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• First in-situ apatite U-Pb ages from pseudotachylytes. • The lower crust fails seismically during the early stages of continental collision. • Earthquakes create long-lived fluid pathways in the lower crust. Determining the precise age of fossil earthquakes is essential for understanding the tectonic setting in which they occurred. Pseudotachylytes are quenched frictional melts that occur along coseismic fractures and represent evidence of fossil earthquakes in the rock record. Here, we present the first in-situ apatite U-Pb ages for three samples from Lofoten in the northern Norwegian Caledonides, that occur within and adjacent to lower-crustal pseudotachylytes: a pristine pseudotachylyte, a mylonitized pseudotachylyte, and a mylonite that contains pseudotachylyte transposed into the shear plane. The apatite U-Pb ages are combined with cathodoluminescence and electron backscatter diffraction analysis to provide microstructural constraints on the interpretation of ages. Apatite shows evidence of either fragmentation and annealing, or dislocation creep in response to coseismic rupture. The results suggest that the short-lived thermal pulse generated by the earthquake is sufficient to reset the apatite U-Pb system. The age of 426 ± 19 Ma is interpreted to date lower-crustal seismicity during the early Caledonian orogeny. Later fluid-rock interaction along the same structures led to partial resetting of the apatite U-Pb systematics, indicating that seismic brittle failure of the lower continental crust occurs at the incipient stages of continental collision, and that the resulting structures are preferential pathways for later fluid flow in the otherwise impermeable anhydrous lower crust. [ABSTRACT FROM AUTHOR]

The January 8, 2022, Ms 6.9 Menyuan earthquake occurred near the junction between the western Leng Long Ling (LLL) and eastern Tuolai Shan (TLS) faults, along the Haiyuan fault system, further illuminating the geometry of that system. Based on unmanned aerial vehicle (UAV) high-resolution images and field investigation, we mapped the fault geometry and surface ruptures, and measured co-seismic offsets in detail. Our results indicate that the event ruptured both a NWW-striking, ∼ 22 km-long segment of the west LLLF and a ∼ EW-striking, ∼ 4 km-long eastern segment of the TLSF, with maximum left-lateral displacements of ∼ 3.4 and 0.95 m, respectively. Both ruptures isolate a 2.8 km-wide, left-stepping, step-over/pull-apart, rather than a releasing bend as previously inferred. However, the two faults may connect continuously at depth, as suggested by both the aftershock distribution and the interferometric synthetic aperture radar (InSAR) data inversion, following the deep Paleozoic Qilian Shan suture, which is marked at the surface by discontinuous, elongated outcrops of ultramaphic rocks. Much larger, longer-term offsets of up to ∼ 71 and 55 m along the TLS and LLL, respectively, may have accrued since the end of the last glacial maximum (LGM, ∼20 ka) and the beginning of the Holocene (∼10 ka), respectively. This may be taken to imply average slip-rates since such times of ∼ 5 ± 0.5 and 3.5 ± 0.4 mm/yr along the LLLF and TLSF, respectively, consistent with extant InSAR/GPS rates. Based on our new UAV-based field survey and recent trenching results (Shi et al., 2019), we infer that neither the LLL nor the Gulang faults ruptured during the 1927, M 8 Gulang earthquake. The co-seismic Coulomb stress increased east of the 2022 Menyuan rupture. This, and the fact that the east-central LLLF does not appear to have ruptured in the last several hundred years may enhance the risk of occurrence of a Mw 7.4 earthquake along it. This might also imply that a large (M 8?) earthquake might rupture that segment, together with the ∼200 km-long Tianzhu seismic gap along the Haiyuan fault farther east, during a pending cascading event. • The 2022 Menyuan earthquake produced a ∼26 km-long, left-lateral surface rupture. • It broke two left-stepping segments of the Leng Long Ling and Tuolai Shan faults. • The average and maximum surface offsets were ∼ 2.5 and 3.4 meters, respectively. • The earthquake rupture followed the Paleozoic Qilian Shan suture zone. • The event increased seismic hazard along the Tianzhu seismic gap. [ABSTRACT FROM AUTHOR]

The elastic strain energy release rate and seismic waves emitted during earthquakes are controlled by the on-fault temporal evolution of the shear stress during rupture propagation. High velocity friction experiments highlighted that shear stress on the fault surface evolves rapidly during seismic slip pulses. This temporal evolution of shear stress is controlled by both fault weakening at seismic slip initiation and re-strengthening rate towards the end of slip. While numerous studies focused on fault weakening, less attention was given to co-seismic re-strengthening processes. Here we performed 53 friction experiments (normal stress ≤30 MPa, slip-rate ≤6.5 m s−1) imposing constant slip acceleration and deceleration (7.8 m s−2), on cohesive Carrara marble (99% calcite) and micro-gabbro (silicate-built rock) under dry, vacuum and water pressurized conditions. Microstructural observations showed that micro-gabbro accommodated seismic slip by bulk melting of the sliding surfaces, whereas Carrara marble by coupled decarbonation and grain-size dependent crystal plastic processes. Under room humidity conditions and low imposed power density (i.e., product of normal stress per slip rate), re-strengthening rate during the deceleration stage was up to ∼ 17 times faster in marble than in micrograbbro. In the latter, the re-strengthening rate increased slightly with the power density. The presence of water enhanced further this trend. On the contrary, in marbles the re-strengthening rate decreased drastically with power density and in the presence of water. Our experimental observations highlighted the first order importance of the mineralogy and rheology of the slip zone materials and, to a second order, of the presence of water in controlling co-seismic re-strengthening of faults during seismic slip deceleration. • Effect of rock composition and water content on fault re-strengthening during earthquakes. • Re-strengthening rate is faster in carbonate-than silicate-bearing rocks at low power density. • In silicate rocks, re-strengthening rate increases with power density and in presence of water. • In carbonate rocks, re-strengthening rate decreases with power density and in presence of water. • Silicate rocks promote intense high frequency radiation during deceleration of the seismic slip. [ABSTRACT FROM AUTHOR]

Seaward dipping reflectors (SDRs) are a key feature within the continent to ocean transition zone of volcanic passive margins. Here we conduct an automated pre-stack depth-migration imaging analysis of commercial seismic data from the volcanic margins of South America. The method used an isotropic, ray-based approach of iterative velocity model building based on the travel time inversion of residual pre-stack depth migration move-out. We find two distinct seismic velocity patterns within the SDRs. While both types show a general increase in velocity with depth consistent with expected compaction and alteration/metamorphic trends, those SDRs that lie within faulted half grabens also have high velocity zones at their down-dip ends. The velocity anomalies are generally concordant with the reflectivity and so we attribute them to the presence of dolerite sills that were injected into the lava pile. The sills therefore result from late-stage melt delivery along the large landward-dipping faults that bound them. In contrast the more outboard SDRs show no velocity anomalies, are more uniform spatially and have unfaulted basal contacts. Our observations imply that the SDRs document a major change in rift architecture, with magmatism linked with early extension and faulting of the upper brittle crust transitioning into more organised, dike-fed eruptions similar to seafloor spreading. • Recognition of two types of SDR from reflectivity, magnetic and seismic velocity patterns. • Operation of both tectonic footwall rotation and flexural-magmatic loading during SDR formation. • Fault-controlled SDRs less voluminous and may be deeply buried by the magmatic-loading controlled ones. [ABSTRACT FROM AUTHOR]

We investigate the fine-scale geometry and structure of the San Andreas Fault near Parkfield, CA, and their role in the development of the 1966 and 2004 ∼M6 earthquakes. Long-term surface fault traces indicate that structural heterogeneities associated with secondary reverse and normal fault structures are present at both rupture tips, near Middle Mountain and Gold Hill. Detailed analysis of almost 50 years of high-resolution seismicity reveals a fault plane that has been twisted into a helicoid between Middle Mountain and Gold Hill. Numerical models support our conclusion that this shape is the result of long-term torqueing of a strong stuck patch surrounded by a weak creeping region. The changes in fault friction behavior and related geometric discontinuities act as barriers to rupture propagation of moderate size earthquakes at Parkfield, and as areas of concentrations where rupture initiates. Our study demonstrates also that smooth strike-slip faults with large cumulative offset can form new fault segments at a late stage in their evolution. • The San Andreas fault plane near Parkfield has been deformed into a helicoid over the long-term. • The torque results from the strength contrast between locked and creeping zones. • Complex structures at surface correspond with the tips of the stuck patch at depth. • Stress concentrations at the tips of the stuck patch control rupture development of M6 events. [ABSTRACT FROM AUTHOR]

Reconstructions of long-term fault activity are essential for understanding both the mechanisms controlling fault behavior and accurate earthquake hazard assessments. Increasing evidence for temporal variations in strain accumulation suggests non-uniform strain rates over a range of historic to geologic timescales. The paucity of long-term records of fault activity has limited our ability to resolve these variations. We present a method for constraining long-term fault activity based on U–Pb dating of fault-related opal from secondary fault segments within the Eastern California Shear Zone (ECSZ). The presence of sheared and breccia-cemented opaline silica within well-exposed faults at near-surface conditions suggest that opal formation is associated with high-magnitude earthquakes capable of surface rupture (>6 M). Temporal constraints from massive and sheared syntectonic opal (n = 74) on related secondary faults from this study provide new insights on the timing of fault initiation, reactivation, and longevity. The oldest dates obtained indicate that ECSZ activity commenced at or before 10 Ma. Multiple deformation events dated within a single structure on episodically deposited and sheared opal (up to six generations), demonstrate that fault reactivation occurred over 105 yr timescales (0.7–0.1 Myr). Relative probabilities of dated deformation events can be used to evaluate changes in fault activity in the past 2.5 Ma (n = 60). This analysis indicates enhanced fault activity starting at 2 Ma and peaking around 1 Ma, possibly due to fault-interactions and distribution of deformation between the ECSZ and the San Andreas Fault. • Time constraint on secondary fault activity within the Eastern California Shear Zone. • Dating fault activity with U–Pb in-situ geochronology of fault-related opal. • Shearing in the ECSZ commenced at or before 10 Ma. • Fault reactivation occurred over 105 yr timescales. • Enhanced fault activity in the ECSZ starts at 2 Ma and peaks around 1 Ma. [ABSTRACT FROM AUTHOR]

