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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]

The Haiyuan fault system plays a crucial role in accommodating the eastward expansion of the Tibetan Plateau (TP) and is currently slipping at a rate of several centimeters per year. However, limited seismic activities have been observed using geodetic techniques in this area, impeding the comprehensive investigation into regional tectonics. In this study, the geometric structure and source models of the 2022 Mw 6.7 and the 2016 Mw 5.9 Menyuan earthquakes were investigated using Sentinel-1A SAR images. By implementing an atmospheric error correction method, the signal-to-noise ratio of the 2016 interferometric synthetic aperture radar (InSAR) coseismic deformation field was significantly improved, enabling InSAR observations with higher accuracy. The results showed that the reliability of the source models for those events was improved following the reduction in observation errors. The Coulomb stress resulting from the 2016 event may have promoted the strike-slip movement of the western segment of the Lenglongling fault zone, potentially expediting the occurrence of the 2022 earthquake. The coseismic slip distribution and the spatial distribution of aftershocks of the 2022 event suggested that the seismogenic fault may connect the western segment of the Lenglongling fault (LLLF) and the eastern segment of the Tuolaishan fault (TLSF). Additionally, the western segment of the surface rupture zone of the northern branch may terminate in the secondary branch close to the Sunan-Qilian fault (SN-QL) strike direction, and the earthquake may have triggered deep aftershocks and accelerated stress release within the deep seismogenic fault. [ABSTRACT FROM AUTHOR]

We successfully detected widely distributed ground displacements for the 2023 Kahramanmaraş, Turkey, earthquakes by conducting interferometric synthetic aperture radar (InSAR) analyses using a ScanSAR observation mode. Major deformation extended approximately 350 and 150 km along the southern and northern strands bifurcating in the west of the East Anatolian Fault, produced by the main shock and the largest aftershock. The deformation map reveals that the ruptures propagated on the Erkenek, Pazarcık and Amanos segments on the southern strand and the Çardak segment on the northern strand. The fault plane of the northern strand bends approximately 45° at both edges with Z-shaped crank geometry. The bending fault at the western edge runs just along the Çardak segment but does not reach the Savrun segment, while at the eastern edge it deviates from known active faults such as Sürgü, Malatya faults and Doğansehır fault zone. A 3-D displacement map demonstrates that almost pure left-lateral fault motions were distributed along the two strands, with little vertical deformation. The moment magnitudes estimated from the slip distribution model were 7.82 and 7.66 for the southern and northern strands, respectively, with the Erkenek and Çardak segments having the largest released seismic moments on each strand, corresponding to approximately 31 and 57 per cent of the total, respectively. The Coulomb Failure Function change values indicate that the main shock can promote the largest aftershock with a standard value of the effective friction coefficient. Additionally, the unclamping effect controlled by the frictional property of the rock was a key factor in pulling the trigger of the seismic event on the northern strand. The historically accumulated and released seismic energies were imbalanced for the Pazarcık and Erkenek segments, suggesting that the 2023 event does not support a simple characteristic earthquake model; rather, it may be consistent with a supercycle model, in which the slip remnants from the characteristic earthquakes have been historically accumulated as coupling on a fault and released as huge earthquakes at longer intervals. [ABSTRACT FROM AUTHOR]

The Mj6.5 (Mw6.2) event that occurred on 5 May 2023 near the northern shoreline of the northeastern tip of the Noto Peninsula, central Japan, is the largest event to date in a long‐lasting, intense earthquake swarm. Here we have created a more precise aftershock catalog associated with the 2023 Mj6.5 and the second‐largest 2022 Mj5.4 sequence to understand the rupture process of this largest earthquake. Most of the aftershocks are aligned along a ∼45° SE‐dipping plane. The mainshock initially ruptured the same deep section of the fault zone that had been ruptured by the 2022 Mj5.4 event, before propagating rapidly to shallow depths and to offshore along the ruptured fault plane. The aftershock front migrated at a speed of ∼20 km/hr. This rapid upward migration of the immediate aftershocks might be driven by upwelling of crustal fluids along the intensely fractured and permeable fault zone via mainshock dynamic rupture. Plain Language Summary: Near the northeastern tip of the Noto Peninsula, central Japan, a long‐lasting, intense earthquake swarm has continued since November 2020. On 5 May 2023, the largest Mj6.5 (Mw6.2) event to date occurred. We precisely located the aftershock distribution following the 2023 Mj6.5 and the second‐largest 2022 Mj5.4 sequence and enhanced the catalog by searching events based on waveform similarity to understand the rupture process of this largest earthquake. The 2023 Mj6.5 event initially ruptured the same deep section of the fault that had been ruptured by the 2022 Mj5.4 event, before propagating rapidly to shallow depths and to offshore along the ruptured fault. We can see that the aftershock front moved at a speed of ∼20 km/hr, which is a rare case that constrains the rapid movement of aftershocks. The rapid upward movement of the aftershocks may have been caused by the upwelling of crustal fluids along the permeable fault zone created by the dynamic rupture of the mainshock. Key Points: We constructed a more precise aftershock catalog associated with the two major ruptures during a long‐lasting intense seismic swarmWe identify a rapid migration of early aftershocks following the largest 2023 Mj6.5 earthquake to date during the seismic swarmUpwelling of crustal fluids along the fractured permeable fault zone could drive the rapid migration of early aftershocks [ABSTRACT FROM AUTHOR]

On 6 February 2023, the MW 7.8 Pazarcik and the MW 7.5 Elbistan earthquakes occurred in southeastern Turkey, close to the Syrian border, causing many deaths and a great deal of property destruction. The Pazarcik earthquake mainly damaged the East Anatolian Fault Zone (EAFZ). The Elbistan earthquake mainly damaged the Cardak fault (CF) and the Doğanşehir fault (DF). In this study, Sentinel-1A ascending (ASC) and descending (DES) orbit image data and pixel offset tracking (POT) were used to derive surface deformation fields in the range and azimuth directions induced by the Pazarcik and Elbistan earthquakes (hereinafter referred to as the Turkey double earthquakes). Utilizing GPS coordinate sequence data, we computed the three-dimensional surface deformation resulting from the Turkey double earthquakes. The surface deformation InSAR and GPS results were combined to invert the coseismic slip distribution of the EAFZ, CF, and DF using a layered earth model. The results show that the coseismic ruptures of the Turkey double earthquakes were dominated by left-lateral strike-slips. The maximum slip was 7.76 m on the EAFZ and about 8.2 m on the CF. Both the earthquakes ruptured the surface. The Coulomb failure stress (CFS) was computed based on the fault slip distribution and the geometric parameters of all the active faults within 300 km of the MW 7.8 Pazarcik earthquake's epicenter. The CFS change resulting from the Pazarcik earthquake suggests that the subsequent Elbistan earthquake was triggered by the Pazarcik earthquake. The Antakya fault experienced an increase in CFS of 8.4 bars during this double-earthquake event. Therefore, the MW 6.3 Uzunbağ earthquake on 20 February 2023 was jointly influenced by the Turkey double earthquakes. Through stress analysis of all the active faults within 300 km of the MW 7.8 Pazarcik earthquake's epicenter, the Ecemis segment, Camliyayla fault, Aadag fault, Ayvali fault, and Pula segment were all found to be under stress loading. Particularly, the Ayvali fault and Pula segment exhibited conspicuous stress loading, signaling a higher risk of future seismic activity. [ABSTRACT FROM AUTHOR]

