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We evaluate seismicity and tectonics along the San Andreas Fault (SAF) in southern California to elucidate ongoing near‐field crustal deformation processes. The principal slip surfaces (PSSs) or the fault core that accommodate major earthquakes, form the boundary between the tectonic plates. We analyze seismicity catalogs extending back to 1857, 1932, and 1981 with progressively improved magnitude of completeness and spatial resolution. The 1857 to present statewide catalog that is complete at M5.5+ documents minimal aftershock activity for the Mw7.9 1857 and 1906 Mw7.8 San Francisco earthquakes. The higher quality 1932 and 1981 catalogs show that the PSSs (the rupture zone) of the 1857 Mw7.9 Fort Tejon earthquake exhibits remarkable seismic quiescence both in the core and in the adjacent extended‐damage zone. Further south, the fault core is still aseismic but the shape of the SAF is more complex, and the rate of adjacent seismicity is much higher. This fault complexity and the seismicity rate are larger the more the strike of the SAF deviates from the Pacific plate velocity‐vector direction. The focal mechanisms of the SAF adjacent earthquakes are also heterogeneous and rarely have strikes and dips that are consistent with slip on the nearby PSSs. We infer that the southern SAF is locked, and a lack of seismicity at the core of the fault may be a standard feature of faults that almost exclusively accommodate high‐slip rates by producing major earthquakes. Correspondingly future aftershock sequences of major earthquakes on the southern SAF will likely have below average aftershock productivity. Plain Language Summary: The fast‐moving San Andreas Fault (SAF) that runs up the spine of California from the Salton Sea to Cape Mendocino, forms the plate boundary between the Pacific and North America plates. It has accommodated California's two largest earthquakes, the 1857 Mw7.9 Fort Tejon and the 1906 Mw7.8 San Francisco, which ruptured the fault for hundreds of kilometers. We focus on analyzing the seismicity of the southern SAF from 1857 to present. Almost no small earthquakes occur along the core of the southern SAF, from south of Parkfield to the Salton Sea. To the north, the 1857 Mw7.9 earthquake was followed by relatively few immediate aftershocks, and since 1932 this section of the fault has been very seismically quiescent. To the south of the 1857 rupture, the rate of extended‐damage zone seismicity inversely correlates with the SAF strike deviation from the Pacific plate vector relative to the SAF strike. In particular, the east‐west striking 28 km long Mill Creek segment may impede SAF slip, which results in abundant off‐fault seismicity and transfer of tectonic strain into the eastern California shear zone and the Peninsular Ranges. Key Points: The near‐field seismicity of the southern San Andreas Fault (SAF) is limited to the adjacent extended‐damage zone and mostly absent from the fault coreStep changes in the 95% depth and in the texture of seismicity depth distribution likely define the presence of the SAF fault coreSeismicity rate in the damage zone positively correlates with SAF deviation from Pacific plate vector except in the quiescent 1857 rupture [ABSTRACT FROM AUTHOR]

Coseismic slip on the Patton Bay splay fault system during the 1964 Mw 9.2 Great Alaska Earthquake contributed to local tsunami generation and vertically uplifted shorelines as much as 11 m on Montague Island in Prince William Sound (PWS). Sudden uplift of 3.7–4.3 m caused coastal lagoons along the island's northwestern coast to gradually drain. The resulting change in depositional environment from marine lagoon to freshwater muskeg created a sharp, laterally continuous stratigraphic contact between silt and overlying peat. Here, we characterize the geomorphology, sedimentology, and diatom ecology across the 1964 earthquake contact and three similar prehistoric contacts within the stratigraphy of the Hidden Lagoons locality. We find that the contacts signal instances of abrupt coastal uplift that, within error, overlap the timing of independently constrained megathrust earthquakes in PWS—1964 Common Era, 760–870 yr BP, 2500–2700 yr BP, and 4120–4500 yr BP. Changes in fossil diatom assemblages across the inferred prehistoric earthquake contacts reflect ecological shifts consistent with repeated draining of a lagoon system caused by >3 m of coseismic uplift. Our observations provide evidence for four instances of combined megathrust‐splay fault ruptures that have occurred in the past ∼4,200 years in PWS. The possibility that 1964‐style combined megathrust‐splay fault ruptures may have repeated in the past warrants their consideration in future seismic and tsunami hazards assessments. Plain Language Summary: During the Mw 9.2 1964 Great Alaska Earthquake, both the subduction zone and the crustal faults connected to the subduction zone (called splay faults) slipped, lifting shorelines up to 11 m on Montague Island in Prince William Sound (PWS) and contributing to local tsunami generation. The sudden uplift cut off tidal connection to coastal lagoons on Montague Island's northwest coast, producing a change to a drier, freshwater environment. The sudden environmental change created a clear sedimentary change from pre‐earthquake lagoon silt to post‐earthquake rooted soil in the stratigraphy of a study site known as Hidden Lagoons. Using the stratigraphic evidence of the 1964 uplift as a guide, we investigated deeper into the subsurface of Hidden Lagoons to see if there were previous instances of subduction zone and splay fault activation. We discovered three previous stratigraphic boundaries marking times when earthquakes uplifted the land. Changes in the species of diatoms across these boundaries show that the marine lagoon system was repeatedly drained by earthquakes causing over 3 m of uplift. These uplift events match the timing of four of the eight known prehistoric subduction zone earthquakes in PWS, suggesting that splay faults have slipped with the subduction zone multiple times over the past ∼4,200 years. Key Points: We find widespread stratigraphic evidence for the 1964 M9.2 earthquake and three prehistoric earthquake contacts on Montague Island, AlaskaStratigraphic and diatom analyses suggest ruptures of splay faults may have occurred in conjunction with 4 of 8 documented Alaska‐Aleutian subduction zone earthquakesHazard assessments may want to consider the tsunami potential with possible future 1964‐style combined megathrust‐splay fault ruptures [ABSTRACT FROM AUTHOR]

Elevated pore fluid pressure is proposed to contribute to slow earthquakes along shallow subduction plate boundaries. However, the processes that create high fluid pressure, disequilibrium compaction and dehydration reactions, lead to different effective stress paths in fault rocks. These paths are predicted by granular mechanics frameworks to lead to different strengths and deformation modes, yet granular mechanics do not predict their effects on fault stability. To evaluate the role of fluid overpressure on shallow megathrust strength and slip behavior, we conducted triaxial shear experiments on chlorite and celadonite rich gouge layers. Experiments were conducted at constant temperature (130 and 100°C), confining pressure (130 and 140 MPa), and pore fluid pressures (between 10 and 120 MPa). Fluid overpressure due to disequilibrium compaction was simulated by increasing confining and pore fluid pressure synchronously without exceeding the target effective pressure, whereas overpressure due to dehydration reactions was simulated by first loading the sample to a target isotropic effective pressure and then increasing pore fluid pressure to reduce the effective pressure. We find that the effects of fluid pressure and stress path on the mechanical behavior of the chlorite and celadonite gouges can generally be described using the critical state soil mechanics (CSSM) framework. However, path effects are more pronounced and persist to greater displacements in chlorite because its microstructure is more influenced by stress path. Due to its effects on microstructure, the stress path also imparts greater control on the rate‐dependence of chlorite strength, which is not predicted by CSSM. Plain Language Summary: High pore fluid pressure has been linked to complex slip modes along subduction plate boundary faults, such as low frequency earthquakes and slow slip. However, the role that pressurized fluids play in controlling these slip modes remains unclear. Along shallow subduction faults (<10 km), the source of pore fluid overpressure transitions with increasing depth from disequilibrium compaction of sediments to fluid release due to dehydration reactions like the smectite to illite transition. Using deformation experiments, we studied how different stress paths simulating these two processes affect the mechanical, frictional, and microstructural properties of shallow subduction fault rock at high fluid pressure. Key Points: Effective stress path and rock composition control effects of high pore fluid pressure on fault constitutive behaviorStress path effects on frictional behavior are more pronounced in simulated chlorite gouge than celadonite gouge due to differences in microstructureChlorite‐rich faults stabilize with increasing displacement at high pore fluid pressure and slow slip speeds [ABSTRACT FROM AUTHOR]

Genuine non‐double‐couple (non‐DC) components of a seismic source, defined here as the non‐DC components that are not due to summation of pure double‐couple (DC) components, provide important insight into special physical processes in non‐earthquake sources such as explosion, volcano eruption and collapse etc. Yet they remain challenging to be resolved. To address the issue and explore the physical mechanism of those special events, we develop a waveform‐polarity‐based moment tensor (WPMT) inversion method and employ it to study physical process in the 2014–2015 Bárðarbunga volcano event sequence. The WPMT method incorporates P‐wave polarity data and seismic waveforms in the source inversion, designs a source simplicity test to check possible complex rupture in the seismic source, and employs a simulated annealing algorithm to search the best source solution. The simplicity test checks consistency of the source processes in the initiation stage of the event and the major energy release process of the event, thus ensuring that the inferred non‐DC components are genuine to the seismic source. Real event and synthetic tests indicate that the WPMT method can identify and resolve genuine non‐DC components in a seismic source. The WPMT inversions of the Bárðarbunga sequence yield many genuine non‐DC source components and reveal that the eruptions are accompanied by seismic activities in depths of 1–5 km with magma migrations out of chambers, collapses of conduits, failures of normal faults, and a magma recharge at a depth of 9 km accompanied by a failure on a nearby normal fault. Plain Language Summary: Special seismic events such as explosion, collapse and volcanic eruption possess source components that are different from fault solutions used to represent a typical tectonic earthquake of simple shear. The identification and quantification of the source components of those special seismic sources thus play an important role in deciphering the physical process during those events. Yet, it remains challenging to resolve the source components of special seismic events due to the ambiguity of distinguishing them from an apparent one due to complex earthquake rupture. We develop a waveform‐polarity‐based moment‐tensor inversion to resolve the source components of a special seismic source and employ the method to study the physical process in the 2014–2015 Bárðarbunga volcano event sequence in Iceland. Real event and synthetic tests confirm method's capability of identifying and constraining the source components of special non‐earthquake events. The study of the Bárðarbunga sequence reveals that the eruptions are accompanied by seismic activities in depths of 1–5 km with magma migrations out of chambers, collapses of conduits, failures of normal faults, and a magma recharge at a depth of 9 km accompanied by a failure on a nearby normal fault. Key Points: We propose a waveform‐polarity‐based moment tensor inversion method to extract non‐double‐couple components of moderate seismic eventsReal event and synthetic tests show the method is effective in distinguishing and resolving genuine non‐double‐couple componentsMethod application reveals magma activities, conduit collapses and induced earthquakes during the 2014–2015 Bárðarbunga eruptions [ABSTRACT FROM AUTHOR]

Large block‐bounding faults on the Tibetan plateau are significant geological structures that accommodate tectonic movements and accumulate stress, leading to large earthquakes. Quantifying the interseismic slip deficit rate helps to better assess the earthquake potential. We combine available InSAR (2015–2020) and interseismic GPS data to determine fault coupling along 14 major block‐bounding faults. Spatially dense InSAR measurements remarkably improve the resolution of the coupling model. Combined with a GPS‐constrained block model, we examine the performance of the inversion approach with the stress constraint and the common Laplacian smoothing based on both synthetic tests and real data. We suggest that, for continental strike‐slip faults, adding the stress constraint can mitigate unphysical coupling distributions due to unreasonable assumptions or modeling artifacts, reducing the model uncertainty and improving the accuracy of the coupling model. This is particularly useful for segments featured by a highly heterogeneous distribution of coupling along the transition zone from locking to creeping region, partially‐coupling segment, and junction zone between main and subsidiary faults. We present a large‐scale fault coupling map along the major block‐bounding faults on the northeastern and eastern Tibetan plateau, highlighting the distinct degrees of fault coupling and lateral variations. The collage of coupling maps along different faults demonstrates the kinematic features over a broad time scale during earthquake cycles ranging from early to late interseismic phases, such as the segments ruptured during the 2001 Kokoxili earthquake and the 1920 Haiyuan earthquake. Plain Language Summary: Large faults at the boundary of blocks accommodate a significant portion of the relative movement between adjacent blocks and accumulate tectonic stress during the interseismic period. Consequently, these faults have a high potential to generate large earthquakes. To evaluate the distribution of strain accumulation, we analyze InSAR (2015–2020) and interseismic GPS measurements to construct a coupling model, which tells us the extent to which two blocks are locked together and accumulate stress before the next earthquake. We focus on 14 major faults on the eastern and northeastern Tibetan plateau. The addition of the spatially dense InSAR data helps to resolve the coupling distribution in greater detail, where GPS observations are limited. We evaluate different approaches to determine fault coupling, including stress constraints and smoothing techniques. Our findings indicate that adding a stress constraint performs better than the smoothing operator and helps mitigate physically unrealistic coupling distributions in the inversion using the spatial smoothing method. The stress constraint improves the accuracy of the coupling model. Based on the stress‐constrained inversion, we present a comprehensive fault coupling map in the northeastern and eastern Tibetan plateau, highlighting the spatial variations of fault coupling, which can be used in earthquake hazard assessment. Key Points: Stress constraints can mitigate unphysical coupling distributions and reduce the uncertainty of the coupling model for continental faultsWe highlight the improvement when using stress constraints in the case of sharp coupling contrasts along the strike‐slip faultWe provide a new coupling model along 14 major block‐bounding faults on the eastern and northeastern Tibetan plateau [ABSTRACT FROM AUTHOR]

All instrumented basaltic caldera collapses have generated Mw > 5 very long period earthquakes. However, previous studies of source dynamics have been limited to lumped models treating the caldera block as rigid, leaving open questions related to how ruptures initiate and propagate around the ring fault, and the seismic expressions of those dynamics. We present the first 3D numerical model capturing the nucleation and propagation of ring fault rupture, the mechanical coupling to the underlying viscoelastic magma, and the associated seismic wavefield. We demonstrate that seismic radiation, neglected in previous models, acts as a damping mechanism reducing coseismic slip by up to half, with effects most pronounced for large magma chamber volume/ring fault radius or highly compliant crust/compressible magma. Viscosity of basaltic magma has negligible effect on collapse dynamics. In contrast, viscosity of silicic magma significantly reduces ring fault slip. We use the model to simulate the 2018 Kı̄lauea caldera collapse. Three stages of collapse, characterized by ring fault rupture initiation and propagation, deceleration of the downward‐moving caldera block and magma column, and post‐collapse resonant oscillations, in addition to chamber pressurization, are identified in simulated and observed (unfiltered) near‐field seismograms. A detailed comparison of simulated and observed displacement waveforms corresponding to collapse earthquakes with hypocenters at various azimuths of the ring fault reveals a complex nucleation phase for earthquakes initiated on the northwest. Our numerical simulation framework will enhance future efforts to reconcile seismic and geodetic observations of caldera collapse with conceptual models of ring fault and magma chamber dynamics. Plain Language Summary: Caldera collapse manifests as the rapid subsidence of a kilometer‐scale block of crust circumscribed by a near‐circular fault on top of a volcano. The subsidence of the caldera block is caused by the eruption‐induced withdrawal of magma and reduction in pressure in the underlying magma chamber. All scientifically instrumented caldera collapses at volcanoes with low‐viscosity magma are accompanied by earthquakes of magnitude 5 and above. How do magma viscosity and the seismic wave radiation influence the amount of slip per earthquake on the fault? What can we learn about the dynamics of these earthquakes from seismic records? We address these questions by performing computer simulations of caldera collapse earthquakes and compare the results to the seismic records from the Kı̄lauea caldera collapse of 2018. Key Points: Seismic wave radiation through ring fault and magma, as well as high magma viscosity, reduce fault slip by up to half during collapseRupture propagation, downward momentum transfer via magma pressure waves, and chamber pressurization are identified in unfiltered seismogramsComparison between simulated and observed near‐field seismograms from Kı̄lauea 2018 reveals complex nucleation phase on the ring fault [ABSTRACT FROM AUTHOR]