Some faults are considered strong because their strength is consistent with the Coulomb criterion under Byerlee's friction, 0.6 < μ < 0.85. In marked contrast, numerous studies have documented significant fault weakening induced by fluid-assisted reaction softening that generally takes place during the long-term evolution of the fault. Reaction softening promotes the replacement of strong minerals with phyllosilicates. Phyllosilicate development within foliated and interconnected fault networks has been documented at different crustal depths, in different tectonic regimes and from a great variety of rock types, nominating fluid-assisted reaction softening as a general weakening mechanism within the seismogenic crust. This weakening originates at the grain-scale and is transmitted to the entire fault zone via the interconnectivity of the phyllosilicate-rich zones resulting in a friction as low as 0.1 < μ < 0.3. Collectively, geological data and results from laboratory experiments provide strong supporting evidence for structural and frictional heterogeneities within crustal faults. In these structures, creep along weak and rate-strengthening fault patches can promote earthquake nucleation within adjacent strong and locked, rate-weakening portions. Some new frontiers on this research topic regard: 1) when and how a seismic rupture nucleating within a strong patch might propagate within a weak velocity strengthening fault portion, and 2) if creep and slow slip can be accurately detected within the earthquake preparatory phase and therefore represent a reliable earthquake precursor. • Fault friction drops from 0.6 to 0.2 for interconnected networks of phyllosilicates. • Fluid-assisted reaction softening is a general weakening mechanism. • Structural and frictional heterogeneities of crustal faults are widespread. [ABSTRACT FROM AUTHOR]

Faults weaken during the propagation of earthquakes due to the onset of thermally-activated mechanisms, which vary depending on the rock type. Recent experimental work suggests that carbonate-hosted faults are lubricated by viscous flow in nano-granular aggregates having ultramylonitic textures. However, their frail nature has often hindered unbiased characterisation of the textures and deformation mechanisms operating at such extreme conditions (strain rates as high as 104), which remain so far poorly investigated and understood. We explore the formation, evolution and deformation mechanisms of coseismic ultramylonites in carbonate-hosted faults generated during high velocity (1.4 m s−1), displacement-controlled shear experiments in a rotary apparatus. Microstructures were analysed using integrated SEM and TEM imaging while detailed crystallographic fabrics were investigated using the electron back-scattered diffraction (EBSD) technique. Mechanical data show that the strength of the experimental fault decays dynamically with slip, according to a characteristic four stage evolution; each stage is associated with characteristic textures. Microstructural observations show that brittle processes dominate when the fault is strong (friction coefficients >0.6). Cataclasis, aided by twinning and crystal plasticity, operates forming an extremely comminuted shear band (mean grain size ∼200 nm). As the fault starts weakening, shear localises within a well-defined principal slip zone. Here, thermally-activated grain size sensitive (GSS) and insensitive (GSI) creep mechanisms compete with brittle processes in controlling fault strength. GSI mechanisms produce strong monoclinic crystallographic preferred orientations in the slip zone, while textures and crystallographic orientations in adjacent locations do not evolve from the previous deformation stage. By the end of the transient weakening stage, the slip zone has reached a steady state thickness (30 μm) and shows a nanogranular ultramylonitic texture. The intensity of the crystallographic preferred orientation in the coseismic ultramylonite is reduced compared to the previous stage, due to grainsize sensitive creep mechanisms becoming gradually more dominant. As the experimental fault re-strengthens, upon deceleration to arrest, the ultramylonite may be partially reworked by brittle deformation. Our findings show that the crystallographic orientations of transient microstructures are preserved in the slip zone of coseismic ultramylonites and in narrow, adjacent deactivated layers, where mirror-like surfaces are located. This shows that EBSD techniques can usefully be employed to determine the deformation mechanisms of coseismic ultramylonites and their evolution during earthquake slip in both experimental and, potentially, natural faults. • Ultramylonites form in calcite gouges sheared at seismic velocities. • Coseismic ultramylonites lubricate the fault during earthquake propagation. • Coseismic creep mechanisms might be similar to those of low strain rate mylonites. • Coseismic ultramylonites (ultra-high strain rates) must operate at the nanoscale. • EBSD analysis could be used to assess the seismic nature of fault microstructures. [ABSTRACT FROM AUTHOR]

Large earthquakes initiate at small nucleation sites and propagate as rupture fronts along the host fault. It is inherently challenging to resolve the complexity of fault rupture at depth, and therefore, the evolution of frictional strength during earthquakes is commonly analyzed in laboratory experiments. We experimentally demonstrate here that this evolution depends simultaneously on all slip kinematic components: displacement, velocity and acceleration. We incorporate these components in shear experiments with slip-histories that resemble the theoretical expectations for earthquake slip. These experiments led to a new friction law that fits fault behavior during high-velocity/long-displacement slip. Our numerical simulations of dynamic rupture along a planar fault that obeys this friction law reproduced a range of earthquake source features including slip-pulse, Yoffe function, Gaussian velocity, and spontaneous slip arrest. Finally, we demonstrate that this experimentally-based friction law can realistically simulate the propagation and arrest of natural earthquakes. • A new friction law based on high-velocity/long-displacement experiments. • Integration of earthquake kinematics (acceleration, velocity and displacement). • Successful implementation of the friction law in earthquake dynamic simulations. • The simulations spontaneously reproduce self-healing pulses and arrest. [ABSTRACT FROM AUTHOR]

Abstract We perform a comparative spatial analysis of inter-seismic earthquake production of rupture area and volume in southern California using observed seismicity and basic scaling relations from earthquake phenomenology and fracture mechanics. The analysis employs background events from a declustered catalog in the magnitude range 2 ≤ M < 4 to get temporally stable results representing activity during a typical inter-seismic period on all faults. Regions of high relative inter-seismic damage production include the San Jacinto fault, South Central Transverse Ranges especially near major fault junctions (Cajon Pass and San Gorgonio Pass), Eastern CA Shear Zone (ECSZ) and the Imperial Valley – Brawley seismic zone area. These regions are correlated with low velocity zones in detailed tomography studies. A quasi-linear zone with ongoing damage production extends between the Imperial fault and ECSZ and may indicate a possible future location of the main plate boundary in the area. The regions around the 1992 M6.1 Joshua Tree, M7.3 Landers and M6.3 Big Bear earthquakes have background seismic activity before 1990. This may represent a regional weakening process by damage production in future rupture zones. The depth of background seismicity and damage production decreases steadily from SW of the coastline to NE of the San Andreas fault, and also to the SE near the US–Mexico border. The seismicity and rock damage become more pronounced and continuous along-strike of main faults with increasing depth. Highlights • We analyze inter-seismic earthquake production of rock damage in southern California. • Regions with ongoing activity beyond fluctuations associated with large events are highlighted. • The results can help separating rock damage and composition in seismic imaging studies. • Regions around several M > 6 earthquakes have background damage production before their occurrence. [ABSTRACT FROM AUTHOR]

Abstract The dynamics of fault ruptures in natural and laboratory earthquakes is governed by a balance of released elastic energy and dissipated local fracture energy. The latter is the result of various friction weakening processes occurring at the fault and is thus often estimated indirectly and from small-scale friction experiments. We analyze high-frequency strain measurements from large-scale laboratory earthquakes with gages positioned slightly away from a granite fault. The strain measurements present rapid fluctuations during fault rupture propagation, as was also observed in other experiments. Characteristics of these strain fluctuations are compared with fracture mechanics theory to estimate local fault properties. We determine fracture energy for secondary rupture fronts, which appear behind the main front where local slip occurred already. Measured fracture energy is consistent with indirect estimates from rupture termination in independent experiments but is orders of magnitude lower than reported values from rotary shear friction experiments, which may be due to large differences in overall slip. Highlights • Secondary rupture fronts propagate in laboratory earthquakes while fault is sliding. • Fracture mechanics analysis of dynamic strain measurements constrain fault properties. • Estimated local fracture energy is consistent with rupture dynamics. • Off-fault stress measurement is remarkably dissimilar to on-fault stress evolution. [ABSTRACT FROM AUTHOR]