The Türkiye-Syria earthquake doublet of 6 February 2023 (Mw 7.8 at 01:17 UTC and Mw 7.6 at 10:24 UTC) resulted in extensive damage and tens of thousands of casualties. We present the surface displacements of the two earthquakes from synthetic aperture radar (SAR) offset tracking measurements. We extracted the geometric parameters of the rupture faults from the surface displacements and early aftershock distribution, based on which we inverted the coseismic slip distributions. We then calculated Coulomb stress to investigate the triggering relationship between the earthquakes and stress transfer to neighbouring faults and regions. The coseismic ruptures of the earthquake doublet were predominantly left-lateral strike-slip motions distributed between 0 and 15 km depth. The maximum fault slip reached > 8 m (Mw 7.8) and almost 10 m (Mw 7.6). The coseismic deformation and fault slip motion are consistent with the overall westward extrusion of the Anatolian Plate relative to the Eurasian and Arabian plates. The Mw 7.8 earthquake increased Coulomb failure stress at the hypocenter of the Mw 7.6 earthquake, implying that the Mw 7.8 event had a strong positive causative effect. Moreover, coseismic stress perturbations revealed a positive Coulomb stress effect on the middle Puturge Fault, northern Dead Sea Fault Zone (DSFZ), Yesemek Fault, Antakya Fault, and Turkoglu Fault, indicating an increasing seismic hazard in these regions. [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]

Since the Mw 7.9 Gorkha earthquake of April 25, 2015, the seismicity of central and western Nepalese Himalaya has been monitored by an increasing number of permanent seismic stations. These instruments contribute to the location of thousands of aftershocks that occur at the western margin of the segment of the Main Himalayan Thrust (MHT) that ruptured in 2015. They also help to constrain the location of seismic clusters that originated at the periphery of the fault ruptured by the Gorkha earthquake, which may indicate a migration of seismicity along the fault system. We report here a seismic crisis that followed the Lamjung earthquake, a moderate Mw 4.7 event (ML 5.8, MLv 5.3) that occurred on May 18, 2021, about 30 km west of the Gorkha earthquake epicenter at the down-dip end of the locked fault zone. The study of the hypocentral location of the mainshock and its first 117 aftershocks confirms mid-crustal depths and supports the activation of a 30–40° dipping fault plane, possibly associated with the rupture of the updip end of the MHT mid-crustal ramp. The cluster of aftershocks occurs near the upper decollement of the thrust system, probably in its hanging wall, and falls on the immediate northern margin of a region of the fault that has not been ruptured since the 1344 or 1505 CE earthquake. The spatio-temporal distribution of the first 117 aftershocks shows a typical decrease in the associated seismicity rate and possible migration of seismic activity. Since then, the local seismicity has returned to the pre-earthquake rate and careful monitoring has not revealed any large-scale migration of seismicity towards the locked fault segments. [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]

The survey and structural analysis of surface coseismic ruptures are essential tools for characterizing seismogenic structures. In this work, a procedure to survey coseismic ruptures using satellite interferometric synthetic aperture radar (InSAR) data, directing the survey using Unmanned Aerial Vehicles (UAV), is proposed together with a field validation of the results. The Sentinel-1 A/B Interferometric Wide (IW) Swath TOPSAR mode offers the possibility of acquiring images with a short revisit time. This huge amount of open data is extremely useful for geohazards monitoring, such as for earthquakes. Interferograms show the deformation field associated with earthquakes. Phase discontinuities appearing on wrapped interferograms or loss-of-coherence areas could represent small ground displacements associated with the fault's ruptures. Low-altitude flight platforms such as UAV permit the acquisition of high resolution images and generate 3D spatial geolocalized clouds of data with centimeter-level accuracy. The generated topography maps and orthomosaic images are the direct products of this technology, allowing the possibility of analyzing geological structures from many viewpoints. We present two case studies. The first one is relative to the 2016 central Italian earthquakes, astride which the InSAR outcomes highlighted quite accurately the field displacement of extensional faults in the Mt. Vettore–M. Bove area. Here, the geological effect of the earthquake is represented by more than 35 km of ground ruptures with a complex pattern composed by subparallel and overlapping synthetic and antithetic fault splays. The second case is relative to the Mt. Etna earthquake of 26 December 2018, following which several ground ruptures were detected. The analysis of the unwrapped phase and the application of edge detector filtering and other discontinuity enhancers allowed the identification of a complex pattern of ground ruptures. In the Pennisi and Fiandaca areas different generation of ruptures can be distinguished, while previously unknown ruptures pertaining to the Acireale and Ragalna faults can be identify and analyzed. [ABSTRACT FROM AUTHOR]

Three-dimensional deformation is an important input to explore seismic mechanisms and geodynamics. The GNSS and InSAR technologies are commonly used to obtain the co-seismic three-dimensional deformation field. This paper focused on the effect of calculation accuracy caused by the deformation correlation between the reference point and the points involved in the solution, to build a high-accuracy three-dimensional deformation field for a detailed geological explanation. Based on the variance component estimation (VCE) method, the InSAR LOS, azimuthal deformation, and the GNSS horizontal and vertical deformation were integrated to solve the three-dimensional displacement of the study area in combination with the elasticity theory. The accuracy of the three-dimensional co-seismic deformation field of the 2021 Maduo MS7.4 earthquake obtained by the method proposed in this paper, was compared with that obtained from the only InSAR measurements obtained using a multi-satellite and multi-technology approach. The results showed the difference in root-mean-square errors (RMSE) of the integration and GNSS displacement was 0.98 cm, 5.64 cm, and 1.37 cm in the east–west, north–south and vertical direction respectively, which was better than the RMSE of the method using only InSAR and GNSS displacement, which was 5.2 cm and 12.2 cm in the east–west, north–south, and no vertical direction. With the geological field survey and aftershocks relocation, the results showed good agreement with the strike and the position of the surface rupture. The maximum slip displacement was about 4 m, which was consistent with the result of the empirical statistical formula. It was firstly found that the pre-existing fault controlled the vertical deformation on the south side of the west end of the main surface rupture caused by the Maduo MS7.4 earthquake, which provided the direct evidence for the theoretical hypothesis that large earthquakes could not only produce surface rupture on seismogenic faults, but also trigger pre-existing faults or new faults to produce surface rupture or weak deformation in areas far from seismogenic faults. An adaptive method was proposed in GNSS and InSAR integration, which could take into account the correlation distance and the efficiency of homogeneous point selection. Meanwhile, deformation information of the decoherent region could be recovered without interpolation of the GNSS displacement. This series of findings formed an essential supplement to the field surface rupture survey and provided a novel idea for the combination of the various spatial measurement technologies to improve the seismic deformation monitoring. [ABSTRACT FROM AUTHOR]

The Turkey-Syria earthquake On 6 February 2023, around 4:15 am local time, a magnitude 7.8 earthquake struck south central Turkey near the Turkey/Syria border. NASA's satellites help with earthquake response Following the magnitude 7.8 and 7.5 earthquakes that struck southern Turkey and western Syria on 6 February 2023, NASA is working to share its aerial views and data from space in ways that can aid relief and recovery workers in the region, as well as improve its ability to model and predict such events (https://www.nasa.gov/feature/nasa-s-satellites-help-with-turkey-syria-earthquake-response). Gl Similar changes were seen in the Turkish town of Gölcük after the 1999 Izmit/Kocaeli earthquake, and more recently along the coast following the 2016 Kaikoura earthquake in New Zealand - both of which were also large strike-slip earthquakes. DEC response to the Turkey-Syria earthquake A series of devastating earthquakes hit southern Turkey (now known as Türkiye) and north-west Syria in February killing over 33 000 people and injuring many more. [Extracted from the article]