The potential for future earthquakes on faults is often inferred from inversions of geodetically derived surface velocities for locking on faults using kinematic models such as block models. This can be challenging in complex deforming zones with many closely spaced faults or where deformation is not readily described with block motions. Furthermore, surface strain rates are more directly related to coupling on faults than surface velocities. We present a methodology for estimating slip deficit rate directly from strain rate and apply it to New Zealand for the purpose of incorporating geodetic data in the 2022 revision of the New Zealand National Seismic Hazard Model. The strain rate inversions imply slightly higher slip deficit rates than the preferred geologic slip rates on sections of the major strike‐slip systems including the Alpine Fault, the Marlborough Fault System and the northern part of the North Island Fault System. Slip deficit rates are significantly lower than even the lowest geologic estimates on some strike‐slip faults in the southern North Island Fault System near Wellington. Over the entire plate boundary, geodetic slip deficit rates are systematically higher than geologic slip rates for faults slipping less than one mm/yr but lower on average for faults with slip rates between about 5 and 25 mm/yr. We show that 70%–80% of the total strain rate field can be attributed to elastic strain due to fault coupling. The remaining 20%–30% shows systematic spatial patterns of strain rate style that is often consistent with local geologic style of faulting. Plain Language Summary: The potential for future earthquakes on faults is often inferred from velocities of the ground surface derived from satellite geodesy, but this approach can be challenging in complex deforming zones with many closely spaced faults. We present a new methodology for estimating the rate at which energy is accumulating on faults using measurements of surface strain rates. The method is applied to New Zealand for the purpose of incorporating geodetic data in the 2022 revision of the New Zealand National Seismic Hazard Model. We show that 70%–80% of the total deformation field can be attributed to energy accumulation on known active faults while the source of the remaining 20%–30% remains unknown. Along some of the major faults in New Zealand we find some important differences in rates of energy accumulation from what is expected from geologic data. Estimated rates are significantly lower than even the lowest geologic estimates on some faults in the fault system near highly‐populated Wellington. Key Points: We develop a method to invert geodetically derived strain rates for slip deficit rates on faultsWe find small but systematic differences between slip deficit rates and geologic slip ratesAbout 70%–80% of the surface strain can be attributed to elastic strain due to coupling on faults [ABSTRACT FROM AUTHOR]

The potential of faults to show earthquake‐generating slip instabilities depends not only on the intrinsic frictional properties of the fault zone, but also on the elasticity of the surrounding material. A velocity‐weakening fault is expected to show increasingly unstable frictional behavior with decreasing local elastic stiffness around the fault zone. Fault zone roughness can cause slip in the shear direction to be accompanied by fault‐normal movement, modulated by fault‐normal elastic properties, however these effects are poorly understood. Here, we systematically vary the stiffness surrounding the fault in both the shear‐parallel and fault‐normal directions, to investigate the origin of slip instabilities and changes in friction constitutive properties. We confirm the transition from stable sliding through slow slip to stick‐slip due to reduced fault‐parallel stiffness, and that the occurrence of different types of slip events can be explained by the ratio between shear and critical stiffness. In contrast, reducing the fault‐normal stiffness produces stick‐slip instabilities under conditions where the conventional critical stiffness criterion predicts stable sliding, and does not produce transitional slow slip events. Our data suggest that: (a) the stability criterion for frictional slip should be modified to incorporate fault‐normal stiffness, and (b) the unexpected slip instabilities may represent wrinkle‐like slip pulses, possibly due to a stiffness asymmetry introduced by lowering the fault normal stiffness on one side of the fault. This implies that earthquakes may occur when the fault‐normal stiffness, or bulk modulus for natural faults, is decreased and/or asymmetric across the fault zone, both of which may be common in nature. Plain Language Summary: Earthquakes occur on fault planes, but whether a fault will show stable sliding or host earthquakes, or their lab equivalent called "stick‐slip", depends only partly on the fault properties but also on how the fault and the surrounding rock respond to stress changes (the stiffness). Previous studies have shown that the transition between stick‐slip instabilities and stable sliding along a fault, can be triggered by changing the stiffness in the direction along the fault plane. However, faults commonly also have movement perpendicular to the fault plane, for example, if the fault surface is very rough, and therefore we study the effect of stiffness changes both perpendicular and parallel to the fault plane. We confirm previous observations when changing the stiffness parallel to the fault plane, but also observe unexpected stick‐slip instabilities when we decrease the stiffness perpendicular to the fault plane. This means that: (a) we have to adjust the stiffness criterion where stick‐slip instabilities are expected to include the fault‐normal stiffness, and (b) the unexpected stick‐slip instabilities at low fault perpendicular stiffness may be a different type of slip instability, called wrinkle‐like slip pulses, that may occur when one side of the fault is stiffer than the other. Key Points: Reduced fault‐normal stiffness alone can cause stick‐slip instabilities, but not slow slip in quartz gougeSystematically reducing both fault‐normal and shear‐parallel stiffness shows highly irregular slip eventsAdapting the critical stiffness criterion for fault‐normal stiffness shows slip instabilities are more likely [ABSTRACT FROM AUTHOR]

The NW Himalayan "seismic gap" lies between the 1905 and 2005 earthquake rupture zones and straddles the meisoseismal zone of the 1555‐Kashmir earthquake. Sufficient elastic strain has now accumulated across this region to drive a mega‐thrust earthquake. We model the crustal structure and geometry of the Main Himalayan Thrust (MHT) using Jammu and Kashmir Seismological NETwork data. P‐wave receiver‐function spatial and common‐conversion‐point (CCP) stacks, and joint inversion with Rayleigh‐wave group‐velocities are used to construct 3D Vs model of the region. This model reveals increasing crustal thickness from foreland‐to‐hinterland, and an under‐thrust Indian crust beneath NW Himalaya. The Moho is at ∼45 km depth beneath Sub‐Himalaya deepening to ∼70 km beneath Higher‐to‐Tethyan Himalaya with a gentle NE dip. The MHT forms a low‐velocity layer and has a flat–ramp geometry. The flat segment is beneath Sub‐to‐Lesser Himalaya at 6–10 km depth, and dips ∼4°. The mid‐crustal (frontal) ramp is beneath Kishtwar Higher‐Himalaya and Zanskar Range at 10–16 km depth, and dips ∼13–17°. Crustal structure varies along‐arc between east (Kishtwar) and west (Kashmir Valley) segments. A Lesser Himalayan duplex (LHD) lies beneath the Kishtwar Window and is bound by MHT and Main Central Thrust. The LHD high‐angle imbricate structures are illuminated by moderate seismicity. Beneath Kashmir Valley, the underthrust crust is ∼10 km thicker, has higher crustal Vs, and a shallower MHT at ∼10 km depth. The westward shallowing of MHT occurs through a lateral ramp. Aftershocks of the 2013‐Kishtwar earthquake concentrate on the frontal–lateral ramp intersection and mark the MHT down‐dip locked‐to‐creep transition. Plain Language Summary: We model the 3D‐seismic‐velocity structure of the NW Himalaya using data from the Jammu and Kashmir Seismological NETwork. The network extends from the Sub‐Himalaya to Tethyan Himalaya, across Himalayan thrust‐systems and rock units. We use distant earthquake body‐wave conversions and reverberations within the crust to construct 2D profiles, and perform joint modeling with surface‐wave dispersion data to compute 3D velocity model. Our results reveal under‐thrust Indian crust beneath NW Himalaya. The Moho at the base of the Indian crust is a boundary with increasing velocity as a function of depth, and deepens from Indo‐gangetic plains (∼45 km) to the Himalayas (∼70 km). The Main Himalayan Thurst (MHT), between the top of the under‐thrust Indian crust and overriding Himalayan wedge, is a low velocity layer with decreasing velocity as a function of depth. The MHT has flat‐ramp geometry beneath Kishtwar Himalaya, with ∼4° dipping flat and ∼13–17° dipping mid‐crustal (frontal) ramp. High‐angle imbricate thrusts overlies the MHT beneath Kishtwar Window and is illuminated by moderate earthquakes. Along‐arc the crust thickens by ∼10 km to the west beneath Kashmir Valley and MHT shallows through a SE‐dipping lateral ramp. Aftershocks of the 2013 Kishtwar earthquake concentrate on MHT frontal and lateral ramp intersection, at the down‐dip locked‐to‐creep transition. Key Points: Indian crust underthrusts the NW Himalaya with a gentle NE dip. Moho deepens from ∼45 to ∼75 km with undulations and southward dipMain Himalayan Thrust beneath Kishtwar has flat‐ramp geometry, overlain by Lesser Himalayan Duplex illuminated by moderate seismicityKashmir Valley has thicker crust, higher Vs, shallow MHT. MHT has SE dipping lateral ramp. Frontal‐lateral ramp intersection is seismogenic [ABSTRACT FROM AUTHOR]

Transient creep of crustal rocks is important to explain time‐dependent geological processes such as postseismic deformation following a large continental earthquake. While the steady‐state creep flow law parameters of quartz and feldspar, major minerals in the upper and lower crust, are well known, the physical mechanism behind transient creep and the corresponding flow law parameters are poorly understood. We quantify the flow law parameters for both quartz and granulite (mixture of plagioclase and pyroxene) under wet conditions with a nonlinear Burgers model using a Markov chain Monte Carlo (MCMC) method. Modeling results yield an activation energy of 70 ± 20 kJ/mol and a stress exponent of 2.0 ± 0.1 for transient creep of quartz aggregates. For granulite/feldspar, we find activation energies of 280 ± 30 and 220 ± 20 kJ/mol and stress exponents of 1.0 ± 0.2 and 0.9 ± 0.1 under mid (1050–1100°C) and high (1125–1150°C), temperature conditions, respectively. The stress exponents and activation energies of transient creep are consistently smaller than those of steady‐state creep for both quartz and granulite/feldspar. Combined with results for transient creep of olivine that were previously obtained (Masuti & Barbot, 2021, https://doi.org/10.1186/s40623-021-01543-9), we suggest that the activation energies and stress exponents of transient creep are smaller than those of steady‐state creep for volumetrically important silicate minerals of the crust and upper mantle. Extrapolation of the estimated flow law parameters of granulite/feldspar to natural conditions suggests that transient creep may dominate during the postseismic period and lasts longer than previously thought. Plain Language Summary: Earthquakes induce intermittent deformation of the solid Earth at rates that are higher or lower than the interseismic strain rates. When an earthquake occurs, it is accompanied by a sudden slip on the fault and a rapid stress change in the lower crust leading to an evolving rock strength. Lower crustal rocks represent a complex assembly of minerals and several different micro‐mechanisms may be activated during deformation. We use experimental data for quartz and feldspar to constrain the mechanical properties of the transient creep (i.e., stress changes with strain under constant strain rate conditions or strain changes nonlinearly with time under constant stress conditions). Based on our results, we conclude that the transient creep following an earthquake may not be as short lived as previously has been thought. Key Points: Transient creep flow law parameters of quartz and granulite are determined using a Markov chain Monte Carlo methodLow stress exponent of the transient creep in silicates could be due to weak dependency of dislocation density on the stressTransient creep could be dominant during the postseismic phase of the earthquake cycle [ABSTRACT FROM AUTHOR]

Variations in fault zone maturity have intermittently been invoked to explain variations in some seismological observations for large earthquakes. However, the lack of a unified geological definition of fault maturity makes quantitative assessment of its importance difficult. We evaluate the degree of empirical correlation between geological and geometric measurements commonly invoked as indicative of fault zone maturity and remotely measured seismological source parameters of 34 MW ≥ 6.0 shallow strike‐slip events. Metrics based on surface rupture segmentation, such as number of segments and surface rupture azimuth changes, correlate best with seismic source attributes while the correlations with cumulative fault slip are weaker. Average rupture velocity shows the strongest correlation with metrics of maturity, followed by relative aftershock productivity. Mature faults have relatively lower aftershock productivity and higher rupture velocity. A more complex relation is found with moment‐scaled radiated energy. There appears to be distinct behavior of very immature events which radiate modest seismic energy, while intermediate mature faults have events with higher moment‐scaled radiated energy and very mature faults with increasing cumulative slip tend to have events with reduced moment‐scaled radiated energy. These empirical comparisons establish that there are relationships between remote seismological observations and fault system maturity that can help to understand variations in seismic hazard among different fault environments and to assess the relative maturity of inaccessible or blind fault systems for which direct observations of maturity are very limited. Plain Language Summary: Earthquakes occur on faults, which are often complex structures. The relationship between the structure of faults and the seismological aspects of an earthquake rupturing that fault is an ongoing puzzle. Here we empirically compare geological and geometrical aspects of strike‐slip fault zones and surface ruptures of major events with seismological attributes of the events. We consider factors such as the total offset on the fault that has accumulated over geological time and the number of segments in maps of earthquake rupture and investigate how they correlate with aspects of the resulting earthquake such as the number of aftershocks or the rupture speed. Several of these factors co‐evolve as a fault accumulates slip and matures, thus the trends can be interpreted as indicative of the type of earthquakes observed on mature versus immature faults. We find that less mature faults tend to generate more aftershocks and have lower rupture velocity. Energy estimated from seismograms is relatively low for very immature faults, increases with fault evolution and then decreases as maturity further increases. These relationships help elucidate seismic hazard for fault systems of different maturity and delineate the important fault zone factors that have bearing on earthquake rupture and aftershock generation process. Key Points: Strike‐slip fault zone relative maturity is inferred from cumulative fault slip and from individual event surface rupture segment geometryMaturity measures are compared with relative aftershock productivity, rupture velocity, and moment‐scaled radiated energyLess mature faults have higher relative aftershock productivity and lower rupture velocity than mature faults [ABSTRACT FROM AUTHOR]

