Search

The M s 5.5 earthquake struck on 24 October 2023, in Subei County, Gansu Province, China, occurring along the eastern segment of the Altyn Tagh fault. It raises the question of whether this earthquake is linked to the ongoing shortening slip rate along this segment or triggered by other seismic events. Analyzing the fault geometry of the Subei earthquake and understanding the significance of the weakening activity rate for seismic hazards in neighboring regions is crucial. The surface deformation from small- and medium-sized earthquakes (magnitudes less than M w5.5) is often subtle, and the coseismic deformation detected by interferometric synthetic aperture radar (InSAR) is vulnerable to atmospheric disturbances, leading to significant measurement errors. Moreover, inaccuracies in the regional crustal velocity structure can cause errors in earthquake localization based on seismic data. These challenges complicate the establishment of a rupture model for seismogenic faults and hinder the inversion of fault slip models. To overcome these limitations, we employed the time-series InSAR stacking method and aftershock relocation to determine the fault geometry of the Subei earthquake. A two-step inversion method was utilized to ascertain both the fault geometry and slip distribution. Our modeling indicates that the 2023 Subei earthquake had a thrust mechanism with a component of strike-slip. The rupture did not reach the surface, with the maximum fault slip measuring 0.45 m at a depth of 2.5–3.5 km. The fault dips westward, and the moment magnitude is calculated at 5.4. This earthquake is associated with the ongoing weakening of the left-lateral strike-slip rupture along the Altyn Tagh fault in the Subei region. Furthermore, retrograde thrust tectonics significantly contribute to the absorption of accumulated stress during this process.Our findings highlight the potential of utilizing time-series InSAR images to enhance earthquake catalogs with geodetic observations, offering valuable data for further studies of the earthquake cycle and active tectonics. This approach is also applicable in other tectonically active regions, enhancing understanding of seismic hazards and risk assessment. [ABSTRACT FROM AUTHOR]

Megathrust geometric properties exhibit some of the strongest correlations with maximum earthquake magnitude in global surveys of large subduction zone earthquakes, but the mechanisms through which fault geometry influences subduction earthquake cycle dynamics remain unresolved. Here, we develop 39 models of sequences of earthquakes and aseismic slip (SEAS) on variably‐dipping planar and variably‐curved nonplanar megathrusts using the volumetric, high‐order accurate code tandem to account for fault curvature. We vary the dip, downdip curvature and width of the seismogenic zone to examine how slab geometry mechanically influences megathrust seismic cycles, including the size, variability, and interevent timing of earthquakes. Dip and curvature control characteristic slip styles primarily through their influence on seismogenic zone width: wider seismogenic zones allow shallowly‐dipping megathrusts to host larger earthquakes than steeply‐dipping ones. Under elevated pore pressure and less strongly velocity‐weakening friction, all modeled fault geometries host uniform periodic ruptures. In contrast, shallowly‐dipping and sharply‐curved megathrusts host multi‐period supercycles of slow‐to‐fast, small‐to‐large slip events under higher effective stresses and more strongly velocity‐weakening friction. We discuss how subduction zones' maximum earthquake magnitudes may be primarily controlled by the dip and dimensions of the seismogenic zone, while second‐order effects from structurally‐derived mechanical heterogeneity modulate the recurrence frequency and timing of these events. Our results suggest that enhanced co‐ and interseismic strength and stress variability along the megathrust, such as induced near areas of high or heterogeneous fault curvature, limits how frequently large ruptures occur and may explain curved faults' tendency to host more frequent, smaller earthquakes than flat faults. Plain Language Summary: Subduction zones, where one tectonic plate dives beneath another, generate the largest earthquakes worldwide. Our study investigates how the shape and tilt of these large offshore underground faults, termed "megathrusts," may determine the size of large earthquakes, how often they happen, and how similar or different subsequent events are. By creating computer simulations of earthquakes in subduction zones, we found that the angles and dimensions of the megathrust may set a limit on how big an earthquake can get. We also find that the presence of bends or curves along these faults can make earthquakes more unpredictable, sometimes leading to more variable series of smaller quakes before the biggest one hits. Our findings may help explain why some areas near subduction zones are prone to larger or more frequent earthquakes than others. Understanding these patterns can improve our ability to prepare for these natural disasters, potentially reducing their damaging effects on nearby communities and infrastructure. Key Points: Systematic earthquake cycle simulations reveal how subduction zone geometry controls megathrust earthquake size and timingDip and curvature affect characteristic slip style: periodic uniform ruptures versus supercycles of variably‐sized slow‐to‐fast eventsSeismogenic zone dip and dimensions limit maximum earthquake size; curvature‐linked stress and strength heterogeneity modulates recurrence [ABSTRACT FROM AUTHOR]

A devastating earthquake with moment magnitude 7.5 occurred in the Noto Peninsula in central Japan on 1 January 2024. We estimate the rupture evolution of this earthquake from teleseismic P‐wave data using the potency‐density tensor inversion method, which provides information on the spatiotemporal slip distribution including fault orientations. The results show a long and quiet initial rupture phase that overlaps with regions of preceding earthquake swarms and associated aseismic deformation. The following three major rupture episodes evolve on segmented, differently oriented faults bounded by the initial rupture region. The irregular initial rupture process followed by the multi‐scale rupture growth is considered to be controlled by the preceding seismic and aseismic processes and the geometric complexity of the fault system. Such a discrete rupture scenario, including the triggering of an isolated fault rupture, adds critical inputs on the assessment of strong ground motion and associated damages for future earthquakes. Plain Language Summary: On 1 January 2024, a moment magnitude 7.5 earthquake occurred in the northern Noto Peninsula, Japan. The strong ground motion and tsunami associated with the earthquake caused severe damage to buildings and infrastructure, resulting in at least 245 causalities in the affected areas. The Noto Peninsula is affected by northwest‐southeast compression, and active reverse faults are known along the northern coast of the peninsula and its offshore region. Before the 2024 earthquake, the source region experienced long‐lasting earthquake swarm activity, which is a set of seismic events without an obvious mainshock‐aftershock pattern. Our seismological analysis found that there was a 10‐s‐long initial rupture episode around the hypocenter that overlapped with the earthquake swarm region. The initial rupture was followed by a series of three different rupture episodes on differently oriented fault segments. This earthquake highlights a multi‐scale rupture growth across a segmented fault network after a very quiet initial rupture process that was controlled by the preceding earthquake swarms and associated aseismic deformation related to fluid injection from depth. The rupture process advances our understanding of earthquake source physics and can lead to a better assessment of future earthquake hazards. Key Points: The 2024 Mw 7.5 Noto Peninsula earthquake involves a multi‐segmented rupture sequence on differently oriented faultsThe long and quiet initial rupture domain coincides with the preceding earthquake swarm regionFluid‐induced earthquake swarms and a segmented fault network control the complex earthquake rupture growth [ABSTRACT FROM AUTHOR]

Physics‐based earthquake simulators have been developed to overcome the relatively short duration and incompleteness of historical earthquake and paleoseismic records, respectively. These simulators have the potential to be a useful addition to seismic hazard assessment as they produce millions of synthetic earthquakes over thousands to millions of years using predefined fault geometries and slip rates. Due to the sparsity of fault data and the computational expense of the modeling, it is common to simplify earthquake simulator input parameters. Fault surfaces are typically characterized by simplified mapped traces and constant dip with depth, with slip rates that are often assumed to be uniform over the fault plane. This study uses the well‐defined 3D geometry of an active normal fault in offshore Aotearoa New Zealand, the Cape Egmont Fault, to demonstrate the impact of using nonuniform slip rates and nonplanar fault geometries on the resulting synthetic earthquakes from the earthquake simulator RSQSim. Adopting variable slip rates that decrease to zero along the fault tipline (rather than uniform slip rates) reduces unrealistically high nucleation rates of seismicity along fault edges. Introduction of complex 3D fault geometries, including fault segmentation and bends on kilometer scales, and variable slip rates produces less characteristic earthquake populations, increasing the number of M6‐7 moderate‐large magnitude events. Incorporation of variable fault geometries and slip rates in physics‐based simulators may modify the seismic hazards estimated from synthetic earthquake catalogs. Plain Language Summary: Earthquake simulators are used to study and anticipate earthquakes. These simulators produce millions of synthetic earthquakes over thousands to millions of years using predefined fault properties. We use a well‐defined active normal fault in offshore Aotearoa New Zealand, to examine the impact of nonuniformly distributed fault slip rates and nonplanar fault geometries on the resulting synthetic earthquake catalogs. We show that adopting variable fault slip rates, with a gradual decrease in slip rate toward fault‐surface edges, reduces the unrealistic occurrence of high concentrations of earthquakes along the fault tip line. The study also shows that utilization of complex 3D fault geometries together with variable slip rates produces populations of earthquakes in a less predictable manner, increasing the number of moderate to large magnitude events. Therefore, details of the input fault geometries and slip rate distributions can significantly affect the resulting synthetic earthquake catalogs from physics‐based earthquake simulators with significant implications for seismic hazard applications. Key Points: Variable, rather than constant, slip rates reduce high seismicity rates along the fault edges and can improve the depth distributions of eventsIntroduction of complex 3D fault geometries produces less characteristic earthquake populationsVariable fault geometries and slip rates in physics‐based simulators can improve the synthetic earthquake catalogs [ABSTRACT FROM AUTHOR]

