strain‐softening, and then steady‐state regimes at displacement rates of 1, 30, and 1,000 µm/s. We developed a machine‐learning‐based automatic boundary detection method to recognize the shear fabrics and quantify displacement partitioning between each fabric element. Our results show fabrics oriented along R1 and Y (including boundary) shears are the two major fabric elements. At rates of 1 and 30 µm/s, the relative amount of displacement on R1 shears is displacement dependent, increasing to ∼20% of the total displacement up to the strain‐softening stage, then decreasing to ∼10%–18% at the steady state. This trend is absent at the high rate where ∼18% of the displacement occurs on R1 shears throughout all investigated stages. At all rates, the relative amount of displacement on Y shears increases linearly with displacement to a total of larger than 50% at the steady state. Our study provides constraints on the development of the active slip zone, which is an important factor controlling heating and weakening associated with small‐magnitude earthquakes with limited displacement (mm‐dm), such as induced seismicity. Plain Language Summary: In the past few decades, several studies have focused on the mechanical behavior of simulated fault zones to understand earthquake nucleation. However, minor attention has been paid to the microstructural characterization of fault‐zone gouges due to the difficulties in quantifying deformation. An understanding of brittle fault‐zone fabrics and their development provides crucial constraints on the mechanical strength and stability of faults. We conducted laboratory experiments on simulated sandstone‐derived fault gouges with a passive strain marker under the conditions relevant to earthquake nucleation to explore the development and evolution of shear zone fabrics and their relations to fault strength. We combined X‐ray Computed Tomography (XCT) and a custom‐designed machine‐learning‐based automatic boundary detection method to analyze the spatial distribution of gouge deformation and to quantify displacement partitioning between deformation features. The results show that our samples have similar evolution of the shear fabrics, partitioning of displacement, and mechanical response with increasing shear strain at all tested nucleation velocities. The evolution of the mechanical behavior from strain‐hardening, to softening, to steady‐state stages is related to the transformation of R1 to shear‐parallel shear bands. Up to 50% of the total displacement can be accommodated within shear‐parallel shear bands, facilitating strain weakening of the materials. Key Points: We performed 3‐D analyses on the evolution of shear zone fabrics within sandstone‐derived fault gouges utilizing the X‐ray CT techniqueOur samples show similar evolution of shear fabrics, slip partitioning, and mechanical response with shear strain at all tested velocitiesUp to 50% of the total imposed displacement can be accommodated within shear‐parallel shear bands during earthquake nucleation [ABSTRACT FROM AUTHOR]

Establishing the absolute age of palaeoearthquakes is of great significance for the assessment of the seismicity and seismic hazards of a region. As such, several different geochronological techniques to date earthquake-related material have been developed to provide answers on the time of past earthquakes. The present study is part of a wider palaeoseismic research project conducted in the Nojima Fault Zone (NFZ), where the 1995 Mw 6.9 Kobe (Japan) earthquake was triggered, to assess the suitability of the isothermal thermoluminescence (ITL) dating technique on fine-grained quartz and medium-grained feldspar and to provide a sequence of ages for fault-rock samples separated from a drilled core that was retrieved from a depth of ~506 m. Our analysis reveals that ITL can produce consistent dating results and can be considered a reliable luminescence technique for the absolute dating of fault-gouge material. The produced ITL ages signified the existence of repeated seismic events within the NFZ that took place through the late Pleistocene period, with gouge ages spanning from 78.6 ± 4.2 to 13.4 ± 1.4 ka; however, overestimation of the produced ITL dating results may be apparent. Nonetheless, even though some degree of overestimation is considered, ITL dating results denote the oldest possible age boundary of formation (or luminescence signal resetting) of the collected fault-gouge layers. [ABSTRACT FROM AUTHOR]

Unraveling the age and kinematics of low temperature deformation events is crucial in understanding the late-stage evolution of orogens. However, accurate age constraints can often be challenging to obtain due to unideal outcrop conditions, large sedimentary hiatuses or the lack of well-defined thermal events. In this study, we show on the example of the Nekézseny Thrust, a poorly exposed late orogenic thrust in the southern Western Carpathians, that a combined approach of structural analysis and multi-method thermochronology can provide the necessary temporal, kinematic and thermal constraints for a detailed reconstruction of the deformation history. While structural mapping revealed that the Late Cretaceous Uppony Gosau Basin in the footwall of the Nekézseny Thrust underwent a significant post-Campanian and pre-Miocene shortening, K/Ar dating of fault gouge samples from the main fault zone constrained the primary thrusting event to the Maastrichtian. Based on the acquired apatite fission-track and (U-Th)/He ages, subsequent heating of the Upper Cretaceous sediments due to tectonic burial was limited to 75-100°C, followed by deformation-related and gradual cooling between the Eocene and Early Miocene. Considering the reconstructed deformation history, as well as the large-scale tectonic affinity of the displaced units in its footwall and hanging wall, the Nekézseny Thrust is a far-traveled (ca. 600 km) segment of the Late Cretaceous Alps-Dinarides contact zone, whose development was linked to the switch from lower plate to upper plate position with respect to the Sava Zone and Alpine Tethys sutures, respectively. [ABSTRACT FROM AUTHOR]