Surface wave amplification measurements have narrower depth sensitivity when compared to more traditional seismic observables such as surface wave dispersion measurements. In particular, Love wave amplification measurements have the advantage of strong sensitivity to the crust. For the first time, we explore the potential of Love wave amplification measurements to image crustal velocity in the western U.S. The effects of overtone interference, radial anisotropy and Moho depth are all explored. Consequently, we present SWUS‐crust, a three‐dimensional shear‐wave velocity model of crustal structure in the western U.S. We use Rayleigh wave amplification measurements in the period range of 38–114 s, along with Love wave amplification measurements in the period range of 38–62 s. We jointly invert over 6,400 multi‐frequency measurements using the Monte‐Carlo based Neighborhood Algorithm, which allows for uncertainty quantification. SWUS‐crust confirms several features observed in previous models, such as high‐velocity anomalies beneath the Columbia basin and low‐velocity anomalies beneath the Basin and Range province. Certain features are sharpened in our model, such as the northern border of the High‐Lava Plains in southern Oregon in the middle crust. Plain Language Summary: When an earthquake ruptures, seismic surface waves called Rayleigh and Love waves travel along the Earth's surface. Seismometers on the Earth's surface record ground motions caused by the passing seismic waves. The amplitude of these waves contains information about the local Earth structure beneath the station, from which we can produce images of the Earth's interior. Whilst Rayleigh waves have previously been used to image the Earth's upper mantle, this study uses Love waves for the first time. We explore how much information Love waves can provide and the advantages and limitations of using both Rayleigh and Love waves. Consequently, we build a new crustal model of the western U.S., called SWUS‐crust. The model correlates with many well‐known surface tectonic features, such as the Columbia Basin, Basin and Range province and Colorado Plateau. We also highlight certain features that have not been seen clearly in previous models, such as the High‐Lava Plains in southern Oregon. Key Points: Love wave amplification measurements are used for the first time and are jointly inverted with Rayleigh wave amplification measurementsWe perform extensive synthetic tests to quantify the advantages and limitations of our approachWe present SWUS‐crust, a new crustal model of the western U.S. [ABSTRACT FROM AUTHOR]

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]

Volcanic glass and its mixture with smectite are commonly observed in shallow parts of subduction zones. As volcanic glass layers often act as glide planes in submarine landslides, and because its alteration product, smectite, is one of the frictionally weakest geological materials, the frictional characteristics of volcanic glass‐smectite mixtures are important for fault slip behavior in shallow parts of subduction zones. We performed a series of friction experiments on volcanic glass‐smectite mixtures with different smectite contents from 0% to 100% at various velocity conditions from 10 μm/s to 1 m/s under an effective normal stress of 5 MPa and pore pressure of 10 MPa. In general, apparent friction coefficients negatively depend on the smectite content at any velocity tested. We found that samples with smectite contents of 15%–30% showed a drastic slip‐weakening behavior at intermediate velocities of 1–3 mm/s. Finite element method modeling shows that thermal pressurization does not contribute to the observed weakening behavior. The critical nucleation length estimated from the slip‐weakening behavior is approximately 1–10 km, which is large enough to prevent the slip to accelerate to seismic slip velocity. Therefore, gouges with minor amount of clay, such as subducting volcanic ash layers, may contribute to the occurrence of the slow earthquakes at shallow depths in subduction zones. Plain Language Summary: Materials erupted from volcanoes deposit on the seafloor and subduct at the trench. One such material, volcanic glass, easily alters into mechanically weak clay minerals such as smectite that can cause an enormous slip during an earthquake in subduction zones. In this study, we experimentally examined the frictional properties of mixtures of volcanic glass and smectite to elucidate fault slip behavior at shallow depths of subduction zones. Experiments with varying smectite content showed that a drastic reduction in fault frictional strength was induced when a small amount of smectite was present at moderately high velocity conditions. This behavior may induce slow earthquakes in shallow subduction zones. Key Points: Small amount of clay minerals in volcanic glass induces a drastic slip‐weakening at ∼1 mm/sThermal processes do not contribute to the weakening behaviorSlip weakening and large critical nucleation length at intermediate slip velocities might cause slow earthquakes [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 San Gabriel, Chino, and San Bernardino sedimentary basins in Southern California amplify earthquake ground motions and prolong the duration of shaking due to the basins' shape and low seismic velocities. In the event of a major earthquake rupture along the southern segment of the San Andreas fault, their connection and physical proximity to Los Angeles (LA) can produce a waveguide effect and amplify strong ground motions. Improved estimates of the shape and depth of the sediment‐basement interface are needed for more accurate ground‐shaking models. We obtain a three‐dimensional basement map of the basins by integrating gravity and seismic measurements. The travel time of the sediment‐basement P‐to‐S conversion, and the Bouguer gravity along 10 seismic lines, are combined to produce a linear relationship that is used to extend the 2D profiles to a 3D basin map. Basement depth is calculated using the predicted travel time constrained by gravity with an S‐wave velocity model of the area. The model is further constrained by the basement depths from 17 boreholes. The basement map shows the south‐central part of the San Gabriel basin is the deepest part and a significant gravity signature is associated with our interpretation of the Raymond fault. The Chino basin deepens toward the south and shallows northeastward. The San Bernardino basin deepens eastward along the edge of the San Jacinto Fault Zone. In addition, we demonstrate the benefit of using gravity data to aid in the interpretation of the sediment‐basement interface in receiver functions. Plain Language Summary: The shaking levels in the Los Angeles (LA) metropolitan area due to an earthquake on the San Andreas fault are underestimated. Northeast of LA, the San Gabriel, Chino, and San Bernardino basins influence the amount of shaking the LA area will experience. Sedimentary basins like these can amplify and trap seismic waves. Understanding these basins' shapes will improve our Earth model of the area and therefore seismic hazard estimates. The Basin Amplification Seismic Investigation project installed several small seismic instruments across these basins to characterize the structure of the basins. Along with gravity measurements, which capture information about the rock's density variations, we determine the basins' depth and shape. The depth model is then combined with a new velocity model of the area to produce an improved Earth model. Future studies of ground shaking should take these improved models into account. Key Points: Passive seismic and gravity measurements are integrated to estimate the 3D depth of the northern Los Angeles basinsThe maximum depth in the San Gabriel basin is ∼4.5 km, and Chino and San Bernardino basins are less than 2 km deepThe use of gravity helps to delineate faults across the basin, which helps our sediment‐basement interpretation in the receiver functions [ABSTRACT FROM AUTHOR]

Complex fault systems are often located in regions with asymmetric topography on one side of a fault, and these systems are very common in Southern California. Along these fault systems, geometrical complexities such as stepovers can impact fault rupture. Previous rupture dynamic studies have investigated the effect of stepover widths on throughgoing rupture, but these studies didn't examine the influence of topography on the rupture behavior. To investigate the effect of asymmetric topography on rupture dynamics at stepovers, I consider three cases: (a) a flat topography, (b) a positive (mountain), and (c) a negative (basin) topography on only one side of the fault system outside of the stepover. In each case, I use the 3D finite element method to compute the rupture dynamics of these fault systems. The results show a significant time dependent variation of the normal stress for the topography cases as opposed to the flat surface case, which can have an important impact on rupture propagation at the stepover. For a positive topography on the right of the rupture propagation, there is a clamping effect behind the rupture front that prevents the rupture to jump a wider extensional stepover. The opposite is observed for a negative topography or for a positive topography on the left side of the rupture propagation, where the rupture can jump over a wider compressional stepover. These results suggest that topography should be considered in dynamic studies with geometric complexities such as stepovers, and perhaps bends and branched fault systems. Plain Language Summary: Topography such as a mountain range or a basin can be found near faults across the world. This work investigates whether a surface topography can affect an earthquake to propagate across fault stepovers which are discontinuous faults with an offset of some distance with one another. I use three‐dimensional dynamic models to generate a number of earthquake scenarios using different types of topography (mountain or basin). I find that the type of topography can sometimes prevent or facilitate an earthquake rupture to propagate across a fault stepover. While numerical experiments without topography have shown it is unlikely for an earthquake to propagate across a stepover width greater than 4 km, adding topography can cause the rupture to jump beyond 4 km. This result could be a potential explanation as to why some past earthquakes are able to jump a stepover width of 4 km or greater while others couldn't and it could have real hazard implications for future earthquakes on stepovers. Key Points: Asymmetric topography can affect rupture propagation across fault stepoversTopography can cause a clamping or unclamping effect that can prevent or facilitate a rupture to jump a wider extensional stepoverWith topography, changing the hypocenter location can have a significant impact on throughgoing rupture across fault stepovers [ABSTRACT FROM AUTHOR]

Gas transport through sediments to the seabed and seepage occurs via advection through pores, faults, and fractures, and as solubility driven gas diffusion. The pore pressure gradient is a key factor in these processes. Yet, in situ measurements for quantitative studies of fluid dynamics and sediment deformation in deep ocean environments remain scarce. In this study, we integrate piezometer data, geotechnical tests, and sediment core analyses to study the pressure regime that controls gas transport along the Vestnesa Ridge in the eastern Fram Strait. The data show a progressive westward decrease in induced pore pressure (i.e., from c. 180 to c. 50 kPa) upon piezometer penetration and undrained shear strength of the sediments, interpreted as a decrease in sediment stiffness. In addition, the data suggest that the upper c. 6 m of sediments may be mechanically damaged due to variations in gas diffusion rates and exsolution. Background pore pressures are mostly at hydrostatic conditions, but localized excess pore pressures (i.e., up to 10 kPa) exist and point toward external controls. When analyzed in conjunction with observations from geophysical data and sediment core analyses, the pore pressure data suggest a spatial change from an advection dominated to a diffusion dominated fluid flow system, influenced by the behavior of sedimentary faults. Understanding gas transport mechanisms and their effect on fine‐grained sediments of deep ocean settings is critical for constraining gas hydrate inventories, seepage phenomena and sub‐seabed sediment deformations and instabilities. Plain Language Summary: Seafloor methane seepage occurs persistently in response to pressure changes. The scarcity of sub‐seabed pressure data limits our understanding of the mechanisms of methane release. Here we document observations from 4 days monitoring of pressure and temperature within the upper 10 m of sediment at a seepage site in the Fram Strait. The survey extends for 60 km between the continental shelf off west‐Svalbard and the mid‐ocean ridge. The data show that the geothermal gradient increases and that sub‐seabed sediments become softer and more susceptible to deformation as they approach the mid‐ocean ridge. When analyzed in conjunction with cross‐disciplinary data it seems plausible that the changes in sediment properties are associated with an increase in the amount of dissolved methane transported to the seafloor. We suggest that changes in the geological setting have resulted in the locking of fractures that would otherwise allow the release of trapped methane to the ocean. Trapped gas in the sediment pores is pumped by tides and causes damages beneath the seafloor. This study is an important step in understanding the mechanisms involved in the transport of methane into the ocean, with implications for climate research, assessment of geological hazards and alternative energy resources. Key Points: This study documents in situ pore pressure measurements along the gas charged Vestnesa sedimentary ridge in the eastern Fram StraitIntegrated piezometer and calypso core analyses indicate spatial variations in sediment stiffness and localized excess pore pressuresGas transport and sub‐seabed sediment stiffness are interrelated and affected by sea‐level changes and fault behavior [ABSTRACT FROM AUTHOR]

Previous experiments and modeling on earthquake nucleation reveal accelerating slip and development of a final patch of fixed or expanding length, but whether the nucleation phase is spatially large enough to be detected in real‐Earth conditions with durations long enough to be helpful is unknown. This study performed a suit of simulations of the nucleation process based on the rate‐and‐state friction law on homogeneous faults started from a "locked" state. Our results reveal the development of a weakening‐zone core (WZcore, defined as a region bounded by two outwardly moving shoulders of stress distribution during the early phase, and later bounded by two inwardly shifting edges between the stress‐releasing region and stress‐increasing exterior) where stress releases continuously, which expands first to a dimension less than the half fault length then shrinks to a final length before re‐expanding. The slip rate within the WZcore is above several times of the driving rate, and the high‐rate region expands constantly up to the instability. The WZcore is similar to a strain‐releasing zone in our new experiment. Under the assumption that much higher seismicity rate can be observed within the WZcore due to higher slip rate and stress‐releasing state as augmented driving force for small asperities, these nucleation features, as well as the slip rate levels in our model, are consistent with step‐like increases in seismicity preceding large subduction earthquakes. On the time scale, duration of the WZcore is weeks before large subduction earthquakes under high driving rate, but years on low‐rate continental faults. Plain Language Summary: As earthquake is understood to be generated on rock surfaces rubbing each other, the precursory behavior is thought to be related to the friction therein, which is now known to depend on both slip speed and its effective contact age. With this type of friction of the fault embedded in an elastic medium, we tried numerical simulations of earthquake preparation to find the features of this process. By observing the evolution of stress distribution, we find the development of a main stress‐releasing zone during the preparation process, which expands first with time then shrinks in its later stage. The main stress‐releasing zone is consistent with a strain‐releasing zone in our new experiment. Assuming that higher rates of seismicity can be observed within the main stress‐releasing zone due to higher slip rates and the stress‐releasing state as the intensified driving force for small asperities, the precursory feature described in the numerical model is consistent with the spatiotemporal migration of microearthquakes observed before large subduction earthquakes. The duration of the main stress‐releasing zone is weeks before large subduction earthquakes under high driving rate, but years for low‐rate continental faults. Key Points: A weakening‐zone core forms in the final stage of simulated earthquake nucleation which expands first and then shrinks to an unstable lengthSpatiotemporal evolution of micro‐seismicity observed before a few large subduction earthquakes is similar to simulated nucleation featuresComparison between aseismic slip rate and the driving rate may help indicate the final stages of mainshock nucleation [ABSTRACT FROM AUTHOR]

Fluids are pervasive in the Earth's crust and saturate fractures and faults. The combination of fluids and gouge layers developing along faults can generate fluids of different viscosities. Such viscous fluids were found to influence the reactivation, frictional stability of faults, and eventually the dynamics of propagating earthquake ruptures. We reproduced laboratory earthquakes on analog material (PMMA) to study the influence of viscous lubricant on fault frictional stability, rupture nucleation, and propagation under mixed lubrication conditions. Experiments were conducted in dry conditions, and with fluids presenting a viscosity ranging from 1 to 1,000 mPa.s. Through photoelasticity, high‐frequency strain gauge sensors, and accelerometer measurements, we obtained new insights about the influence of lubricant on a characteristic nucleation length, rupture propagation velocity, and local slip and slip rate evolution during the reproduced frictional ruptures. Our experiments show that the presence of a lubricant generating mixed lubricated conditions along the fault induces, (a) a reduction of the frictional resistance, (b) an increase in nucleation length, (c) a decrease in the fracture energy. In addition, the larger the viscosity of the fluids, the larger the reduction of frictional strength and the increase in the nucleation length. Moreover, ruptures occurring under mixed lubricated conditions showed a pulse‐like rather than crack‐like behavior, suggesting that viscous lubrication can induce the transition from crack‐like to pulse‐like rupture along natural faults. We demonstrate, supported by existing theory, that this transition is mainly governed by the stress acting on the fault at the onset of nucleation, which is drastically reduced in presence of a lubricant. Plain Language Summary: Fluids naturally permeating the Earth's crust or being injected for reservoir stimulation can promote fault reactivation, resulting in natural or so‐called induced earthquakes. It is important to comprehend how the presence of these fluids, as well as their properties, affect the activation and magnitude of seismic events. In particular, we investigated the role of fluid viscosity under controlled experimental conditions through laboratory experiments on analog material. We show that for the same applied normal stress, faults under lubricated conditions (i.e., where a thin film of viscous lubricant had been spread) can be activated at much lower shear stresses than dry faults. At the same time, the generated events will be much smaller in magnitude (i.e., smaller stress drop) and propagate more stably. Key Points: Frictional strength of lubricated artificial interfaces is dramatically reduced under mixed lubrication conditionsNucleation length of rupture under lubricated conditions is larger than the one of ruptures under dry conditionsViscous lubrication (under mixed conditions) can affect the nature of ruptures that would propagated pulse‐like rather than crack‐like [ABSTRACT FROM AUTHOR]