Rupture dynamics along a heterogeneous fault is studied in the framework of the recently developed damage‐breakage rheological model. The model utilizes a fault structure accommodating shear and wear in a heterogeneous weak fault‐core a few centimeters thick, separating two blocks with damage level gradually decreasing toward the host rock. We first demonstrate the similarity of the static stress field around a fault, represented by a three‐body tribosystem, to the one developed around a rough frictional interface. We show that both models predict heterogeneous stress field pattern. We then apply simulations of rupture on heterogeneously damaged faults. We show that increasing initial heterogeneity amplitudes is associated with smaller events with lower slip rates. The simulations further allow us to quantify the amount of accumulated damage correlative with wearing. During the propagating rupture, the strength of the fault‐core evolves, leading to higher wear generation along relatively strong zones or barriers. The total wear production in a given event is strongly dominated by the initial damage heterogeneity. Processing of synthetic seismograms shows excess energy radiation of high‐frequency seismic waves comparing to the expected radiation from planar faults. This radiation is enhanced with the increase in the variability of fault strength heterogeneity. Further calculations of the scaled energy show good fit with previous seismological and laboratory observations and demonstrate the impact of fault heterogeneity on radiated energy during dynamic rupture. Therefore, initial fault heterogeneity, manifested by fault‐core strength or by geometrical irregularity controls many aspects of earthquake rupture, including slip displacement and velocity. Plain Language Summary: Earthquakes are triggered when the stress build‐up on rocks exceeds the resistance to fracture of fault material. When faulting occurs, energy is generated in the fault zone and radiated through the rock body to the surface. The ground motions and their durations strongly depend on the earthquake rupture characteristics such as the amount of displacement, velocity, and directivity. Many studies modeled natural faults as planar frictional interfaces that failed and were displaced during earthquakes. Yet, geological observations and seismological measurements demonstrated that this is probably a naive picture, and fault zones are very heterogeneous, including complicated geometry and variation of strength along and across them. Here we examine the effect of such initial heterogeneities on earthquake and show that the heterogeneity of the rock strength in the fault core itself strongly affects earthquake dynamics. In particular, larger magnitude earthquakes are predicted for the more homogenous fault. We also find that higher fault heterogeneity would result in excess high‐frequency radiation. Considering these effects on ground motion around the fault, we show that seismological records may capture these differences in fault characteristics during earthquakes. Key Points: Initial damage level and its heterogeneity along the fault core control dynamics and seismic moment of simulated rupture eventsWear production is affected by the initial damage distribution. During slip, higher wear accumulation is calculated for barriers along faultsHigh variability of fault damage heterogeneity significantly enhances high‐frequency radiation [ABSTRACT FROM AUTHOR]

Following the July 2019 Ridgecrest, California earthquakes, multiple field investigators noted that pebble- to boulder-sized rocks had been displaced from their position in the desert pavement within a stepover along the right-lateral strike-slip M7.1 rupture trace, without evidence of dragging or shearing. This implies localized ground motions in excess of 1 g, in contrast to the instrumentally recorded peak of 0.57 g. Similar rock displacement occurred in a stepover in the predominantly strike-slip 2010 M7.2 El Mayor-Cucapah earthquake. Together, these examples suggest that some aspect of how earthquake rupture negotiates a strike-slip fault stepover produces extremely localized strong ground acceleration. Here, we use the 3D finite element method to investigate how rupture through a variety of strike-slip stepover geometries influences strong ground acceleration. For subshear ruptures, we find that the presence of a stepover in general matters more than its dimensions; the strongest ground acceleration always occurs at the end of the first fault. For supershear ruptures, the stepover is effectively irrelevant, since the strongest particle acceleration occurs at the point of the supershear transition on the first fault. Our model subshear and supershear ruptures alike do produce horizontal particle acceleration above 1 g, but over a region so close to the fault (< 1 km) that a seismic network may not catch it. We suggest that the physics of rupture through a fault stepover could have been responsible for the displaced rocks in the Ridgecrest and El Mayor-Cucapah earthquakes, and that stepover regions may have particularly high ground motion hazard. Our study suggests that ground motion predictions and local hazard assessments should account for much stronger accelerations in the immediate near field of active faults, especially around stepovers and other geometrical discontinuities. • Rupture dynamics can provide a mechanism for displaced rocks in fault stepovers. • Abrupt rupture stops at fault ends can cause PGA > 1 g within 1 km of the endpoint. • Rupture velocity strongly affects the pattern and intensity of ground acceleration. • Most instrumental records are not close enough to faults to capture these effects. • Small rocks may be useful crude strong motion sensors in the extreme near field. [ABSTRACT FROM AUTHOR]

The East Anatolian Fault Zone (EAFZ) is a left-lateral transform fault zone located between the Anatolian and Arabian plates. In this study, in order to image the upper crustal structure beneath the eastern segments of EAFZ, 3D seismic velocity variations are computed using local earthquake tomography. The initial catalog for the tomography process consists of 2200 well-located earthquakes recorded at 49 seismic stations around the study region between 2007 and 2020. 1D initial velocity model is constructed based on previous studies in the region. The maximum number of iterations and the velocity perturbations which sustain the linearity of the inversion are determined based on the detailed tests. Reliable zones of the final model are decided based on the Derivative Weighted Sum and Hit Count distribution. The resulting velocity model displays a clear velocity contrast across the surface trace of the EAFZ down to a depth of 12 km. While the Anatolian side of the fault displays higher velocities associated with the ophiolitic units in the region, the south of the fault zone is represented by lower velocities due to sedimentary deposits. The vertical cross-sections of tomographic models show a north dipping fault between Palu and Çelikhan. The complete earthquake catalog is relocated using the 3D velocity model. Together with the obtained velocity model, the relocated hypocenters indicate that the dip of the EAFZ is not uniform, the Palu segment dips to the north with an angle of ~80°, while the Pütürge and Erkenek segments dip to the north with a lower angle of ~60-70°. [ABSTRACT FROM AUTHOR]

A precise finite‐fault model including the fault geometry and slip distribution is essential to understand the physics of an earthquake. However, the conventional linear inversion of geodetic data for a finite‐fault model cannot fully resolve the fault geometry. In this study, we developed a Bayesian inversion framework that can comprehensively solve a non‐planar fault geometry, the corresponding fault slip distribution with spatially variable directions, and objective weighting for multiple data types. In the proposed framework, the probability distributions of all the model parameters are sampled using the Monte Carlo method. The developed methodology removes the requirement for manual intervention for the fault geometry and data weighting and can provide an ensemble of plausible model parameters. The performance of the developed method is tested and demonstrated through inversions for synthetic oblique‐slip faulting models. The results show that the constant rake assumption can significantly bias the estimates of fault geometry and data weighting, whereas additional consideration of the variability of slip orientations can allow plausible estimates of a non‐planar fault geometry with objective data weighting. We applied the method to the 2013 Mw 6.5 Lushan earthquake in Sichuan province, China. The result reveals dominant thrust slips with left‐lateral components and a curved fault geometry, with the confidence interval of the dip angles being between 20°–25° and 56°–58°. The proposed method provides useful insights into the scope of imaging a non‐planar fault geometry, and could help to interpret more complex earthquake sources in the future. Plain Language Summary: An earthquake originates from a rapid relative movement between two blocks on both sides of a fault. The earthquake can produce ground deformation, which can be measured by geodetic instruments deployed on the ground. Geophysicists can thus utilize the geodetic measurements to invert the earthquake attributes, including the fault geometry and the associated slips (i.e., the zone and amount of the relative movement between two blocks). Simultaneous estimation of the fault geometry parameters and the slip parameters is a nonlinear problem. The conventional inversion of geodetic data uses a linear adjustment method that cannot fully resolve the fault geometry. In addition, geodetic measurements have different precisions in terms of the types of instruments. We developed a nonlinear inversion based Bayesian inference framework to solve both these problems. The proposed method can correctly interpret an earthquake source with a non‐planar fault geometry. We applied the method to the 2013 Mw 6.5 Lushan earthquake in southwest China and determined a statistical estimate of the fault geometry and slips. The results indicate that the proposed method could help to improve our knowledge of complicated earthquake sources in the future. Key Points: We develop a Bayesian finite‐fault inversion for non‐planar fault geometry and spatially variable slip amplitude and rakeAccounting for variable directions of fault slips allows to better constrain non‐planar fault geometry and tune objective data weightingAccounting for the non‐planar fault geometry, the 2013 Lushan earthquake modeling reveals a dominant thrust slip with sinistral component [ABSTRACT FROM AUTHOR]