Whether tectonic strain from the early stage India‐Asia collision has synchronously affected the far‐field margin of the Tibetan Plateau is crucial for understanding plateau deformation and growth processes. However, direct evidence for early far‐field deformation remains scarce. Utilizing illite K‐Ar dating of three fault gouge samples, we established the faulting history of the Leibo fault zone (LFZ) at the southeastern margin of the Tibetan Plateau (SEMTP). Consistent authigenic illite ages of 52 ± 2, 54 ± 12 and 55 ± 6 Ma suggest the reactivated thrust faulting of the LFZ in the Early Cenozoic. Positioned ∼700 km east of the collisional boundary and at the intersection of three blocks with distinct lithospheric rheology in strength/viscosity, this event suggests a quasi‐synchronous far‐field tectonic response in the SEMTP to the India‐Asia collision. Plain Language Summary: Understanding how tectonic strain resulting from the collision between the Indian and Eurasian plates in the Early Cenozoic can spread to the edges of the Tibetan Plateau is crucial for comprehending the plateau's tectonic development. To investigate this question, we used fault gouge, which resulted from the sliding of brittle faults, to directly reveal faulting information. By conducting illite K‐Ar dating on three samples from the Leibo fault zone (LFZ) at the southeastern margin of the Tibetan Plateau (SEMTP), we found consistent ages of newly formed illite, 52 ± 2, 54 ± 12, and 55 ± 6 million years ago. Since this type of minerals form simultaneously with faulting, these ages reveal a reactivated thrust faulting event of the LFZ, suggesting a nearly simultaneous response of faulting in the SEMTP to the early stage of the India‐Asia collision. Key Points: Fault gouge illite K‐Ar dating is used to establish the faulting history of the Leibo fault zone, SE margin of the Tibetan PlateauThree consistent authigenic illite ages of ∼55, ∼54, and ∼52 Ma suggest a reactivated thrust faulting event of the Leibo fault zoneThis event indicates a quasi‐synchronous far‐field response of the SE margin of the Tibetan Plateau to the India‐Asia collision [ABSTRACT FROM AUTHOR]

Thermal pressurization of the trapped pore fluid is considered to be a widespread fault weakening during fluid injection in confined geological formations. Tremendous amounts of heats will be generated within the narrow shear zone during fault slip. Considering the melting of the fault gouge, the fault seal zone is adopted to construct the fluid pressurization model, thermal pressurization implemented in Comsol Multiphysics is established to illustrate the fault weaken coefficient and effective normal stress during fluid injection. The friction weakening coefficients mT and mF are proposed to research the performance of fault weakening during fluid injection. The results indicate that, the friction coefficients mT and mF both exhibit the initially decreasing and then increasing tendency, and thermal-induced fault weakening of CO2 injection occurs earlier than that of water injection. It was found that initial pore pressure and fault sealing porosity have a negligible influence on the evolutions of friction weakening and effective normal stress. Initial normal stress and fault sealing permeability have certain obvious influences on fault weakening during CO2 injection. Fault thickness is the primary factor influencing the friction weakening coefficient. When the fault thickness is over 1 mm, the variation of fault weakening is totally different from that when the thickness is less than 1 mm. This investigation of friction weakening during fault slip provides an effective reference for fluid injection. [ABSTRACT FROM AUTHOR]