Abstract The 2013 Craig, Alaska M W 7.5 earthquake ruptured along ∼150 km of the Queen Charlotte Fault (QCF), a right-lateral strike-slip plate boundary fault separating the Pacific and North American plates. Regional shear wave analyses suggest that the Craig earthquake rupture propagated in the northward direction faster than the S-wave (supershear). Theoretical studies suggest that a bimaterial interface, such as that along the QCF, which separates oceanic and continental crust with differing elastic properties, can promote supershear rupture propagation. We deployed short-period ocean-bottom seismometers (OBS) as a part of a rapid-response effort less than four months after the Craig earthquake mainshock. During a 21-day period, 1,133 aftershocks were recorded by 8 OBS instruments. Aftershock spatial distribution indicates that the base of the seismogenic zone along the QCF approaches ∼25 km depth, consistent with a thermally-controlled fault rheology expected for igneous rocks at oceanic transform faults. The spatial distribution also provides supporting evidence for a previously hypothesized active strand of the QCF system within the Pacific Plate. Tomographic traveltime inversion for velocity structure indicates a low-velocity (V P and V S) zone on the Pacific side of the plate boundary at 5–20 km depths, where Neogene Pacific crust and upper mantle seismic velocities average ∼3–11% slower than the North American side, where the Paleozoic North American crust is seismically faster. Our results suggest that elastic properties along the studied portion of the QCF are different than those of a simple oceanic–continental plate boundary fault. In our study region, velocity structure across the QCF, while bimaterial, does not support faster material on the west side of the fault, which has been proposed as one possible explanation for northward supershear propagation during the Craig earthquake. Instead, we image low-velocity material on the west side of the fault. Explanations could include that part of the rupture was subshear, or that fault damage zone properties or fault smoothness are more important controls on supershear rupture than a bimaterial contrast. Highlights • Pacific velocities are slower than North American velocities at seismogenic depths. • Base of seismogenic zone approaches 25 km, consistent with oceanic deformation. • Pacific crust may be weaker than North American crust in the Craig earthquake area. • The bimaterial contrast was not likely a control on Craig supershear directionality. [ABSTRACT FROM AUTHOR]

Abstract Dyke intrusion is a highly dynamic process with seismicity preceding and accompanying magma emplacement on timescales of hours to days. Recent surveys of microseismicity indicate shear failure along fault planes parallel to the orientation of intruding dykes. However, the precision of earthquake hypocentre locations is typically limited to tens or hundreds of meters and cannot resolve whether the hypocentres relate to strain of wall rock fragments within the dykes, fault damage around the intrusions or peripherally in the country rock. Here we present high-resolution three-dimensional (3D) reconstructions of outstanding coastal exposures of a swarm of 19 dolerite dykes, near Albany, Western Australia using an unmanned aerial vehicle and Structure-from-Motion photogrammetry. It is observed that the number of faults and joints increases towards the dyke swarm, which, alongside mutually overprinting relationships, indicate that dyke emplacement and faulting were coeval. The faults contain cataclasites and are parallel to the dykes. In contrast, Mohr–Coulomb theory predicts shear failure on strike-parallel faults inclined ∼30° to the dyke plane. The faults and joints form a damage zone associated with the dyke swarm, even though the dykes themselves occupy Mode I extension fractures. These results confirm the recent geophysical evidence for dyke-parallel shear failure that can occur in the host rocks around intruding dykes. We suggest that coeval dyke-parallel seismicity both reactivated existing fracture networks and nucleated new fractures. Based on the premise that host-rock fracturing induces changes in elastic properties, remote stresses can reorientate locally leading to shear failure. This model provides for the first time an explanation for the origin of double-couple failure that is parallel in 3D (strike and dip) to dykes during their emplacement. Highlights • Exceptional exposures show dyke-parallel shear coeval with dyke emplacement. • Outcrop data are compatible the moment tensor solutions from Icelandic seismicity. • Dykes lie within a corridor of fractured host rock about twice the background intensity. • Dyke propagation in a non-coaxial strain field triggers shear on dyke-parallel planes. [ABSTRACT FROM AUTHOR]

Abstract Subduction systems globally terminate, allowing plate-motion to be transferred from the oceanic megathrust onto continental and/or oceanic transform faults. The mechanism of this kinematic transition over earthquake timescales is, however, poorly understood due to a lack of relevant data. Here, we study the 2016 M w 7.8 Kaikōura Earthquake in New Zealand, the first large instrumentally-recorded earthquake across a subduction-termination, to investigate this transfer mechanism in detail. We find that the Kaikōura Earthquake, unlike standard subduction earthquakes globally, involved a predominance (∼80%) of coseismic-slip on upper-plate faults and minor triggered-slip on the underlying oceanic subduction-thrust. In the months following the earthquake, the subduction-thrust accommodated most of the earthquake's afterslip down-dip of its co-seismic rupture zone. This top-down strain-release mechanism is in accord with local geological, geodetic and historical seismicity data which suggest that the bulk of plate-convergence (>75%) is accommodated in the upper-plate. We suggest, therefore, that this alternative strain-release mechanism, which is distinct from standard plate-boundary earthquakes, is characteristic of subduction-terminations and results in the majority of seismic/tsunami hazard being on steep near-surface faults. Highlights • The first large instrumental earthquake to have ruptured a subduction termination. • Primary slip on upper-plate faults triggers minor slip on the subduction-thrust. • Afterslip on the plate-interface released down-dip of the co-seismic asperities. • Plate-interface and offshore splay-fault key elements in transferring coseismic slip. • A top-down strain-release mechanism may typify subduction-terminations globally. [ABSTRACT FROM AUTHOR]

Seafloor spreading at slow and ultraslow rates is often taken up by extension on large-offset faults called detachments, which exhume lower crustal and mantle rocks, and in some cases make up domed oceanic core complexes. The exposed footwall may reveal a characteristic pattern of spreading-parallel corrugations, the largest of which are clearly visible in multibeam bathymetric data, and whose nature and origin have been the subject of controversy. In order to tackle this debate, we use available near-bottom bathymetric surveys recently acquired with autonomous deep-sea vehicles over five corrugated detachments along the Mid-Atlantic Ridge. With a spatial resolution of 2 m, these data allow us to compare the geometry of corrugations on oceanic detachments that are characterized by differing fault zone lithologies, and accommodate varying amounts of slip. The fault surfaces host corrugations with wavelengths of 10–250 m, while individual corrugations are finite in length, typically 100–500 m. Power spectra of profiles calculated across the corrugated fault surfaces reveal a common level of roughness, and indicate that the fault surfaces are not fractal. Since systematic variation in roughness with fault offset is not evident, we propose that portions of the exposed footwalls analyzed here record constant brittle strain. We assess three competing hypotheses for corrugation formation and find that the continuous casting and varying depth to brittle–ductile transition models cannot explain the observed corrugation geometry nor available geological observations. We suggest a model involving brittle strain localization on a network of linked fractures within a zone of finite thickness is a better explanation for the observations. This model explains corrugations on oceanic detachment faults exposed at the seafloor and on normal faults in the continents, and is consistent with recently imaged corrugations on a subduction zone megathrust. Hence fracture linkage and coalescence may give rise to corrugated fault zones, regardless of earlier deformation history and tectonic setting. [ABSTRACT FROM AUTHOR]

The November 2017 M w 7.3 Darbandikhan earthquake in the northern Zagros Mountains of Iran/Iraq was the largest instrumentally recorded earthquake to date in this mountain belt. Despite extensive research in this region, the nature of active faults in the Arabian basement beneath the Zagros Mountains is largely unresolved. Additionally, there is active debate regarding the interaction of thin- and thick-skin shortening of the Zagros, and whether these processes are accommodated seismogenically. Here, we use interferometric synthetic aperture radar (InSAR) of co- and post-seismic surface displacements to document the source of the Darbandikhan earthquake and the first 120 days of afterslip following the earthquake. We apply a fault inversion approach that finds permissible geometries, locations, and slip depths as a means to image subsurface structure with remote sensing geodesy. We find that the Darbandikhan earthquake ruptured the shallowly dipping (14°–19°) Mountain Front Fault within the Arabian crystalline basement (∼12–22 km deep). We additionally find that afterslip propagated up-dip of the earthquake onto the nearly horizontal (dip = 1°–5°) basal decollement of the Zagros Mountains, indicating that the earthquake and its initial afterslip activated a ramp-flat structure. We also inverted co-seismic InSAR displacements of the January 11, 2018 Mandali earthquake sequence that occurred ∼80 km south of the Darbandikhan earthquake and find that these moderate magnitude events (M w 5.0–5.5) ruptured the lower Phanerozoic cover of the Zagros. Together, the Darbandikhan and Mandali sequences demonstrate that the Zagros Mountains actively shorten via both thin- and thick-skin styles, and that these modes of shortening are accommodated in part by earthquakes. Moreover, the Darbandikhan earthquake and afterslip highlight the presence of seismically active low-angle thrust faults beneath the Zagros that were not previously recognized, highlighting sources for potential future large earthquakes. We compare these results to the 2015 Gorkha, Nepal earthquake to argue that frictional characteristics of decollement govern spatial relationships between seismic and aseismic slip in ramp-flat geometries. [ABSTRACT FROM AUTHOR]

On 25 November 2016, a M w 6.6 earthquake ruptured the Muji fault in western Xinjiang, China. We investigate the earthquake rupture independently using geodetic observations from Interferometric Synthetic Aperture Radar (InSAR) and regional seismic recordings. To constrain the fault geometry and slip distribution, we test different combinations of fault dip and slip direction to reproduce InSAR observations. Both InSAR observations and optimal distributed slip model suggest buried rupture of two asperities separated by a gap of greater than 5 km. Additional seismic gaps exist at the end of both asperities that failed in the 2016 earthquake. To reveal the dynamic history of asperity failure, we inverted regional seismic waveforms for multiple centroid moment tensors and construct a moment rate function. The results show a small centroid time gap of 2.6 s between the two sub-events. Considering the >5 km gap between the two asperities and short time interval, we propose that the two asperities failed near-simultaneously, rather than in a cascading rupture propagation style. The second sub-event locates ∼39 km to the east of the epicenter and the centroid time is at 10.7 s. It leads to an estimate of average velocity of 3.7 km/s as an upper bound, consistent with upper crust shear wave velocity in this region. We interpret that the rupture front is propagating at sub-shear wave velocities, but that the second sub-event has a reduced or asymmetric rupture time, leading to the apparent near-simultaneous moment release of the two asperities. [ABSTRACT FROM AUTHOR]