The 2008 Mw 7.9 Wenchuan earthquake is the most disastrous seismic event in China since 1976. Both field and seismological investigations suggest a multi‐stage coseismic rupture with most damage associated with the Yingxiu‐Beichuan Fault (YBF) of spatially variable fault strike and dip angles. To investigate the effect of fault geometric complexity on coseismic rupture and paleoseismic pattern on the YBF, we perform earthquake sequence modeling on 3D fault geometry in the framework of rate‐and‐state friction law. Our model produces a long‐term earthquake sequence with quasi‐regular recurrences of whole‐fault ruptures and segmented ruptures. The along‐strike rupture segmentation, earthquake propagation speed, and slip rate are mainly controlled by along‐strike variations of fault dip and strike angles. Particularly, the YBF can be divided into two segments: the southern segment featured by large events (Mw > 8.0) and the northern segment with smaller events (Mw < 7.5), with recurrence intervals primarily determined by the tectonic loading rate. In a whole‐rupture event, the rupture speed mainly correlates with the fault dip angle; a smaller dip angle results in a wider seismogenic zone and higher rupture speeds, whereas the small‐scale variation in rupture speed is regulated by the fault strike angle. The effect of strike variation on the total coseismic slip amount is more pronounced due to the large strike angle gradient along the YBF. After varying the slip vector to reflect the northward transition from the dominant thrust‐slip to strike‐slip faulting during the Wenchuan coseismic rupture, we obtain reasonably good agreement of model simulated coseismic surface displacements with GPS observations. Plain Language Summary: The Yingxiu‐Beichuan Fault (YBF) hosting the 2008 Mw 7.9 Wenchuan earthquake has a complex nonplanar interface. Seismic studies after the Wenchuan earthquake have unveiled a complex rupture process. Field trenching studies find a complex recurrence interval and rupture pattern of paleoseismic events. Here, we conduct a numerical simulation of earthquake sequences on the 3D YBF to study the effect of fault geometry on the coseismic rupture and paleoseismic behaviors. Our modeling results are quantitatively comparable with most field observations. Both whole‐fault ruptures and partial rupture events are produced. The larger events (magnitude >8.0) initiate near the southern end of the YBF. The smaller events (magnitude <7.5) initiates near the northern end of the YBF. Their repeating time is mainly controlled by the background tectonic loading rate. In the partial rupture events, coseismic rupture stops near Nanba area when encountering a geometrical barrier due to a sharp change of fault dip. While the lateral rupture propagation speed is controlled by variation of both fault dip and strike angles, coseismic slip distribution is more controlled by the variation of fault strike (lateral fault orientation) than dip. We find that the orientation of slip vector is important in coseismic rupture modeling of Wenchuan‐type event. Key Points: A long‐term earthquake sequence with quasi‐regular recurrences is simulated, including whole‐fault and segmented rupturesFault dip angle and hence the seismogenic zone width has first‐order control on the along‐strike rupture segmentationFault strike angle controls small‐scale variations in rupture propagation speed and slip rate, which affect the cumulative coseismic slip [ABSTRACT FROM AUTHOR]

Paleoseismic evidence from the late‐interseismic Alpine Fault suggests key section boundaries conditionally inhibit rupture. We utilize a year of data from a two‐part seismometer network (Dense Westland Arrays Researching Fault Segmentation) to characterize ∼7,500 earthquakes (−0.7 ≤ MLv ≤ 4.2) and ∼800 focal mechanisms, producing high‐resolution structural images of these boundaries to study effects of material and structural heterogeneities on mode‐switching rupture behavior. Lithologically‐controlled frictional behavior and crustal strength appear to influence lateral and vertical on‐fault seismicity distributions. Ultramafic hanging‐wall serpentinite and serpentinite‐related fault core minerals along the South Westland (SW) boundary, result in a locally shallow seismogenic cuttoff (∼8 km) and abundant on‐fault seismicity. Maximum horizontal compressive stress rotations (14° anti‐clockwise and 20° clockwise near the SW and North Westland (NW) boundaries, respectively, relative to the Central Section), coupled with spatially variable fault frictional properties, are more important than geometry alone in controlling Sections' relative frictional stability. Whereas the SW and Central Sections are well‐oriented for failure, the NW Section is severely misoriented compared with favorably oriented faults of the Marlborough Fault Zone, which possibly facilitate a preferred rupture route. Geometrically, a 40° dip change at the SW boundary may be accommodated either by a single through‐going fault plane ‐ a difficult geometry across which to obtain multi‐segment earthquakes when considering rupture dynamics ‐ or by a deeper vertical fault strand truncated by a shallower listric plane. Our new observations have implications for Alpine Fault rupture scenarios and highlight the need to consider a range of spatially heterogeneous, interdependent physical factors when evaluating controls on rupture segmentation. Plain Language Summary: New Zealand's Alpine Fault last ruptured in 1717 AD and the likelihood of it producing a major earthquake in the next 50 years is estimated to be 75%. The magnitude of the next earthquake will be strongly determined by the area of fault rupture. Two key localities along the fault, which correspond to the boundaries between sections of the fault with differing geometry, slip rate and frictional behavior, have been proposed to sometimes, but not always, act as barriers to fault rupture, limiting the sizes of the resulting earthquakes. However, it remains unclear what physical properties control this process. Using seismic data collected from dense seismometer networks, we construct detailed catalogs of small earthquakes occurring near these localities, at Martyr River and Inchbonnie, and precisely map the distribution of active seismicity near the fault. From these data we improve estimates of (a) the subsurface geometry of the fault and how it changes along length, (b) how seismicity is distributed with depth, to understand possible maximum depth of rupture in conjunction with observations of crustal temperatures and structure, and (c) the orientation of the stresses acting on the fault, to understand its stability to tectonic loading and its frictional behavior. Key Points: Dense microseismicity catalogs constrain fault structure, seismogenic depths and the stress field bookending the fault's Central SectionOn‐fault seismicity, a 40° dip change and a shallow, rheologically controlled cutoff depth are observed at the South Westland boundaryStress analyses indicate South and North Westland Sections are favorably and unfavorably oriented for frictional failure, respectively [ABSTRACT FROM AUTHOR]

Intersecting orthogonal strike‐slip faults with opposite senses of slip pose the question of what allows rupture to propagate through the junction and through both faults versus confining rupture to a single fault. I conduct dynamic rupture simulations on simplified orthogonal strike‐slip fault systems, to determine which conditions produce rupture on both component faults. In models with uniform initial tractions on both faults, slip on the first fault must reduce normal stress on the second fault for it to rupture. If the first fault ends at the cross fault, a stopping phase causes the cross fault to rupture. In models where I resolve a uniform regional stress field on the faults, only a narrow range of stress orientations allow multifault ruptures. These results will be helpful for evaluating hazard near orthogonal strike‐slip faults. Plain Language Summary: There are many examples around the world where two strike‐slip earthquake faults cross each other at nearly 90° angles. This is not remarkable when only one of the faults in a pair causes an earthquake, but it becomes notable when two or more crossing faults move at the same time. This raises the question of what causes the second fault to get involved, or not. To address this question, I use computer simulations of the physics of the earthquake process to test dozens of different fault configurations and earthquake starting points. I find that the location where the earthquake starts on the first fault controls whether the second fault is made stronger versus weaker, and therefore whether both faults can move together in one earthquake. These results can help us understand earthquake hazard around crossing faults. Key Points: Nucleation location effectively controls whether multifault rupture occurs on orthogonal strike‐slip fault systemsA stopping phase from rupture reaching the end of one fault is often required to initiate rupture on the cross faultOnly a narrow range of regional stress orientations allows both cross‐faults to rupture [ABSTRACT FROM AUTHOR]

The 700‐km‐long Chaman fault (CF) marks the western edge of the plate boundary between India and Eurasia. Although global plate models predict 2.3–3.6 cm/yr left‐lateral motion between both plates, the fault is known to have hosted few earthquakes in historical times. Recent geodetic measurements attested the presence of aseismic slip locally. To detail the interplay between fast and slow slip along the CF, we build three Interferometric Synthetic‐Aperture Radar time series of ground deformation covering the whole fault length over 5 years (2014–2019). We find that most of the active fault trace slips aseismically and continuously. From south to north, we identify three creeping fault portions: the Nushki, Central, and Qalat segments of lengths between 80 and 130 km. The loading rate is 1.2 ± 0.3 cm/yr for the two southernmost portions, while it is about 0.7 ± 0.2 cm/yr for the Qalat segment. The Central segment and the nearby locked segments have hosted the largest known historical earthquakes on the CF, and three moderate magnitude earthquakes in our observation period. We image these earthquakes for which modeled slip at depth (Mw 5–5.6), time series of surface slip and deformation patterns argue toward large triggered aseismic slip. The June 2018 event displays postseismic moment 3–15 times greater than coseismic moment. Over the two decades covered by geodetic observations, continuous or triggered aseismic slip dominates along most of the fault and co‐locates with earthquakes. We observe that fault geometrical complexities delimit active segments and may be responsible for the kilometer‐scale intertwining between seismic and aseismic events. Plain Language Summary: The Chaman fault (CF) separates Indian and Eurasian tectonic plates moving at about 3 cm/yr with respect to each other. This fault is known to have hosted very few earthquakes (sudden and rapid slip) in historical times, and appears to slip slowly. We measure slip on the CF with radar satellite images covering 2014–2019, and assess the average amount of slip attributed to earthquakes over the past century. We identify three 80–130‐km‐long portions of the 700‐km‐long fault that slip silently, without radiating seismic waves, at rates reaching 1.2 ± 0.3 cm/year along the southern half, and 0.7 ± 0.2 cm/year in the north. The largest historical earthquakes and three earthquakes of moderate size in our observation period occurred on the central part of the CF. We image the surface deformation induced by these three earthquakes and find that they were followed by significant slow slip on the fault, especially the third event in June 2018. Since the 2000s space‐based measurements indicate that the slow mode of slip dominates on the CF plane and co‐locates with earthquakes. Changes of fault orientation may delimit portions of the fault which either slip slowly or break into fast earthquakes. Key Points: Aseismic slip along the Chaman fault (CF) reaches 12 mm/yr with three large distinct aseismic sections, despite marked elastic strain increaseThree earthquakes are imaged along the central CF and two exhibit large post‐seismic afterslipAlong‐strike distribution of slip is compared with historical seismicity and fault geometry [ABSTRACT FROM AUTHOR]