On June 14, 2020, Mw 5.9 Kaynarpınar earthquake occurred in the easternmost part of the North Anatolian Fault Zone, along the Yedisu Seismic Gap near the Karlıova Triple Junction. Following the event, various agencies reported different epicenter locations. This issue and the complex fault pattern around the triple junction caused uncertainties about the ruptured fault and the rupture mechanism. To define the geometry of the ruptured fault, we (1) held a field survey within a week of the mainshock, (2) correlated our field data with the multitrack InSAR data, (3) relocated the epicenter of the mainshock, and its aftershocks, and (4) implemented moment tensor inversion analyses to the earthquakes larger than M 3.3. We defined a ~ N70E-striking surface rupture and measured the average co-seismic dextral slip magnitude as ~ 16 ± 1 cm in the field which is compatible with the horizontal displacement magnitude measured by InSAR (13–15 cm). Since no ~ N70E-striking faults were reported in the existing active fault database, we examined the prominent geomorphological structures along the rupture area. We determined two dextral fault families along the eastern part of the Yedisu Seismic Gap; (1) NW–SE-striking faults of the North Anatolian Fault Zone, and (2) N70E-striking faults forming the Kaynarpınar–Yuvaklı Fault Zone. This fault zone, defined and named in this study, is a part of the N70E-striking faults of the Turkish–Iranian High Plateau and the North Anatolian Fault Zone. The fault zone must have carried some, if not most, of the dextral sense-of-slip in the easternmost part of the North Anatolian Fault Zone during the Quaternary. [ABSTRACT FROM AUTHOR]

Precise earthquake locations and InSAR (Interferometric Synthetic Aperture Radar) deformation observation are the major methods to understand the earthquake occurrence and disaster-causing process. This paper proposes a processing framework for analyzing strong earthquake mechanisms from one-dimensional velocity inversion to precise earthquake locations combined with InSAR deformation observation, and discusses earthquake-generating fault and dynamic mechanisms of tectonic deformation. We analyzed the Menyuan Ms 6.9 earthquake in 2022 and discuss the historical seismic activities and corresponding stress adjustment processes in the research region. To analyze and study the seismogenic structure and mechanism of the earthquake, we investigated the spatial and temporal distribution characteristics of the Menyuan earthquake sequence and analyzed the InSAR coseismic deformation field. We obtained the precise locations of the main shock and aftershocks and the coseismic InSAR deformation field of the main shock. It was confirmed that the Ms 6.9 earthquake was a shallow sinistral strike-slip earthquake, which led to the sequential activation of the Tuolaishan and Lenglongling faults. The main seismogenic fault of the mainshock was the northwestern end of the Lenglongling fault, and the earthquake rupture was segmented. It can be inferred that the earthquake was a stress-adjusted event triggered in the Qilian-Haiyuan tectonic belt caused by the northeasterly push of the Qinghai-Tibet Plateau. The risk of moderate to high earthquakes in the region remains high in the future, requiring enhanced seismic observations. [ABSTRACT FROM AUTHOR]

The 2010 Beni-Ilmane earthquake sequence, with 3 Mw ≥ 5.2 mainshocks, occurred in the southern Tellian Atlas approximately 200 km southeast of the Algiers. We offer updated 3D Vp and Vs models, and we outline the space and time variations in the Vp/Vs ratio of the Beni-Ilmane crust based on 2297 well-located aftershocks representing the complete aftershock sequence. Two main P-wave low-velocity patterns correspond to fault alignments determined from geological studies: the NE–SW thrust fault located at the foot of Djebel Choukchot and the NNW–SSE oriented Samma strike-slip corridor. The former crosscuts the two P-wave high-velocity anomalies of the Kef Ouagnane and Kef Kherrat summits of the Choukchot Mountains. The third high-velocity anomaly represents the Jebel Affroun Mountain in the sub-Bibanic domain. Two high-Vp/Vs ratio patterns were revealed in the central aftershock part and along the NE–SW Ouanougha syncline, which constitutes a fluid reservoir and a fluid-saturated fault zone filled after aftershock activity. The 4D Vp/Vs models monitored fluid migration, and the modeled pore-pressure diffusion revealed that earthquake rupture along an ~ N–S blind strike-slip fault fractured the closed reservoir and released high-pressure fluids, provoking a notable and long-lasting aftershock sequence. Our study underlines that local earthquake tomography (LET) can provide useful insights into the upward migration of fluids from a compartmentalized hydrocarbon reservoir. [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]

We provide high‐resolution seismic imaging of the central Garlock fault using data recorded by two dense seismic arrays that cross the Ridgecrest rupture zone (B4) and the Garlock fault (A5). Analyses of fault zone head waves and P‐wave delay times at array A5 show that the Garlock fault is a sharp bimaterial interface with P waves traveling ∼5% faster in the northern crustal block. The across‐fault velocity contrast agrees with regional tomography models and generates clear P‐wave reflections in waveforms recorded by array B4. Kirchhoff migration of the reflected waves indicates a near‐vertical fault between 2 and 6 km depth. The P‐wave delay times imply a ∼300‐m‐wide transition zone near the Garlock fault surface trace beneath array A5, offset to the side with faster velocities. The results provide important constraints for derivations of earthquake properties, simulations of ruptures and ground motion, and future imaging studies associated with the Garlock fault. Plain Language Summary: Along the northern edge of the Mojave Desert, the Garlock fault intersects the San Andreas fault and is the second largest (∼300 km long) fault in Southern California. It can host M > 7 earthquakes that pose significant seismic hazard to densely populated communities. However, the subsurface structure of the Garlock fault is not well understood due to the sparse seismic network and lack of seismic activity nearby. The 2019 Ridgecrest earthquake sequence in the Eastern California Shear Zone led to a rapid deployment of several dense linear arrays with ∼100 m spacing and apertures of a few kilometers, which cross the Ridgecrest rupture zone and also the Garlock fault. The recorded seismic data is used here to illuminate the internal structure of the central Garlock fault. Analyses of P‐wave delay times and waves refracted along and reflected by the fault interface indicate a near‐vertical Garlock fault that separates two distinctive crustal blocks with different wave speeds. The resolved high‐resolution fault zone image can have important implications for multiple studies associated with the Garlock fault. Key Points: We image the central Garlock fault using data of aftershocks of the 2019 Ridgecrest earthquake recorded by two dense linear arraysA P‐wave velocity contrast across the fault (∼5% faster in the north) generates clear fault zone head and reflected wavesKirchhoff migration of P waves reflected by the fault indicates a near‐vertical interface with a sharp impedance contrast [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]

An Mw 5.7 earthquake occurred in Xegar, Tingri Basin, Tibetan Plateau, on 20 March 2020. Determining the precise focal mechanism solution is helpful for understanding the seismogenic mechanism and the geodynamic process of this earthquake. Here, we used Sentinel-1A data to obtain line-of-sight coseismic deformation. Fault geometric parameters and coseismic slip distribution can be estimated with the Bayesian method and the steepest descent method, respectively. The inversions show that the strike of the seismogenic fault is ~ 330.0°, with a dip angle of ~ 62.7°. The main rupture zone covers an area of ~ 5 × 5 km2. Only one slip asperity appears at a depth of 1.9–5.0 km, the maximum slip on the fault is 0.98 m at a depth of 3.26 km, with a centroid location of 87.40° E, 28.66° N, and the mean rake angle is ~ −104.6°. The results reveal that this earthquake is dominated by normal faulting, and the derived seismic moment is ~ 3.23 × 1017 Nm (Mw 5.6). Furthermore, it is found that the 2015 Mw 7.9 Gorkha earthquake played a key role in triggering the 2020 Mw 5.7 Xegar earthquake based on calculation of the coseismic and postseismic Coulomb failure stress with different viscosities and depths. [ABSTRACT FROM AUTHOR]