Mature fault cores are comprised of extremely fine, low permeability, clay‐bearing gouges. Saturated granular fault materials are known to dilate in response to increases in sliding velocity, resulting in significant pore pressure drops that can suppress instability. Up to now, dilatancy has been measured predominantly in clay‐poor gouges. Clay minerals have low frictional strengths and, in previous experiments, even small proportions of clay minerals were shown to affect the frictional properties of a fault. It is important, therefore, to document in detail the impact of the proportion of clay on the frictional behavior and dilatancy of fault rocks. In this work, a suite of triaxial deformation experiments elucidated the frictional behavior of saturated, synthetic quartz‐clay (kaolinite) fault gouges at effective normal stresses of 60, 25, and 10 MPa. Upon a 10‐fold velocity increase, gouges of all clay‐quartz contents displayed measurable dilatancy with clay‐poor samples yielding comparable changes to previous studies. Peak dilation did not occur in the pure quartz gouges, but rather in gouges containing 10 to 40 wt% clay. The clay content of the simulated gouges was found to control the gouge frictional strength and the stability of slip. A transition occurred at ∼40 wt% clay from strong, unstably sliding quartz‐dominated gouges to weak but stably sliding clay‐dominated gouges. These results indicate that in a low permeability, clay‐rich fault zone, the increases in pore volume could generate pore‐fluid pressure transients, contributing to the arrest of earthquake nucleation or potentially the promotion of sustained slow slip. Plain Language Summary: The strength of faults is dependent on the fluid pressure contained within the pore space of fine‐grained fault rocks on which fault slip events such as earthquakes occur. When slip velocity increases at the start of an earthquake, the fault rocks dilate, thereby reducing the fluid pressure and consequently strengthening the fault. If this strengthening is sufficient, an earthquake may arrest. Clay minerals are commonly present within faults, but previous measurements of fault volume changes have focused on clay‐poor materials. In this study, a series of experiments simulated the conditions within a tectonic fault in the Earth's crust. The experiments focused on the effect of changing the proportion of clay on the strength, slip behavior and volume changes of the fault material. All of the experiment materials experienced measurable volume changes that would change the pressure, and therefore the strength, of the fault. Fault composition was found to control both the strength and slip behavior of the fault. Clay‐poor materials were stronger and more likely to initiate earthquake slip, whereas the clay‐rich materials were weaker and slip via steady creep. This study provides essential insight and data toward understanding the slip behavior of faults, from slow fault creep to earthquakes. Key Points: Dilatancy with increasing slip velocity is seen for fault gouges ranging from 100 wt% quartz to 100 wt% clayDilatancy is at a maximum in the quartz‐rich, clay‐poor gouges, rather than in the end member compositionsIncreasing clay content systematically decreases the gouge frictional strength and promotes velocity strengthening behavior [ABSTRACT FROM AUTHOR]

Faults are composed of a damage zone and a fault core with wear material that has accumulated during sliding. Fault zone width and structural complexity have been suggested as key factors that dictate the stability of fault slip and the transition from stable to unstable motion. Here we address the role of fault rock wear rate and the presence of gouge on the stability of frictional sliding. We performed biaxial shear experiments on quartz‐rich bare rock surfaces having different resistances to abrasion (i.e., wear production with slip), as well as simulated quartz gouge. To characterize the influence of shear displacement on fault stability, constant velocity and velocity step experiments were performed to displacements up to 30 mm. Mechanical data are analyzed within the rate‐and‐state framework, and integrated with post‐mortem microscopic analyses of the samples. For initially bare surfaces a shear displacement threshold is required to transition from stable to unstable sliding. Stick‐slip events (laboratory earthquakes) evolve systematically as a function of fault slip. Shear displacement has a clear effect on the rate‐and‐state parameters (a‐b) and Dc. For all our tests, (a‐b) decreases with increasing shear displacement, implying enhanced velocity weakening and potential unstable behavior for larger fault slip. For high wear rates and simulated gouge, Dc decreases with increasing slip. However, for low wear rate faults, changes in Dc are negligible. Our results demonstrate that, fault stability varies systematically with fault slip and in particular that shear displacement and strain localization are dominant parameters controlling fault stability. Plain Language Summary: Large earthquakes mainly occur on faults having large cumulative slip. In the field, fault structure consists of a fault core made of wear product (rock flour) that is surrounded by fractured rocks (damage zone). Most of laboratory studies on fault stability, that is, the capability of fault to nucleate earthquake, consider faults either as only intact rock interfaces or wear product only. Here, we address the role of fault rock wear rate (wear development with slip) and the presence of fault gouge on the mode of frictional sliding (stable sliding vs. earthquake). Three fault systems were tested in the laboratory: intact rock surfaces having a low wear rate, intact rock surfaces having a high wear rate and wear product alone. We found that for all the tested faults, earthquake nucleation requires a certain amount of slip. Microstructural analyses of the fault after the experiment show that cumulative slip favors localization of the deformation necessary for earthquake nucleation. We show that the fault stability is not constant in time and space but depends on the fault history (cumulative displacement, shear localization, presence or absence of gouge). Key Points: Laboratory investigation of fault stability with varying wear rateShear displacement has a dominant impact on fault stabilityIndependently of the fault wear rate, the rate‐and‐state stability parameters evolve continuously with shear displacement [ABSTRACT FROM AUTHOR]

A large earthquake unlocks a fault‐zone via dynamic rupture while releasing part of the elastic energy stored during the interseismic stage. As earthquakes occur at depth, the analyses of earthquake physics rely primarily on experimental observations and conceptual models. A common view is that the earthquake instability is necessarily related to the frictional weakening that is commonly observed in shear experiments under seismic slip velocities. However, recent experiments with frictional interfaces in brittle acrylics (e.g., Svetlizky & Fineberg, 2014, https://doi.org/10.1038/nature13202) and rocks (e.g., Passelegue et al., 2020, https://doi.org/10.1038/s41467-020-18937-0) have explicitly demonstrated that no characteristic frictional strength exists. Namely, prior to nucleation, frictional interfaces can sustain a wide range of applied stresses ("overstresses") that exceed the residual stresses, which are the stresses along the interface that remain after the sliding. Moreover, the experimentally observed singular stress‐fields and rupture dynamics are precisely those predicted by fracture mechanics (Freund, 1998). We therefore argue here that earthquake dynamics are best understood in terms of dynamic fracture mechanics and not governed by the frictional properties of faults. In this view, rupture dynamics are driven by the release of the elastic energy due to overstresses, whereas the values of the residual stresses and the energy dissipation are determined by fault frictional properties. Plain Language Summary: A large earthquake occurs when a "locked" fault becomes unlocked and starts slipping rapidly while releasing stored elastic energy. As earthquakes occur at depth, earthquake analyses rely primarily on experimental observations and conceptual models. One common view attributes the earthquake instability to the transition from the strong "static friction" to the weaker "dynamic friction." Recent observations of experimental earthquakes along brittle faults cause us to challenge this common view. These experiments have explicitly demonstrated that faults may stay locked under a wide range of imposed stress levels, thereby casting doubt on the concept of a characteristic "static friction" for a fault. Moreover, the features of these experimental earthquakes fit precisely the predictions of fracture mechanics theory (Freund, 1998). We therefore argue here that earthquake dynamics is best understood in terms of dynamic fracture mechanics, a process not directly related to the fault frictional properties. Key Points: Earthquakes as dynamic rupture phenomenaDynamic weakening by breakdown at earthquake rupture frontFracture mechanics as controlling earthquake instability regardless of frictional properties [ABSTRACT FROM AUTHOR]

Ocean transform faults often generate characteristic earthquakes that repeatedly rupture the same fault patches. The westernmost Gofar transform fault quasi‐periodically hosts ∼M6 earthquakes every ∼5 years, and microseismicity suggests that the fault is segmented into five distinct zones, including a rupture barrier zone that may have modulated the rupture of adjacent M6 earthquakes. However, the relationship between the systematic slip behavior of the Gofar fault and the fault material properties is still poorly known. Specifically, the role of pore fluids in regulating the slip of the Gofar fault is unclear. Here, we use differential travel times between nearby earthquakes to estimate the in‐situ Vp/Vs of the fault‐zone materials. We apply this technique to the dataset collected by an ocean‐bottom‐seismometer network deployed around the Gofar fault in 2008, which recorded abundant microearthquakes, and find a moderate Vp/Vs of 1.75–1.80 in the rupture barrier zone and a low Vp/Vs of 1.61–1.69 in the down‐dip edge of the 2008 M6 rupture zone. This lateral variation in Vp/Vs may be caused by both pore fluids and chemical alteration. We also find a 5%–10% increase in Vp/Vs in the barrier zone during the 9 months before the mainshock. This increase may have been caused by fluid migrations or slip transients in the barrier zone. Plain Language Summary: Oceanic transform faults are ideal natural laboratories for studying earthquake physics due to their regular behaviors. The westernmost Gofar transform fault near the East Pacific Rise generates magnitude six earthquakes every 5–6 years on two rupture zones, respectively, which are separated by a barrier zone repeatedly stopping ruptures on the adjacent segments. The distinct behavior of the barrier zone might be due to differences in material properties from the rupture zones. To explore this hypothesis, we analyze thousands of small earthquakes in 2008 and find that the barrier zone has a higher ratio between P and S velocities than the rupture zone. This difference indicates that the barrier and rupture zones differ in their fluid content and chemical composition, which may have regulated their distinct slip behaviors. We also find an increase in the ratio between P and S velocities in the barrier zone during the 9 months before the magnitude six earthquake in 2008, which may reflect fluid flows or aseismic fault slips. Our findings suggest that pore fluids and the chemical composition of fault‐zone materials likely play a crucial role in regulating the slip behaviors of oceanic transform faults. Key Points: Rupture barrier zone has a moderate Vp/Vs of 1.75–1.80Down‐dip edge of the 2008 M6 mainshock has a low Vp/Vs of 1.61–1.69Vp/Vs in the rupture barrier zone increased in the 9 months before the mainshock [ABSTRACT FROM AUTHOR]

Oceanic transform faults are intriguing in that they do not produce earthquakes as large as might be expected given their dimensions. We use 1‐year of local seismicity (370 events above MC = 2.3) recorded on an array of ocean bottom seismometers (OBSs) and geophysical data to study the seismotectonic properties of the Chain transform, located in the equatorial Mid‐Atlantic. We extend our analysis back in time by considering stronger earthquakes (MW ≥ 5.0) from global catalogs. We divide Chain into three areas (east, central, and west) based on historical event distribution, morphology, and multidimensional OBS seismicity cluster analysis. Seismic activity recorded by the OBS is the highest at the eastern area of Chain where there is a lozenge‐shaped topographic high, a negative rMBA gravity anomaly, and only a few historical MW ≥ 5.5 events. OBS seismicity rates are lower in the western and central areas. However, these areas accommodate the majority of seismic moment release, as inferred from both OBS and historical data. Higher b‐values are significantly correlated with lower rMBA and with shallower bathymetry, potentially related to thickened crust. Our results suggest high lateral heterogeneity along Chain. Patches with moderate to low OBS seismicity rates that occasionally host MW ≥ 6.0 earthquakes are interrupted by segments with abundant OBS activity but few historical events with 5.5 ≤ MW < 6.0. This segmentation is possibly due to variable fluid circulation and alteration, which may also change in time. Plain Language Summary: Oceanic transform faults (TFs) typically host earthquakes much smaller than expected based on their total seismogenic area. We study the seismotectonic properties of the Chain TF by combining 1‐year of seismicity recorded by an ocean bottom seismometer (OBS) array with geophysical data (bathymetry, tidal height, gravity anomalies). We supplement our analysis with strong historical earthquakes (M ≥ 5.0) from global catalogs. Our analysis divides Chain into three areas: The east area is characterized by the highest OBS seismicity rates, negative gravity anomalies, large topographic highs, and few historical events with 5.5 ≤ M < 6.0. The west and central areas demonstrate lower OBS seismicity rates. However, they occasionally produce M ≥ 6.0, being responsible for most of the total seismic energy release. Higher numbers of stronger events occur in areas with negative gravity anomalies and shallower water depths. Our results suggest high lateral heterogeneity along Chain, with alternating seismic and aseismic patches, variable crustal thickness, different degrees of hydrothermal circulation/alteration, and potentially time‐dependent behavior. Key Points: Segmentation along Chain transform fault (Mid‐Atlantic Ocean) is studied by means of seismicity cluster analysis and geophysical dataChain demonstrates lateral heterogeneity with high moment‐release patches interrupted by segments with abundant but small magnitude eventsSpatiotemporal variations of seismicity are possibly manifested by variable hydrothermal circulation and alteration [ABSTRACT FROM AUTHOR]

The hydrous mineral talc is stable over a relatively large P‐T field and can form due to fluid migration and metamorphic reactions in mafic and ultramafic rocks and in faults along plate boundary interfaces. Talc is known to be one of the weakest minerals, making it potentially important for the deformation dynamics and seismic characteristics of faults. However, little is known about talc's mechanical properties at high temperatures under confining pressures greater than 0.5 GPa. We present results of deformation experiments on natural talc cylinders exploring talc rheology under 0.5–1.5 GPa and 400–700°C, P‐T conditions simulating conditions at deep faults and subducted slab interface. At these pressures, the strength of talc is highly temperature‐dependent where the thermal weakening is associated with an increased tendency for localization. The strength of talc and friction coefficient inferred from Mohr circle analysis is between 0.13 at 400°C to ∼0.01 at 700°C. Strength comparison with other phyllosilicates highlights talc as the weakest mineral, a factor of ∼3–4 weaker than antigorite and a factor of ∼2 weaker than chlorite. The observed friction coefficients for talc are consistent with those inferred for subducted slabs and the San Andreas fault. We conclude that the presence of talc may explain the low strength of faults and of subducted slab interface at depths where transient slow slip events occur. Plain Language Summary: Whether faults creep by slow continuous and stable motion, generate slow slip or low‐frequency seismicity, or rupture by an unstable spontaneous event (i.e., an earthquake) is related to the mechanical properties of the minerals in place. Talc is one of the weakest minerals and is expected to form in fault interfaces. There is a lack of experimental data on the mechanical properties of talc under the pressures and temperatures that exist along deep faults and subduction zones. Here, we present results from a set of deformation experiments on talc under pressure‐temperature conditions that simulate deep faults and subducted slab interface. Results show a transition from pressure‐dependent to pressure‐independent strength at high temperatures (∼700°C). In addition, at increased temperature (≥600°C) talc shows an increased tendency for strain localization. These extremely low frictional strengths are about an order of magnitude lower than most rocks and are consistent with inferred weak interfaces along the San Andreas fault and various subducted slab interfaces at depths where creep and/or slow slip events occur. Key Points: Talc strength is pressure‐ and temperature‐dependent under conditions of deep faults and subduction zones (0.5–1.5 GPa and 400–700°C)Talc microstructure shows fracturing under all conditions tested associated with thermal weakening and increased localizationThe extremely low friction coefficient (<0.13) is consistent with the inferred strength of different subducted slab interfaces and the San Andreas fault [ABSTRACT FROM AUTHOR]