The 2021 Mw7.3 Maduo (Qinghai, China) earthquake ruptured ∼160 km along a poorly known secondary fault inside the Bayanhar block on the northern Tibetan plateau, which is generally parallel to the Kunlun fault. Here we integrate the interseismic (2015–2020) and coseismic geodetic observations to quantify the interseismic strain rate, fault geometry and coseismic slip distribution. Our results reveal that the seismogenic fault is featured by the low (<20 nanostrain/yr) and nearly undetectable interseismic strain rate. Three‐dimensional displacement fields and coseismic strain maps demonstrate the spatial variations of rupture kinematics due to the change of fault geometry. Our study reveals the steeply north‐dipping fault geometry of the seismogenic fault. The majority of coseismic slip occurred between 0 and 15 km with slight shallow slip deficit, which not penetrates through the inferred elastic upper crust (∼20–25 km). Our study highlights the tectonic and geometric control on fault kinematics of the Maduo earthquake. Plain Language Summary: The Bayanhar block on the north‐central Tibetan plateau experienced several major earthquakes with Mw > 6.5 on the boundary faults since the 1997 Mw7.6 Manyi earthquake, which is consistent with fast long‐term slip rates and large locking depth. However, the potential of generating large earthquakes on the faults inside the Bayanhar block is typically considered to be insignificant due to the estimated low fault slip rate (<5 mm/yr) and limited fault length. The 2021 Mw7.3 Maduo earthquake occurred on a poorly known subsidiary fault inside the Bayanhar block. Here, we use InSAR measurements prior to (2015–2020) and during the earthquake to investigate interseismic strain accumulation, to map coseismic deformation and to constrain the fault geometry as well as coseismic slip distribution of the Maduo earthquake. The three‐dimensional displacement fields and the relocated aftershocks support a slightly off vertical, dipping to the north, fault geometry. Our inversion results show that the coseismic slip is mainly distributed at a depth of 0–15 km. The kinematic analysis of the Maduo earthquake demonstrates that long‐term quantification of earthquake hazard of the subsidiary strike‐slip fault inside the Bayanhar block is challenging, because the interseismic strain rate is remarkably low and is largely undetectable by the geodetic observations. Key Points: Interseismic strain rate, coseismic three‐dimensional displacement field, finite strain and short‐term postseismic deformation of the Maduo earthquakeThe 2021 Maduo earthquake occurred on a subsidiary fault with a low (<20 nanostrain/yr) interseismic shear strain rateLong (∼160 km) rupture of the Maduo earthquake features shallow slip deficit with multi‐segment slip not penetrating through the upper crust [ABSTRACT FROM AUTHOR]

Eastern Bangladesh sits on the seismically active Chittagong‐Myanmar fold and thrust belt (CMFB), a north‐trending accretionary wedge on the eastern side of the India‐Eurasia collision. Earthquakes on the basal décollement and associated thrusts within the CMFB present a hazard to this densely populated region. In this study, we interpret 28 seismic reflection profiles from both published and unpublished sources to constrain the depth of the basal décollement. To convert profiles from the time domain to the depth domain, we integrate sonic log and seismic stacking velocity data to generate time‐velocity relationships for different parts of the CMFB. Our analysis reveals that the décollement is ∼9 km deep in northeast and southeast Bangladesh, but shallows to ∼5 km in east‐central Bangladesh. The décollement has an area of 7.25 × 104 km2 (∼150 × 450 km), making it capable of an Mw 8.5 earthquake. However, the warped geometry of this fault might act as a rupture barrier were a large earthquake to occur on the décollement. Our combined velocity and fault model lay the groundwork for future studies to address seismic segmentation, ground shaking, and rupture modeling in the CMFB. Finally, we use our compiled data set to analyze the evolution of fold kinematics in the CMFB. We observe that folding style and failure mode varies, from mainly ductile deformation in the foreland to mainly brittle in the hinterland. The dual‐failure modes within the CMFB support the hypothesis that a region with ductile deformation may still be capable of seismic behavior. Plain Language Summary: In Bangladesh, the interface between two tectonic plates has created a large, nearly flat, earthquake‐producing fault called a décollement. Above the décollement, sedimentary layers are compressed to create a series of north‐south trending folds and thrust faults that extend for hundreds of kilometers. This study uses 28 seismic reflection data sets (a technology similar to ultrasounds), originally collected by the oil and gas industry, and reinterprets the subsurface structure of the folds as a way to determine the décollement depth. We find that the décollement has a curved shape, with a depth of ∼9 km in northeast and southeast Bangladesh, and ∼5 km in east‐central Bangladesh. We hypothesize that the greater amount of sediment deposited in north and south Bangladesh has weighed down the surface of the earth and warped the décollement. This is the first study to constrain the geometry of this décollement, and we find that it has the potential to host a magnitude 8.5+ earthquake. The fault model presented here can be incorporated into studies of the rupture patterns of earthquakes on this décollement to better understand the earthquake hazard in a region inhabited by over 160 million people. Key Points: We present a data‐based seismic velocity model for Bangladesh to 10 km depthThe décollement underlying the Chittagong‐Myanmar fold and thrust belt forms a broad arch at depths of 5–9 kmThis morphology is likely caused by lithospheric flexure due to sediment deposition and may impact rupture propagation [ABSTRACT FROM AUTHOR]

Interferometric synthetic aperture radar and teleseismic P‐wave data were combined to investigate the source rupture characteristics of the 2020 Mw 6.3 Yutian, China, earthquake. We first utilized the near‐field displacements together with 29 broadband teleseismic P waveforms to investigate the uniform slip model by using a Bayesian bootstrap optimization nonlinear inversion method to resolve the nucleation point location, origin time and fault geometrical parameters. Based on these results, the kinematic rupture process of the earthquake was inverted. We conclude that the 2020 Yutian earthquake occurred on a west‐dipping blind normal fault with a dip of ∼60° at the junction of Bayan Har block (BHB) and western Kunlun block (WKB). The rupture nucleation point was located at longitude = 82.435°E, latitude = 35.619°N, and a depth of 6.377 km, making it shallower than estimated from other point source inversions. The earthquake started at 21:05:20 UTC on June 25, 2020 and was delayed by ∼2.07 s compared with GeoForschungsZentrum. The rupture lasted for ∼12 s, with a total seismic moment of ∼3.28 × 1018 Nm, corresponding to a moment magnitude of 6.3. The slip was mainly confined between ∼3.0 and 9.0 km in depth and the peak slip was ∼1.40 m, which occurred at a depth of ∼6.377 km. The slip was predominantly normal slip with slight right‐lateral strike‐slip components, which agrees with the southeastward movement of the BHB relative to the WKB. Plain Language Summary: The June 25, 2020 Mw 6.3 Yutian earthquake occurred on a normal fault within the northwestern Tibetan Plateau, a region characterized by EW extension. Because the rough focal mechanism solutions and source locations based on seismic wave data reported by several institutes are different, they cannot be used to create a reliable fault model for inverting the kinematic rupture process of this moderate earthquake. The initial rupture point and origin time, which are the prior information needed for revealing the slip characteristics, are also unclear. To resolve these rupture parameters, both Sentinel‐1 interferometric synthetic aperture radar and teleseismic P‐wave data were collected. We found that the rupture mainly occurred in the shallow part of a ∼60° west‐dipping normal fault, the nucleation point was near the point of maximum slip, and the slip lasted ∼12 s and extended northward and southward. Our research has important significance for understanding earthquake disasters and deformation mechanisms of faults within the northwestern Tibetan Plateau. Key Points: Both interferometric synthetic aperture radar and teleseismic P wave are used to resolving the ambiguity of fault geometryThe rupture nucleation point and origin time were clarified by a Bayesian bootstrap optimization inversion methodRupture process of this earthquake is predominantly normal‐sip with slight right‐lateral strike‐slip components [ABSTRACT FROM AUTHOR]