Seismic rupture of the shallow plate-boundary can result in large tsunamis with tragic socio-economic consequences, as exemplified by the 2011 Tohoku-Oki earthquake. To better understand the processes involved in shallow earthquake rupture in seismic gaps (where megathrust earthquakes are expected), and investigate the tsunami hazard, it is important to assess whether the region experienced shallow earthquake rupture in the past. However, there are currently no established methods to elucidate whether a margin segment has repeatedly experienced shallow earthquake rupture, with the exception of mechanical studies on subducted fault-rocks. Here we combine new swath bathymetric data, unpublished seismic reflection images, and inter-seismic seismicity to evaluate if the pattern of permanent deformation in the marine forearc of the Northern Chile seismic gap allows inferences on past earthquake behavior. While the tectonic configuration of the middle and upper slope remains similar over hundreds of kilometers along the North Chilean margin, we document permanent extensional deformation of the lower slope localized to the region 20.8°S–22°S. Critical taper analyses, the comparison of permanent deformation to inter-seismic seismicity and plate-coupling models, as well as recent observations from other subduction-zones, including the area that ruptured during the 2011 Tohoku-Oki earthquake, suggest that the normal faults at the lower slope may have resulted from shallow, possibly near-trench breaking earthquake ruptures in the past. In the adjacent margin segments, the 1995 Antofagasta, 2007 Tocopilla, and 2014 Iquique earthquakes were limited to the middle and upper-slope and the terrestrial forearc, and so are upper-plate normal faults. Our findings suggest a seismo-tectonic segmentation of the North Chilean margin that seems to be stable over multiple earthquake cycles. If our interpretations are correct, they indicate a high tsunami hazard posed by the yet un-ruptured southern segment of the seismic gap. [ABSTRACT FROM AUTHOR]

Earthquake swarms represent a sudden increase in seismicity that may indicate a heterogeneous fault-zone, the involvement of crustal fluids and/or slow fault slip. Swarms sometimes precede major earthquake ruptures. An earthquake swarm occurred in October 2015 near San Ramon, California in an extensional right step-over region between the northern Calaveras Fault and the Concord–Mt. Diablo fault zone, which has hosted ten major swarms since 1970. The 2015 San Ramon swarm is examined here from 11 October through 18 November using template matching analysis. The relocated seismicity catalog contains ∼4000 events with magnitudes between − 0.2 < M d < 3.6 . The swarm illuminated three sub-parallel, southwest striking and northwest dipping fault segments of km-scale dimension and thickness of up to 200 m. The segments contain coexisting populations of different focal-mechanisms, suggesting a complex fault zone structure with several sets of en échelon fault orientations. The migration of events along the three planar structures indicates a complex fluid and faulting interaction processes. We searched for correlations between seismic activity and tidal stresses and found some suggestive features, but nothing that we can be confident is statistically significant. [ABSTRACT FROM AUTHOR]

For some megathrust earthquakes, the rupture extends to the solid Earth's surface, at the ocean floor. This unexpected behaviour holds strong implications for the tsunami potential of subduction zones and for the physical conditions governing earthquakes, but such ruptures occur in underwater areas which are hard to observe, even with current instrumentation and imaging techniques. Here, we evidence that aftershocks occurring ocean-ward from the trench are conditioned by near-surface rupture of the megathrust fault. Comparison to well constrained earthquake slip models further reveals that for each event the number of aftershocks is proportional to the amount of shallow slip, a link likely related to static stress transfer. Hence, the spatial distribution of these specific aftershock sequences could provide independent constrains on the coseismic shallow slip of future events. It also offers the prospect to be able to reassess the rupture of many large subduction earthquakes back to the beginning of the instrumental era. [ABSTRACT FROM AUTHOR]

The distribution of slip during an earthquake and how it propagates among faults in the subduction system play a major role in seismic and tsunami hazards, yet they are poorly understood because offshore observations are often lacking. Here we derive the slip distribution and rupture evolution during the 2016 M w 7.9 Kaikōura (New Zealand) earthquake that reconcile the surface rupture, space geodetic measurements, seismological and tsunami waveform records. We use twelve fault segments, with eleven in the crust and one on the megathrust interface, to model the geodetic data and match the major features of the complex surface ruptures. Our modeling result indicates that a large portion of the moment is distributed on the subduction interface, making a significant contribution to the far field surface deformation and teleseismic body waves. The inclusion of local strong motion and teleseismic waveform data in the joint inversion reveals a unilateral rupture towards northeast with a relatively low averaged rupture speed of ∼1.5 km/s. The first 30 s of the rupture took place on the crustal faults with oblique slip motion and jumped between fault segments that have large differences in strike and dip. The peak moment release occurred at ∼65 s, corresponding to simultaneous rupture of both plate interface and the overlying splay faults with rake angle changes progressively from thrust to strike-slip. The slip on the Papatea fault produced more than 2 m of offshore uplift, making a major contribution to the tsunami at the Kaikōura station, while the northeastern end of the rupture can explain the main features at the Wellington station. Our inversions and simulations illuminate complex up-dip rupture behavior that should be taken into consideration in both seismic and tsunami hazard assessment. The extreme complex rupture behavior also brings new challenges to the earthquake dynamic simulations and understanding the physics of earthquakes. [ABSTRACT FROM AUTHOR]

The M7.8 Kaikoura Earthquake that struck the northeastern South Island, New Zealand, on November 14, 2016 (local time), is one of the largest ever instrumentally recorded earthquakes in New Zealand. It occurred at the southern termination of the Hikurangi subduction margin, where the subducting Pacific Plate transitions into the dextral Alpine transform fault. The earthquake produced significant distributed uplift along the north-eastern part of the South Island, reaching a peak amplitude of ∼8 m, which was accompanied by large (≥10 m) horizontal coseismic displacements at the ground surface along discrete active faults. The seismic waveforms' expression of the main shock indicate a complex rupture process. Early automated centroid moment tensor solutions indicated a strong non-double-couple term, which supports a complex rupture involving multiple faults. The hypocentral distribution of aftershocks, which appears diffuse over a broad region, clusters spatially along lineaments with different orientations. A key question of global interest is to shed light on the mechanism with which such a complex rupture occurred, and whether the underlying plate-interface was involved in the rupture. The consequences for seismic hazard of such a distributed, shallow faulting is important to be assessed. We perform a broad seismological analysis, combining regional and teleseismic seismograms, GPS and InSAR, to determine the rupture process of the main shock and moment tensors of 118 aftershocks down to Mw 4.2. The joint interpretation of the main rupture and aftershock sequence allow reconstruction of the geometry, and suggests sequential activation and slip distribution on at least three major active fault domains. We find that the rupture nucleated as a weak strike-slip event along the Humps Fault, which progressively propagated northward onto a shallow reverse fault, where most of the seismic moment was released, before it triggered slip on a second set of strike-slip faults at the northern end of the rupture. The northern and southern strike-slip fault domains have the same orientation but are spatially separated by >15 km. In our model, the low angle splay thrust fault is located above the slab and connects the strike-slip faults kinematically. During the aftershock phase, the entire fault system remained active. [ABSTRACT FROM AUTHOR]

Precursory signals, manifestations of microscale damage that precedes dynamic faulting, are key to earthquake forecasting and risk mitigation. Detections of precursors have primarily relied on measurements performed using sensors installed at some distance away from the rupture area in both field and laboratory experiments. Direct observations of continuous microscale damage accumulated during fault nucleation and propagation are scarce. Using an X-ray transparent triaxial deformation apparatus, we show the first quantitative high resolution three-dimensional (3D) information about damage evolution of rocks undergoing brittle failure. The dynamic microtomography images documented a spectrum of damage characteristics and different fault growth patterns. The interplay between various deformation mechanisms can result in either a positive, negative, or constant net volume change. Consequently, changes in rock density and acoustic wave velocities before faulting are expected to vary in different tectonics settings, hence making failure forecasting intrinsically dependent on rock type at depth. [ABSTRACT FROM AUTHOR]

Principal slip zones often contain highly reflective surfaces referred to as fault mirrors, shown to consist of a nanogranular coating. There is currently no consensus on how the nanograins form, or why they survive weathering on a geological time-scale. To simplify the complex system of a natural fault zone, where slip and heat generation are inherently coupled, we investigated the effect of elevated temperatures on carbonate rock surfaces, as well as their resistance to water exposure. This allows us to isolate the role of the decarbonation process in the formation of nanograins. We used cleaved crystals of Iceland spar calcite, manually polished dolomite protolith, as well as natural dolomite fault mirror surfaces. The samples were heated to 200–800 °C in a ∼5 h heating cycle, followed by slow cooling (∼12 h) to room temperature. Subsequently, we imaged the samples using scanning electron microscopy and atomic force microscopy. Nanograin formation on all sample surfaces was pervasive at and above 600 °C. The Foiana fault mirror samples were initially coated with aligned naturally-formed nanograins, but display a non-directional nanogranular coating after heating. The nanograins that were formed by heating rapidly recrystallized to bladed hydroxides upon exposure to deionized water, whereas the nanograins on unheated fault mirror samples remained unchanged in water. This shows that the nanograins formed by heating alone are different from those formed in fault zones, and calls for a better characterization of nanograins and their formation mechanisms. Furthermore, we find a characteristic star-shaped crack pattern associated with reacted regions of the carbonate surfaces. The existence of this pattern implies that the mechanical stresses set up by the decarbonation reaction can be sufficiently large to drive fracturing in these systems. We propose that this mechanism may contribute to grain size reduction in fault zones. [ABSTRACT FROM AUTHOR]