The Brawley Seismic Zone (BSZ) is a ∼58-km-long section of the Pacific–North America Plate boundary that connects the southernmost San Andreas Fault (SAF) and the Imperial Fault in southern California. We analyse the BSZ as two segments: a north segment, dominated by SAF tectonics, and a south segment that accommodates a higher level of seismicity. The south segment includes a ∼6 km wide right lateral step-over that includes the Salton Sea geothermal field (SSGF) and Holocene subaerial lava domes at the south end of the Salton Sea, called Salton Buttes. In general, the 40 yr (1981–2021) of BSZ seismicity is characterized by transitory sequences that often are accommodated simultaneously or in quick succession on intricate ladder-like faults. These sequences rarely reactivate faults associated with previous seismicity alignments but rather activate adjacent faults, sometimes located within less than 1 km. They can include several events of similar size as the mainshock, followed by bursts of aftershocks sometimes located away from the mainshock rupture. The seismicity rate and the spatial geometrical complexity varies between the BSZ-north and BSZ-south segments. The 24-km-long BSZ-north accommodates a ∼12-km-long linear trend of seismicity that extends from the SAF terminus into the Salton Sea, where moderate-sized northeast striking sequences form the rungs in a ladder-structure in a weak crust. The seismicity in this area is most likely influenced by the stress state of the SAF. In contrast, the 34 km long BSZ-south segment, which also has a weak crust, has accommodated larger sequences that illuminate irregular ladder-type faulting, with aftershocks defining linear distributions striking either north or northeast. The focal mechanisms exhibit a mostly strike-slip style of faulting with minor dip-slip faulting in the south Salton Sea area as well as the Mesquite basin to the south. The state of stress, as determined from focal mechanisms, consists of almost horizontal σ 1, and σ 3 with vertical σ 2. The activation angle between the trend of σ 1 and the preferred nodal plane of the largest nearby event decreases systematically from north to south along the long-axis of the BSZ. In the step-over zone, the Holocene volcanism and the frequent seismicity sequences suggest crustal extension as well as associated reduced crustal strength as compared to other parts of the BSZ. The presence of weak, thin, hot crust and distributed ladder-like or conjugate predominantly strike-slip faulting suggests that the whole BSZ acts as a several kilometre wide fault zone where no prominent principal slip surfaces are required to accommodate moderate-sized (M 6+) earthquakes. [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]

• The seismogenic fault is a typical sinistral strike-slip fault with a rupture length of over 160 km. • The displacement in E-W is the biggest among three-dimensional displacements with the RMSE 3.01 mm. • The peak slip is 5.53 m with the seismic moment of Mw 7.3 in the homogenous half-space model. • The peak slip is 5.96 m with the seismic moment of Mw 7.4 in the layered half-space model. On May 22, 2021, an Mw 7.4 earthquake occurred on the Maduo-Jiangcuo fault, located in Maduo County, Qinghai Province (China). According to the co-seismic deformation and the three-dimensional displacement reached from the interferometric synthetic aperture radar (InSAR) and the global navigation satellite system (GNSS), the rupture trace in the NWW direction was mapped, spanning a length of approximately 160 km and the earthquake happened on a typical sinistral strike-slip fault. In three-dimensional displacement, the displacement in E-W was bigger than that in N-S and vertical, with the RMSE 3.01 mm, 15.28 mm, and 5.16 mm respectively. Moreover, the slip distribution inverted with the homogeneous half-space model and layered half-space model, indicated that the corresponding magnitude was 7.3 Mw and 7.4 Mw, and the corresponding peak slip was 5.53 m and 5.96 m. The correlation coefficients of observed and predicted both exceeded 0.791, meeting the need for slip inversion. What is more, the peak inversion depth was no more than 15 km with strike angle and dip angle of 269° and 82.9° in the homogeneous half-space model and 265° and 83.7° in the layered half-space model, respectively. In addition, the Coulomb stress changes decreased on the Maduo-Jiangcuo fault and accumulated on the East Kunlun fault, which might cause aftershocks and future earthquakes were still possible. In short, the 2021 Mw7.4 Maduo earthquake made it possible to unravel seismic behavior in the Tibet plateau, which is of great significance for seismic hazard research. [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]

Objectives: On January 8th 2022, an Ms 6.9 earthquake struck Menyuan, Qinghai Province, China. The epicenter is located in the middle section of Qilian-Haiyuan fault in the northeast margin of the Tibetan Plateau, which is a seismic gap. It is of great significance to study this earthquake for under-standing the tectonic deformation mechanism, strain release process and seismic hazards of the NE Tibetan Plateau.Methods: Differential synthetic aperture radar interferometry (D-InSAR) is used to obtain the coseismic deformation along the line of sight (LOS) of the Menyuan earthquake from Sentinel-1 satellite images. The co-registration of optically sensed images and correlation are used to obtain the EW coseismic deformation from the multi-temporal Sentinel-2 data. Based on the coseismic deformation field derived from InSAR interferograms, we invert the slip distribution. Results: The LOS deformation fields showed that the Menyuan earthquake caused a surface rupture of about 20 km long and the maximum LOS displacement was 0.75 m. The maximum EW displacement derived from optical images was 2.5 m. The slip distribution model showed that the Menyuan earthquake was dominated by a sinistral strike-slip. The coseismic slip is concentrated at a depth of 0 - 10 km. The maximum slip was 3.25 m at a depth of 4.89 km and the rake angle was 10.44°. The estimated total seismic moment is 1.07x1019 N.m, corresponding to a magnitude of Mw 6.6. Conclusions: According to the results from aftershock relocation, field investigation and tectonic background, we infer that the seismogenic fault is the Lenglongling fault and the eastern segment of Tuolaishan fault is also ruptured. The stress on the eastern segment of the Lenglongling fault and the western segment of the Tuolaishan fault is enhanced, implying that there is the possibility of strong earthquakes in this region in the future. [ABSTRACT FROM AUTHOR]

A Mw 6.6 earthquake struck Menyuan, Qinghai, China, on 7 January 2022. To determine the rupture parameters of this event, the coseismic InSAR deformation fields were mapped and further employed to estimate the focal mechanism. The best-fitting solution emphasized that the 2022 Menyuan earthquake ruptured at the junction of the Tuolaishan fault and the Lenglongling fault. Both rupturing faults were dominated by sinistral strike-slip, and the main slip was concentrated on the shallow part of the rupture plane. The latter was the main rupture segment with a strike of 106° and a dip of 86°. The slip mainly occurred at depths of 0–8 km, and the rupture was exposed at the surface. The maximum slip reached ~3.5 m, which occurred mainly at a depth of 4 km. Joint analysis of the optimal slip model, relocated aftershocks, Coulomb stress change, and field observation suggested that the strain energy in the Tuolaishan fault may not have been fully released and needs further attention. Moreover, the 2022 Mw6.6 Menyuan earthquake caused a significant stress loading effect on the western Tuolaishan fault and eastern Lenglongling fault, which implies that the 2022 event increased the seismic hazard in these regions. [ABSTRACT FROM AUTHOR]