The orientation of deep-seated orebodies was observed shifted from the northwest to north at the Xincheng deposit in the Jiaoxibei gold field during the mining operations. To make informed decisions, understanding the geological factors influencing this directional change is crucial for mining engineers. Prior research suggested that the ore-controlling fault morphology might correlate with the orebody positions and the thickness of alteration zones. Hence, the morphology of fault structures emerges as a key point in this work. First, we constructed a three-dimensional (3D) fault surface model. A large fault framework was interpreted using gravity-magnetic-electrical geophysical datasets, and then, the local details of the fault surfaces were characterized using an interpolation method based on the drillhole fault gouge data. Subsequently, we statistically assessed the spatial association between fault morphology and orebody positioning using the standardized C-values method and demarcated the ore-controlling structures on fault surfaces using quantitative indicators. The indicators of fault dips, fault azimuths, and the second-order trend residuals of fault surfaces represented the fault morphology. The drillhole grade data ^sampling grade*sampling length) were used to describe the scale of mineralization zones, and the cumulative thickness of the pyrite-sericite altered rocks indicated the intensity of alteration zones. Finally, a geostatistical structural analysis was used to depict the mineralization orientation at different locations. Through these methods, we aim to propose a potential pattern of how ore-controlling fault morphology spatially influences the positions and orientations of orebodies. This comprehensive approach offers valuable insights into exploring deep-seated mineralization at the Xincheng deposit, aiding in a better understanding of the deposit's geological dynamics for future mining endeavors. [ABSTRACT FROM AUTHOR]

Atera Fault clay gouges were collected for age dating near Kawaue, Nakatsugawa City, Central Japan, and the results integrated within its complex geological history. The results form an internally consistent data set constrained by extensive geochronological data (AFTA, ZFTA, CHIME) and support the application of gouge dating in constraining timing of brittle deformation in Central Japan. The Atera illite age data complete recently obtained limited illite fault gouge age data from underground exposure in the Toki Granite; the new illite age data are identical within error. The age of the heterogenous welded tuff breccia zone (Atera 1) ranges from 40.6 ± 1.0 Ma to 60.0 ± 1.4 Ma, whereas ages of the fault core gouge sample (Atera 2) range from 41.8 ± 1.0 Ma to 52.7 ± 1.2 Ma. The finest < 0.1 µm fraction of the fault breccia and fault core gouge yield ages around 41 Ma, identical within error. The new illite age data indicate brittle faulting and a following geothermal event occurring in the Paleogene–Eocene, similar to the nearby Toki Granite area and confirm they were both synchronous with a post-intrusive pluton exhumation. The Atera Fault illite age data provide additional insights into an integrated, regional-scale record of the tectonic displacement of Central Japan and might be influenced by large-scale tectonic processes such as the Emperor sea mount kink around 55 to 46 Ma with fault initiation around 50 Ma and brittle fault cessation or reactivation around 40 Ma in the Eocene. [ABSTRACT FROM AUTHOR]

The neotectonic processes are crucial for identification of active faults in a seismic unstable area. The Aїn Smara active fault runs along the Middle Rhumel Valley located in Constantine area of Algerian Eastern Tell Atlas mountain chain and it emerges in the south‒western part of the epicentral zone of the earthquake occurred on October 27, 1985 (Mw = 5.8) within a corridor displaying morphological features and deformed Pliocene‒Quaternary layers, which are the consequence of transpressive tectonic regime. In our study the morpho-tectonic and structural research supported by the fieldwork, was performed. The thick fault gouge indicating simple shear deformation, fault breccias and minor faults affecting the Pliocene limestone and Quaternary alluvial terraces, were found. The active fault splits into northern and southern segments. The northern segment corresponds to the El Aria Fault that ruptured during the earthquake in 1985 and the southern segment with the studied Aĭn Smara Fault. Our research showed the significant extent of the active fault and including its parameters for improved seismic hazard assessment of the Constantine area. [ABSTRACT FROM AUTHOR]

The rate and location at depth of fault creep are important, but difficult to characterize, parameters needed to assess seismic hazard. Here we take advantage of the magnetic properties of serpentinite, a rock type commonly associated with fault creep, to model its depth extent along the Bartlett Springs fault zone, an important part of the San Andreas fault system north of the San Francisco Bay, California (western United States). We model aero-magnetic and gravity anomalies using geologic constraints along 14 cross sections over a distance of 120 km along the fault zone. Our results predict that the fault zone has more serpentinite at depth than inferred by geologic relationships at the surface. Existing geodetic models are inconsistent and predict different patterns of creep along the fault. Our results favor models with more extensive creep at depth. The source of the serpentinite appears to be ophiolite thrust westward and beneath the Franciscan Complex, an interpretation supported by the presence of antigorite, a high- temperature serpentine mineral stable at depth, in fault gouge near Lake Pillsbury. [ABSTRACT FROM AUTHOR]