We use ocean bottom seismometer data from the Endeavour segment of the Juan de Fuca ridge to construct a long‐term earthquake catalog for an intermediate spreading rate mid‐ocean ridge. We present >50,000 new earthquake locations for 2016–2021 from the Ocean Networks Canada NEPTUNE cabled observatory and relocate earthquakes from two autonomous networks in 1995 and 2003–2006. The catalog comprises >85,000 earthquakes located using three‐dimensional segment‐scale P and S wave velocity models from a prior tomography experiment. Despite the small footprints of networks near the segment center, locations show good agreement with geologic features at segment ends. The improved locations show that the northern Endeavour segment ruptured southwards from 48.3°N to 48.05°N during two diking events in early 2005, possibly accompanied by diking on the West Valley (WV) propagator. Persistent off‐axis seismicity near the segment center appears to be related to the WV and Cobb propagating rifts which we infer extend ∼10 km closer to the Endeavour segment center than is apparent in bathymetry. We suggest that the proximity of the propagators to the Endeavour vent fields (VFs) contributes to the localization, vigor, and longevity of the fields by focusing permeability through ongoing fracturing and by limiting extrusive magmatism through degassing of the axial magma lens. Increasing rates of seismicity beneath the VFs beginning in late 2018 and a deepening of earthquakes in 2020 indicate that the central portion of the segment may be entering the later stages of the eruptive cycle. Plain Language Summary: Seafloor spreading centers are boundaries where tectonic plates move apart and ocean crust is created by volcanism. Small earthquakes in these settings result from extensional faulting, magma injection, and cooling of the crust by circulating fluids. Seafloor spreading is cyclical: strain steadily builds up as the plates move apart and is relieved episodically when magma is injected into the crust. Earthquake activity must be observed on the order of decades to fully capture this cycle. Until recently, however, batteries have limited earthquake observations from autonomous seafloor seismometers to about a year. We combine observations from a new long‐term undersea cabled observatory and older experiments to present an earthquake catalog that spans 20 years for the Endeavour segment of the Juan de Fuca ridge. We improve earthquake locations relative to past studies and relate evolving earthquake activity to seafloor features. Our results suggest that the complicated geometry of nearby plate boundaries, through focused fracture creation, affects how fluids circulate through the crust. Earthquake patterns preceding the last observed magmatic event in 2005 indicate that the increases in earthquake rates and depths we observe in 2018–2020 may be signs of the center of the segment nearing the next spreading event. Key Points: Three‐dimensional velocity models facilitate reliable earthquake localization in the marine setting despite limited seismometer coveragePropagating rift tectonics enable long‐lived high‐temperature hydrothermal venting through amplified and spatially focused deformationEarthquakes across 20 years reflect strain that evolves cyclically as extension is accumulated and released on overlapping spreading axes [ABSTRACT FROM AUTHOR]

Key factors controlling earthquake ruptures include fault geometry, continuity, and seismic velocity structure around the fault. We present a novel tool that better informs deep bimaterial fault geometry embedded in distributed damage and seismicity, associated velocity contrasts across the fault, and their correlations with surface complexities. The method employs fault zone head and direct body waves and is applied to recordings from five spatiotemporally different seismic arrays along the complex San Jacinto fault zone (SJFZ) in southern California. We detect and distinguish these signals based on instantaneous phase coherence and relative energy in a cascading manner from one scale array to another. The analysis reveals a >70‐km long continuous bimaterial interface within the SJFZ with several deep northeast dipping fault segments. The northern SJFZ, for instance, locates ∼7 km northeast of its surface expression at 18‐km depth. P‐wave velocity contrasts range from near 0% to >15%, consistent with other bimaterial faults, and differ by a few % depending on fault‐array azimuth, implying directional‐dependent velocity contrasts. S‐wave head waves and velocity contrasts are also imaged for the first time at the southern SJFZ, averaging to 2.9% in agreement with tomography results. The imaged geometry and continuity suggest the SJFZ initiated along remnant tectonic structures and translates to a rupture potential of M > 7.2, i.e., the sizes of its largest paleo‐earthquakes. The P and S contrasts, and their ratios, have important implications for earthquake rupture speed, mode, directivity, and frictional heating along the SJFZ and other major faults globally. Plain Language Summary: Large earthquakes occur on faults that are structurally complex, leading to earthquakes that are themselves structurally and dynamically complex. More accurate information on fault complexities is key to quantifying the potential size and shaking from large earthquakes along them. Here, we introduce a novel tool that resolves the deep geometry, continuity, and contrast in seismic velocity along major transform fault systems using the complex San Jacinto fault zone (SJFZ) in southern California as an exemplary case. It employs unique signals that travel along, and illuminate, continuous faults contrasting rock volumes of different seismic properties. With this tool, we reveal a >70‐km long continuous fault within the SJFZ that is nonsimple and nonplanar, curving laterally, and in depth. For example, a deep northern section of the SJFZ dips extensively toward the northeast. The seismic velocity contrast across the entire fault varies from 0% to >15% and depends on the angle of observation from the surface. This long continuous fault is capable of an M > 7.2 earthquake and likely initiated along older structures that predate the SJFZ. The observed geometries and velocity contrasts have implications for the direction, speed, distribution, and other key defining features of large SJFZ earthquakes. Key Points: Image the San Jacinto fault zone with direct body and fault zone head waves recorded by several dense arrays along the faultReveal a ∼72‐km long continuous bimaterial interface within the core fault zone with variable geometry and P‐ and S‐wave velocity contrastsEstimated bimaterial properties have important implications for earthquake rupture properties along the San Jacinto and other faults [ABSTRACT FROM AUTHOR]

Repeating earthquakes, or repeaters, which repeatedly rupture the same location and release the strain energy, are interpreted as repeated ruptures of an isolated asperity patch surrounded by a stable sliding regime. While repeaters are frequently observed along plate‐boundary faults, the occurrence of repeater sequences are reported in various tectonic settings. Here we systematically investigate whether repeaters occur in the continental crust and subducting slabs beneath the Japanese Islands and show a prevalence of repeaters in both locations. The crustal and intraslab repeaters show the power‐law decay of seismicity rates that is identical to that along plate‐boundary faults and occur on well‐defined fault planes coincidentally with non‐repeating earthquakes. These observations suggest that repeaters share a common generation process independent of the tectonics regime and are not distinct from non‐repeaters in their source locations and occurrence times. We thus infer that repeaters and non‐repeaters are both generated by ruptures of locked asperities in response to stress loading by aseismic slip in the surrounding stable regime. Repeater sequences can be observed when aseismic slip is large enough to rupture an isolated asperity more than twice, while no repeaters are generated when the amount of aseismic slip is not sufficient to cause multiple ruptures at an isolated asperity. Since an earthquake cannot be triggered unless the amount of the aseismic stress loading exceeds the strength of asperities at the fault, hidden aseismic slip probably occurs repeatedly and frequently during an interseismic period. Plain Language Summary: The majority of earthquakes occur at three places in the Earth; along plate boundaries where two plates are adjacent, in downgoing plates (slabs), and in continental plates. However, whether the same generation mechanism works for the earthquakes in the different tectonic regimes is still debatable. Earthquakes with very similar waveforms have been recognized since ∼1900, and we recognize at present that many of such similar earthquakes are repeating earthquakes, or repeaters. The repeaters are caused by repeated ruptures of an isolated, frictionally locked asperity patch by stress loading due to aseismic slip in the surrounding stable regime. This study reveals that these repeaters occur prevalently in both the continental crust and slabs beneath the Japanese Islands. We find that there are no substantial differences in the hypocenter locations and occurrence time between repeaters and non‐repeaters. These observations suggest that all earthquakes share a common generation mechanism. We hypothesize that aseismic slip plays an essential role in stress loading to frictionally locked areas and an earthquake occur when the amount of aseismic slip exceeds the strength of faults. Our idea, although it needs further verification, will help better understanding of how frequent aseismic slip occurs and how earthquakes are triggered. Key Points: Repeating earthquakes are more common in the continental crust and subducting slabs than previously consideredRepeaters are not distinct from non‐repeaters in their source locations and occurrence timesRepeaters and non‐repeaters are triggered by ruptures of locked asperities by stress loading of aseismic slip [ABSTRACT FROM AUTHOR]

The strength and sliding behavior of faults in the crust is largely controlled by friction and effective stress, which is itself modulated by fluid pressure. Most earthquake models assume a fixed pore fluid pressure despite widespread evidence that is varies strongly in time due to changes in permeability. Here we explore how dynamic changes in pore pressure influence the properties of earthquakes in the upper crust. To study this problem we develop a two dimensional model that incorporates slow tectonic loading and fluid pressure generation during the interseismic period with frictional sliding on a thrust fault whose permeability evolves with slip. We find that the presence of relatively modest fluid overpressures tends to reduce coseismic slip, stress drop, maximum sliding velocity, rupture velocity and the earthquake recurrence time compared to models without fluids. Our model produces a wide range of sliding velocities from rapid to slow earthquakes, which occur due to the presence of high pore pressures prior to rupture. The models also show evidence for aftershocks that are driven by fluid transfer along the fault plane after the mainshock. Overall, this study shows that fluids can exert an important influence on earthquakes in the crust, which is mostly due to modulation of the effective stress and variations in permeability, and to a lesser extent to poro‐elastic coupling. Plain Language Summary: In this study we use a numerical model to investigate how fluid pressures vary over the seismic cycle and how they interact with and influence the properties of earthquakes that occur in the upper crust. In the model, fluid overpressures are generated slowly during the interseismic period by phenomena such as dehydration reactions while they are episodically released during earthquakes due to fracturing and a dramatic increase in permeability. The models show that the presence of high fluid pressures has an important influence on earthquakes. High fluid pressures favor smaller, more frequent earthquakes. Also high fluid pressures may sometimes be responsible for aftershocks and for anomalously slow earthquakes that involve slip over several months rather than several seconds. Overall, we show that the presence of fluids in the crust plays an integral part in the earthquake process. Key Points: Properties of earthquakes are markedly influenced by the presence of fluid overpressuresRuptures in wet models have relatively low stress drops, sliding speed and rupture velocitySlow slip events and aftershocks might be a fingerprint of fluid overpressures [ABSTRACT FROM AUTHOR]

A variety of mechanisms have been proposed to explain how a fault might deform relatively slowly, rather than manifesting as earthquakes. Several rely on the frictional properties of the fault interface. In this study I analyze the slip stability of a fluid‐infiltrated fault in the framework of a microphysically‐based friction model, which has shear zone porosity as its state variable. Linear stability analysis of the model, incorporating the evolution of fluid pressure, gives the critical stiffness (kc ${k}_{c}$) and frequency (wc ${w}_{c}$) as a function of normalized dilatancy (E/Ec $E/{E}_{c}$) and diffusivity (U $U$) factors. The theoretical results are similar to those derived from the previous friction laws, except that a positive kc ${k}_{c}$ always exists, even for a highly‐dilatant, impermeable fault where the value is proportional to the diffusivity. This implies that dilatant faults in impermeable media are potentially seismogenic, while the previous models predict negative kc ${k}_{c}$, implying a stable fault regardless of loading stiffness (k). Adopting a spring‐slider fault analogue, the analytical results are verified numerically under a wide span of spring stiffness, dilatancy, and diffusivity factors. The numerical results further reveal that four different modes of periodic slow slips can emerge in the model, without invoking inertia, which is distinct from previous friction models. Translating kc ${k}_{c}$ into the scenario of an elastically‐deformable medium, the critical wavelength on a dilatant fault can be as long as tens of kilometers. Gaining insights from all these features, I posit that the microphysical model is inherently favorable for generating slow slips. Plain Language Summary: As stress accumulates, a frictional interface can move either suddenly or slowly. The same phenomenon also occurs in natural fault zones. Until very recently, the slow motion of the fault interface (so‐called "slow slips") was observed in the subduction zones such as those in Japan. For the purpose of modeling these slow fault motion processes, a useful result would be a physically‐based constitutive relation that well characterizes the relevant observations and underlying mechanisms. This study analyzes the motion behavior of a fluid‐infiltrated fault zone in the framework of a microphysically‐based friction model, that is, slow and continuous sliding versus episodic acceleration of slip, like earthquakes. Interestingly, four mechanisms that have been previously proposed and widely used in the simulations to damp an accelerated fault emerge naturally from the analyzed model. The theoretical analysis further tells that the microphysical model is inherently favorable for the generation of slow fault slips. Key Points: Various damping regimes emerge as slip accelerates in a fluid‐infiltrated faultEven highly dilatant and low‐permeability faults are potentially unstableFour distinct types of slow slip events are predicted by the microphysical friction model [ABSTRACT FROM AUTHOR]

Subduction megathrusts exhibit a spectrum of slip modes, including catastrophic earthquakes. Although the mechanical and frictional properties of materials sampled from subduction zones have been studied extensively, few datasets have been collected for compositions and at pressure and temperature conditions representative of those in situ. The Nankai subduction zone in southwest Japan is a well‐studied margin, and abundant data provide an opportunity to advance our understanding of fault and earthquake physics. Here, we use samples exhumed in the Shimanto and Sanbagawa Belts on Shikoku Island of southwest Japan that represent analogs for materials along the present‐day megathrust at depths of ∼5–>25 km, and we shear these at their peak in situ pressure‐temperature (P‐T) conditions. Effective normal stresses range from 28 to 192 MPa, and temperatures from 105°C to 470°C. We used pore fluid pressures of 45–240 MPa, corresponding to fluid overpressure ratios λ of 0.65 and 0.90. Slip velocities of 0.1–100 μm/s were used, in order to focus on the nucleation of instability and earthquakes. We found predominantly velocity‐strengthening (inherently stable) behavior under all conditions for λ = 0.65. For λ = 0.90, velocity‐weakening behavior was observed at 350°C, with velocity‐strengthening behavior at lower and higher temperatures. The rate/state frictional stability parameter (a‐b) increases with slip velocity at temperatures up to ∼200°C and remains constant or decreases with slip velocity at higher temperatures. Overall, our results demonstrate the potentially important roles of both temperature and slip velocity in controlling the distribution of stress and frictional rheology along subduction thrusts. Plain Language Summary: The largest earthquakes on Earth occur along fault zones at subduction plate boundaries. These earthquakes are controlled, in large part, by the frictional properties of the fault zone materials. However, those properties are not fully understood, and laboratory experiments are key to advancing our knowledge. One example of a subduction zone is the Nankai Trough in southwest Japan. This subduction zone has been the subject of several drilling expeditions, recovering samples from the subsurface, and providing materials used in laboratory experiments. However, the experiments to date focus on materials recovered from depths shallower than those where earthquakes nucleate, and have been performed at temperatures lower than those in earthquake source regions. In this study, we used samples known to come from depths where earthquakes nucleate, and we performed friction experiments at temperatures and pressures relevant for these depths. We find that the frictional properties are primarily dependent on temperature and on the velocity of sliding. Key Points: Friction experiments on samples exhumed in the Shimanto Belt and Sanbagawa Belt at sample‐specific in situ peak PT conditionsBehavior predominantly velocity‐strengthening with velocity‐weakening at 350°CFrictional slip stability shows temperature‐dependent trend with slip velocity [ABSTRACT FROM AUTHOR]

