Coseismic Rupture Research Articles

Seismic versus aseismic slip for the 2023 Kahramanmaraş earthquake doublet

Interplay between seismic and aseismic slip could shed light on the frictional properties and seismic potential of faults. The well-recorded 2023 Kahramanmaraş earthquake doublet provides an excellent opportunity to understand their partitioning on strike-slip faults. Here, we utilize InSAR and strong motion data to derive the coseismic rupture during the doublet, ~4-month postseismic afterslip, and slip distributions of two Mw>6.0 aftershocks. Our results show that afterslip appears to be complementary to coseismic slip and aftershocks, accounting for ~11.3% of the coseismic moment. Aftershocks mainly fall within the regions of positive Coulomb stresses caused by afterslip and follow a temporal decay similar to that of afterslip, indicating that aftershock production is the failure of small asperities loaded by the afterslip. The early postseismic afterslip is released ~93.7% aseismically and ~6.3% seismically by aftershocks. Our modeling results thus depict a complex fault system with highly variable slip patterns and stresses.

Distribution of Interseismic Coupling Along the Maidan Fault in Tianshan Before the 2024 Mw 7.0 Wushi Earthquake

AbstractDespite historically low seismic activity on the Maidan Fault (MDF), the sudden Mw 7.0 Wushi earthquake in early 2024 underscores the urgent need to assess the seismic risk on this fault. This study utilizes Global Navigation Satellite System and Interferometric Synthetic Aperture Radar data to image the spatial distribution of interseismic coupling before the Wushi event along the MDF. The four asperities we identified indicate a high seismic risk for the MDF, with potential magnitudes of Mw 7.5–7.6, Mw 7.5–7.7, Mw 7.0–7.2, and Mw 6.8–7.0. Specifically, the Maierkaiqi segment has already accumulated seismic moment comparable to previous event. Furthermore, our comparison of the interseismic coupling model with the coseismic rupture of the Wushi earthquake reveals that the shallow compliant zone, creeping patches, and irregular fault geometry collectively constrained the rupture propagation. The consistency between the rupture and high coupling area also suggests that the obtained coupling distribution may represent potential earthquake‐prone zones along the MDF.

Coseismic slip model and early post-seismic deformation processes of the 2024 M7.5 Noto Peninsula, Japan earthquake revealed by InSAR and GPS observations

SUMMARY On 2024 January 1, an Mw7.5 earthquake ruptured the shallow reverse fault in the Noto Peninsula, which provides opportunities to better constrain the distributed coseismic slip of this earthquake and explore the early post-seismic deformation processes following this earthquake. We first utilized the coseismic displacements to invert the preferred coseismic rupture, including the optimal fault geometry and slip distribution. Our results indicated that the distributed slip mode on the simple fault plane with the dip and strike angles of 35o and 51o of this event is mainly featured by thrust slip. Three main rupture zones with ∼100 km length and 0–20 km depths have averaged slip of ∼3 m and maximum slip of ∼5.2 m, which has a seismic moment of 2.18 × 1020 Nm (Mw7.49). We first estimate the 3-D surface deformation due to the viscoelastic relaxation in the lower crust and upper mantle through the finite-element simulation method, and obtain the residual displacements at these GPS stations through removing its deformation effects from the observed post-seismic displacements. Based on the above fault geometry, we then inverted the first 138-day afterslip evolution following this earthquake by fitting the above residual displacements. The afterslip model mainly occurred the gap between two shallow slip asperities, which has a peak slip of up to ∼1.2 m and a seismic moment of 2.36 × 1019 Nm (Mw6.85). A number of aftershocks mainly ruptured on the surroundings of the coseismic slip and afterslip zones, suggesting that the aftershocks are mostly driven by the combined stress changes from the coseismic and post-seismic slip of the fault. The viscoelastic relaxation in the lower crust and upper mantle and the afterslips of the fault play the dominant roles in the early deformation processes, which also contribute to the post-seismic surface deformation following this earthquake. We simulate the deformation due to the poroelastic rebound in the top of upper crust. Model results indicate that the poroelastic rebound produces the centimetre-scale surface subsidence in the near-field area within ∼50 km around the hypocentre of the earthquake.