Crustal deformation is characterized by brittle and ductile faults that accommodate at different scales the strain imposed by plate tectonics. The aim of this contribution is to show with the help of different examples how the in situ 40Ar/39Ar dating of synkinematic neocrystallized minerals in ductile shear zones and the step-heating 40Ar/39Ar dating of synkinematic authigenic clays in fault gouges can bring information on the timing of fault activity. However, due to their complex evolution, interpretation of the argon signature in fault zones requires consideration of several effects among which re- or neocrystallization, inheritance and fluid interaction processes are dominant. [ABSTRACT FROM AUTHOR]

Metallogenic studies carried out in the Holy Cross Mountains indicate a relationship between mineralization and fault tectonics in Devonian formations. The impact of fault formation on the geochemistry of host rocks has not yet been studied. Mineralogical and geochemical studies of fault core gouges and damage zones in the fault walls of Devonian dolomites in the Budy and Jurkowice quarries were carried out. In the clay-carbonate filling of the fault fissure, the presence of two generations of Fe sulfides, the increased content of Zr, Nb, U and Th in relation to the surrounding rocks was noted. In the fault walls of the dolomites, iron sulfide and hematite mineralization were found. Research on the REE content indicates that it is lower in the fault walls than in those located far from it, while it is clearly higher in the fault gouge, especially in terms of the content of "heavy" elements (HREE). This indicates both the supply of some components to the fault zone from external sources and their displacement from the surrounding rocks. It was also found that the fault was renewed before and after the Neogene at least twice (Badenian). [ABSTRACT FROM AUTHOR]

The Astaneh Fault is one of the active and seismic faults in the Shahroud sinistral fault system. Fault gouges can be seen in a completely pristine and unchanged form in the portions of this fault that have affected the carbonate rocks. Fault gouges are the direct product of lithology and are formed as a result of the fault slip from the host rocks. Studying these rocks for type, composition, mineralogical phases and structure can provide results on how the fault slip and gouge process occurs. The result of a seismic slip in faults, and especially in gouges, are different types of deformations, such as mineralogical and compositional deformations, depending on the conditions of their formation. From this deformation model, we can mention the thermal decomposition of calcite and dolomite, as well as the presence of siderite in gouges. In this article, by studying the combined [aggregated] microstructures in the Astaneh fault gouge, their relationship with seismic slip has been investigated. According to previous research, the Astaneh Fault is certainly a seismic fault. The presence of the resulting thermal decomposition of calcite and dolomite (calcium and magnesium oxide or periclase) and the welding of particles due to heat may be the key to seismic slip in this fault. [ABSTRACT FROM AUTHOR]

Abstract Whether tectonic strain from the early stage India‐Asia collision has synchronously affected the far‐field margin of the Tibetan Plateau is crucial for understanding plateau deformation and growth processes. However, direct evidence for early far‐field deformation remains scarce. Utilizing illite K‐Ar dating of three fault gouge samples, we established the faulting history of the Leibo fault zone (LFZ) at the southeastern margin of the Tibetan Plateau (SEMTP). Consistent authigenic illite ages of 52 ± 2, 54 ± 12 and 55 ± 6 Ma suggest the reactivated thrust faulting of the LFZ in the Early Cenozoic. Positioned ∼700 km east of the collisional boundary and at the intersection of three blocks with distinct lithospheric rheology in strength/viscosity, this event suggests a quasi‐synchronous far‐field tectonic response in the SEMTP to the India‐Asia collision.

Frictional healing is fundamental to the seismic cycle and plays a role in the energy balance, dynamics, and recurrence interval of earthquakes and slow slip events. Although the healing behavior of quartz has been studied extensively, the role of clay content is less understood. We tested synthetic mixtures of quartz and smectite (10%–100% smectite) in a double‐direct shear configuration to measure frictional healing. We show that the magnitude and rate of healing decreases systematically with higher clay content (from 0.008/decade at 10% smectite to 0.002/decade at 100% smectite). Healing scales with both the magnitude of stress relaxation during holds and layer‐normal compaction of the gouge. We suggest this reflects the alignment of clay minerals, leading to saturation of the real area of contact that limits restrengthening during holds. The low healing rates of clay‐rich faults—together with rate‐neutral to rate‐strengthening friction—should promote frequent, small failures or stable sliding. Plain Language Summary: Healing is a time‐dependent strengthening process that makes it possible for faults to regain strength after an earthquake and fail again in future events. Faults are commonly hosted in clay‐rich, fine‐grained gouges. While there is a wealth of data on the frictional behavior of these gouges, there is a relative lack of data regarding their healing behavior. Here, we measure the change in frictional healing with increasing clay content using experimental deformation of synthetic fault gouges. We find that the amount and rates of healing decrease with clay content. This decrease is likely caused by the tendency for clay grains, which are elongate and plate‐shaped, to align and limit the ability for the fault gouge to increase the area of contact between grains, thus limiting healing. This small healing, together with their low strength and tendency toward stable behavior, may promote small frequent earthquakes—or inhibit earthquakes—on faults hosted in clay‐rich gouges. Key Points: We conducted experiments on a suite of quartz‐smectite mixtures to investigate the role of clay in frictional healingIncreasing smectite content reduces both healing and frictional strength, interpreted to result from saturation of real area of contactLow healing rates in clay‐rich gouges may favor more frequent and smaller failures than in quartz‐dominated gouges [ABSTRACT FROM AUTHOR]