A large strike‐slip earthquake occurred in the Caribbean Sea on January 28, 2020. We inverted teleseismic P waveforms from the earthquake to construct a finite‐fault model by a new method of inversion that simultaneously resolves the spatiotemporal evolution of fault geometry and slip. The model showed almost‐unilateral rupture propagation westward from the epicenter along a 300 km section of the Oriente transform fault with two episodes of rupture at speeds exceeding the local shear‐wave velocity. Our modeling indicated that the 2020 Caribbean earthquake rupture encountered a bend in the fault system associated with a bathymetric feature near the source region. The geometric complexity of the fault system triggered multiple rupture episodes and a complex rupture evolution. Our analysis of the earthquake revealed complexity of rupture process and fault geometry previously unrecognized for an oceanic transform fault that was thought to be part of a simple linear transform fault system. Plain Language Summary: On 28 January 2020, a large earthquake occurred on the Oriente fault, an oceanic transform fault in the Caribbean Sea between Jamaica and Cuba. The Oriente fault forms the boundary between the North America and Caribbean tectonic plates. The 2020 Caribbean earthquake was caused by horizontal sliding between the two plates. We used waveforms of the earthquake that were recorded around the world to build a model of the earthquake‐rupture process. The model showed that rupture during the earthquake was complex, featuring multiple rupture episodes with various rupture speeds and in various directions. Our model suggests that a bend in the fault was responsible for the changes of rupture speed and direction and the triggering of successive rupture episodes. Our analysis of the 2020 Caribbean earthquake has revealed complexity of both fault geometry and rupture process that were previously unknown in oceanic transform fault earthquakes. Key Points: We built a kinematic source model of the 2020 Caribbean earthquake to analyze the spatiotemporal evolution of fault geometry and slipA fault bend disturbed supershear rupture along the linear fault section and triggered subsequent ruptureOceanic transform faults can have geometric complexity that controls rupture evolution [ABSTRACT FROM AUTHOR]

Fault source models inverted from surface displacement measurements are a common tool to contextualize the seismotectonic significance of earthquakes. However, these models commonly invoke a simplified homogeneous elastic half‐space (flat Earth) description of Earth, which may bias the resulting fault geometries and slip distributions in the presence of spatially varying Earth's elastic structure and topography. Here, we developed both forward and inverse three‐dimensional finite‐element models with planar faults to systematically assess the impacts of topography, crustal elastic structure, and fault geometry on surface deformation and earthquake source inversions, with a focus on the 2015 Gorkha, Nepal earthquake. Our forward models with fixed fault slip confirm the impacts of topography and crustal heterogeneity on surface deformation. These impacts, however, contribute only maximally ∼10% of the total surface displacement, and the fault depth and dip dominantly control the coseismic deformation for the Gorkha case. When using homogeneous half‐space models to invert the synthetic displacement fields generated by forward finite‐element models, we find that ignoring crustal heterogeneity primarily biases inferred dip angle, while ignoring relief primarily biases inferred fault depth. Our inversions based on finite‐element models suggest that topography and crustal heterogeneity introduce negligible bias on recovering the Gorkha line of sight observations. Fault dip angle and depth can systematically trade off with slip area and slip magnitude, regardless of the presence of relief and crustal heterogeneities. Taken together, our findings highlight the importance of fault geometry on inverted fault kinematics with geodetic observations in the central Himalayan arc. Key Points: We explore the impacts of unmodeled relief and crustal heterogeneity on coseismic deformation and inversions for fault geometry and slip for the Gorkha earthquakeExcluding relief and crustal heterogeneity biases the determination of fault depth and dip, respectivelyInverted fault slip distribution trades off significantly with fault depth and dip angle, regardless of the presence of relief and/or crustal heterogeneity [ABSTRACT FROM AUTHOR]

For understanding the physical condition of earthquake generation, we estimated the deviatoric stress fields and fault planes of the large earthquakes in the hypocentral area of the 2016 Kumamoto earthquake sequence, central Kyushu, Japan. We analyzed precise hypocenters and their focal mechanism tensors from 1993 to 2017 and found the complex fault planes of the largest foreshock and the mainshock during the sequence. The faults of large earthquakes could not be detected from the preceding high seismic activity in the area to the mainshock. The analyses of the deviatoric stress tensors obtained from pre‐ and post‐sequence seismic moment tensor data revealed the temporal change in the deviatoric stress field at the vicinity of the hypocenters of the largest foreshock and the mainshock. Because of this temporal change, we found that the stress level of the deviatoric stress field in this region is the same level as the co‐seismic stress change caused by the two large earthquakes. We attempted to estimate the differential stress at the region using a new method, which can be applied to the heterogeneous stress fields. The average value of the estimated differential stresses in the region was 15.4 ± 10 MPa. The rotation of the principal stress axes and the low differential stress are important information to understand the physical processes of the earthquake generation and the state of the crustal stress. Key Points: We investigated the characteristics of the hypocenters and their focal mechanism tensors before and after the 2016 Kumamoto earthquakeWe revealed the temporal change in the deviatoric stress field before and after the two large earthquakes around their faultsThe differential stress at the hypocentral area was 15.4 ± 10 MPa and did not significantly change after the events [ABSTRACT FROM AUTHOR]

We explore the three‐dimensional structure of the 2016–2017 Central Italy sequence using ~34,000 ML ≥ 1.5 earthquakes that occurred between August 2016 and January 2018. We applied cross‐correlation and double‐difference location methods to waveform and parametric data routinely produced at the Italian National Institute of Geophysics and Volcanology. The sequence activated an 80 km long system of normal faults and near‐horizontal detachment faults through the MW 6.0 Amatrice, the MW 5.9 Visso, and the MW 6.5 Norcia mainshocks and aftershocks. The system has an average strike of N155°E and dips 38°–55° southwestward and is segmented into 15–30 km long faults individually activated by the cascade of MW ≥ 5.0 shocks. The two main normal fault segments, Mt. Vettore‐Mt. Bove to the North and Mt. della Laga to the South, are separated by an NNE‐SSW‐trending lateral ramp of the Sibillini thrust, a regional structure inherited from the previous compressional tectonic phase putting into contact diverse lithologies with different seismicity patterns. Space‐time reconstruction of the fault system supports a composite rupture scenario previously proposed for the MW 6.5 Norcia earthquake, where the rupture possibly propagated also along an oblique portion of the Sibillini thrust. This dissected set of normal fault segments is bounded at 8–10 km depth by a continuous 2 km thick seismicity layer of extensional nature slightly dipping eastward and interpreted as a shear zone. All three mainshocks in the sequence nucleated along the high‐angle planes at significant distance from the shear zone, thus complicating the interpretation of the mechanisms driving strain partitioning between these structures. Key Points: 3D fine‐scale geometry and kinematics of the 2016–2017 Central Italy Sequence by HypoDD locations of data produced in monitoring environmentSpace‐time evolution of the seismic activity and its relation with a shallow shear zone bounding the system at depthFocus on faults segmentation and role of the preexisting compressional structures in modulating the extensional deformation pattern [ABSTRACT FROM AUTHOR]

Highlights • A nonlinear inversion of InSAR-observed deformation is developed to map fault dip. • Fault geometry of Wenchuan earthquake is retrieved with varying dips along depth. • Coseismic slip distribution reveals variable motion features on fault surface. Abstract Satellite Interferometric Synthetic Aperture Radar (InSAR) is playing an increasingly important role in the observation of coseismic surface deformation caused by earthquakes, and has been used to invert for subsurface fault structure and reveal earthquake source mechanisms. However, the mapping of complex non-planar or curved (e.g., listric-shaped) faults still remains a challenging task due to variable dips along the underground depth and the impenetrability of the deep crust. Here, we develop a set of new inversion algorithms to determine the listric fault geometry with InSAR- and GPS-observed surface deformation as the significant constraints. The fault surface with variable dip angles is discretized into consecutive sub-fault layers along the down-dip direction. A nonlinear iteration algorithm is used to minimize the objective function to determine the dip angle for each sub-fault layer. The proposed method is first tested using synthetic data to show its effectiveness for retrieval of varying fault geometry dips, and then applied to the 2008 Mw 7.9 Wenchuan earthquake that ruptured the Yingxiu-Beichuan fault for over 320 km along the southwest-northeast strike. The inversion shows that the dip angle of the seismogenic fault is up to 76° near the surface layer, and gradually decreases along the down-dip direction. A significant decrease in dip occurs within the depths of 6–15 km with a dip of 32° at a depth of 15 km. The dip angle decreases to 2° at a depth of 20 km, and finally merges with the subparallel PengGuan fault, which is basically consistent with geological investigations and seismic waveform data inversion. Using the inferred fault geometry, the slip model associated with the event is estimated. Five high-slip concentrations along the strike of the Yingxiu-Beichuan fault are recognized. The inversion misfit of InSAR data is reduced to 7.1 cm with a significant improvement compared to previous studies. [ABSTRACT FROM AUTHOR]