The Linked Complexity of Coseismic and Postseismic Faulting Revealed by Seismo‐Geodetic Dynamic Inversion of the 2004 Parkfield Earthquake

AbstractSeveral regularly recurring moderate‐size earthquakes motivated dense instrumentation of the Parkfield section of the San Andreas fault (SAF), providing an invaluable near‐fault observatory. We present a seismo‐geodetic dynamic inversion of the 2004 Parkfield earthquake, which illuminates the interlinked complexity of faulting across time scales. Using fast‐velocity‐weakening rate‐and‐state friction, we jointly model coseismic dynamic rupture and the 90‐day evolution of postseismic slip in a 3D domain. We utilize a parallel tempering Markov chain Monte Carlo approach to solve this non‐linear high‐dimensional inverse problem, constraining spatially varying prestress and fault friction parameters by 30 strong motion and 12 GPS stations. From visiting 2 million models, we discern complex coseismic rupture dynamics that transition from a strongly radiating pulse‐like phase to a mildly radiating crack‐like phase. Both coseismic phases are separated by a shallow strength barrier that nearly arrests rupture and leads to a gap in the afterslip, reflecting the geologic heterogeneity along this segment of the SAF. Coseismic rupture termination involves distinct arrest mechanisms that imprint on afterslip kinematics. A backward propagating afterslip front may drive delayed aftershock activity above the hypocenter. Trade‐off analysis of the 10,500 best‐fitting models uncovers local correlations between prestress levels and the reference friction coefficient, alongside an anticorrelation between prestress and rate‐state parameters . We find that a complex, fault‐local interplay of dynamic parameters determines the nucleation, propagation, and arrest of both, co‐ and postseismic faulting. This study demonstrates the potential of inverse physics‐based modeling to reveal novel insights and detailed characterizations of well‐recorded earthquakes.

The Effects of Characteristic Weakening Distance on Earthquake Nucleation Styles in Fully Dynamic Seismic Cycle Simulations

AbstractEarthquake nucleation is a crucial preparation process of the following coseismic rupture propagation. Under the framework of rate‐and‐state friction (RSF), it was found that the ratios of to parameters control whether earthquakes nucleate as an expanding crack or with a fixed length prior to the dynamic instability. However, the characteristic weakening distance controls the weakening efficiency of state variables in RSF and can influence the nucleation styles as well. Here we investigate the effects of on nucleation styles in the context of fully dynamic seismic cycles by evaluating the evolution of the nucleation zone quantitatively when it accelerates from the tectonic loading rate to seismic slip velocity. A larger (>0.75) is needed to produce expanding crack nucleation styles for relatively small , which suggests that fixed length nucleation styles may dominate on natural and laboratory faults. Furthermore, we find a more complex nucleation style when the nucleation site is not in the center of the asperity and identify a twin‐like nucleation style which includes two initial acceleration phases. We conclude that the earthquake nucleation style is strongly controlled by the value of . The possible dominance of fixed length nucleation styles suggests that the minimum size of earthquake rupture may be estimated at the early stage of the nucleation phase.

D-InSAR-Based Analysis of Slip Distribution and Coulomb Stress Implications from the 2024 Mw 7.01 Wushi Earthquake

On 23 January 2024, an Mw 7.01 earthquake struck the Wushi County, Xinjiang Uygur Autonomous Region, China. The occurrence of this earthquake provides an opportunity to gain a deeper understanding of the rupture behavior and tectonic activity of the fault system in the Tianshan seismic belt. The coseismic deformation field of the Wushi earthquake was derived from Sentinel-1A ascending and descending track data using Differential Interferometric Synthetic Aperture Radar (D-InSAR) technology. The findings reveal a maximum line-of-sight (LOS) displacement of 81.1 cm in the uplift direction and 16 cm in subsidence. Source parameters were determined using an elastic half-space dislocation model. The slip distribution on the fault plane for the Mw 7.01 Wushi earthquake was further refined through a coseismic slip model, and Coulomb stress changes on nearby faults were calculated to evaluate seismic hazards in surrounding areas. Results indicate that the coseismic rupture in the Mw 7.01 Wushi earthquake sequence was mainly characterized by left-lateral strike-slip motion. The peak fault slip was 3.2 m, with a strike of 228.34° and a dip of 61.80°, concentrated primarily at depths between 5 and 25 km. The focal depth is 13 km. This is consistent with findings reported by organizations like the United States Geological Survey (USGS). The fault rupture extended to the surface, consistent with field investigations by the Xinjiang Uygur Autonomous Region Earthquake Bureau. Coulomb stress results suggest that several fault zones, including the Kuokesale, Dashixia, Piqiang North, Karaitike, southeastern sections of the Wensu, northwestern sections of the Tuoergan, and the Maidan-Sayram Fault Zone, are within regions of stress loading. These areas show an increased risk of future seismic activity and warrant close monitoring.