The fault gouges have been worldwide proven sources of recording significant information on fault slippage and timing of activity. This paper presents the period and mode of fault activity inferred by observing the characteristics of quartz micromorphology along the Yishu Fault, Eastern China. Yishu Fault is the northward extension of Tanlu Fault where the fault gouge is well-developed. Based on the field investigation, four sections were selected in Changyi and Juxian. The quartz particles were observed by the scanning electron microscope（SEM） and analyzed by photoluminescence dating. The results show that the quartz is mainly composed of moderate dissolution type II and Ib+Ic micromorphology, and also type III, IV, Ia and Ru, indicating that the Yishu Fault has the characteristics of multi-stage activity. It is inferred from the obtained results that the last strong movement occurred in the early Middle Pleistocene and has been slightly active since the Late Pleistocene. According to the statistics of the collision, fracture and morphological characteristics of quartz in the fault gouge, the fault has been dominated by creep-slip and transition type since the Late Pleistocene, and the stick-slip was weak in general. The obtained results might provide strong support for monitoring the fault activity in the study. [ABSTRACT FROM AUTHOR]

Active serpentinite mud volcanoes in the forearc region of the Izu‐Bonin‐Mariana system represent an excellent natural laboratory for studying the geochemical processes along convergent plate margins and the associated forearc. During IODP Expedition 366, serpentinite mud with lithic clasts from the underlying forearc crust and mantle as well as from the subducting Pacific Plate was recovered. Ultramafic clasts from Fantangisña Seamount reveal very high degrees of serpentinization with mesh and bastite textures as well as development of late lizardite and chrysotile veins, which suggests serpentinization temperatures below 200°C. On the other hand, recovered harzburgites and, on occasion, dunites from Asùt Tesoru Seamount show a well‐preserved primary assemblage with low degrees of serpentinization and forearc peridotite characteristics. Fine‐grained antigorite associating with lizardite has been identified throughout the serpentine mud matrix, suggesting an alteration temperature of c. 340°C. Furthermore, alteration conditions during rodingitization point to temperatures of at least 228°C, estimated via chlorite geothermometry. Additionally, a rare ophicarbonate clast containing andraditic as well as Cr‐rich hydrogarnets from Asút Tesoru Seamount indicates crystallization temperatures of at least 230°C. Hence, a trend of lower temperature of serpentinization and higher degree of alteration closer to the trench. The detailed characterization of the fluid‐rock alteration conditions as well as fluids composition and transport permits a better constraining of the fluid–rock interactions and related mass transfers within subduction zones and during ascent of serpentinite fault gouge within mud volcano conduits and in mudflows after their emplacement on the flanks of the edifices. Plain Language Summary: The fluid migration and circulation in subduction zones play a crucial role in the geochemical cycling as well as the physical and mechanical processes taking place there. Characterizing their nature, source and pathways would contribute to a better understanding not only of the rheology and fluid recycling but also of the tectonic and metamorphic processes operating deep within the Earth's lithosphere as a whole. During an International Ocean Discovery Program (IODP) Expedition, 366 serpentinite mud volcanoes located on the fractured forearc of the Mariana subduction system were drilled. The recovered material consists of highly hydrated rocks that experienced varying degrees of metamorphism and alteration. Mineralogical and chemical composition study of these rocks showed that they are former mantle rocks formed by the infiltration of fluids within the subduction zone. Some of them experienced additional transformation during their ascent to the seafloor by acquiring substantial amount of CO2‐rich minerals. Furthermore, the results show a trend in which alteration temperature is decreasing but transformation degree is increasing with proximity to the subduction channel. Key Points: Serpentinization and rodingitization in the subduction channel as well as carbonation in mud volcano conduits were identifiedOverall evolution of the slab‐derived fluid was narrowed down to temperatures from c. 350° to c. 100°C and belowSerpentinization degree increases with proximity to the trench [ABSTRACT FROM AUTHOR]