Fault bends, and associated changes in fault dip, play a key role in explaining the scatter in maximum offset versus surface rupture length fault scaling relationships. Detailed field measurements of the fault geometry and magnitude of slip in the 2016–2017 Central Italy earthquake sequence, alongside three examples from large historical normal‐faulting earthquakes in different tectonic settings, provide multiple examples in which coseismic throw increases across bends in fault strike where dip also increases beyond what is necessary to accommodate a uniform slip vector. Coseismic surface ruptures produced by two mainshocks of the 2016–2017 Central Italy earthquake sequence (24 August 2016 Mw 6.0 and 30 October 2016 Mw 6.5) cross a ~0.83‐km amplitude along‐strike bend, and the coseismic throws for both earthquakes increase by a factor of 2–3, where the strike of the fault changes by ~28o and the dip increases by 20–25o. We present similar examples from historical normal faulting earthquakes (1887, Sonora earthquake, Mw 7.5; 1981, Corinth earthquakes, Mw 6.7–6.4; and 1983, Borah Peak earthquake, Mw 7.3). We demonstrate that it is possible to estimate the expected change in throw across a bend by applying equations that relate strike, dip, and slip vector to horizontal strain conservation along a nonplanar fault for a single earthquake rupture. The calculated slip enhancement in bends can explain much of the scatter in maximum displacement (Dmax) versus surface rupture length scaling relationships. If fault bends are unrecognized, they can introduce variation in Dmax that may lead to erroneous inferences of stress drop variability for earthquakes, and exaggerate maximum earthquake magnitudes derived from vertical offsets in paleoseismic data sets. Key Points: In throughgoing normal fault ruptures, such as the 2016 central Italy earthquakes, throw maxima occur at fault bendsFault growth and linkage result in steeper dips within bends, conservation of strain along strike results in throw maxima across bendsEnhanced slip at bends contributes to scatter in fault scaling relationships and overestimation bias of magnitude, moment and stress drop [ABSTRACT FROM AUTHOR]

Using seismic data recorded by a dense temporary seismic array, we apply a waveform matched‐filter technique and obtain an early aftershock catalog around the mainshock rupture zone of the 2008 Wenchuan, Sichuan, China, earthquake. Specifically, we use 1,273 events in a relocated aftershock catalog as templates and scan through the waveforms within 7 days following the mainshock. We obtain up to ∼5.3 and ∼10 times more aftershocks than the standard and relocated catalogs, respectively. We find that early aftershocks mostly occurred below 10 km depth, downdip to large coseismic slip areas. Early aftershocks also show minor along‐strike migration with time since the mainshock, indicating triggering of afterslip following the mainshock. From the epicentral region to Beichuan where the mainshock slip was primarily reverse faulting, early aftershocks illustrated the Yingxiu‐Beichuan and Guanxian‐Jiangyou faults as high‐angle listric feature and rooted to a gentle dipping plane at the depth of ∼20 km. The aftershocks in the northeast segment of the Longmen Shan faults mostly occurred along a near‐vertical dipping fault, consistent with the primary strike‐slip component of mainshock slip in this region. In addition, differences in early and long‐term aftershock distribution indicate significant postseismic deformation along the conjugate Xiaoyudong fault. Our results help to illuminate the deep geometry of the Longmen Shan fault zone and the early evolution after the Wenchuan mainshock, which provides additional constraints to the mechanism of plateau uplifting. Key Points: Wenchuan aftershocks occurred below the coseismic rupture zoneWenchuan early aftershocks illuminated two listric reverse faults connecting at ∼20 Km depthWenchuan aftershocks expanded slightly along the fault strike with time since the mainshock [ABSTRACT FROM AUTHOR]

Geodetic observations and seismic inversions suggest that the 2021 Mw7.4 Maduo earthquake's rupture velocity differs in two directions. However, the dynamic mechanisms underlying these differences remain unclear. To address this issue, we constructed a three-dimensional finite element model implementing heterogeneous fault geometry and real topography and simulated the Maduo earthquake's dynamic rupture process, with the tectonic stress field varying with depth. The results show that the uneven distribution of normal and shear stresses on the fault plane, due to the complex geometry of the seismogenic fault, is the primary factor controlling rupture velocity and slips on the fault plane of the Maduo earthquake. The modelled results reproduced basic kinematic characteristics as indicated by the observation and inversion results of the Maduo earthquake. Our study also suggests that if an earthquake occurs in a region without significant changes in velocity structure and stress field distribution, the complex geometry of the seismogenic fault is responsible for the dynamic rupture process. These findings provide important insights into the rupture behaviour of geometrically complex faults and have important implications for seismic hazard assessments of these complicated fault systems. • The dynamics rupture process of the 2021 Maduo earthquake was modelled. • The modelled results reproduced the rupture process indicated by the geodetic observations and seismic inversions. • Stress heterogeneity due to the complex fault geometry is responsible for the asymmetric and uneven rupture process. [ABSTRACT FROM AUTHOR]

Two earthquake sequences that affected the Mentawai islands offshore of central Sumatra in 2005 (Mw 6.9) and 2009 (Mw 6.7) have been highlighted as evidence for active backthrusting of the Sumatran accretionary wedge. However, the geometry of the activated fault planes is not well resolved due to large uncertainties in the locations of the mainshocks and aftershocks. We refine the locations and focal mechanisms of medium size events (Mw > 4.5) of these two earthquake sequences through broadband waveform modeling. In addition to modeling the depth-phases for accurate centroid depths, we use teleseismic surface wave cross-correlation to precisely relocate the relative horizontal locations of the earthquakes. The refined catalog shows that the 2005 and 2009 “backthrust” sequences in Mentawai region actually occurred on steeply (∼60 degrees) landward-dipping faults (Masilo Fault Zone) that intersect the Sunda megathrust beneath the deepest part of the forearc basin, contradicting previous studies that inferred slip on a shallowly seaward-dipping backthrust. Static slip inversion on the newly-proposed fault fits the coseismic GPS offsets for the 2009 mainshock equally well as previous studies, but with a slip distribution more consistent with the mainshock centroid depth (∼20 km) constrained from teleseismic waveform inversion. Rupture of such steeply dipping reverse faults within the forearc crust is rare along the Sumatra–Java margin. We interpret these earthquakes as ‘unsticking’ of the Sumatran accretionary wedge along a backstop fault separating imbricated material from the stronger Sunda lithosphere. Alternatively, the reverse faults may have originated as pre-Miocene normal faults of the extended continental crust of the western Sunda margin. Our waveform modeling approach can be used to further refine global earthquake catalogs in order to clarify the geometries of active faults. [ABSTRACT FROM AUTHOR]

Understanding the effect of the complex fault geometry on the dynamic rupture process and discriminating it from the complexity originating from the rheological properties of faults, is an essential subject in earthquake science. The 2014 Northern Nagano earthquake, which occurred near the end zone of a major active fault system, provided unique observations that enabled us to investigate both the detailed geometrical fault structure and surface deformation patterns as well as the temporal sequence led up from a prominent foreshock activity. We first develop a geometrical fault model with a substantial variation along strike, and a model for the regional stress field, which is well constrained by the observations. This significant along-strike variation in fault geometry probably reflects the difference of fault maturity at the end zone of the complex fault system. We used this model in order to conduct a set of dynamic rupture simulations using the highly efficient spatiotemporal boundary integral equation method. Based on our simulations, we show that the observed surface deformation can be reasonably explained as the effect of the non-planar fault geometry with a number of branch faults and bends. [ABSTRACT FROM AUTHOR]

The 2015 M w 7.8 Gorkha earthquake sequence that unzipped the lower edge of the Main Himalayan Thrust (MHT) in central Nepal provides an exceptional opportunity to understand the fault geometry in this region. However, the limited number of focal mechanisms and the poor horizontal locations and depths of earthquakes in the global catalog impede us from clearly imaging the ruptured MHT. In this study, we generalized the Amplitude Amplification Factor (AAF) method to teleseismic distance that allows us to model the teleseismic P-waves up to 1.5 Hz. We used well-constrained medium-sized earthquakes to establish AAF corrections for teleseismic stations that were later used to invert the high-frequency waveforms of other nearby events. This new approach enables us to invert the focal mechanisms of some early aftershocks, which is challenging by using other long-period methods. With this method, we obtained 12 focal mechanisms more than that in the GCMT catalog. We also modeled the high-frequency teleseismic P-waves and the surface reflection phases (pP and sP) to precisely constrain the depths of the earthquakes. Our results indicate that the uncertainty of the depth estimation is as small as 1–2 km. Finally, we refined the horizontal locations of these aftershocks using carefully hand-picked arrivals. The refined aftershock mechanisms and locations delineate a clear double-ramp geometry of the MHT, with an almost flat décollement sandwiched in between. The flat (dip ∼7 degrees) portion of the MHT is consistent with the coseismic rupture of the mainshock, which has a well-constrained slip distribution. The fault morphology suggests that the ramps, both along the up-dip and down-dip directions, play a significant role in stopping the rupture of the 2015 Gorkha earthquake. Our method can be applied to general subduction zone earthquakes and fault geometry studies. [ABSTRACT FROM AUTHOR]