Anthropogenic fluid injections at depth induce seismicity which is generally organized as swarms, clustered in time and space, with moderate magnitudes. Earthquake swarms also occur in various geological contexts such as subduction zones, mountain ranges, volcanic, and geothermal areas. While some similarities between anthropogenic and natural swarms have already been observed, whether they are driven by the same mechanism, or by different factors, is still an open question. Fluid pressure diffusion or aseismic deformation processes are often proposed to explain observations of hypocenters migration during swarms, while recent models suggest that swarm seismicity is rather triggered by fluid‐induced aseismic fault slip. Here, using a global compilation of 22 natural and anthropogenic swarms, we observe that duration, migration velocity, and total moment scale similarly for all swarms. This supports a common driving process for both natural and induced swarms. The scaling relations are similar to those found for slow slip events. These observations highlight the prevalence of fluid‐induced aseismic slip as main driver of earthquakes migration during swarms. After quantifying aseismic slip released in the swarms, we propose an approach to estimate the seismic‐to‐total moment ratio, which we then compare to a theoretical estimation that depends on the migration velocity of the swarm and the effective stress drop. Our findings lead to a generic explanation of the process driving earthquake swarms that might open new possibilities to monitor seismic swarms. Plain Language Summary: Earthquake swarms are a particular type of seismic activity, during which a sequence of many earthquakes occurs without being initiated by a larger one. Swarms can be induced by anthropic hydraulic injections at depth, like during geothermal power exploitation and massive storage of diverse fluids (i.e., wastewater, CO2) in porous reservoir formations. Natural earthquake swarms are also observed in a large variety of geological contexts. Previous works showed that natural and injection‐induced swarms share some similarities, like the migration of seismicity. However, their underlying processes remain unclear. Here, we explain the observed similarities in both types of swarms by a model in which earthquakes are triggered by the propagation of an aseismic slip transient, which in turn is induced by pressurized fluid circulation. This model reconciles a suite of independent observations made over different length and time scales and provides a generic explanation of the driving process for the migration of earthquake swarms. Key Points: Scaling laws support that injection‐induced and natural earthquake swarms have the same driving mechanismAseismic slip is a key feature of earthquake swarms, although its contribution differs from one swarm to anotherWe introduce a simple model based on fluid‐induced aseismic slip propagation to relate observables to physical parameters [ABSTRACT FROM AUTHOR]

The constitutive behavior of faults is central to many interconnected aspects of earthquake science, from fault dynamics to induced seismicity, to seismic hazards characterization. Yet, a friction law applicable to the range of temperatures found in the brittle crust and upper mantle is still missing. In particular, rocks often exhibit a transition from steady‐state velocity‐strengthening at room temperature to velocity‐weakening in warmer conditions that is poorly understood. Here, we investigate the effect of competing healing mechanisms on the evolution of frictional resistance in a physical model of rate‐, state‐, and temperature‐dependent friction. The yield strength for fault slip depends on the real area of contact, which is modulated by the competition between the growth and erosion of interfacial micro‐asperities. Incorporating multiple healing mechanisms and rock‐forming minerals with different thermodynamic properties allows a transition of the velocity‐ and temperature‐dependence of friction at steady‐state with varying temperatures. We explain the mechanical data for granite, pyroxene, amphibole, shale, and natural fault gouges with activation energies and stress power exponent for weakening of 10–50 kJ/mol and 55–150, respectively, compatible with subcritical crack growth and inter‐granular flow in the active slip zone. Activation energies for the time‐dependent healing process in the range 90–130 kJ/mol in dry conditions and 20–65 kJ/mol in wet conditions indicate the prominence of viscoelastic collapse of microasperities in the absence of water and of pressure‐solution creep, crack healing, and cementation when assisted by pore fluids. Plain Language Summary: Predictions of seismicity on faults depend on our understanding of how rocks break under stress and heal over time. Although this is well documented in laboratory experiments, a physical description of these observations from first principle is still incomplete, preventing extrapolation outside the experimental conditions. Here, we describe the frictional resistance of rocks based on the size of the small asperities that form the real area of contact in a fault zone. While the erosion of asperities during sliding weakens the frictional resistance, any mechanism at the micro‐scale that flattens these contacts strengthens the fault. The temperature dependence of friction and the microstructure of fault gouge representative of different tectonic contexts indicate that rapid sliding occurs by the growth of micro‐cracks and by granular flow in ultrafine‐grained shear zones. Time‐dependent strength recovery occurs by different mechanisms depending on the ambient temperature and water content. In dry conditions, healing occurs by internal viscous deformation of the micro‐asperities. The presence of water enables more efficient healing mechanisms at low temperature, including dissolution and precipitation of minerals and filling of pre‐existing cracks. The different thermodynamic properties of these mechanisms make faults unstable in a specific range of temperature, allowing the initiation of earthquakes. Key Points: A rate‐, state‐, and temperature‐dependent friction law based on the real area of contact explains fault gouge mechanical dataCompeting healing mechanisms enable a transition between velocity‐strengthening and velocity‐weakening friction at steady‐stateTime‐dependent healing is accommodated by low‐temperature plasticity, for example, viscoelastic flow, pressure‐solution creep, or crack healing [ABSTRACT FROM AUTHOR]

Earthquake simulation and hazard prediction are strongly hampered by insufficient physical knowledge of the constitutive behavior of faults. Laboratory studies of carbonate fault friction suggest that seismogenic rupture on faults in carbonate terrains can be explained by a transition from high friction at low initial sliding velocities (V) to low friction at seismic slip velocities, that is, by rapid dynamic weakening. One proposed explanation for this weakening, invokes frictional heating resulting in deformation by grain boundary sliding accommodated by solid‐state diffusion (sometimes referred to as "viscous" or "superplastic" flow). We recently added this dynamic weakening mechanism to a microphysically based model addressing the (rate‐and‐state) frictional behavior of granular gouges undergoing low V shear characteristic of rupture nucleation and arrest. In the present study, we applied the full model to simulate seismic slip in laboratory carbonate faults. Assuming that slip localizes in a principal shear band within the fault (gouge) zone, and accounting for grain size evolution with velocity and temperature, the model reproduces the frictional, thermal and (micro‐)structural evolution observed during seismic slip experiments. In particular, it predicts spatial and temporal evolutions of grain size, porosity, and dominant deformation mechanisms, within and outside the assumed shear band, consistent with trends identified in the laboratory and natural fault zones. Plain Language Summary: Improving the basis for earthquake hazard assessment relies in part on structural observation of natural fault zones, on laboratory experiments, and on theoretical developments. While quantitative models that reproduce the laboratory data on frictional slip on faults is the minimum necessary to simulate their mechanical behavior in nature, convincing models must ultimately also account for all key fault zone observations. Taking a fault in carbonate rock as an example, and using fundamental data on the microscale processes that lead to fault friction, this study takes a first step toward simulating the dynamic evolution and self‐organization of internal fault zone (micro)structure, processes and properties during a seismic slip event. The results show that the model captures many key observations on seismic fault behavior, paving the way for still further improvements in developing a physics‐based understanding of earthquake rupture in future. [ABSTRACT FROM AUTHOR]

Earthquake surface deformation provides key constraints on the geometry, kinematics, and displacements of fault rupture. However, deriving these characteristics from past earthquakes is complicated by insufficient knowledge of the pre‐event landscape and its post‐event modification. The 1987 Mw 6.5 Edgecumbe earthquake in the northern Taupō volcanic zone (TVZ) in New Zealand represents a moderate‐magnitude earthquake with distributed surface rupture that occurred before widespread high‐resolution topographic data were available. We use historical aerial photos to build pre‐ and post‐earthquake digital surface models (DSMs) using structure‐from‐motion techniques. We measured discrete and distributed deformation from differenced DSMs and compared the effectiveness of the technique to traditional field‐ and lidar‐based studies. We identified most fault traces recognized by 1987 field mapping, mapped newly identified traces, and made dense remote slip measurements with a vertical separation resolution of ∼0.3 m. Our maximum and average vertical separation measurements on the Edgecumbe fault trace (2.5 ± 0.3 and 1.2 m, respectively), are similar to field‐based values of 2.4 and 1.1 m, respectively. Importantly, this technique can discern between new and pre‐existing fault scarps better than field techniques or post‐earthquake lidar‐based measurements alone. Our surface displacement results are used to refine subsurface fault geometries and slip distributions at depth, which are further used to investigate potential magmatic‐tectonic stress interactions in the northern TVZ. Our results suggest the Edgecumbe fault dips more gently at depth than at the surface, hosted shallow slip in 1987, and may be advanced toward failure by interactions with nearby magma bodies. Plain Language Summary: Understanding earthquake behavior relies heavily on information about how past earthquakes changed the landscape. Detailed information about the surface topography before the earthquake is often limited, creating challenges for accurately measuring earthquake surface deformation. Aerial photos are widely available and can be used to create 3D digital surface models in places where other topographic information is lacking. We use historical aerial photos to make 3D datasets of the surface topography before and after the 1987 Edgecumbe earthquake in New Zealand. We calculated the difference between the two 3D datasets to identify and measure how the earthquake changed the landscape, and compared our results to previous measurements. We found that this method generally works as well as field methods for identifying and measuring fault movement, and has some advantages over other techniques. In particular, this approach can separate deformation from individual earthquakes, which had previously been a challenge. The results refine our understanding of the faults below the surface, provide insight into the earthquake's relationship to surrounding faults and volcanic systems, and allow us to better characterize seismic hazard both here and in other similar geologic settings. Key Points: Historical aerial photos can be used to generate pre‐ and post‐earthquake digital surface models for measuring displacementThis method performs at least as well as field surveys, and better captures single‐event displacements to constrain earthquake behaviorThe Edgecumbe earthquake is important for understanding low dip‐angle normal fault ruptures and possibly magma‐tectonic interactions [ABSTRACT FROM AUTHOR]

Observations of recent earthquake surface ruptures show that ground deformations include a localized component occurring on faults, and an off‐fault component affecting the surrounding medium. This second component is also referred to as off‐fault deformation (OFD). The localized component generally occurs on complex networks of faults that connect at depth onto a unique fault plane, whereas OFD consists of distributed fracturing and diffuse deformation of the bulk volume, and occurs over scales of hundreds of meters to kilometers around the faults. High‐resolution optical image correlation presents a unique potential to characterize the complexity of the surface displacements, including on‐fault displacements and OFDs. In this study, we used sub‐pixel correlation of 0.5‐m resolution optical images to measure the surface displacement field with a <20 cm accuracy for a 30‐km long section of the 2013 Mw7.7 Baluchistan, Pakistan, rupture. Our results document significant variability in the fault displacements, associated with large proportions of OFD in regions of fault geometrical complexity. Conversely, in regions where the fault geometry is simple, surface deformation is entirely accommodated by the primary faults with 0% OFD. When combining the localized deformation on faults with the OFD, we show that the total surface displacement budget is constant along the strike of the rupture, despite strong variations observed in the rupture geometry. Based on this analysis, we propose an idealized scenario of earthquake surface deformation as a function of the rupture geometrical variations. Plain Language Summary: The 2013 Mw7.7 Baluchistan earthquake produced a 220 km‐long fault rupture in the Makran region in Pakistan. Because it is a military conflict zone, no field observations could be made, and the earthquake was studied exclusively using seismic, GNSS, and satellite data. In this work, we correlate high‐resolution (0.5 m) pre‐ and post‐earthquake optical images to measure the 3D ground displacement field for a 30 km‐long section of the rupture. This allows us to perform a detailed budget of surface deformation, including displacements on the different faults, and low‐amplitude deformation (10−2 to 10−3) of the medium up to 1 km from the faults. We show that the distant deformation occurs around the two major fault geometrical complexities present in our study area. In fact, we measure lower fault displacements across these complexities than across regions of simpler fault geometry. However, when considering both the displacements occurring on the faults and the distant deformation of the medium, we obtain a total surface displacement that is constant along the entire study area. This shows that, in this case, fault geometrical complexity did not prevent the surface deformation from occurring, but favored the distribution of the deformation over wider regions around the faults. Key Points: East‐west, north‐south, and vertical surface displacement maps are calculated at 0.5 m ground resolutionWe show the role of off‐fault deformation in accommodating surface displacements across km‐scale fault geometrical complexitiesTotal coseismic surface displacement is constant along the study area, despite strong variations in the rupture geometry [ABSTRACT FROM AUTHOR]

Following great subduction earthquakes, postseismic deformation of coastal areas shows consistent seaward motion but complex vertical deformation. Understanding both the horizontal and vertical components in the same geodynamic framework presents challenges. Here, by modeling short‐term (a few years) postseismic viscoelastic relaxation (VER) and afterslip following synthetic and real subduction earthquakes, we demonstrate that the complexity of the vertical deformation can be explained in simple terms. Along a margin‐normal profile, VER results in an up‐down‐up trisegment, long‐wavelength pattern common to most megathrust earthquakes, including near‐trench uplift, midway subsidence, and near‐arc uplift, with locations controlled by coseismic fault slip. The magnitude of the first two segments is controlled mainly by oceanic mantle viscosity, and the third by mantle wedge viscosity. In contrast with VER, afterslip results in an up‐down bimodal pattern of variable wavelengths specific to individual earthquakes. Its site‐specific and heterogeneous nature is primarily responsible for the complexity in vertical deformation, but its effect can be adequately modeled using a simple elastic model. If the coast is near the megathrust rupture zone, variable combinations of the VER and afterslip effects lead to either uplift or subsidence. If the coast is in the near‐arc segment of VER deformation, uplift usually occurs. Modeling the common VER process enables the identification of site‐specific afterslip, which helps to understand the mechanism of afterslip in the context of the broad spectrum of fault slip behavior. Our results also have important implications to deciphering coastal paleoseismic records to constrain coseismic versus postseismic deformation of ancient earthquakes. Plain Language Summary: How the Earth surface deforms after great subduction earthquakes yields important information on the physical properties of the mantle and the megathrust fault. A few years after these earthquakes, postseismic deformation of coastal areas shows consistent seaward motion but complex vertical deformation. Our work is focused on understanding this complexity. The postseismic deformation consists of two components. One is due to the gradual response of the mantle, called viscoelastic relaxation, and the other is due to the continuing slip of the megathrust around the rupture zone, called afterslip. We show that the two components have different spatiotemporal characteristics governed by different parameters of the subduction zone system. The combined effects of the two components at different distances from the earthquake rupture zone explain the observed complexity. Understanding the mechanisms of postseismic deformation helps us to understand mantle and fault properties and to properly interpret geological data that constrain deformation due to ancient earthquakes. Key Points: Viscoelastic relaxation (VER) after great subduction earthquakes generates a common up‐down‐up postseismic deformation pattern from trench to arcSite‐specific, heterogenous afterslip causes complexity of coastal vertical postseismic deformation, depending on coast‐rupture distanceSeparately modeling VER and afterslip helps to understand rheology, fault slip behavior, and paleoseismic records [ABSTRACT FROM AUTHOR]