On May 5, 2014, an earthquake with a magnitude of Mw 6.1 (the largest earthquake in Thailand so far) occurred in Chiang Rai of the Golden Triangle area in northern Thailand. We had an opportunity to conduct field survey immediately after the earthquake. Serious damage to buildings and casualties of lives were observed, and the estimated Maximum Mercalli Intensity (MMI) of the earthquake is VIII (evaluated according to the MMI scale of the Chinese Standard). No long continuous ground ruptures were produced during the earthquake, but in the epicenter (commonly within MMI VIII extent), massive small linear ruptures (usually several tens of meters long) developed and displayed intriguing structural features, offsetting many roads several centimeters left laterally on NE trending cracks or offsetting right laterally on NW trending ones. The focal mechanism solution of earthquake shows that this is a pure strike‐slip event, and two nodal planes in NW and NE directions had the same motion senses respectively as those of breakage associated with the earthquake. The long axis of the isoseismals and aftershock distributions are in NE direction, which is consistent with the strike of Luang Namtha fault. The 230‐km‐long Luang Namtha fault which starts from the border of China and Laos, runs through northern Laos, and terminates at Chiang Rai of Thailand is predominated by left‐lateral strike‐slip and active in late Quaternary, and two earthquakes over Ms 6.0 occurred along the fault in 1925 and 2007 respectively. This Mw 6.1 earthquake occurred at the southwestern end of the fault. All related features such as evident structural rupturing, elongated orientation of MMI and aftershock distribution, as well as the location of the epicenter, suggest that the Luang Namtha fault may be responsible for the 2014 Northern Thailand earthquake. [ABSTRACT FROM AUTHOR]

The 2016 M w 7.8 Kaikoura, New Zealand, earthquake occurred along the eastern margin of the transition region between active subduction in the North Island and oblique collision in the South Island. To infer crustal properties, we imaged Q (1/seismic attenuation) by combining selected M > 3.5 aftershocks with data from previous Q models. For 158 distributed aftershocks, we fit spectral decay on temporary stations and all Geonet stations, providing 6194 t * p and 19 497 t * s. Considering the varied rheology and faults, we also used 2.5-D numerical models to study ductile strain development. The complex earthquake ruptured an ∼180-km-long zone of multiple faults, which involved jumping around the complicated eastern end of the Hope fault, without significant slip on the Hope fault. The Qs and Qp results show features in the upper and lower crust which correlate to the distribution and types of fault rupture. This earthquake involved numerous faults over a region of greywacke crust, where the underlying high Q Cretaceous slab is about 30-km depth. It initiated with ∼5-m slip on the Humps fault in a region of background seismicity and low Q lower crust, adjacent to the Hope fault. The central region near Kaikoura shows a high Q crustal block, which appears to have inhibited rupture; as the rupture progressed over several small faults to jump offshore of the apparently strong block. Underlying the Kaikoura greywacke crust, below 20-km depth, there is a 40-km-long region of increased Vp, Qs and gravity, which likely represents an intraplate plutonic complex emplaced into the Hikurangi Plateau, forming an elevated section which influences deformation. In the northern section, in a region with relatively uniform moderate to low Q , the earthquake evolved into the relatively continuous ∼80-km-long major rupture along the Jordan, Kekerengu and Needles faults, with ∼6–20 m dextral slip at depth and surface displacements of ∼10 m dextral and ∼2 m vertical. The northern progression of the rupture stopped when it approached an abrupt change to high Q crust across Cook Strait. At 20–30-km depth northwest of the rupture, deeper zones with low Q are consistent with regions of distributed ductile shear and creep where the observed afterslip may have occurred, where the underlying slab is 25–40 km deep. The numerical model shows that ductile deformation localizes in this area of lower crust above the relatively strong slab, connecting outer faults (Kekerengu) to inland faults (Clarence, Awatere, Wairau), and demonstrates that no subduction thrust is required under the Marlborough region. [ABSTRACT FROM AUTHOR]

Three devastating earthquakes of MW ≥ 5.9 activated a complex system of high‐angle normal, antithetic, and sub‐horizontal detachment faults during the 2016–2017 central Italy seismic sequence. Waveform cross‐correlation based double‐difference location of nearly 400,000 aftershocks illuminate complex, fine‐scale structures of interacting fault zones. The Mt. Vettore–Mt. Bove (VB) normal fault exhibits wide and complex damage zones, including a system of bookshelf faults that intersects the detachment zone. In the Laga domain, a comparatively narrow, shallow dipping segment of the deep Mt. Gorzano fault progressively ruptures through the detachment zone in four subsequent MW ∼ 5.4 events. Reconstructed fault planes show that the detachment zone is fragmented in four sub‐horizontal, partly overlaying shear planes that correlated with the extent of the mainshock ruptures. We find a new, deep reaching seismic barrier that coincides with a bend in the VB fault and may play a role in controlling rupture evolution. Plain Language Summary: In 2016–2017, a sequence of three devastating earthquakes with magnitudes ∼6 near the towns of Amatrice, Visso, and Norcia in central Italy triggered hundreds of thousands of aftershocks. We compute high‐precision locations for these aftershocks that have the power to illuminate the fine‐scale structure of the complex system of intersecting and interacting faults that were activate during the sequence. The normal faults that ruptured during the three mainshocks are capped at ∼9 km depth by a sub‐horizontal detachment zone. The normal faults intersect the detachment at an acute angle, causing complex fault zone structures and fragmentation of the detachment where they intersect. We observe a deep reaching seismic barrier that we conclude may play a role in controlling rupture extent and evolution of large earthquakes along this section of the Apennines. In general, fault complexity here is higher than in similar sequences in 2009 near L'Aquila to the south and in 1997 near Colfiorito to the north. Key Points: Precise locations of 390,334 aftershocks of the 2016–2017 Central Italy sequence illuminate the complex normal fault systemA system of high‐angle bookshelf faults accommodates slip at the intersection between normal and detachment faultsA deep‐reaching seismic barrier is identified north of the Sibillini Thrust that may control the ruptures of large earthquakes [ABSTRACT FROM AUTHOR]

We develop finite element models of the coseismic displacement field accounting for the 3D elastic structures surrounding the epicentral area of the 2019 Ridgecrest earthquake sequence containing two major events of Mw7.1 and Mw6.4. The coseismic slip distribution is inferred from the surface displacement field recorded by interferometric synthetic aperture radar. The rupture dip geometry is further optimized using a novel nonlinear‐crossover‐linear inversion approach. It is found that accounting for elastic heterogeneity and fault along‐strike curvilinearity improves the fit to the observed displacement field and yields a more accurate estimate of geodetic moment and Coulomb stress changes. We observe spatial correlations among the locations of aftershocks and patches of high slip, and rock anomalous elastic properties, suggesting that the shallow crust's elastic structures possibly controlled the Ridgecrest earthquake sequence. Most of the coseismic slip with a peak slip of 7.4 m at 3.6 km depth occurred above a zone of reduced S‐wave velocity and significant post‐Mw7.1 afterslip. This implies that viscous materials or fluid presence might have contributed to the low rupture velocity of the mainshock. Moreover, the zone of high slip on the northwest‐trending fault segment is laterally bounded by two aftershock clusters, whose location is characterized by intermediate rock rigidity. Notably, some minor orthogonal faults consistently end above a subsurface rigid body. Overall, these observations of structural controls improve our understandings of the seismogenesis within incipient fault systems. Plain Language Summary: On July 4 and 5, 2019, a magnitude 7.1 and 6.4 earthquake occurred near the town, Ridgecrest, California. The permanent surface movements induced by these events were captured by satellite images. Here, we develop realistic numerical models and innovational optimization algorithms to investigate the displacement fields and image the distribution of the earthquake slip. These events were hosted by a pair of orthogonal faults and their resolved orientations are similar to the nearby known fault structures. We discover that the fault slip and aftershock locations could be related to the local rock materials, suggesting that the variable elastic properties of the shallow crust might have influenced the development of the Ridgecrest earthquake sequence. As also supported by other seismic and geophysical datasets, most of the coseismic slip appeared over a zone of elastic weakness where could be relevant to subsurface viscous or fluid bodies, meanwhile, aftershocks tended to cluster within a shallow rigid body. Overall, the findings of this study provide insights into potential structural controls over the seismic occurrence and help improve our understandings of the seismogenesis within the western Mojave desert. Key Points: Heterogeneous velocity structures in the epicentral area are simulated in elastic modelsPeak slip occurred above a zone of reduced S‐wave velocity and documented afterslipMost aftershocks are located outside slip asperities and within a shallow brittle zone [ABSTRACT FROM AUTHOR]