Establishing the absolute age of palaeoearthquakes is of great significance for the assessment of the seismicity and seismic hazards of a region. As such, several different geochronological techniques to date earthquake-related material have been developed to provide answers on the time of past earthquakes. The present study is part of a wider palaeoseismic research project conducted in the Nojima Fault Zone (NFZ), where the 1995 Mw 6.9 Kobe (Japan) earthquake was triggered, to assess the suitability of the isothermal thermoluminescence (ITL) dating technique on fine-grained quartz and medium-grained feldspar and to provide a sequence of ages for fault-rock samples separated from a drilled core that was retrieved from a depth of ~506 m. Our analysis reveals that ITL can produce consistent dating results and can be considered a reliable luminescence technique for the absolute dating of fault-gouge material. The produced ITL ages signified the existence of repeated seismic events within the NFZ that took place through the late Pleistocene period, with gouge ages spanning from 78.6 ± 4.2 to 13.4 ± 1.4 ka; however, overestimation of the produced ITL dating results may be apparent. Nonetheless, even though some degree of overestimation is considered, ITL dating results denote the oldest possible age boundary of formation (or luminescence signal resetting) of the collected fault-gouge layers.

Thermal pressurization (TP) is predicted to be an important dynamic frictional weakening mechanism during earthquakes. The prevailing models, though, assume that the physical, thermal and hydraulic properties of the fluid‐saturated rock in the fault zone are constant during TP (the constant case) which inherently involves temperature and pressure changes. We solve the governing equations for TP in their general form with pressure‐ and temperature‐dependent physical, thermal and hydraulic parameters of a saturated fault zone with a one‐dimensional numerical model (the variable case). The model considers slip on a plane at a constant rate, and so does not account for dynamic frictional rupture scenario. We test a wide range of medium permeabilities of 10−22–10−16 m2 and porosities 0.5%–17%, based on experimental data for Frederick diabase, Westerly granite and the Hanaore Fault gouge. We find that the predicted shear stress drop and temperature rise is similar between the two cases for low permeability (<10−20 m2) and low porosity (<1%) fault rocks, owing to an increase in the fluid pressurization factor and hydraulic diffusivity. Differences between the two models are evident in more permeable and porous fault rocks. The increase in hydraulic diffusivity during TP results in a diffusional length which scales with time0.7 in the variable case. In addition, our calculations show that it is important to apply the constant case model with the ambient initial conditions for the modeled fault zone and not time‐averaged values that aim to represent the temperature and pressure changes that occur during TP. Plain Language Summary: Thermal pressurization (TP) is a dynamic frictional weakening mechanism that is predicted to operate during earthquakes. TP activates when frictional heat causes the fluids in the pores of the rocks to pressurize due to differences in thermal expansivity between fluids and rocks. The elevated pore pressure decreases the frictional resistance during sliding. The current models for TP assume that the properties of the fault rocks and fluids are constant (the constant case), whereas in practice, these properties vary due to temperature and pressure changes during TP. We solve the general form of the governing equations for TP with a one‐dimensional numerical model with pressure‐ and temperature‐dependent fluid and rock properties (the variable case). We compare the two models for different fault rocks that span a wide range of hydraulic properties. Our results show that for low permeability (<10−20 m2) and low porosity (<1%) rocks the two cases are similar. However, for more permeable and porous rocks the two models differ with continuous fault slip. Furthermore, we find that it is important to apply ambient initial conditions in the constant case model, rather than some time‐averaged conditions that aim to compensate for the lack of sensitivity to pressure and temperature changes. Key Points: Changes in hydraulic diffusivity and pressurization factor during thermal pressurization (TP) balance each other in low permeability and low porosity fault rocksHydraulic diffusional length scales as time0.7 when considering TP parameters that depend on temperature and pressureThe constant case model should be considered with ambient initial conditions and not time‐averaged ones [ABSTRACT FROM AUTHOR]