The 2019 Ridgecrest earthquake sequence manifested as one of the most complex fault surface ruptures observed in California in modern times. The M6.4 foreshock and M7.1 mainshock occurred on an intricate network of orthogonal and sub‐parallel faults resulting in observable surface displacement and surface rupture captured by geodetic data. Here we present the application of a high‐resolution 3D finite element model (FEM) approach to invert for the detailed fault slip of the entire sequence using complex rheology and fused coseismic Global Navigation Satellite System (GNSS) data with Sentinel‐1 differential interferometric synthetic aperture radar and pixel offset data. The heterogeneous FEM and the fused geodetic data set of pixel offsets, interferograms, and GNSS data results in our optimal inversion solution. This preferred solution is a complex, high‐resolution non‐planar slip model of both the M6.4 and M7.1 events that features three main regions of large slip (6.9+ m), with depths ranging from 2 to 10 km. The regions of slip are bounded by the mainshock hypocenter and the mainshock aftershocks and appear to be related to spatially varying rheological properties. We successfully reproduce a localized region of observed subsidence in the northern portion of the primary fault through the inclusion of a curved fault strand with a significant dip‐slip component. The curved fault strand is the site of our maximum slip of 7.4 m at a depth of 4.2 km. The results demonstrate a robust fit from a more complete, detailed model for the entire seismogenic zone with reasonable computational cost, providing new insights into the governing rheologic and structural processes. Plain Language Summary: The 2019 Ridgecrest, California earthquake sequence consisted of two main earthquakes: the M6.4 foreshock and the M7.1 mainshock. Understanding the structure and behavior of the faults responsible for these earthquakes is important to earthquake hazard and forecasting. We construct a complex 3D model representing the earth's interior at the location of the Ridgecrest earthquake using triangular building blocks. By combining Global Positioning System and different microwave remote sensing data, we produce a detailed map of how the ground deformed, even close to the faults. We use this deformation map to fine tune our 3D model and determine in which direction and by how much the faults moved during the M6.4 and M7.1 earthquakes. We find that the faults had large movement of almost 7 m in three regions at depths of 2–10 km. The largest movement of 7.4 m occurs north of the mainshock at an offshoot of the main fault where our mapped deformation shows the ground sinking. The locations of large fault movement coincide with regions of low aftershock activity and particular earth material. Key Points: High resolution finite element model with complex rheology and fused synthetic aperture radar‐Global Navigation Satellite System data produces a more complete fault slip model at all depthsMagnitude and extent of maximum slip regions surrounding the hypocenter persist for both homogeneous and heterogeneous rheology typesMaximum slip is bounded by mainshock hypocenter, aftershocks, decreased Poisson's ratio bodies, and the seismogenic zone [ABSTRACT FROM AUTHOR]

Knowledge of the fault kinematics underlying the Papuan Fold Belt is important for better understanding the evolution of the orogen, but the active and long‐term tectonics of the region remain widely debated. The 2018 Mw 7.5 Papua New Guinea earthquake provides an unprecedented opportunity to probe the active fault structure deep in the Papuan Fold Belt. Here, we use Interferometric Synthetic Aperture Radar data from four ALOS‐2 acquisitions to study coseismic and postseismic ground deformation and invert for fault slip models. The results show that the oblique reverse earthquake reactivated a flat‐ramp structure and ruptured through most of the crust with the majority of coseismic slip confined between 5 and 25 km. Additionally, we found three separated postseismic slip zones with variable spatial complementarity between coseismic and postseismic slip, dip‐slip/strike‐slip ratio, and seismic/aseismic budget at three separated postseismic slip zones. Our results demonstrate that thick‐skinned tectonics dominate the current state of Papua New Guinea frontal orogen evolution. Plain Language Summary: On 25 February 2018, an Mw 7.5 earthquake struck the central Papua New Guinea Highland. We use Interferometric Synthetic Aperture Radar data (ALOS‐2) to study the surface displacements associated with the great earthquake, seismogenic fault geometry, and coseismic and postseismic slip. We find complex seismic and aseismic slip occurred on a flat‐ramp structure, which ruptured through most of the crust indicating thick‐skinned basement‐involved deformation during the earthquake. We also compare the uplift displacements and the topography and local folds, and highlight that thick‐skinned tectonics play a significant role in the current frontal orogen evolution of Papua New Guinea. Key Points: Coseismic and postseismic surface displacements of the 2018 Mw 7.5 Papua New Guinea earthquake are captured by InSARThe depth extent of coseismic and postseismic slip on a flat‐ramp fault suggests thick‐skinned deformationFault frictional heterogeneity is found based on variable seismic and aseismic slip behaviors of three separated postseismic slip zones [ABSTRACT FROM AUTHOR]

Fault surfaces are rough at all scales, and this significantly affects fault‐slip behavior. However, roughness is only occasionally considered experimentally and then often in experiments imposing a low‐slip velocity, corresponding to the initiation stage of the earthquake cycle. Here, the effect of roughness on earthquake nucleation up to runaway slip is investigated through a series of dry load‐stepping biaxial experiments performed on bare rock surfaces with a variety of roughnesses. These laboratory faults reached slip velocities of at least 100 mm/s. Acoustic emissions were located during deformation on bare rock surfaces in a biaxial apparatus during load‐stepping experiments for the first time. Smooth surfaces showed more frequent slip instabilities accompanied by slip bursts and larger stress drops than rough faults. Smooth surfaces reached higher slip velocities and were less inclined to display velocity‐strengthening behavior. The recorded and localized acoustic emissions were characterized by a greater proportion of large‐magnitude events, and therefore likely a higher Gutenberg‐Richter bGR‐value, for smoother samples, while the cumulative seismic moment was similar for all roughnesses. These experiments shed light on how local microscopic heterogeneity associated with surface topography can influence the macroscopic stability of frictional interfaces and the associated microseismicity. They further provide a laboratory demonstration of roughness' ability to induce stress barriers, which can halt rupture, a phenomenon previously shown numerically. Plain Language Summary: Earthquakes occur when slip develops on a fault. However, the geometry of the fault's surface can affect how this slip occurs and whether or not the fault will radiate large amounts of seismic energy. By performing experiments on bare rock samples with imposed surface geometries, the load experienced by the laboratory fault is steadily increased. The laboratory faults' slip velocities develop across seven orders of magnitude, providing insight across a large portion of the earthquake cycle. By analyzing the slip events that occur along the faults as well as the stiffnesses during the recovery phase of these slip events, it is demonstrated that rough surfaces result in the arrest of propagating slip fronts such that the laboratory‐scale fault does not completely slip. Specifically, increased microscopic surface heterogeneity causes stress barriers that are less prone to begin slipping, ultimately resulting in relative macroscopic stability of the fault. Key Points: Experimental‐fault roughness affects fault stabilityStress heterogeneity related to roughness inferred to halt seismic rupture on millimetric scaleAll stages of seismic cycle observed across seven‐orders of slip velocity magnitude [ABSTRACT FROM AUTHOR]

We investigate the relationship between the long‐term (Quaternary) interplate coupling and the short‐term geodetically derived interseismic coupling at the Central Ecuador subduction zone. At this nonaccretionary margin, the Cabo Pasado shelf promontory and coastal area are associated with two inter‐plate geodetically locked patches. The deepest patch ruptured co‐seismically during the Mw7.8‐2016 Pedernales earthquake, while the shallowest underwent dominantly after‐slip. Marine geophysical and chronostratigraphic data allow reconstructing the Quaternary tectonic evolution of the shelf promontory and substantiating variation of the long‐term inter‐plate coupling that led to the geodetically locked patches. Prior to ∼1.8 Ma, the outer‐wedge inter‐plate coupling was strong enough to activate trench‐subparallel strike‐slip faults. Then, between ∼1.8 and 0.79 Ma, shortening and uplift affected the shelf promontory, implying a locally increased inter‐plate coupling. After a short, post‐0.79 Ma period of subsidence, shortening and uplift resumed denoting a high inter‐plate coupling that endured up to the present. The synchronicity of the structural evolution of the shelf promontory with the subduction chronology of two reliefs of the Carnegie Ridge crest suggests that the locked patches are caused by a geometrical resistance to subduction that propagates landward causing permanent deformation. In 2016, the deepest subducted relief localized stress accumulation and high seismic slip, while the shallowest relief, which is associated with a weakened outer‐wedge, prevented updip rupture propagation. Thus, at nonaccretionary margins, active outer‐wedge strike‐slip faults might be considered a proxy of near‐trench coupling, and subducted relief a cause of plate coupling but an obstacle to the tsunami genesis when the relief is shallow. Plain Language Summary: The 2016‐Ecuador earthquake ruptured a subduction fault segment previously locked for decades beneath the coastline. The rupture was arrested updip by another locked fault segment called locked patch, which instead slipped slowly. To understand the cause of the locked patches, their rupture behaviors, and whether the decadal fault locking and long‐term subduction processes are related, we reconstructed the Quaternary tectonic evolution of the margin offshore Central Ecuador using geophysical data. We consider that tectonic deformation reflects the long‐term inter‐plate coupling, which is the ability of the fault to transfer long‐term stress and strain to the margin. Prior to ∼1.8 Ma, a trench‐subparallel fault accommodating lateral displacement indicates a shallow plate coupling, which increased locally between ∼1.8 and 0.79 Ma as shown by margin shortening. After a brief subsidence, shortening resumed, denoting a strong coupling that persisted until today in the form of the locked patches. Although many physical factors have been proposed to control plate coupling, here we find that the locked patches are caused by the subduction of two reliefs of a submarine ridge. Remarkably, in 2016, the deepest relief released high elastic strain, while the shallower relief, thrust under a weakened outer‐margin, damped updip rupture propagation, impeding a significant tsunami. Key Points: A trench‐parallel strike‐slip fault and its earthquake‐controlled fault scarps substantiate a pre‐1.8 Ma, outer‐wedge inter‐plate couplingFrom 1.8 Ma, a robust shelf uplift caused by subducted reliefs highlights a long‐term coupling that led to geodetically locked patchesThe shallowest subducted relief likely impeded the generation of a major tsunami during the Mw 7.8, 2016 event [ABSTRACT FROM AUTHOR]

Fluids injection for hydraulic stimulation and fracturing, typical in the development of enhanced geothermal systems (EGS) in granites, can reactivate deep faults and induce seismicity. Such faults typically contain chlorite coatings as an alteration product that may impact styles of deformation—aseismic through seismic. We performed low velocity shear experiments on simulated granite fault gouges under conditions typifying a geothermal reservoir at ∼4‐km depth with a confining pressure of 110 MPa, a temperature of 150°C, fluid pressures of 21–80 MPa, and chlorite contents of 0–100%, to investigate the influence of variation in effective stress and mineral composition on fault strength and stability. Our results show a transition from velocity‐strengthening to velocity‐weakening behavior in simulated granite gouge when the effective confining pressure was reduced from 89 to 30 MPa, characterized by a transition from fault compaction to dilation—as revealed by microstructural observations—with implications in enabling unstable failure. Conversely, increasing chlorite content stabilizes slip but reduces frictional strength. The microstructures of these mixed gouges exhibit shear localized on chlorite‐enriched planes and promoting fault sliding. These results suggest that earthquake ruptures occurring during fluid injection can be facilitated by effective stress variations and that both controlling fluid overpressures (effective stresses) and being aware of the presence of alteration minerals are both important controls in mitigating such injection‐induced seismic risks. Plain Language Summary: Enhanced geothermal systems (EGS) host an increasing number of induced earthquakes potentially linked to hydraulic stimulation. Rock cores recovered from geothermal reservoirs worldwide show an abundance of chlorite coatings on fault surfaces—present both natively and as a result of fluid circulation. To understand whether slip on deep fault will result in earthquakes, we measure the frictional properties of powdered granite fault rocks from the Pohang geothermal reservoir where an earthquake has occurred. We vary chlorite content and effective confining pressures and observe stable slip at higher effective confining pressures that transitions to unstable slip at lower effective confining pressures. Reducing the effective confining pressure destabilizes the fault behavior at in situ stress and temperature representative of the reservoir where an earthquake was observed. The addition of chlorite in the simulated gouge results in the opposite result, reducing the frictional strength but resulting in stable (aseismic) slip. Our results highlight the importance of effective stress variation and mineral composition in controlling fault frictional strength and stability, with their potential contributions to earthquake triggering. Key Points: Effective stress variation and chloritization exert competing controls on friction and stability of granite faultsLower effective stress accentuates velocity‐weakening behavior but retains high strengthConversely, chloritization reduces frictional strength but promotes stable failure [ABSTRACT FROM AUTHOR]

Earthquake rupture in strong, anhydrous lower continental crust requires high brittle failure stresses unless high pore fluid pressures are present. Several mechanisms proposed to generate high stresses at depth imply transient loading driven by a spectrum of stress changes, ranging from highly localized stress amplifications to crustal‐scale stress transfers. High transient stresses up to GPa magnitude are proposed by field and modeling studies, but the evidence for transient prerupture loading is often difficult to extract from the geological record due to overprinting by coseismic damage and slip. However, the local preservation of deformation microstructures indicative of crystal‐plastic and brittle deformation associated with the seismic cycle in the lower crust offers the opportunity to constrain the progression of deformation before, during and after rupture, including stress and temperature evolution. Here, detailed study of pyroxene microstructures characterizes the short‐term evolution of high‐stress deformation and temperature changes experienced before and during lower crustal earthquake rupture. Pyroxenes are sampled from pseudotachylyte‐bearing faults and damage zones of lower crustal earthquakes recorded in the exhumed granulite facies terrane of Lofoten, northern Norway. The progressive sequence of microstructures indicates localized high‐stress (at the GPa level) prerupture loading accommodated by low‐temperature plasticity, followed by coseismic pulverization‐style fragmentation and subsequent grain growth triggered by the short‐term heat pulse associated with frictional sliding. Thus, up to GPa‐level transient high stress (both differential and shear) leading to earthquake nucleation in the dry lower crust can occur in nature, and be preserved in the fault rock microstructure. Plain Language Summary: Earthquake initiation within strong, dry rock types in the Earth's lower continental crust requires high driving stresses if fluids are absent. Several mechanisms have been proposed to generate these unusually high stresses deep in the lower crust, many of which imply very short‐term stress increases. High short‐term stresses are proposed by field‐based studies and numerical modeling, but the geological record for these stress increases occurring in the build‐up to earthquakes is not always obvious because the effects of the subsequent earthquake normally will overprint the evidence. However, in some cases a complete record of stress change before, during and after an earthquake may survive. In this study, detailed observations of the deformation of pyroxene (a silicate mineral) characterizes the short‐term changes in stress and temperature experienced before and during earthquakes recorded in the lower crust. Pyroxene crystals close to faults exhibiting ancient earthquake‐generated frictional melts (pseudotachylytes) are investigated from an exhumed fault zone in Lofoten, northern Norway. The microstructures imply high stress increases prior to the earthquake, followed by widespread fragmentation and grain growth linked to the passage of earthquake slip. These results support very high stresses localized within the lower crust and show that crystal deformation can capture such changes. Key Points: Microstructures capable of recording transient stress changes prior to and during earthquake rupture in the lower crustLocalized differential stresses exceeding 1 GPa during prerupture loading accommodated in dislocation glide of pyroxenesCoseismic deformation represented by pulverization style fragmentation and thermally activated grain growth within orthopyroxene [ABSTRACT FROM AUTHOR]