The 2011 M w 9.0 Tohoku-oki earthquake ruptured the subduction megathrust fault in the central Japan Trench. We investigated the aftershock activity in the southern Japan Trench to the south of the main rupture area using ocean bottom seismographs deployed both landward and seaward of the trench. In the trench-outer rise region seaward of the trench axis, we identified several ∼100-km-long linear earthquake trends both parallel and oblique to the southern Japan Trench. The earthquake trend oblique to the southern Japan Trench is a southward extension of the trench-parallel linear earthquake trend in the central to northern Japan Trench. The trench-parallel normal-faults in the trench-outer rise region could extend linearly, despite the change of the trench strike from N–S to NNE–SSW to the south of the main rupture area. Normal-faults oblique to the trench should be considered as substantial parts of large intraplate normal-faulting earthquakes. In addition, intraplate seismicity coinciding with the lower velocity oceanic mantle suggest that the structure heterogeneity would be indicative of normal-faults extending into the mantle. In the trench landward area, earthquake activity showed along-trench variations. Earthquakes along the shallow megathrust interface near the trench were observed south of 37°N. These shallow near-trench regular earthquakes, which are located close to the episodic tremors and temporally correlated with the tremor activities, suggest that the afterslip on the plate interface likely extended to the shallow plate interface close to the trench axis. Smaller spatial scale structure heterogeneity, such as the thickness variation in the channel-like low-velocity sedimentary unit, likely relate to the proximity of the regular earthquakes and slow slip which results in the formation of diverse slip behaviours in the shallow subduction zone of the southern Japan Trench. [ABSTRACT FROM AUTHOR]

We analyse the instrumental seismicity in a sector of the Po Plain (Italy) with the aim of defining the baseline for seismic monitoring of a new underground gas storage plant that will use the depleted gas reservoir of Cornegliano Laudense, near Lodi. The target area - a square approximately 80 x 80 km wide - is commonly considered aseismic. The analysed period, 1951-2019, includes all available instrumental data. We gathered the P- and S-phase readings collected by various agencies for more than 300 events, approximately located inside the target area. We processed the earthquakes in a uniform way, using absolute location algorithms and velocity models adopted by the regional and national monitoring networks. The relocated earthquake dataset depicts an image of weak and deep seismicity for this central sector of the Po Plain, which is quite different from the initial one derived from the existing earthquake catalogues. Within a distance of approximately 30 km from Lodi, earthquakes are extremely rare (on average 0.5 earthquake/yr, assuming a completeness magnitude Mc = 2.7 from the 1980s); only 2 weak events fall at less than 15 km distance from the reservoir in the whole period 1951-2019. The strongest events instrumentally recorded are related to the seismic sequence of Caviaga in 1951 that represent the first instrumental recordings for that area. Confirming the hypocentral depths recently proposed by Caciagli et al., 2015, the events are far from the gas reservoir; we suggest a common tectonic stress of the main shock of 1951 and the M4.2 earthquake of Dec 17, 2020, on the basis of the similarities in depth, location and focal mechanism. While it is clear that the deep seismicity corresponds to the collision between the Northern Apennine and Southern Alps, it is much less clear, however, which geological structures are capable of generating earthquakes. Our results and the improvement in the observational capabilities of the very last years will help refining the seismogenic sources hypothesized for this area. [ABSTRACT FROM AUTHOR]

We analyse the instrumental seismicity in a sector of the Po Plain (Italy) with the aim of defining the baseline for seismic monitoring of a new underground gas storage plant that will use the depleted gas reservoir of Cornegliano Laudense, near Lodi. The target area - a square approximately 80 x 80 km wide - is commonly considered aseismic. The analysed period, 1951-2019, includes all available instrumental data. We gathered the P- and S-phase readings collected by various agencies for more than 300 events, approximately located inside the target area. We processed the earthquakes in a uniform way, using absolute location algorithms and velocity models adopted by the regional and national monitoring networks. The relocated earthquake dataset depicts an image of weak and deep seismicity for this central sector of the Po Plain, which is quite different from the initial one derived from the existing earthquake catalogues. Within a distance of approximately 30 km from Lodi, earthquakes are extremely rare (on average 0.5 earthquake/yr, assuming a completeness magnitude Mc=2.7 from the 1980s); only 2 weak events fall at less than 15 km distance from the reservoir in the whole period 1951-2019. The strongest events instrumentally recorded are related to the seismic sequence of Caviaga in 1951 that represent the first instrumental recordings for that area. Confirming the hypocentral depths recently proposed by Caciagli et al., 2015, the events are far from the gas reservoir; we suggest a common tectonic stress of the main shock of 1951 and the M4.2 earthquake of Dec 17, 2020, on the basis of the similarities in depth, location and focal mechanism.While it is clear that the deep seismicity corresponds to the collision between the Northern Apennine and Southern Alps, it is much less clear, however, which geological structures are capable of generating earthquakes. Our results and the improvement in the observational capabilities of the very last years will help refining the seismogenic sources hypothesized for this area. [ABSTRACT FROM AUTHOR]

The article presents a study of the semibrittle seismic deformation related to a high-temperature mantle below the brittle lithosphere along the Romanche oceanic transform fault (OTF) in the equatorial Atlantic Ocean. Topics discussed include the inconsistency of the maximum depths of microseismicity along the OTF and a fixed temperature isotherm, influence of local dynamic processes such as hydrothermal circulation in microseismicity, and the common location of the microearthquakes.

This study introduces an improved hypocentral version of the space‐time Epidemic‐Type Aftershock Sequence (ETAS) model, entitled as the 3D‐finite source (3D‐FS) ETAS model, and applies it to the analysis of the Southern California earthquake catalog. By stochastic reconstruction, we are able to reconstruct the patterns of aftershock productivity density along the mainshock ruptures. Detailed analysis of the productivity patterns reveals that: (1) Directly triggered aftershocks make up 21% to 41% of all earthquakes within the mainshock rupture areas, and show significant spatial heterogeneity and temporal migrations; (2) Major aftershocks tend to locate in low productivity areas, at the edges of clusters formed by small aftershocks; (3) Large slip areas are depleted of aftershocks, over 60% of all productivity distributes in areas with slip less than 0.3 times of the maximum slip, and the trajectory of the productivity pattern on the fault plane demonstrates apparent compensation to coseismic slip. We relate the difference in triggering abilities of four mainshocks to the heat flow in corresponding regions. Simulation results suggest that the 3D‐FS ETAS model has apparent advantages of improving the performance of short‐term aftershock forecast. Moreover, the later aftershocks are more correlated with the locations of subsequent events than earlier aftershocks, suggesting that the migration of aftershocks is important for mitigating aftershock hazard. Plain Language Summary: How many aftershocks can be produced at each part along the rupture of a large earthquake is crucial for us to understand how aftershocks are generated and how the strain energy in the crust relaxes afterward. It is difficult to know whether an earthquake is an aftershock triggered by some previous earthquake or a background earthquake that has no ancestry since earthquake clusters not only overlap with each other but also with background seismicity in space and time. We proposed a new earthquake clustering model to disentangle such overlapping and quantify family trees among the earthquakes. Our study on the earthquake data in Southern California shows that aftershocks tend to occur at low slip areas in the mainshock rupture and that large aftershocks occur farther away near the boundary of such concentration of aftershocks. Moreover, this model improves the short‐term aftershock forecasting. Key Points: Large aftershocks occur in the edge of areas with high aftershock productivityAftershock productivity exhibit compensating patterns in association with the coseismic slipThe mainshock ruptures constrained from early aftershocks help improve aftershock forecasting [ABSTRACT FROM AUTHOR]