Stress accumulation and release reflected by acoustic emissions (AEs) during shearing of granular materials provide important information on failure mechanisms in seismic faults and landslides controlled by stick‐slip. Among many characteristics (amplitude, energy, counts, and frequency) of AE signals generated by stick‐slip, stress changes corresponding to various frequency AEs in different stages of the stick‐slip process are not clear, which limits our knowledge of the characteristics of precursory signals before stick‐slip failure. To better understand the physical mechanisms of granular stick‐slip, we monitored the mechanical and AE signals using high‐frequency (2 MHz) synchronous acquisition during constant‐speed shear of packs of uniform glass beads with different sizes at different normal stresses. The release rate of AE energy was found to accelerate with the dilatation of the sample volume, and the stress drop of stick‐slip was augmented with the increase of normal stress and particle size. Three characteristic events of single cycle stick‐slip were observed in this study: main slip, minor slip, and microslip. We analyzed the AE frequency spectra of these three event types. Both main slip and minor slip corresponded to stress drop and generated high‐frequency AEs (about several hundred kHz), while the AE frequencies generated by microslip were lower (about tens of kHz) and exhibited stress strengthening, which were not apparent in previous studies due to the low frequency of acquisition. We propose that the microslip is mainly due to sliding on grain contacts, while the main slip and minor slip resulted from breakage and reforming of force chains. Low‐frequency AEs from microslip may suggest a crucial precursor of seismic faults and landslides. Plain Language Summary: The occurrence of earthquakes of similar magnitude (quasi‐periodic earthquakes) in a fault zone every few decades is thought to be controlled by stick‐slip of fault gouges. The mechanical evolution of fault gouges prior to earthquake occurrence is extremely complex and difficult to predict. However, many studies have observed the presence of significant microslip prior to stick‐slip failure of seismic faults, which may provide precursor information for an impending earthquake. Laboratory study of stick‐slip nucleation and failure mechanisms in granular materials provides important understanding of the physical mechanisms of quasi‐periodic earthquakes. Our experimental results of high‐frequency acoustic emission (AE) and ring‐shear simultaneous acquisition showed that stick‐slip failure (corresponding to the occurrence of an earthquake) leads to a decrease in the shear resistance of a granular system, generating high‐frequency acoustic signals. Increments of microslip increased the shear resistance of the granular system, and were recorded as low‐frequency acoustic signals before failure occurred. We suggest that the stick‐slip nucleation and failure were controlled by grain friction and collision, respectively. The different AE frequencies of the two may shed light on the precursor monitoring of quasi‐periodic earthquakes. Key Points: Stick‐slip failure was associated with high‐frequency (∼105 Hz) acoustic emissions (AEs) and micro‐slip during stick‐slip nucleation was associated with low‐frequency (∼104 Hz) AEsHigh‐frequency AEs may be caused by particle collisions after force‐chain breakageLow‐frequency AEs may be caused by particle friction during force‐chain adjustment and may suggest important precursors of laboratory earthquakes [ABSTRACT FROM AUTHOR]

The use of quartz is critical to the electron spin resonance (ESR) dating of sediments and fault gouges. The germanium center (Ge center) in quartz is a commonly used color center employed in ESR signal measurement. Although Ge center signals in quartz can be detected using an ESR spectrometer, they are weak and sometimes undetectable. Impurities in quartz grains can further aggravate these deficiencies and may even invalidate any attempt to repeat ESR ages due to the deficiencies evident in the processes used in sample preparation. Using sieving, carbonate-organic matter removal, water flotation, magnetic separation, heavy liquid separation, and HF solution etching, we separated quartz, feldspar (plagioclase), mica, and the heavy minerals and measured the ESR signal in each of them to examine the impact of impurities within mineral grains on the ESR signals. From the ESR spectra, we observed intense ESR signals in feldspar and the heavy minerals and weak signals in mica at the same positions as the Ge center in the quartz. The ESR signals in the feldspar proved to be the most intense and overrode the peaks exhibited by the Ge center in the quartz. Feldspar is commonly associated with quartz in sediments, but it is difficult to separate it from quartz as their colors, specific gravities, and magnetic properties are similar. Any ESR signal in quartz containing foreign minerals would therefore be likely to be heavily disturbed by the signals from these foreign minerals, particularly from the feldspar and heavy minerals. ESR signals in the feldspar decreased similarly to those in the quartz when exposed to sunlight, declining by 70%–80% after being exposed for 50 h. Such declination was more obvious in high-altitude areas than in low-altitude areas. Our results indicate that highly pure quartz is required when dating; we would therefore strongly suggest that the procedures for the purification of quartz grains should include floating, heavy liquid separation, and HF solution etching to remove the feldspar, heavy minerals, and any remnant magnetite. Such a procedure would guarantee that the signal that is measured comes from the quartz alone, allowing repeatable measurements using the same spectrometer as well as valid comparisons between the spectrometries derived from different spectrometers. [ABSTRACT FROM AUTHOR]