Fault stability is inherently linked to the frictional and healing properties of fault rocks and associated fabrics. Their complex interaction controls how the stored elastic energy is dissipated, that is, through creep or seismic motion. In this work, we focus on the relevance of fault fabrics in controlling the reactivation and slip behavior of dolomite‐anhydrite analog faults. We designed a set of laboratory experiments where we first develop fault rocks characterized by different grain size reduction and localization at normal stresses of σN = 15, 35, 60, and 100 MPa and second, we reload and reactivate these fault rocks at the frictional stability transition, achieved at σN = 35 MPa by reducing the machine stiffness. If normal stress is lowered this way, reactivation occurs with relatively large stress drops and large peak‐slip velocities. Subsequent unstable behavior produces slow stick‐slip events with low stress drop and with either asymmetric or Gaussian slip velocity function depending on the inherited fault fabric. If normal stress is raised, deformation is accommodated within angular cataclasites promoting stable slip. The integration of microstructural data (showing brittle reworking of preexisting textures) with mechanical data (documenting restrengthening and dilation upon reactivation) suggests that frictional and chemically assisted healing, which is common in natural faults during the interseismic phase, can be a relevant process in developing large instabilities. We also conclude that fault rock heterogeneity (fault fabric) modulates the slip velocity function and thus the dynamics of repeating stick‐slip cycles. Plain Language Summary: Displacement is accommodated within the Earth's upper crust through motion along fault zones. These fault zones are composed of a wide variety of deformed rocks, which are characterized by different geometric (e.g., inner textures) and physico‐chemical properties. Such complexity is key in controlling how the elastic energy stored in the loading medium surrounding the fault will be dissipated in time, for example, through slow (aseismic creep) or fast slip (earthquakes). This study presents a series of laboratory experiments where: (a) a range of pressures are applied to form fault rocks characterized by different inner textures; and then (b) the fault rocks are reactivated at the same experimental conditions, chosen to favor unstable sliding as documented in previous experiments in the same material. The observed slip behavior of reactivated faults indicates that the fault's inner textures greatly influence how our experimental faults move. For instance, fault motion is observed to occur constantly in presence of heterogeneous slip zones, and rhythmically (a behavior known as stick‐slip) in homogenous, fine‐grained slip zones. Chemically assisted processes, favored by high pressures and the presence of fluids, promote rehealing of the fault, favoring reactivation with energetic slip events that can be as fast as earthquakes. Key Points: We explored the effect of inherited fault fabric on the mechanical behavior of simulated faults during shear experimentsFaults inner fabric and fault slip behavior are intimately relatedFrictional and chemical healing facilitates laboratory earthquakes [ABSTRACT FROM AUTHOR]

In 2016, a series of large normal faulting earthquakes in the Apennines filled the seismic gap between the 1997 Colfiorito and 2009 L'Aquila earthquakes. These earthquakes, known as the Amatrice‐Visso‐Norcia (AVN) sequence, spawned hundreds of thousand of aftershocks in the first year, extending about 60 km along strike. The Colfiorito and L'Aquila aftershocks showed a significant high fluid‐pressure component in their aftershock genesis, and here we show evidence that the AVN aftershocks also include a significant high pressure CO2 component. This CO2 is both deeply‐derived, and internally generated by thermal decomposition. Using a simple model of non‐linear diffusion with a source term, we compare model results with the observed cumulative number of aftershocks, and also spatial comparisons between the calculated fluid pressure and the hypocenters of about 35,000 well‐located events. The good comparisons between model and observations provide evidence that the AVN sequence includes a contribution (additional fluid source) from co‐seismic decarbonization. Our results suggest that internal fluid generation through devolitization or decarbonization is responsible for non‐Omori type aftershock behavior such as observed for the AVN sequence and for the 2011 Mw 9.1 Tohoku (Japan) and the 2014 Iquique (Chile) Mw 8.2 megathrust subduction zone earthquakes. Plain Language Summary: The 2016 Amatrice‐Visso‐Norcia (AVN) earthquake sequence (Italian Apennines) spawned hundreds of thousands of aftershocks in the first year. In this work, we show using a physical and numerical model that the aftershocks are generated by high pressure CO2 thermally decomposed from the carbonates (decarbonization), in addition to deeply‐derived CO2 already shown to be a major contributing factor in Apenninian earthquake sequences. Model results also suggest that aftershock sequences that differ from the standard Omori‐type decay rate (e.g., The 2016 AVN‐sequence, the 2011 Tohoku earthquake, Japan and the 2014 Iquique earthquake, Chile) indicate the internal generation of fluids from either dehydration or de‐carbonization. Key Points: Amatrice‐Visso‐Norcia earthquakes sequence modeled as a fluid‐pressure diffusion process with a source termModel shows that this sequence is driven by deeply‐derived and co‐seismically generated CO2 through thermal decompositionGood spatial and temporal agreement for all depth profiles between modeled and well‐located aftershocks of about 35,000 events [ABSTRACT FROM AUTHOR]

The theory of rate and state friction unifies field, laboratory, and theoretical analysis of the evolution of slip on natural faults. While the observational study of earthquakes and aseismic fault slip is hampered by its strong multi‐scale character in space and time, numerical simulations are well‐positioned to link the laboratory study of grain‐scale processes to the scale at which rock masses move. However, challenges remain in accurately representing the complex and permanently evolving sub‐surface fault networks that exist in nature. Additionally, the common representation of faults as interfaces may miss important physical aspects governing volumetric fault system behavior. In response, we propose a transient viscous rheology that produces shear bands that closely mimic the rate‐ and state‐dependent sliding behavior of equivalent fault interfaces. Critically, we show that the expected tendency of the continuum rheology for runaway localization and mesh dependence can be halted by including an artificial diffusion‐type regularization of anelastic strain rate in the softening law. We demonstrate analytically and numerically using a simplified fault transect that important aspects of the frictional behavior are not significantly affected by the introduced regularization. Any discrepancies with respect to the interfacial description of fault behavior are critically evaluated using one dimensional numerical velocity stepping and spring‐slider experiments. Since no new physical parameters are introduced, our model may be straightforwardly used to extend the existing modeling techniques. The model predicts the emergence of complex patterns of shear localization and delocalization that may inform the interpretation of complex damage distributions observed around faults in nature. Plain Language Summary: How, where, and when earthquakes nucleate is one of the great questions in science and society that, despite steady progress, has hardly been answered to any practical degree. Based on field observations, laboratory experiments, and theoretical work it is believed that a cocktail of escalating mechanical, chemical, and thermal grain‐scale processes cause the sudden and rapid onset of earthquakes. The net effect of these processes are characterized by an immediate strengthening and a gradual weakening response to deformation and are unified in simplified form in the theory of "rate and state friction." This theory is commonly used in computer simulations of earthquake sequences. We point out that rate and state friction, unlike some physical theories of earthquake rupture, does not incorporate a diffusion process such as for example heat conduction. We show the introduction of an artificial diffusion process can prevent the mathematical reduction of a fault zone to a two‐dimensional interface while retaining the properties of the original friction law. This in turn enables simulation techniques that rely on an interface‐free description of the earth and promise to provide new insights into the spontaneous organization of seismic and aseismic phenomena in developing fault zones. Key Points: We reformulate the empirical rate and state friction law as a bulk viscous flow law in terms of anelastic shear strain rateWe show how mesh independence is achieved by including a gradient‐like nonlocal anelastic shear strain rate equivalentWe show analytically and numerically that the proposed continuum model closely reproduces existing results of rate and state friction [ABSTRACT FROM AUTHOR]

Rupture size is a fundamental earthquake source parameter that is challenging to infer independently from far‐field seismological observations. Here, we develop a novel observational constraint on source size based on the decay rate of wavefield coherence across a seismic array. For a given earthquake, waveform coherence decays with increasing interstation distance or, more precisely, with increasing projection difference defined as the difference between the takeoff vectors associated to the two stations projected along the rupture direction. We find that coherence generally falls off with projection difference faster for earthquakes of larger magnitudes. The magnitude dependence of the coherence decay rate can be explained by a finite source effect: larger source sizes cause larger differences of phase delays between waves arriving from different parts of the rupture at different stations, hence a stronger spatial variability of the wavefield, resulting in a breakdown of waveform coherence. Assuming a 1‐D Haskell's source model, the rupture size can be estimated from the coherence decay rate. We apply this method to USArray data of earthquakes in three subduction zones, the Sea of Okhotsk, South America and Japan. The source sizes inferred from the coherence decay patterns are consistent with scaling relations intermediate between width‐saturated L‐models and quasi equi‐dimensional rupture models. Our observation captures a unique pattern of array waveform coherence and demonstrates the potential of utilizing waveform coherence to study earthquake source parameters. Plain Language Summary: In this paper, we observe that for a given earthquake, the similarity of the waveform (coherence) recorded by stations in a far‐field array decays with increasing interstation distance. The waveform coherence generally falls off with interstation distance faster for earthquakes of larger magnitudes and therefore, larger rupture sizes. We develop a novel method using array waveform coherence to estimate the earthquake rupture size, and apply this method to USArray data of earthquakes in three geological regions: the Sea of Okhotsk, South America and Japan. The source sizes inferred from the coherence decay patterns are consistent with previous studies using different methods. According to our observation, there is a potential to infer earthquake rupture independently from far‐field observations by waveform coherence. Key Points: Waveform coherence decays as a function of interstation distance faster for large earthquakes (Mw > 7)The waveform coherence pattern is governed by earthquake source sizeRupture sizes inferred from waveform coherence satisfy empirical scaling relations [ABSTRACT FROM AUTHOR]

The 2008 Mw 7.9 Wenchuan earthquake generated ∼270 and ∼80 km long surface ruptures along the Longmenshan fault belt, namely the Yingxiu‐Beichuan fault (YBF) and the Guanxian‐Anxian faults (GAF), respectively. So far, most of the frictional investigations were performed on the YBF gouge materials. Here, we present the results of rotary shear friction experiments performed on the GAF gouges recovered from the depth of ∼1.25 km of the Wenchuan Earthquake Fault Scientific Drilling project‐3 along the GAF. The fault gouges, mainly composed of quartz, illite, chlorite, and kaolinite, were sheared at slip velocities V ranging from 10−5 to 2 m/s and normal stresses from 8.5 to 10 MPa under both room humidity and wet conditions. At any imposed slip velocity, the wet gouges have an apparent friction coefficient lower than the room humidity one. In addition, enhanced velocity‐strengthening behavior at intermediate velocities (10−2 m/s < V ≤ 10−1 m/s) was recognized. We characterized the products using field‐emission scanning electron microscopy combined with synchrotron X‐ray diffraction analysis. These microanalytical investigations evidence the formation of size‐reduced particles (without mineral phase changes) and R‐ and Y‐shears in the principal slip zone (PSZ). Regardless of the ambient conditions, the width of PSZ was proportional to the input frictional work density (the product of shear stress times displacement). Our results support the hypothesis that the GAF preferentially ruptures through wet fault gouges; however, the enhanced velocity‐strengthening regime at intermediate velocities may act as a barrier to slip acceleration during fault rupture propagation. Plain Language Summary: During the 2008 Mw 7.9 Wenchuan earthquake, two surface ruptures were generated, namely the ∼270 km long Yingxiu‐Beichuan fault (YBF) and the ∼80 km long Guanxian‐Anxian faults (GAF), respectively. This earthquake triggers mechanisms that weaken fault‐zone rocks, allowing the rupture to propagate. While the frictional behavior of the YBF fault gouges and the possible mechanisms were investigated in detail, the GAF materials have not been studied yet. Here, we shear fault gouges of the GAF from low to high velocities and characterize the products. Results show that at any imposed slip velocity, the wet gouges are weaker than the room humidity ones. In addition, GAF fault gouges show high frictional resistance at intermediate velocities (10−2 m/s < V ≤ 10−1 m/s). The microanalytical investigations evidence the similarity between the experimentally formed principal slip zone and the reported natural observation of GAF fault gouges. Our results support the hypothesis that the GAF preferentially ruptures through wet fault gouges; however, the high frictional resistance at intermediate velocities may act as a barrier to slip acceleration during the 2008 Wenchuan Mw 7.9 earthquake. Key Points: Fault gouges of Wenchuan Earthquake Fault Scientific Drilling‐3 show three velocity friction regimes from low to high velocitiesAt any imposed slip velocity, the wet gouges have an apparent friction coefficient lower than the room humidity oneHigh frictional resistance at intermediate velocities may act as a barrier to slip acceleration during the 2008 Wenchuan Mw 7.9 earthquake [ABSTRACT FROM AUTHOR]

The 2015 M7.2 Sarez (Pamir) earthquake occurred at the north‐west margin of the Tibetan Plateau. We use Sentinel‐1 and ALOS‐2 Synthetic Aperture Radar and Global Navigation Satellite System data to investigate coseismic and postseismic deformation due to the Sarez earthquake. Kinematic inversions show that the earthquake ruptured a ∼80 km long, sub‐vertical fault producing the maximum surface offset of 3–4 m on the south‐west and central fault segments. In contrast, the largest postseismic displacements are observed at the north‐east end of the earthquake rupture, predominantly on the west (hanging wall) side of the fault with an average rate of 20–30 mm/yr in the satellite line of sight. We use the derived coseismic and postseismic slip models to investigate mechanisms of time‐dependent relaxation, stress transfer and possible triggering relationships between the Sarez earthquake and a sequence of strong M6+ events that occurred within ∼100 km of the 2015 earthquake. We find that the near‐field postseismic displacements are best explained by shallow afterslip driven by the coseismic stress changes. The data also allow some contribution from poroelastic rebound, but do not show a clear signature of viscoelastic relaxation in the lower crust and upper mantle during the observation period, suggesting a lower bound on the effective viscosity of ∼1019 Pa s. A pair of M6+ events that occurred within 100 km and several months of the 2015 mainshock have experienced near‐zero and in some cases negative static Coulomb stress changes, suggesting either delayed dynamic triggering, or no relation to the mainshock. Plain Language Summary: Large earthquakes are often followed by slow deformation that results from redistribution and relaxation of coseismic stress changes in the host rocks. The patterns and rates of postseismic deformation can be used to learn about the properties of rocks at depth. We analyzed surface deformation that occurred during and after a major (magnitude 7.2) earthquake that occurred in the Pamir orogen at the north‐west margin of Tibet. We used radar imagery from satellites of the European and Japanese space agencies, as well as data from the Global Navigational Satellite System to measure subtle (centimeter‐scale) displacements of the Earth's surface that occurred within 100 km from the earthquake epicenter. A combination of coseismic and postseismic displacement data reveals that the earthquake rupture split into two branches as it was propagating from south‐west to north‐east. We found evidence of slow aseismic creep ("afterslip") at the north‐east end of the earthquake rupture. In contrast, we do not observe a broad pattern of displacements expected from enhanced viscous flow in the lower crust and/or upper mantle. The lack of the respective signal argues for a relatively strong "ductile" portion of the Tibetan lithosphere. Key Points: Near‐field postseismic deformation is dominated by shallow afterslip and poroelastic relaxation at the NE end of the earthquake ruptureData do not show a clear signal expected of viscoelastic relaxation, indicating effective viscosity of the lower crust >1019 Pa sWe investigate triggering relationships between the M7.2 and subsequent M6+ events that occurred within 1 year and 100 km of the mainshock [ABSTRACT FROM AUTHOR]