Recent deployments of dense seismic and geodetic observation networks have revealed a detailed pattern of crustal stress and strain rate in tectonically active regions all over the world. Since the DSW origin is located between the large cluster of earthquakes and a low-velocity zone, they concluded that the DSW origin was composed of geofluid accumulated midway in the upward fluid movement from the low-velocity zone to the earthquake cluster. 2020; 72: 19. 10.1186/s40623-020-1147-y 11 Iio Y, Matsumoto S, Yamashita Y, Sakai S, Tomisaka K, Sawada M, Iidaka T, Iwasaki T, Kamizono M, Katao H, Kato A, Kurashimo E, Teguri Y, Tsuda H, Ueno T. Stationarity of aftershock activities of the 2016 Central Tottori Prefecture earthquake revealed by dense seismic observation. Earth Planets Space 71:94. https://doi.org/https://doi.org/10.1186/s40623-019-1073-z 17 Matsumoto S, Iio Y, Sakai S, Kato A. Inelastic strain in the hypocentral region of the 2000 Western Tottori earthquake (M 7.3) inferred from aftershock seismic moment tensors. [Extracted from the article]

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 article discusses temperature plays a critical role in defining the seismogenic zone, the area of the crust earthquakes commonly occur; thermal controls on fault ruptures have rarely observed directly. Topics include the maximum magnitude of an earthquake on a fault has determined by the area of the fault surface located above the transition from brittle to visco-plastic deformation; and earthquake segmentation due to thermal structure has occured temperature gradients exist on a fault.

We investigate the coseismic and post-seismic deformation due to the 6 February 2018 M w 6.4 Hualien earthquake to gain improved insights into the fault geometries and complex regional tectonics in this structural transition zone. We generate coseismic deformation fields using ascending and descending Sentinel-1A/B InSAR data and GPS data. Analysis of the aftershocks and InSAR measurements reveal complex multifault rupture during this event. We compare two fault model joint inversions of SAR, GPS and teleseismic body waves data to illuminate the involved seismogenic faults, coseismic slip distributions and rupture processes. Our preferred fault model suggests that both well-known active faults, the dominantly left-lateral Milun and Lingding faults, and previously unrecognized oblique-reverse west-dipping and north-dipping detachment faults, ruptured during this event. The maximum slip of ∼1.6 m occurred on the Milun fault at a depth of ∼2–5 km. We compute post-seismic displacement time series using the persistent scatterer method. The post-seismic range-change fields reveal large surface displacements mainly in the near-field of the Milun fault. Kinematic inversions constrained by cumulative InSAR displacements along two tracks indicate that the afterslip occurred on the Milun and Lingding faults and the west-dipping fault just to the east. The maximum cumulative afterslip of 0.4–0.6 m occurred along the Milun fault within ∼7 months of the main shock. The main shock-induced static Coulomb stress changes may have played an important role in driving the afterslip adjacent to coseismic high-slip zones on the Milun, Lingding and west-dipping faults. [ABSTRACT FROM AUTHOR]

The unbroken section of the North Anatolian Fault beneath the Sea of Marmara is a major source of seismic hazard for the city of İstanbul. The northern and currently the most active branch, the Main Marmara Fault (MMF), is segmented within a shear zone and exhibits both partially creeping and locked behaviour along its 150 km length. In 2019 September, a seismic activity initiated near MMF, off-coast the town of Silivri, generating 14 earthquakes ≥  M w 3.5 in a week. The M w 5.8 Silivri earthquake, is the largest in the Marmara Sea since the 1963 M w 6.3 Çınarcık earthquake. Our analyses reveal that the activity started in a narrow zone (∼100 m) and spread to ∼7 km following an M w 4.7 foreshock within ∼2 d. The distribution of relocated aftershocks and the focal mechanisms computed from regional waveforms reveal that the M w 5.8 earthquake did not occur on the MMF, but it ruptured ∼60° north-dipping oblique strike-slip fault with significant thrust component located on the north of the MMF. Finite-fault slip model of the main shock shows 8 km long rupture with directivity toward east, where the ruptured fault merges to the MMF. The narrow depth range of the slip distribution (10–13 km) and the aftershock zone imply that the causative fault is below the deep sedimentary cover of the Marmara Basin. The distribution of aftershocks of the M w 5.8 event is consistent with Coulomb stress increase. The stress changes along MMF include zones of both stress decrease due to clamping and right-lateral slip, and stress increase due to loading. [ABSTRACT FROM AUTHOR]

Aseismic deformation has been suggested as a mechanism to release the accumulated strain in rifts. However, the fraction and the spatial distribution of the aseismic strain are poorly constrained during amagmatic episodes. Using Sentinel‐1 interferograms, we identify the surface deformation associated with the 2014 Mw5.2 Karonga earthquake, Malawi, and perform inversions for fault geometry. We also analyze aftershocks and find a variety of source mechanisms within short timescales. A significant discrepancy in the earthquake depth determined by geodesy (3–6 km) and seismology (11–13 km) exists, although both methods indicate Mw5.2. We propose that the surface deformation is caused by aseismic slip from a shallow depth. This vertical partitioning from seismic to aseismic strain is accommodated by intersecting dilatational faults in the shallow upper crust and sedimentary basin, highlighting the importance of considering aseismic deformation in active tectonics and time‐averaged strain patterns, even in rifts with little volcanism. Plain Language Summary: During the early stages of continental rifting, some accumulated tectonic energy is released with little or no earthquake activity (i.e., aseismic strain). This is most often observed in places where magma intrusion occurs. However, in rift basins lacking evidence of magma intrusion, how much and where the aseismic strain is released remains largely unknown. We investigate a magnitude 5.2 earthquake that occurred in 2014 near Karonga, Malawi, using both local seismic data and satellite ground deformation data. Our analyses show that although the main earthquake and other aftershocks ruptured at ∼12 km deep, the surface deformation is caused by an aseismic source from a shallower region (3–6 km deep). The aftershocks occur seconds to minutes apart and have different rupture sources, suggesting a highly damaged zone. The depth discrepancy between earthquake locations and the aseismic energy source might be caused by a weak, damaged layer where many faults intersect. Since intersecting faults are common in rift systems, our study suggests that this depth discrepancy might be common as well, indicating a large portion of energy released as aseismic strain in a continental rifting system. Key Points: InSAR implies deformation from a shallow depth (3–6 km), while seismicity suggests a deeper origin (11–13 km)Aftershock source mechanisms suggest fault intersection along steep east and west dipping fault planesFractured zone might separate shallow aseismic and deeper seismic strain beneath sedimentary basin [ABSTRACT FROM AUTHOR]