In-plane dynamic analysis of supershear rupture transition due to poroelastic pressurization

Summary Although pore fluid has received increased attention in the problems of earthquake rupture, its effects on coseismic rupture are not fully understood. In this paper, we numerically simulate dynamic rupture in saturated poroelastic media to study the interplay between poroelastic pressurization and rupture evolution. Particular attention is paid to the physical process of the supershear transition. The spontaneous rupture is assumed to be governed by slip weakening law and the fault is treated as a leaky interface where pore pressure equilibrium is delayed. Results show that the pore pressure induced by coseismic mechanical deformation can strengthen the fault on the extensional side but weaken the fault on the compressional side. As a result, the poroelastic pressurization on the compressional side can promote the occurrence of the transition to supershear rupture while its strengthening effect on the extensional side can suppress this transition even in the state of high initial shear stress. Meanwhile, it is found that the permeability distribution along the fault plane also affects rupture evolution. Faults with nucleation zones located in low permeability regions are more likely to undergo a supershear rupture.

Rheological Structure and Stress Triggered Megathrust Slip Constrained From the 2016 Mw 7.8 Kaikōura Crustal Earthquake

AbstractUnderstanding postseismic processes following the 2016 Mw 7.8 Kaikōura earthquake remains challenging due to the time‐dependent afterslip over the complex forearc system including crustal faults and megathrust, and the viscoelastic relaxation of the upper mantle. How the 2016 Mw 7.8 Kaikōura crustal earthquake interacts with the megathrust has yet to be better understood. Here we have derived the first 5‐year postseismic displacements from Global Positioning System (GPS) time series of 75 stations to study postseismic processes through a three‐dimensional viscoelastic finite element model. The optimal steady state viscosities of the crustal shear zone, megathrust shear zone, Australian upper mantle and Pacific upper mantle in the lowest‐misfit model among test models are 1018 Pa s, 4 × 1017 Pa s, 2 × 1019 Pa s and 1020 Pa s, respectively. The stress‐driven afterslip within the first 5 years after the earthquake is up to 80 cm over crustal faults, and up to 70 cm over the megathrust. A Kapiti slow slip sequence is probably promoted with a shorter interval by the 2016 earthquake, and is up to ∼11 cm within the first year after the earthquake. Afterslip over crustal faults and the megathrust are both required to reproduce the first‐order pattern of horizontal GPS observations. Coseismic rupture over the megathrust enhances shallow megathrust afterslip, which better fit the eastward postseismic displacement of sites near the rupture area. The southern end of Hikurangi megathrust may be activated during the 2016 earthquake and undergo continuous aseismic slip after the event.

Co-Seismic and Post-Seismic Slip Properties Associated with the 2024 M 7.5 Noto Peninsula, Japan Earthquake Determined by GNSS Observations

Based on GNSS observations, the co-seismic and post-seismic slip of the 2024 Noto Peninsula earthquake and the spatio-temporal pattern of afterslip are investigated in this paper. The co-seismic slip is mainly distributed in the depth range of 2 to 15 km with the maximum value of 5.94 m. Compared with the co-seismic rupture pattern, a shallow afterslip can be observed after the earthquake, and the afterslip patch is formed northeast of the epicenter. The maximum value of afterslip during the post-seismic 180 days is 1.13 m, which is situated at the longitude of 137.53°, latitude of 37.75°, and epth of 5.43 km. The spatio-temporal evolution of afterslip indicates that the fault activity has continued throughout the post-seismic 180 days, and the coverage and magnitude of afterslip have gradually increased. As time goes on, the fault activity tends to weaken, as evidenced by a decrease in slip rate. The daily images of afterslip demonstrate that the fault activity is particularly strong in the early time period following the earthquake. The maximum value of afterslip in the first week accounts for about 18% of that in the post-seismic 180 days, and the maximum slip rate reaches 0.043 m/day. In addition, the Coulomb stress analysis indicates that afterslip and most aftershocks appear in the positive Coulomb stress region, suggesting that co-seismic Coulomb stress changes may be the driving mechanism of afterslip and aftershocks.

Effects of the 2022 Mw 7.0 Chihshang Earthquake on the Shallow Creeping of the Central Longitudinal Valley Fault in Eastern Taiwan: Insights from Precise Leveling Measurements