We constructed and analyzed full 3-D ground surface displacement field associated with the 2016 Kumamoto (Japan) earthquake using satellite radar images from ALOS-2. Displacements reflect not only tectonic crustal deformation caused by main earthquake faults but also non-tectonic surface deformations. The largest deformations in the earthquake sequence were found in the Aso Valley, along the NW outer rim of the Mt. Aso caldera. Large, independent surface deformations occurred in three areas with diameters of 500 m–2 km in the Aso Valley, and each area was horizontally displaced by more than 2 m to the NNW. These areas are underlain by thick lake-bottom deposits of saturated silt with low penetration resistance. As the direction of the displacements was parallel to the ground slope in each area, the strong seismic motion of the earthquake most probably induced liquefaction in the lake-bottom deposits and the ground surface slid horizontally along the slope on the liquefied silt. Because the areas of the large displacements are closely related to the thickness and shape of the lake-bottom deposits, amplification of the seismic wave in these deposits likely contributed to the liquefaction. A seismograph installed at the Aso Valley recorded co-seismic movement within several seconds of the main shock, indicating a possibility of the strong seismic motion amplifying the horizontal displacement of this area. On the SSE side of the deformed areas, numerous graben-like ruptures developed. As these ruptures are caused by surface tension during the large horizontal displacement, they are not earthquake fault traces and the process of this deformation is non-tectonic. [ABSTRACT FROM AUTHOR]

The rupture, localization, and slip of faults in serpentinite were studied under varying pore fluid pressure conditions to understand deformation mechanisms potentially responsible for slow slip in fault zones. Experiments were conducted at a constant effective confining pressure of 10 MPa and under pore fluid pressures from 0 to 120 MPa and at temperatures from 23 to 110 °C. With no fluid pressure, faulting occurs rapidly and audibly, and the duration of failure increases monotonically with increasing fluid pressure and temperature. Although non-dilatant during initial strain hardening, the serpentinite dilates during strain weakening concomitant with fault rupture and slip. Non-dilatant strain hardening occurs by microcracking along serpentine basal planes and grain boundaries and rarely in mode I orientations, consistent with previous studies. Dilatant fault rupture produces a network of transgranular shear fractures in conjugate orientations, generally with one dominant fracture. Structural observations show that as fluid pressure increases, the number of transgranular fractures increases. We propose that when faulting occurs over a distributed zone rather than a pre-existing principal slip surface, dilatant hardening causes deformation to migrate. This process causes an increase in slip weakening distance and fracture energy at elevated fluid pressures that can lead to more stable failure. Further, thermally-activated processes caused deformation at propagating crack tips, which also increases the slip weakening distance and the effective fracture energy with increasing temperature. Given the geologic settings for slow slip, our results indicate that high fluid pressure, distributed deformation, and thermally-activated processes may all contribute to slow fault rupture and slip. [ABSTRACT FROM AUTHOR]

In the stick-slip cycles of faults, the evolving contact state inside the gouge is a major contribution to the evolution of fault friction. However, the microscopic contact changes within stick-slip cycles associated with the possible existence of multiple fault slip regimes are still obscure. By simulating a sheared fault containing a granular gouge, we examine the particle-level contact evolution inside the gouge and the corresponding gouge stress field heterogeneity at fault slip. We find two regimes of fault slip driven by the heterogeneity of the gouge stress field: in the case of weak gouge stress field heterogeneity, local contact rearrangements give rise to macroscopic friction coefficient drop and accumulated gouge stress field heterogeneity (dispersive regime); in the instance of strong stress field heterogeneity, catastrophic failure occurs via contact rearrangement sweeping the whole fault and homogenizes the stress field distribution in the gouge (pervasive regime). In the language of self-organized criticality, the accumulation of gouge stress field heterogeneity is an inevitable consequence of increased entropy in the preparation stage of a large earthquake, which eventually destroys the long-range stress field correlations in the gouge and resets at the beginning of a new cycle of a large earthquake. • Particle-level contact evolution in the fault gouge is numerically examined in stick-slip cycles. • Two regimes of fault slip driven by the heterogeneity of gouge stress field are reported. • Gouge stress field heterogeneity degree correlates well with consecutive stick-slip cycles. [ABSTRACT FROM AUTHOR]

Pseudotachylyte, a fault rock formed through coseismic frictional melting, provides an important record of coseismic mechanics. In particular, injection veins formed at a high angle to the fault surface have been used to estimate rupture directivity, velocity, pulse length, stress drop, as well as slip weakening distance and wall rock stiffness. These studies have generally treated injection vein formation as a purely elastic process and have assumed that processes of melt generation, transport, and solidification have little influence on the final vein geometry. Using a pressurized crack model, an analytical approximation of injection vein formation based on dike intrusion, we find that the timescales of quenching and flow propagation may be similar for a subset of injection veins compiled from the Asbestos Mountain Fault, USA, Gole Larghe Fault Zone, Italy, and the Fort Foster Brittle Zone, USA under minimum melt temperature conditions. 34% of the veins are found to be flow limited, with a final geometry that may reflect cooling of the vein before it reaches an elastic equilibrium with the wall rock. Formation of these veins is a dynamic process whose behavior is not fully captured by the analytical approach. To assess the applicability of simplifying assumptions of the pressurized crack we employ a time-dependent finite-element model of injection vein formation that couples elastic deformation of the wall rock with the fluid dynamics and heat transfer of the frictional melt. This finite element model reveals that two basic assumptions of the pressurized crack model, self-similar growth and a uniform pressure gradient, are false. The pressurized crack model thus underestimates flow propagation time by 2–3 orders of magnitude. Flow limiting may therefore occur under a wider range of conditions than previously thought. Flow-limited veins may be recognizable in the field where veins have tapered profiles or smaller aspect ratios than expected. The occurrence and shape of injection veins can be coupled with modeling to provide an independent estimate of minimum melt temperature. Finally, the large aspect ratio observed for all three populations of injection veins may be best explained by a large reduction in stiffness associated with coseismic damage, as injection vein growth is likely to far exceed the lifetime of dynamic stresses at any location along a fault. [ABSTRACT FROM AUTHOR]

Deep earthquake b values appear to vary with slab thermal state and earthquake magnitude. The physical reason for the variations and the relation with deep rupture mechanisms are still unclear. Here I confirm the spatial variations of b value and the dependence on slab temperature using about 40 yr of data from the Global Centroid Moment Tensor catalog. A new bimodal pattern is observed for the 500–700 km depth range: b is close to 1 in the cold Tonga slab, while in warmer slabs (e.g., South America, Japan–Kuril, Izu–Bonin–Mariana), b is close to 0.5 for intermediate magnitudes ( M w 5.3–6.5) and increases to ∼1 for large magnitudes ( M w > 6.5 ). To explain these observations, I propose a dual-mechanism hypothesis in which deep earthquakes nucleate only within the metastable olivine wedge (MOW), but can rupture outside MOW by a different mechanism. The fractal dimension of earthquake size distribution changes from 2 to 1 as the thermally controlled MOW thickness decreases, and back to 2 as the mechanism outside MOW dominates. [ABSTRACT FROM AUTHOR]

Earthquake recurrence models assume that major surface-rupturing earthquakes are followed by periods of reduced rupture probability as stress rebuilds. Although purely periodic, time- or slip-predictable rupture models are known to be oversimplifications, a paucity of long records of fault slip clouds understanding of fault behavior and earthquake recurrence over multiple ruptures. Here, we report a 16 kyr history of fault slip—including a pulse of accelerated slip from 6.4 to 6.0 ka—determined using a Monte Carlo analysis of well-dated offset landforms along the central Altyn Tagh strike-slip fault (ATF) in northwest China. This pulse punctuates a median rate of 8.1 + 1.2 / − 0.9 mm / a and likely resulted from either a flurry of temporally clustered ∼Mw 7.5 ground-rupturing earthquakes or a single large >Mw 8.2 earthquake. The clustered earthquake scenario implies rapid re-rupture of a fault reach >195 km long and indicates decoupled rates of elastic strain energy accumulation versus dissipation, conceptualized as a crustal stress battery. If the pulse reflects a single event, slip-magnitude scaling implies that it ruptured much of the ATF with slip similar to, or exceeding, the largest documented historical ruptures. Both scenarios indicate fault rupture behavior that deviates from classic time- or slip-predictable models. [ABSTRACT FROM AUTHOR]