One of the three narrow rift belts that mark the southern end of the East African Rift System (EARS) intersects the Makgadikgadi Basin of northeastern Botswana. Although tectonic activity in the region is known to have influenced the evolution of these pans, the interrelationship between shoreline geometry, fault strikes, and the intersection of the underlying tectonic terranes has yet to be fully realized. We analyzed faults and subsurface structures in the region of the pans using a field investigation in combination with satellite imagery and geophysical data, to constrain the influence that the regional tectonic regime has had on the formation of the present-day pan geometry. We find that pan shorelines are controlled by the intersection of three preferred fault orientations which can be understood in the context of the "older" terranes they overlie, namely the Magondi Belt and the Limpopo Belt. We propose that the pronounced curvature of the southern Magondi Belt has influenced the eastward curvature of the rift-related faults and was likely produced by the impingement of the developing fold belt on the Zimbabwe Craton. Furthermore, limited focal mechanism solutions data from earthquakes north and south of the pans suggests a change in regional extension direction from NW-SE to NE-SW. Determining the relationship between these fault orientations and the underlying tectonic terrains is an important step in understanding the formation of the Makgadikgadi Basin, and more broadly the current tectonic regime of Botswana. The evidence of fault-controlled shorelines within an evaporitic environment may also have implications for regional groundwater activity. • Shoreline geometry of the Makgadikgadi Pans is largely fault controlled. • The fault pattern in the Makgadikgadi Pans is controlled by underlying structures in several intersecting Archean-Proterozoic terranes. • High relief lineaments within the pans are collinear with identified faults and possibly formed from groundwater processes. • The Makgadikgadi Basin marks a regional change in extension direction from NW-SE in the north to NE-SW in the south. [ABSTRACT FROM AUTHOR]

Investigating the fine structure of fault zones is essential for understanding the mechanism of large earthquake nucleation. In this paper we study the complex fault geometry of the Tangshan fault zone in North China using a new seismic data set from 2010 to 2020 that spans ∼10 years. Based on the well-located hypocenters and focal mechanism solutions, we obtained more detailed information on the fault zone structural features: (1) the fault zone is divided into five segments according to different dip angles of the fault plane; (2) the spatial distribution of seismicity reveals conjugate fault branches and a flower-shaped structure at the northern part of the fault zone; and (3) the fault zone is dominated by a strike-slip type, but their intersecting segments are characterized by a normal-fault type. We further obtained high-resolution seismic images of Vp, Vs and Vp/Vs around the Tangshan source region. Combining the fault geometry with the low-velocity anomalies below 10 km depth, we infer that the ascending of hot mantle material into the lower crust may have weakened the Tangshan fault zone and generate the 1976 Ms. 7.8 Tangshan earthquake. • The Tangshan fault zone is composed of five fault segments according to different dip angles of the fault plane. • Low velocity anomalies are imaged in the middle-lower crust of the Tangshan source region. • The Tangshan Ms. 7.8 earthquake is associated with the thermomechanical weakening of the fault zone. [ABSTRACT FROM AUTHOR]

The mechanical behavior of continental lithosphere of Aegean region and western Turkey is one of the foremost interesting geological disputes in earth sciences. This region provides complex tectonic events which produced a strong heterogeneity in the crust as such in among most continental regions. The reasons of the ongoing lithospheric-scale extension within the Aegean region can be revealed by the correlation with the prevailing kinematic and dynamic factors such as roll-back of the subduction slab and back arc extension, westward extrusion of the Anatolian micro-plate, block rotations and transtensional transform faults. Seismological studies of earthquake source mechanisms and slip inversions play important roles on deciphering the current deformation and seismotectonic characteristics of the region. In recent years, several moderate earthquakes have occurred in the Gulfs of Gökova, Sığacık, Kuşadası, and Simav Graben. We studied source mechanisms and rupture histories of those earthquakes to retrieve the geometry of active faulting, source characteristics, kinematic and dynamic fault parameters and current deformations in western Turkey. We used teleseismic body-waveform inversions of long-period P- and SH-, and broad-band P-waveforms. We also checked first motion polarities of P-waves recorded at both regional and teleseismic stations. Inversion results revealed E–W striking normal faulting mechanisms with small amount of left-lateral strike-slip components in the Gulf of Gökova, and NE–SW oriented right-lateral strike-slip faulting mechanisms in the Gulf of Sığacık. In Simav Graben, earthquake source parameters show dominantly normal faulting mechanisms with strike-slip components. Our inversions resulted in focal depths for the earthquake ranging from 10 to 15 km and NE–SW trending T-axes directions. The finite-fault slip distribution and rupture propagation models exhibit seismic moment releases and large displacement values which essentially occurred at hypocenters of the earthquakes. The overall results exhibited uniform and circular-shaped rupture propagations along dip directions of fault planes. Although most of the focal mechanism solutions show dominantly normal faulting mechanisms associated with the E–W oriented horst–graben structures in western Turkey, there are also strike-slip faulting mechanisms related to remarkable strike-slip faults which are capable of generating damaging earthquakes, particularly in the Gulf of Sığacık and Karaburun Peninsula. Thus, we suggest that present-day deformation in the Gulfs of Gökova, Sığacık, Kuşadası, and Simav Graben (western Turkey) is still mainly driven by the N–S extensional tectonics, but we tentatively further emphasize the importance of strike-slip faults in shaping tectonic structures in the Aegean region. [ABSTRACT FROM AUTHOR]

Highlights: [•] Study of the second mainshock of the 2006 Taitung sequence. [•] Computation of focal mechanism and fault slip inversion in extended source. [•] Statistical determination of the fault geometry from waveform adjustment. [•] Relation between the 2006 Taitung sequence and the 2003 Chengkung earthquake. [•] Earthquake initiation. [ABSTRACT FROM AUTHOR]

Abstract: Earthquakes are the result of a combination of (1) physico-chemical processes operating in fault zones, which allow ruptures to nucleate and rock friction to decrease with increasing slip or slip rate, and (2) of the geometrical complexity of fault zones. In this review paper, we summarize recent experimental findings from high velocity (conducted at about 1 m/s slip rate, or typical seismic slip rates) rock friction experiments with an emphasis on potential dynamic weakening mechanisms (melt lubrication, nanopowder lubrication, etc.) and how these mechanisms might be recognized by means of microstructural and mineralogical studies in exhumed fault zones. We discuss how earthquake source parameters (coseismic fault strength, weakening distances, energy budgets, etc.) might be derived from the field and laboratory experiments. Additionally, we discuss what needs to be considered in terms of fault zone geometry and morphology (focusing on fault surface roughness) in order to develop models of realistic fault surfaces and present theoretical considerations for microphysical modeling of laboratory data at seismic slip rates, with an emphasis on the case of melt lubrication. All experimental data and, in the case of melt lubrication, microphysical models indicate that faults must be very weak (μ < 0.1) during coseismic slip. Moreover, experiments have shown that the slip weakening distance during coseismic slip is on the order of a few tens of centimeters at most under natural conditions, consistent with inferences from field observations. Finally, we discuss open questions, future challenges and opportunities in the field of earthquake mechanics. [Copyright &y& Elsevier]

Abstract: Recent observations of large continental strike-slip earthquakes show that different fault segments may rupture at different speeds and that rupture may propagate faster than the shear wave velocity of surrounding rocks. We report that all the documented occurrences of supershear rupture are associated with faults which have simple geometry. The surface trace of these faults is described in the field or seen on satellite images as remarkably linear, continuous and narrow. Segmentation features along these segments are small or absent and the deformation is highly localized. As theoretically predicted, supershear is clearly associated with Mode II rupture. [ABSTRACT FROM AUTHOR]

Abstract: The northern segment of the Chelungpu Fault shows an unusually large co-seismic displacement from the event of the Mw 7.6 Chi-Chi earthquake in western Taiwan. Part of the northern segment near the Fengyuan City provides an excellent opportunity for characterizing active thrust-related structures due to a dense geodetic-benchmark network. We reproduced co-seismic deformation patterns of a small segment of this Chelungpu Fault using 924 geodetic benchmarks. According to the estimated displacement vectors, we identified secondary deformations, such as local rigid-block rotation and significant shortening within the hanging wall. The data set also allows us to determine accurately a 3D model of the thrust fault geometry in the shallow subsurface by assuming simple relations between the fault slip, and the horizontal and vertical displacements at the surface. The predicted thrust geometry is in good agreement with borehole data derived from two drilling sites close to the study area. The successful prediction supports our assumptions of rigid displacement and control of displacement in the hanging wall by the fault geometry being useful first approximations. [Copyright &y& Elsevier]