To investigate the role of atomic-scale structure on the frictional properties of gibbsite, a dioctahedral-type aluminum hydroxide, we calculated the atomic-scale interlayer shear properties using the first-principles method based on density functional theory. We found that the presence of vacant sites within the octahedral sheet of gibbsite enables hydroxyls to move to more stable positions and reduce the repulsive force, leading to a lower atomic-scale shear stress of gibbsite compared with brucite, a trioctahedral-type magnesium hydroxide. We also estimated the macroscopic single-crystal friction coefficient of gibbsite with the assumption that only the atomic-scale interlayer friction controls macroscopic friction. The estimated single-crystal friction coeficient for gibbsite is 0.36(6), which is clearly lower than the experimentally obtained friction coeficient of the powdered gouge of gibbsite (0.74). This diference between the interlayer friction coeficient and gouge friction coeficient suggests the presence of additional mechanisms that afect the frictional strength, such as microstructures within a fault gouge. [ABSTRACT FROM AUTHOR]

While the Xianshuihe fault displays continuous creeping behavior, it is also the most seismically active fault in the eastern Tibetan Plateau, and its earthquake mechanisms remain unclear. Here, we aim at the stick‐slip portion of the creeping Qianning segment of the Xianshuihe fault to determine the characteristics of fault rocks and how fluids at depth influence fault behavior. Field survey, optical and scanning electron microscope observations, X‐ray diffraction and fluorescence analyses, as well as carbon and oxygen isotope analyses were performed on the collected samples. The fault core consists of 3–5 cm‐thick black fault gouge and ∼2.5 m‐thick breccia, surrounded by ∼12 m damage zone. In contrast to fault breccia (1–8 cm in diameter), the black fault gouge, which represents the principal slip zone of repeated seismic events, contains angular quartz particles (∼10 μm on average) and clays dominated by illite. The fluid‐rock interactions altering silica minerals into illite, and the thermal decomposition of carbonate minerals, passively increase the relative content of quartz and feldspar (total 63%–73%) in the fault gouge. The deeply sourced CO2 (from mantle and metamorphic degassing) within the hydrothermal fluids causes carbonate precipitation in breccias (21%–53%), composed of calcite, dolomite, and aragonite. These fluid‐assisted reactions lead to more abundant strong mineral phases (quartz, feldspar, and carbonates, 64%–87%) than weak clays (12%–36%) within the fault core, and locally strengthen the fault, which inhibits slow release of stress at shallow depth and promotes seismic rupture of the fault. Plain Language Summary: Large earthquakes are less likely to occur along active faults with slow slip that can release tectonic stress continuously (thus called "creeping" fault). However, numerous large earthquakes still occur along a typical creeping section of the Xianshuihe fault in eastern Tibet. Here, we better constrain the structural, mineralogical, and geochemical properties of fault rocks formed by earthquakes in the creeping Qianning segment of the Xianshuihe fault to understand its earthquake mechanism. The study outcrop consists of 3–5 cm‐thick black fault gouge, ∼2.5 m‐thick breccia, and ∼12 m damage zone. Intense fluid‐assisted alteration of silica minerals and thermal decomposition of carbonate minerals result in the passive enrichment of quartz and feldspar (total 63%–73%) in the black fault gouge. The fault breccia contains abundant carbonate minerals (21%–53%), resulting from carbonate precipitation associated with the influx of hydrothermal fluids enriched in CO2 at depth. Therefore, these fluid‐assisted chemical reactions lead to more abundant strong mineral phases (quartz, feldspar, and carbonates, 64%–87%) than weak clays (12%–36%) within the fault core, which locally strengthen the fault and promote the occurrence of earthquakes on this creeping fault. Key Points: Fault gouge at Cunielongba exposure of long‐term creeping Xianshuihe fault represents principal slip zone of repeated seismic eventsFluid‐rock interactions caused by hydrothermal fluids at depth increase relative abundance of strong mineral phases in fault coreFluid influx locally strengthens the creeping Xianshuihe fault and thus promotes fault stress accumulation [ABSTRACT FROM AUTHOR]

This paper presents a mortar-based finite element formulation for modelling earthquake cycles and wear on rough faults governed by rate and state friction. The method allows large sliding on the fault and accounts for all stages in the earthquake cycle, using a variable time step size with a transition between quasi-static and fully dynamic time discretizations. Wear laws with linear and power law forms are discretized and implemented into the mortar method, as well as a minimum level of normal traction constraint to treat fault opening. We examine the effect of wear laws on the slip behaviour and near-fault stresses during simulations of earthquake cycles on rough faults. The simulations demonstrate that the implementation of wear allows more realistic modelling of the earthquake cycle without the development of unrealistically large stresses and with less reduction of earthquake magnitude with total fault slip. Moreover, the method enables to study the effects of roughness and fault slip on the gouge zone thickness. [ABSTRACT FROM AUTHOR]