Earthquakes generate tsunamis by coseismically deforming the seafloor, and that deformation is largely controlled by the shallow rupture process. Therefore, in order to better understand how earthquakes generate tsunamis, one must consider the material structure and frictional properties of the shallowest part of the subduction zone, where ruptures often encounter compliant sedimentary prisms. Compliant prisms have been associated with enhanced shallow slip, seafloor deformation, and tsunami heights, particularly in the context of tsunami earthquakes. To rigorously quantify the role compliant prisms play in generating tsunamis, we perform a series of numerical simulations that directly couple dynamic rupture on a dipping thrust fault to the elastodynamic response of the Earth and the acoustic response of the ocean. Gravity is included in our simulations in the context of a linearized Eulerian description of the ocean, which allows us to model tsunami generation and propagation, including dispersion and related nonhydrostatic effects. Our simulations span a three-dimensional parameter space of prism size, prism compliance, and sub-prism friction – specifically, the rate-and-state parameter b − a that determines velocity-weakening or velocity-strengthening behavior. We find that compliant prisms generally slow rupture velocity and, for larger prisms, generate tsunamis more efficiently than subduction zones without prisms. In most but not all cases, larger, more compliant prisms cause greater amounts of shallow slip and larger tsunamis. Furthermore, shallow friction is also quite important in determining overall slip; increasing sub-prism b − a enhances slip everywhere along the fault. Counterintuitively, we find that in simulations with large prisms and velocity-strengthening friction at the base of the prism, increasing prism compliance reduces rather than enhances shallow slip and tsunami wave height. [ABSTRACT FROM AUTHOR]

The Alpine Fault is inferred on paleoseismological grounds to produce magnitude 8 earthquakes approximately every 330 yrs and to have last ruptured almost 300 yrs ago in 1717 AD. Despite approximately 90% of its typical interseismic period having elapsed since the last major earthquake, the Alpine Fault exhibits little present-day microseismicity and no geodetic evidence for shallow creep. Determining the mechanical state of the fault ahead of a future earthquake is a key objective of several studies, including the Deep Fault Drilling Project (DFDP). Here we use a network of borehole seismometers installed in conjunction with DFDP to detect and characterise low-magnitude seismicity adjacent to the central section of the Alpine Fault. We employ matched-filter detection techniques, automated cross-correlation phase picking, and singular value decomposition-derived magnitude estimation to construct a high-precision catalogue of 283 earthquakes within 5 km of the fault trace in an otherwise seismically quiet zone. The newly recognised seismicity occurs in non-repeating, spatially and temporally limited sequences, similar to sequences previously documented using standard methods but at significantly lower magnitudes of M L < 1.8 . These earthquakes are not clustered on a single distinctive structure, and we infer that they are distributed throughout a highly fractured zone surrounding the Alpine Fault. Focal mechanisms computed for 13 earthquakes using manual polarity picks exhibit predominantly strike-slip faulting, consistent with focal mechanisms observed further from the fault. We conclude that the Alpine Fault is locked and accumulating strain throughout the seismogenic zone at this location. [ABSTRACT FROM AUTHOR]

The 2015 Gorkha earthquake produced displacement on the lower half of a shallow decollement that extends 100 km south, and upward from beneath the High Himalaya and Kathmandu to where it breaks the surface to form the trace of the Himalayan Frontal Thrust (HFT), leaving unruptured the shallowest ∼50 km of the decollement. To address the potential of future earthquakes along this section of the HFT, we examine structural, stratigraphic, and radiocarbon relationships in exposures created by emplacement of trenches across the HFT where it has produced scarps in young alluvium at the mouths of major rivers at Tribeni and Bagmati. The Bagmati site is located south of Kathmandu and directly up dip from the Gorkha rupture, whereas the Tribeni site is located ∼200 km to the west and outside the up dip projection of the Gorkha earthquake rupture plane. The most recent rupture at Tribeni occurred 1221–1262 AD to produce a scarp of ∼7 m vertical separation. Vertical separation across the scarp at Bagmati registers ∼10 m, possibly greater, and formed between 1031–1321 AD. The temporal constraints and large displacements allow the interpretation that the two sites separated by ∼200 km each ruptured simultaneously, possibly during 1255 AD, the year of a historically reported earthquake that produced damage in Kathmandu. In light of geodetic data that show ∼20 mm/yr of crustal shortening is occurring across the Himalayan front, the sum of observations is interpreted to suggest that the HFT extending from Tribeni to Bagmati may rupture simultaneously, that the next great earthquake near Kathmandu may rupture an area significantly greater than the section of HFT up dip from the Gorkha earthquake, and that it is prudent to consider that the HFT near Kathmandu is well along in a strain accumulation cycle prior to a great thrust earthquake, most likely much greater than occurred in 2015. [ABSTRACT FROM AUTHOR]

On 16 September 2015, the Mw 8.4 Illapel earthquake ruptured a section of the subduction thrust on the west coast of central Chile. The mainshock was followed by numerous aftershocks including some normal-faulting events near the trench. We apply a template matching approach to improve the completeness of early aftershocks within one month of the mainshock. To constrain the distribution of afterslip, we utilize repeating earthquakes among the aftershocks and perform a joint slip inversion of postseismic GPS and InSAR data. The results show that the aftershock zone abruptly expands to the south ∼14 h after the mainshock while growing relatively continuously to the north within the first day. The repeating earthquakes accompanying the early expansion suggest that aseismic afterslip on the subduction thrust surrounding the coseismic rupture is an important triggering mechanism of aftershocks in addition to stress transfer or poroelastic effects. An energetic earthquake sequence near the trench initiated with a M 4.6 event ∼3.5 h after the mainshock, suggesting delayed triggering by the static or dynamic stress changes induced by the mainshock. The spatial distribution of repeating earthquakes and the geodetic-inverted afterslip are consistent and appear to wrap around the large coseismic slip patch. Both data sets suggest that the largest cumulative afterslip is located at ∼30.5°S to the north of the mainshock rupture zone. The estimated postseismic moment released in the first ∼24 days of afterslip is equivalent to an earthquake of Mw 7.5. The afterslip illuminates the velocity strengthening sections of the plate interface that surround the mainshock rupture, consistent with plate coupling models inferred from interseismic GPS velocities. [ABSTRACT FROM AUTHOR]

Coseismic faulting may activate complicated physicochemical processes in the fault zone through both mechanical deformation and frictional heating. While plenty of work emphasizes the great importance of thermal effects on dynamic fault weakening, less attention has been paid to the mechanochemical effects. We study the effects of mechanochemical changes of chlorite on its dynamic weakening process via low- and high-velocity friction experiments, simultaneous thermal analysis, X-ray diffraction, microstructural observation, and numerical modeling in this work. With higher degrees of deformation, the experimentally sheared chlorite samples generally show more pronounced reductions in the onset temperature of thermal decomposition (T d 0 ; from 561 °C to 189 °C in the extreme case) and more striking increases in the reaction rate. The exponential-decay formula can well describe the relation between T d 0 and the mechanical work (integral of shear stress with respect to displacement) in the shear deformation. Moreover, the presence of pore water and the variation in chlorite content could significantly affect the mechanochemical changes of the sheared chlorite-bearing samples. Taking the markedly lowered T d 0 as the characteristic temperature leading to thermal softening of asperity contacts, numerical modeling shows that flash weakening of chlorite-rich samples can be significantly promoted, dwarfing the thermochemical pressurization that is also enhanced by mechanochemical effects though. This is largely supported by high-velocity experiments (∼m/s) performed on the pre-deformed chlorite samples, which show both lower peak friction and fracture energy, and earlier activation of dynamic weakening. Clearly, shear deformation plays a key role in aiding thermally-activated mechanisms in weakening faults and facilitating rupture propagation during earthquakes. • Shear deformation markedly reduces thermal decomposition temperature of chlorite. • Pre-deformed chlorite shows lower fracture energy and quicker dynamic weakening. • Shear deformation may significantly promote flash weakening in clay-rich faults. • Shear-enhanced thermochemical pressurization aids dynamic fault weakening. [ABSTRACT FROM AUTHOR]

The origin of plate tectonics is one of the most fundamental issues in earth and planetary sciences. Laboratory experiments indicate that the viscosity of silicate rocks is so strongly temperature-dependent that the entire surface of the Earth should be one immobile rigid plate. The rheology of oceanic lithosphere is, however, still poorly understood, and there exist few constraints on the temperature dependency of viscosity on the field scale. Here we report a new kind of observational constraint based on the geoid along oceanic fracture zones. We identify a large number of conspicuous small-scale geoid anomalies, which cannot be explained by the standard evolution model of oceanic lithosphere, and estimate their source density perturbations using a new Bayesian inversion method. Our results suggest that they are caused most likely by small-scale convection involving temperature perturbations of ∼ 300 K ± 100 K . Such thermal contrast requires the activation energy of mantle viscosity to be as low as 100 ± 50 kJ mol − 1 in case of diffusion creep, and 225 ± 112 kJ mol − 1 in case of dislocation creep, substantially reducing the thickness of the stiffest part of oceanic lithosphere. Oceanic lithosphere may thus be broken and bent much more easily than previously thought, facilitating the operation of plate tectonics. [ABSTRACT FROM AUTHOR]