Here, we present the results of a kinematic slip model of the 2020 M w 6.7 Doğanyol-Sivrice, Turkey Earthquake, the most important event in the last 50 yr on the East Anatolian Fault Zone. Our slip model is constrained by two Sentinel-1 interferograms and by 5 three-component high-rate GNSS (Global Navigation Satellite System) recordings close to the earthquake source. We find that most of the slip occurs predominantly in three regions, two of them at between 2 and 10 km depth and a deeper slip region extending down to 20 km depth. We also relocate the first two weeks of aftershocks and find a distribution of events that agrees with these slip features. The HR-GNSS recordings suggest a predominantly unilateral rupture with the effects of a directivity pulse clearly seen in the waveforms and in the measure peak ground velocities. The slip model supports rupture propagation from northeast to southwest at a relatively slow speed of 2.2 km s−1 and a total source duration of ∼20 s. In the absence of near-source seismic stations, space geodetic data provide the best constraint on the spatial distribution of slip and on its time evolution. [ABSTRACT FROM AUTHOR]

Here, we show that the 2019 Mw7.0 Ridgecrest mainshock as well as its Mw6.5 foreshock ruptured orthogonal conjugate faults. We invert the waveforms recorded by the dense strong motion network at relatively high frequencies (up to 1 Hz for P; 0.25 Hz for S) to derive multiple‐point source models for both events, aided by path calibrations from a Mw5.4 and a Mw5.5 earthquake. We demonstrate that the mainshock started from a shallow (3 km) depth with a Mw5.2 event and ruptured the main fault branches oriented in the NW‐SE direction. At ~11 s, two Mw6.2 subevents took place on the SW‐NE oriented fault branches that conjugate to the main fault to the NE and SW. The SW branch rupture partially overlapped with the foreshock rupture. We suggest the coseismic rupture on nearly orthogonal faults was enabled by high pore fluid pressure, which greatly weakened the immature fault system in a heterogeneous way. Plain Language Summary: Earthquakes, which are caused by shear dislocation processes on faults, often rupture single faults or multiple faults oriented at acute angles. However, rupture of orthogonal faults (i.e., faults oriented at 90° to each other) has until now been considered unfavorable based on the basic Mohr circle stress analysis. Here, we show that two large July 2019 earthquakes (Mw6.5 and Mw7.0) ruptured fault segments that are perpendicular to each other, with one NW trending and the other SW trending. We suggest that the complex fault slip is the result of a young fault system, aided by high heterogeneous pore fluid pressure. Key Points: The rupture process of the Ridgecrest Mw7.0 earthquake is represented by six subevents whose source parameters are constrained by local strong motion dataTwo subevents of the mainshock occurred on the SW striking conjugate fault, while the major rupture propagated on the NW striking fault systemSubevents and seismicity demonstrate the highly complex fault geometry, and the orthogonal fault coseismic rupture is most likely the result of high pore fluid pressure [ABSTRACT FROM AUTHOR]

To improve our understanding of the interactions between regular and slow earthquakes along the Japan Trench, we investigated the spatial relationship of slow-earthquake activity with the preseismic, coseismic, and postseismic fault ruptures of interplate earthquakes off the Iwate and Ibaraki coasts, Japan, including two large interplate aftershocks of the 2011 Tohoku earthquake: the 2011 off Iwate earthquake (MJMA 7.4) and the 2011 off Ibaraki earthquake (MJMA 7.6). We found that the coseismic ruptures of these earthquakes did not overlap with the active areas of slow earthquakes, while their foreshocks and aftershocks occurred in slow-earthquake-prone areas. Moreover, the 2011 off Iwate earthquake and the previous M7-class events shared common fault rupture characteristics: coseismic rupture occurred in a common asperity area, and afterslip with many aftershocks was triggered in the active area of slow earthquakes. Off the Ibaraki coast, tremor activity on a subducting seamount located updip of the coseismic rupture of the 2011 off Ibaraki earthquake implies that the seamount acted as a soft barrier to the coseismic rupture of the 2011 off Ibaraki earthquake. This study demonstrates that large earthquakes off the Iwate and Ibaraki coasts feature similar rupture behaviors, spatially complementary distributions of coseismic ruptures with slow-earthquake activity and foreshock and aftershock activities within and around slow-earthquake-prone areas. This information is useful in considering future large earthquakes along the Japan Trench. [ABSTRACT FROM AUTHOR]

In the southern Sichuan Basin, China, the recent increase in the seismic activity has been suspected to be related to hydraulic fracturing stimulation for producing the shale gas. In this study, we used the monitoring data from a local seismic network within the shale gas blocks to study the earthquakes near the shale gas production wells that have detailed injection data. Comparison of the timing of earthquakes and stimulation schedule of the studied well pads indicates an apparent correlation between the seismic activity and hydraulic fracturing. The results of seismic velocity tomography reveal that the reactivation of preexisting faults due to fluid diffusion is the primary cause of the observed earthquakes. Focal mechanism analysis combined with geomechanical modeling indicates that the increased pore pressures resulted from hydraulic fracturing are sufficient to trigger seismic slip on the faults. Plain Language Summary: Starting in 2014, felt earthquakes have increased significantly in the southern Sichuan Basin, China. The hydraulic fracturing stimulation used for producing the shale gas in this area is suspected to be responsible for the increase of the seismic rate. This study analyzed the seismic monitoring and well injection data to investigate the relationship between the recorded earthquakes and hydraulic fracturing at the shale gas well pads. The results presented in this study indicate that fluid diffusion from hydraulic fracturing triggers the slip of preexisting faults and causes the occurrence of small earthquakes. Key Points: Stimulation data are available to study the relationship between seismicity and shale gas hydraulic fracturing in southern Sichuan BasinThe earthquakes are spatiotemporally correlated with hydraulic fracturing and are associated with low‐velocity anomaliesFocal mechanism and stress analysis suggest that the increased pore fluid pressure by hydraulic fracturing is enough to trigger the fault slip [ABSTRACT FROM AUTHOR]

We constrained the rupture process of the 25 October 2018 Mw6.8 Zakynthos earthquake by jointly inverting near‐fault strong motion records, static, and high‐rate GPS displacements along with teleseismic waveforms. The inverted slip distribution is characterized by two asperities with peak slip amplitude of 1.4 m, which is located at 17 km depth. The rupture essentially propagated unilaterally in the downdip direction after the initiation and to the north‐northeast toward Zakynthos island. The earthquake produced a deep rupture whose downdip extent approached 38 km, indicating that this event broke the entire underthrust Ionian oceanic crust beneath the megathrust and penetrated into the top few kilometers of the solid uppermost mantle. The extent of the rupture implied the presence of a low coefficient of dynamic friction. The mainshock triggered a counterclockwise rotation of the stress field with respect to the long‐term regional stress. Aftershocks occurred at shallower depths than the coseismic slip zone. Plain Language Summary: On 25 October 2018 a major earthquake occurred 45 km SSW of Zakynthos Island at Western Greece. The region where the earthquake struck is one of the most active seismic zones in the country. It is where the active Hellenic subduction, that is, the interface along which the Nubia plate subducts under the Aegean plate, reaches the Cephalonia Transform Fault. We jointly used seismic and geodetic data sets to study the 2018 Zakynthos earthquake and revealed that it was not a typical subduction or a transform fault event; it was something different. By constraining the rupture process of the earthquake, we showed that this event did not slip the megathrust surface of the subduction zone but instead fractured the entire underthrust Ionian oceanic crust as it descends beneath the Aegean plate suggesting intraslab deformation and probably tearing of the subducting slab. Furthermore, the main shock induced a counterclockwise rotation of the stress field with respect to the long‐term regional stress. Key Points: The slip distribution is characterized by two asperities and the rupture propagated unilaterally to the NNE and from 10 to 38 km in depthThe 2018 Zakynthos earthquake broke the entire underthrust Ionian oceanic crust beneath the megathrustThe mainshock triggered a counterclockwise rotation of the stress field with respect to the long‐term regional stress [ABSTRACT FROM AUTHOR]