Abstract: We investigated the fault geometry effects and the corresponding coseismic slip distribution using various proposed earthquake fault models for the Chi-Chi earthquake of 21 September 1999. The types of fault geometries are threefold: a simple planar fault plane, a two segmented planar fault plane and a three dimensional (3D) curved fault surface rupture propagation model. We derived the estimated spatial slip distribution from an inversion analysis of GPS coseismic displacement data and show that the 3D fault model is the preferred solution. The simple and segmented fault models lead to significant artificial slip distributions associated with the pre-defined fault geometry and the spatial distribution of GPS stations. The spatial distribution of coseismic slip deduced from the 3D fault model has three observable features: (1) the overall slip is concentrated at depth of less than 12 km, which may well correspond to a shallow-dipping detachment; (2) the maximum slip of about 10 m is located 45 km to the north of the epicenter; and (3) the slip vector is dominated by the dip-slip component. In addition, the results from the inversion of GPS data are consistent with those from the inversion analysis of teleseismic broadband data. A resolution analysis, further, demonstrates that the results are highly correlated with field GPS data studies when we used synthetic test data. The inversion of spatially distributed GPS data is highly sensitive to fault geometry. We conclude that the use of the 3D fault model is not only necessary but also certainly competent enough to well explain the inferred slip style and the observed static coseismic displacements. [Copyright &y& Elsevier]

Abstract: We combined field mapping and structural analysis of Landsat imagery in order to identify active faults in the broader area of the Simitli graben and to the east towards the cities of Razlog and Bansko, in southwest Bulgaria. We mapped five large active fault segments with normal-slip kinematics and down-to-north displacement and three smaller, antithetic faults near Razlog. Our work suggests that: (a) present-day deformation in SW Bulgaria is extensional and is accommodated by seismic slip along E–W, NE–SW and WNW–ESE normal faults; (b) inversion of fault slip data shows a σ 3 axis oriented 336–356°; (c) the Krupnik fault comprises one earthquake segment with a general NE–SW strike and dip to the N–NW; its length is about 20km so its earthquake potential is of the order of M w =6.7±0.3; (d) as the 4 April, 1904 earthquake comprised two events, a static stress triggering hypothesis may apply, which is also compatible with the fault segmentation, geomorphology and the macroseismic reports. Source faults for the first event (10:02a.m.) may have been either the 12km long Gradevo fault or the 11km long Elovitsa fault. We estimate a moment magnitude of 6.3 for this event. The first event triggered the second one (10:28a.m.) on the Krupnik fault. [Copyright &y& Elsevier]

The Parnitha mountain range lies between two Quaternary rift systems in central Greece: the Gulf of Corinth Rift and the Gulf of Evia rift. We suggest that the range was formed by footwall uplift on active normal faults striking WNW–ESE and NE–SW. We investigated the scarp appearance, geometry and slip rates of three normal faults bounding this mountain range by field mapping at 1:5000 scale. Active faults studied include the 8.5 km long Fili Fault, the 4.7 km long Maliza Fault and the 4 km long Thrakomakedones Fault. We calculated comparable mean slip rates for all mapped faults (Fili: 0.18 mm/yr, Avlon: 0.2 mm/yr, Thrakomakedones: 0.24 mm/yr); however, we suggest that the WNW–ESE structures are more active during the Late Quaternary because of abundant field evidence of recent movements along slip surfaces (fresh basal stripes and slickenlines). In addition, stress axes analysis shows a N7°E–N25°E (NNE–SSW) oriented, extensional stress field, which is compatible with the focal mechanism of the Athens 1999 earthquake. The fault-slip data from the Parnitha faults show orientations similar to other low-strain areas in central Greece, such as the Gulf of Evia Rift to the north. Our slip rate estimates may explain the low recurrence of large earthquakes in Attica as opposed to high slip rate areas in central Greece such as the neighbouring Gulf of Corinth. [Copyright &y& Elsevier]

• NE-striking Aras Fault Zone (AFZ) bounds the Talesh and Lesser Caucasus blocks. • The AFZ is characterized by well-developed geological and geomorphological features. • Paleostress results along the AFZ indicate a similar tectonic regime since Pliocene. • The transpressive tectonic regime controls the structural evolution of the AFZ. • Transpressive tectonic regime leads to strain partitioning along the AFZ. Aras Fault Zone (AFZ) is one of the most important intra-plate fault zones within the Turkish-Iranian Plateau. The AFZ is divided into five fault branches based on geometrical and structural properties called the Aras Main, Horadiz, Khodafarin, Asadkandi, and Aslanduz fault branches. Our geometric and kinematic analysis indicates that these faults exhibit compressional left-lateral strike-slip fault characteristics with general directions of NE-SW and dipping to SE. Structural mapping has revealed that the NE section of the AFZ shows typical horsetail fault geometry. Few meters to approximately 17 km of left-lateral displacements were observed along the AFZ, indicating the occurrence of numerous historical earthquake events. Paleostress analysis of fault slip data reveals that the AFZ has developed in the dominant NNE-SSW and NE-SW oriented compressional stress regime. Evaluating the Ratio of Principal Stress Differences (R), Stress Regime Index (R'), and principal stress directions show that the zone is under the transpressional tectonic regime. Paleostress orientations are consistent with orientations of the GPS velocity vectors, which indicates that the region, including the AFZ, is under the effect of the same tectonic regime, at least since Pliocene. The amounts of strike-slip partitioning on discrete strike-slip faults are determined as 33–41%, which means at least 41% of total wrench components in the region have been accommodated by discrete strike-slip faulting along the AFZ. Results indicate that the simple shear component of total stress applied to the area is consistent with the GPS slip velocities. [ABSTRACT FROM AUTHOR]

Beneath the Marmara Sea, Turkey, the Main Marmara Fault (MMF), the offshore part of the North Anatolian Fault (NAF), is a well-known seismic gap for future M > 7 earthquakes. However, its detailed fault geometry and microearthquake activity have been debated for several decades. Using data acquired from long-term ocean bottom seismograph (OBS) observations, we made precise hypocenter estimations based on 3-D Vp and Vs velocity structures and assessed the fault geometry beneath the western and central parts of the MMF. The results indicate a segmentation boundary between the near-vertical western part and the south-dipping eastern part located around 28.10°E. Enriched OBS locations indicate microseismicity along both the inner and outer boundary faults of the Central Basin, especially on the western side. A comparison with previously published results suggests that the seismicity pattern has not changed for at least two years, between 2014 and 2016. Using a combined dataset of this and previous studies, lateral variations in the dip angle along the MMF fault segment from 27.4°E to 28.8°E were investigated. Based on this, we depicted on-fault seismicity along the MMF and defined three inactive areas of microseismicity. Two are located in the western segment, corresponding to the rupture area of the 1912 Ms. 7.4 earthquake, and the other, the largest, is located on the eastern segment. From a comparison of previous seismic and geodetic studies, it is considered that this area is a fully locked zone and has the potential for large earthquakes. Having compared the difference between hypocenter locations determined from OBSs and land-based stations, it is proposed that the epicentral locations of the mainshock and aftershocks of the September 26, 2019, M 5.7 earthquake are located much closer to the MMF than locations reported from only land-based results. • A segmentation boundary on the Main Marmara Fault is identified beneath the Central Basin. • Both inner and outer boundary faults of the Central Basin are developed in the western Central Basin. • Zones of no seismicity beneath the Kumburgaz Basin have the potential for a large earthquake. • The focal area of the September 2019 M 5.7 event was close to the Main Marmara Fault. [ABSTRACT FROM AUTHOR]

The 2016 Kaikoura (New Zealand) earthquake is characterized as one of the most complex multifault rupture events ever observed. We perform dynamic rupture simulations to evaluate to what extent relatively simple forward models accounting for realistic fault geometry can explain the characteristics of coseismic observations. Without fine parameter tuning, our model reproduces many observed features including the multifault rupture, overall slip distribution, and the locations of the maximum slip and rupture arrest. In particular, our model shows spontaneous arrest of dynamic rupture at the both ends of the ruptured fault system due to smaller prestress levels expected from a regional tectonic stress field. Both the simulated and the observationally inferred source time functions show similar double peaks with a larger second peak. The results illuminate the importance of the 3‐D fault geometry in understanding the dynamics of complex multifault rupture. Plain Language Summary: The 2016 Kaikoura earthquake in the northern South Island of New Zealand was one of the most complex faulting events ever observed. Previous studies identified surface ruptures along at least a dozen major faults extending over 150 km. Here we use computer simulations to investigate how the complex three‐dimensional geometry of faults plays a role in the rupture propagation and termination during the Kaikoura earthquake. We find that our model can explain a number of observations associated with the earthquake. In particular, the model suggests that the termination of propagating rupture at the ends of faults was caused by the unfavorable orientations of these faults with respect to regional tectonic stresses. The results illuminate the importance of three‐dimensional fault geometry in understanding the dynamics of multifault earthquakes. Key Points: Dynamic model considering realistic fault geometry and regional stress field reproduces multifault rupture during the Kaikoura earthquakeModel shows spontaneous rupture arrest on the western Humps and Needles faults, which are unfavorably oriented in a regional stress fieldDynamic triggering of fault slip may have played an important role in transferring the rupture from the southern to northern faults [ABSTRACT FROM AUTHOR]