We report detailed field measurements of the 2021, M w 7.4 Maduo earthquake surface rupture south of the Kunlun fault, near the northern boundary of Tibet's Bayan Har block. In the field, the dominantly left-lateral surface rupture length was ∼151 km, along the poorly known Jiangcuo fault, with maximum horizontal and vertical displacements of ∼2.1±0.2 to 2.9±0.2 m (up to 3.6±0.2 m at one site) and ∼1.2±0.2 to 1.8±0.2 m, respectively. The rupture, which propagated bilaterally from a roughly central epicenter, terminated in well-defined, ∼20 km-long horsetail splays, as predicted by Coulomb stress failure, consistent with the very young age of the still growing Jiangcuo fault, which may be trying to bypass the Anyemaqen restraining bend in order to connect directly the Kokoxili and Maqen segments of the main Kunlun fault. 14C dating at one site (Cuoerjiala) implies long return times (≥6500 years) for M7+, Maduo-type earthquakes, in keeping with a slow horizontal slip rate of ∼0.55±0.03 mm/yr and the subdued geomorphic expression of the fault. In the broader Tibetan tectonics framework, the Jiangcuo fault may be better interpreted as a kinematic streamlining of the Kunlun fault bend, rather than a result of diffuse deformation across the Bayan Har terrane. • EQ rupture length and surface offsets were 151 km, max 3.6 (horiz) and 1.8 m (vert). • Symmetrical, 20 km-long horsetail swings mark growing fault rupture extremities. • Return time of M7+ earthquakes is ≥6500 years, consistent with ∼0.55 mm/yr slip-rate. • Nascent (<3Ma?) Jiangcuo fault is short-cutting ∼70km-wide Anyemaqen restraining bend. [ABSTRACT FROM AUTHOR]

Relative to more mature fault zones, immature fault zones that have accumulated smaller total displacement are characterized by less efficient strain localization and more complicated earthquake ruptures. How differences in maturation are reflected in regional-scale upper-crustal fracturing is not well known. Recently, complicated earthquake ruptures associated with immature fault zones, such as the 2016 Kaikōura earthquake in New Zealand, have occurred in areas that are in regional proximity (<100 km away) to more mature faults. Here we examine whether inefficient strain localization in less mature fault zones is associated with a broader distribution and anomalously elevated concentration of fractures over distances of tens of kilometers. We use regional seismic arrival-time tomography in a broad area around the Kaikōura earthquake to investigate lateral variations in Vp and Vp/Vs. Focusing on the extensively faulted but compositionally uniform Torlesse-Pahau terrane in the Marlborough region where the earthquake occurred, we attribute lateral variations in Vp and Vp/Vs to differences in concentration of fluid-filled fractures. Using numerical models relating seismic velocities and fracturing, we solve for the lateral variation in concentration of ∼0.01 aspect ratio fluid-filled fractures. We find that areas near the Kaikōura rupture have >3% elevated fracture porosity compared to the adjacent area to the north. The elevated regional fracturing in the Kaikōura area is interpreted to result from more distributed deformation, and broader distribution of earthquakes, due to inefficient localization of strain from a regionally uniform strain rate field, highlighting the relationship between relative maturity of upper-crustal fault zones and lateral variability of regional upper-crustal properties. Plain language summary: When earthquakes occur in immature fault zones (areas that have accumulated small total displacement), they tend to rupture multiple poorly developed faults of diverse orientations. In contrast, more mature fault zones are associated with more developed, smoother faults and more localized earthquake activity. How these differences in maturation are reflected in the regional distribution of fractures is not well known. Here we use the arrival times of P and S waves from earthquakes to constrain seismic velocities (Vp and Vp/Vs) and use numerical models to relate seismic velocities to fracture concentration. We focus on the Marlborough region of the South Island, New Zealand, where immature fault zones recently ruptured during the 2016 Kaikōura earthquake but which also has more mature fault zones <100 km away. We find elevated fracture concentrations (3% higher fracture porosity), indicative of more distributed deformation along immature fault zones relative to more mature fault zones. In immature settings, fracturing is not as effectively localized along individual fault traces, leading to a broadly distributed fracture network. • We image upper-crustal Vp and Vp/Vs, and interpret their variations in terms of amount of fluid-filled fractures. • The Marlborough region, which experiences near-uniform shear, has variable fluid-filled fracture concentration. • Within the Marlborough region, areas of less mature faulting have more fluid-filled fractures than more mature areas. [ABSTRACT FROM AUTHOR]

Seismic cycles emerge in a broad range of rupture styles, from slow-slip events to pulse-like earthquake sequences. Meanwhile, large earthquakes in paleoseismic and instrumental catalogues exhibit various recurrence patterns going from periodic to chaotic cycles with characteristic or dissimilar ruptures. The potential connection between these observations is still poorly known. Here, we investigate the link between rupture styles and recurrence patterns in quasi-dynamic models of seismicity in two-dimensional faults embedded in a compliant zone, exploring a wide range of frictional and fault zone properties. The recurrence patterns evolve from purely periodic to multiple-periodic time- and slip-predictable cycles, to chaotic sequences of super-cycles with full and partial ruptures with an increasing number of aftershocks. This transition is accompanied by changes of rupture styles from crack-like to pulse-like ruptures. These behaviors can be obtained either by a more compliant fault zone or a reduced characteristic weakening distance of friction. The effects of the compliant zone and other physical characteristics can be conflated into a single non-dimensional number, such that seismic cycles with similar behaviors can be obtained with or without a compliant fault zone in quasi-dynamic simulations. The connection between recurrence patterns and rupture styles implies that the paleoseismic record can bring useful constraints to rupture characteristics and fault zone properties. • Rupture styles and recurrence patterns of earthquakes are linked. • The nucleation size and fault zone fabric control the rupture style. • Recurrence patterns evolve from periodic to chaotic with varying rupture style. • The paleoseismic record may help constrain rupture characteristics. [ABSTRACT FROM AUTHOR]

Studying rupture speeds of shallow earthquakes is of broad interest because it has a large effect on the strong near-field shaking that causes damage during earthquakes, and it is an important parameter that reflects stress levels and energy on a slipping fault. However, resolving rupture speed is difficult in standard waveform inversion methods due to limited near-field observations and the tradeoff between rupture speed and fault size for teleseismic observations. Here we applied back-projection methods to estimate the rupture speeds of 15 M w ≥ 7.8 dip-slip and 8 M w ≥ 7.5 strike-slip earthquakes for which direct P waves are well recorded in Japan on Hi-net, or in North America on USArray. We found that all strike-slip events had very fast average rupture speeds of 3.0–5.0 km/s, which are near or greater than the local shear wave velocity (supershear). These values are faster than for thrust and normal faulting earthquakes that generally rupture with speeds of 1.0–3.0 km/s. [ABSTRACT FROM AUTHOR]

The chronology of the seismic ruptures along the active faults of Western Nepal remains almost unconstrained despite their high seismogenic potential. We present here a slip history of one of these structures, a 120 km-long reactivated segment of the Main Boundary Thrust named the Surkhet–Gorahi fault. This slip history is based on geomorphologic and neotectonic mapping of active faults deduced from the analysis of a high resolution total station digital elevation model and 15 detrital charcoals radiocarbon ages constraining the age of deposition or abandonment of 4 alluvial terraces of the Bheri river in Botechaur. Our results show that the last two earthquakes occurred on this fault after 1860 and 640 BP, respectively, and accommodated slip greater than 8 m each, a value corresponding to the incremental vertical offset of the terraces. Such events released a significant part of the slip deficit accumulated on the Main Himalayan thrust fault. However, given the geometry of this fault system as well as the date of occurrence of the last events, the ruptures could be associated with major earthquakes also rupturing the Main Frontal Thrust, such as the great 1505 earthquake. [ABSTRACT FROM AUTHOR]

The potential for strong ground shaking in large earthquakes partly depends on how fast the earthquake rupture propagates. It is observed that strike-slip earthquakes usually propagate at speeds slower than the Rayleigh wave speed ( v R ) but occasionally jump to speeds faster than the S wave speed ( v s ), or supershear speeds. Supershear earthquakes can be more catastrophic and cause unusually large ground motions at long distances. Here we use both fully dynamic rupture simulations and high-resolution seismic observations to show that supershear earthquakes can be induced by damaged fault zones, the low-velocity layers of damaged rocks that typically exist around major faults and serve as waveguides for high-frequency energy. In contrast to supershear ruptures in homogeneous media, supershear ruptures in damaged fault zones can occur under relatively low fault stress and propagate stably at speeds within the range usually considered as unstable. [ABSTRACT FROM AUTHOR]

Fault zone head waves (FZHWs) travel along the bimaterial fault interface and can be used to provide high-resolution imaging of fault zone properties. Here we use aftershock data following the 2010/04/13 Mw6.9 Yushu earthquake to quantify velocity contrast along the Garzê–Yushu Fault (GYF). FZHWs are identified as low-amplitude, long-period seismic signals with polarities opposite to the direct P arrivals. We observe FZHWs at 4 stations near faults that bound both sides of the Jielong basin, which is a pull-apart basin that is partially filled by Longbao Lake. The time delays between the direct P and FZHWs increase systematically with along-strike distances, and the corresponding observed velocity contrast is on the order of 5–8%. However, we do not observe a clear increase of time delay with hypocentral depth, suggesting that the pull-apart basin forms a thick low-velocity zone that likely exists in the top few kilometers of the crust. The velocity contrast southeast of the mainshock epicenter is very small (1–3%), consistent with both field observations and seismic tomographic results. [ABSTRACT FROM AUTHOR]

The transition between seismic rupture and aseismic creep is of central interest to better understand the mechanics of subduction processes. A Mw 8.2 earthquake occurred on April 1st, 2014 in the Iquique seismic gap of northern Chile. This event was preceded by a long foreshock sequence including a 2-week-long migration of seismicity initiated by a Mw 6.7 earthquake. Repeating earthquakes were found among the foreshock sequence that migrated towards the mainshock hypocenter, suggesting a large-scale slow-slip event on the megathrust preceding the mainshock. The variations of the recurrence times of the repeating earthquakes highlight the diverse seismic and aseismic slip behaviors on different megathrust segments. The repeaters that were active only before the mainshock recurred more often and were distributed in areas of substantial coseismic slip, while repeaters that occurred both before and after the mainshock were in the area complementary to the mainshock rupture. The spatiotemporal distribution of the repeating earthquakes illustrates the essential role of propagating aseismic slip leading up to the mainshock and illuminates the distribution of postseismic afterslip. Various finite fault models indicate that the largest coseismic slip generally occurred down-dip from the foreshock activity and the mainshock hypocenter. Source imaging by teleseismic back-projection indicates an initial down-dip propagation stage followed by a rupture-expansion stage. In the first stage, the finite fault models show an emergent onset of moment rate at low frequency ( < 0.1 Hz ), while back-projection shows a steady increase of high frequency power ( > 0.5 Hz ). This indicates frequency-dependent manifestations of seismic radiation in the low-stress foreshock region. In the second stage, the rupture expands in rich bursts along the rim of a semi-elliptical region with episodes of re-ruptures, suggesting delayed failure of asperities. The high-frequency rupture remains within an area of local high trench-parallel gravity anomaly (TPGA), suggesting the presence of subducting seamounts that promote high-frequency generation. Our results highlight the complexity of the interactions between large-scale aseismic slow-slip and dynamic ruptures of megathrust earthquakes. [ABSTRACT FROM AUTHOR]

Slow slip events are now well-established in fault and glacier systems, though the processes controlling slow rupture remain poorly understood. The Whillans Ice Plain provides a window into these processes through bi-daily stick-slip seismic events that displace an ice mass over 100 km long with a variety of rupture speeds observed at a single location. We compare the glacier events with laboratory experiments that have analogous loading conditions. Both systems exhibit average rupture velocities that increase systematically with the pre-rupture stresses, with local rupture velocities exhibiting large variability that correlates well with local interfacial stresses. The slip events in both cases are not time-predictable, but clearly slip-predictable. Local pre-stress may control rupture behavior in a range of frictional failure events, including earthquakes. [ABSTRACT FROM AUTHOR]

Complexities in the rupture process during a megathrust earthquake can be attributed to the combined effect of inhomogeneous distribution of stress accumulated during the interseismic period and inhomogeneous rheology of the seismogenic contact. We modeled the free-air gravity field of the southern Central Chile convergent margin along five 2-D profiles that cross the patch of highest slip during the Chilean 2010 megathrust earthquake in order to analyze variability in the density and shape of the continental wedge and its relationship with seismotectonics. We also analyzed the bathymetry to derive the long-term interplate friction coefficient. The results show that the high slip patch during the Maule earthquake corresponds to a segment of the margin characterized by (1) low densities in the continental wedge, (2) low vertical loading over the inter-plate contact, (3) a well-developed shelf basin and, (4) low taper angles consistent with a low effective basal friction coefficient. We interpret the correlation between these parameters in terms of the total potential energy change during the earthquake and conclude that if the normal stress or frictional coefficient are low, then a large slip does not necessarily imply a large amount of coseismic work. Heterogeneities in density of the continental basement can therefore be related to complexities in the pattern of coseismic slip and in the aftershock distribution. Locally, a subducted seamount or seaward spur of high-density continental crust may be present near the high slip patch. [ABSTRACT FROM AUTHOR]

We are mapping the topography of upper mantle seismic discontinuities beneath the North Atlantic and surrounding regions by using precursor arrivals to PP and SS seismic waves that reflect off the seismic discontinuities. Numerous source–receiver combinations have been used in order to collect a large dataset of reflection points beneath our investigation area. We analysed over 1700 seismograms from M W > 5.8 events using array seismic methods to enhance the signal to noise ratio. The measured time lag between PP (SS) arrivals and their corresponding precursors on robust stacks are used to measure the depth of the transition zone boundaries. The reflectors' depths show a correlation between the location of known hotspots and a significantly depressed 410 km discontinuity indicating a temperature increase of 50–300 K compared to the surrounding mantle. For the 660 km discontinuity three distinct behaviours are visible: (i) normal depths beneath Greenland and at a distance of a few hundred kilometres away from known hotspots, (ii) shallower 660 km discontinuity compared with the global average value near hotspots closer to the Mid-Atlantic Ridge, and (iii) very few observations of a 660 km discontinuity at the hotspot locations. We interpret our observations as a large upwelling beneath the southern parts of our study region, possibly due to the South Atlantic convection cell. The thermal anomaly may be ponding beneath the endothermic 660 km phase transformation and likely does not extend through the top of the transition zone as a whole, except for those branches which appear as the thinner upwellings of Azores, Canaries and Cape Verde hotspots at the surface. [ABSTRACT FROM AUTHOR]

During the decade from mid-2004 to mid-2014 18 great ( M w ≥ 8.0 ) earthquakes occurred globally (∼1.8 per year), compared to 71 from 1900 to mid-2004 (∼0.68 per year), yielding a short-term rate increase of 265%. Six events had M w ≥ 8.5 , larger than any prior event since the 1965 Rat Islands earthquake. The December 26, 2004 M w 9.2 Sumatra earthquake had the longest recorded rupture length of 1300+ km and a rupture duration exceeding 450 s. The largest recorded strike-slip earthquake ( M w 8.7) occurred in the Indo–Australian plate on April 11, 2012. The largest recorded deep focus earthquake ( M w 8.3) occurred under the Sea of Okhotsk on May 24, 2013. While this overall surge of activity has not been demonstrated to be causally linked, regional spatio-temporal clustering is clearly evident for great events along the Sumatra, Kuril and Tonga subduction zones, and longer-range interactions have been established for global seismicity and seismic tremor at lower magnitudes following some of the events. This recent decade of intense great earthquake activity coincided with vastly expanded global networks of seismometers, GPS stations, tsunami gauges, and new satellite imaging capabilities such as InSAR and LandSAT interferometry and gravity measurements by GRACE and GOCE, enabling unprecedented analyses of precursory, co-seismic and post-seismic processes around the subduction zone environments where most of the events occurred. Individual events such as the March 11, 2011, Tohoku, Japan M w 9.0 earthquake produced more ground motion and tsunami recordings than available for all great earthquakes of the last century collectively. Joint inversion and modeling of the diverse data sets exploit complementary sensitivity of the signals to different aspects of the earthquake processes. Major advances have been achieved in quantifying frictional locking and strain accumulation prior to some great events and in relating it to co-seismic slip heterogeneity. Many surprising aspects of these well-quantified great earthquakes have been manifested, associated with their rupture dimensions, tectonic location, compound faulting, triggering interactions, slow slip and foreshock migration precursors, aftershock complexity, and depth-varying seismic radiation characteristics. Regions with potential for near-future great ruptures include mature seismic gaps along the Mentawai Islands and northern Chile, as well as western North America and the Himalayan front, so more great earthquake activity can certainly be anticipated. [ABSTRACT FROM AUTHOR]

Seismic analyses show that slip during large earthquakes evolves in a slip-pulse mode that is characterized by abrupt, intense acceleration followed by moderate deceleration. We experimentally analyze the friction evolution under slip-pulse proxy of a large earthquake, and compare it with the evolution at loading modes of constant-velocity and changing-velocity. The experiments were conducted on room-dry, solid granite samples at slip-velocities of 0.0006–1 m/s, and normal stress of 1–11.5 MPa. The analysis demonstrates that (1) the strength evolution and constitutive parameters of the granite fault strongly depend on the loading mode, and (2) the slip-pulse mode is energy efficient relatively to the constant-velocity mode as manifested by faster, more intense weakening and 50–90% lower energy dissipation. The results suggest that the frictional strength determined in slip-pulse experiments, is more relevant to simulations of earthquake rupture than frictional strength determined in constant-velocity experiments. Further, for a finite amount of crustal elastic energy, the efficiency of slip-pulse would amplify earthquake instability. [ABSTRACT FROM AUTHOR]

Fault slip may involve slow aseismic creep and fast seismic rupture, radiating seismic waves manifested as earthquakes. These two complementary behaviors accommodate the long-term plate convergence of major subduction zones and are attributed to fault frictional properties. It is conventionally assumed that zones capable of seismic rupture on the subduction megathrust are confined to between about 10 to 50 km depth; however, the actual spatiotemporal distribution of fault mechanical parameters remains elusive for most subduction zones. The 2011 Tohoku M w 9.0 earthquake ruptured with > 50 m slip up to the trench, thus challenging this conventional assumption, and provides a unique opportunity to probe the mechanical properties of the Japan subduction zone. Drawing on the inferred distribution of coseismic and postseismic slip, it has recently been suggested that portions of the megathrust are capable of switching between seismic and aseismic behavior. Kinematic models of the coseismic rupture and 15-month postseismic afterslip of this event suggest that the coseismic rupture triggered widespread frictional afterslip with equivalent moment magnitude of 8.17–8.53, in addition to viscoelastic relaxation in the underlying mantle. The identified linear relation between modeled afterslip, slip inferred from repeating earthquakes on the plate interface, and the cumulative number of aftershocks within 15 km distance of the subduction thrust suggests that most aftershocks are a direct result of afterslip. We constrain heterogeneous rate-state friction parameters of the subduction thrust from the computed coseismic stress changes and afterslip response. Our results indicate a variable pattern along dip and strike, characterizing areas down-dip and south of the main rupture zone as having velocity-strengthening properties. In agreement with seismic tomographic models of plate boundary elastic properties and geologic evidence for previous M > 8.5 megathrust earthquakes on this section of the plate boundary, we suggest that the obtained pattern of the frictional properties is characteristic of subducted material and thus persistent in time and space. [ABSTRACT FROM AUTHOR]