Abstract This study investigated the effects of the 2022 Mw 7.0 Chihshang earthquake in eastern Taiwan on the shallow creeping of the central Longitudinal Valley fault (LVF). Precise leveling surveys were conducted along the Ruisui-Dewu, Yuli, and Dongzhu routes over the LVF. Before the earthquake, previous leveling surveys had detected deformations caused by stable fault creeping of the LVF. Interseismic linear trends and coseismic deformations were removed from the leveling data. Two shallow faults at 1 km were estimated from the corrected-coseismic deformation for the Yuli route. A shallow fault at 1 km and a deep fault at 5 km were estimated from the interseismic deformation. A positive delta Coulomb failure function change was calculated for shallow faults due to the creeping movement on the deep fault before the earthquake, and the creeping movement on the deep fault before the earthquake may have promoted the rupture of the shallow fault. The strong Chihshang earthquake most likely triggered the rupture of the shallow fault, on which strain had been concentrated by the interseismic creeping of the deeper fault. In contrast, the accumulated total strain at the deeper part might be quite low due to a strain release process for a steady creeping movement on deep fault in the interseismic period. The significant slip was not induced in the coseismic period. Although significant slip occurred in the shallower part, the slip might not be induced in the deeper part adjacent to the asperity of the LVF. In the Dewu route, the observed large west-side-uplift is attributed to the coseismic rupture of the Chihshang earthquake. In the Dongzhu route, an interseismic east-side-uplift by a creeping fault and a coseismic west-side-uplift were observed at the same location, indicating a local tensile stress field by the Chihshang earthquake.

The 2024 Mw 7.0 Wushi Earthquake in Southern Tianshan Convergent Zone: Finite-Fault Model for the Coseismic Rupture and Aftershock

Abstract On 22 January 2024, an Mw 7.0 earthquake struck the oblique thrust Maidan fault (MDF) at the boundary between the Southern Tianshan and Tarim basin, making it the largest earthquake on the fault in the last 100 yr. Here, we use Interferometric Synthetic Aperture Radar (InSAR) to retrieve coseismic and aftershock deformation caused by the earthquake and then constrain fault geometry and slip distribution. Line-of-sight InSAR observations revealed a coseismic deformation area of 70 km × 60 km with a maximum value of ∼0.8 m and also captured the deformation from the aftershock event seven days after the mainshock. Our preferred two-segment coseismic fault model ruptures the MDF locked segment and deeper regions, exhibiting a homogeneous elliptical rupture with a maximum slip of ∼2.7 m on a rectangular plane with a dip angle of ∼60°, a length of ∼55 km, and a depth between 5 and 20 km. The aftershock formed a rupture plane of 10×8 km2 with the maximum slip of ∼0.3 m, causing slip in the shallow area of the fault where the coseismic ruptures were smaller, and supplementarily released the stress in the shallow part of the fault. The strong earthquake (Mw 7) return period on MDF inferred from the interseismic slip rate is 170–200 yr. Oblique thrust slip revealed by the slip distribution model is the result of long-term absorption of oblique convergence strain in the Southern Tianshan by the MDF and then concentrated release. The 2024 event resulted in a 60 km unruptured segment on the MDF being in a stress-triggering zone, increasing the potential seismic hazard, in contrast to the delayed seismicities on 100 km region of the southern Kalping fault (KPF).

Postseismic deformation due to the 2021 MW 7.4 Maduo (China) earthquake and implications for regional rheology and seismic hazards around the Bayan Har block

GPS and InSAR observations of the first ∼1.5 years of postseismic deformation caused by the 2021 MW 7.4 Maduo earthquake provide a valuable opportunity to investigate fault interactions and regional rheological structure, as well as the future seismic potential around the Bayan Har block, northeastern Tibetan Plateau. We develop an integrated model to simulate the afterslip and viscoelastic relaxation contributions to the observed postseismic displacements, and found that afterslip driven by the coseismic stress is concentrated downdip of rupture, and dominates the postseismic deformation in the early stage (∼0.4 year after the event). Because afterslip decays quickly over time, viscoelastic relaxation should become the main postseismic mechanism as time goes on. The two mechanisms produce similar displacements during 0.4–1.5 years after the earthquake, but at 1.5 years after the earthquake the velocity caused by viscoelastic relaxation is larger than that caused by afterslip. Viscoelastic models assuming either a Burgers body or power-law rheology produce very similar predictions, with the Burgers body model having a slightly lower overall misfit. The rheological structure constrained by the postseismic observations supports the 35-km thick elastic upper crust overlying a Burgers body viscoelastic lower curst with a Maxwell viscosity of 3 × 1019 Pa s (5 - 50 × 1018 Pa s at 95% confidence), assuming the Kelvin viscosity is equal to 10% of that value. This is different from the regional rheology inferred by the postseismic investigations on the 2001 MW 7.8 Kokoxili and the 2008 MW 7.8 Wenchuan events, and the preferred thickness of the elastic crust is also different from that inferred from magnetotelluric profiles deployed in previous studies. We thus infer that the rheological structure within the Bayan Har block is possibly heterogeneous from west to east. Finally, the normal stress changes triggered by the coseismic rupture and postseismic process are estimated to be negative, but the shear stress changes to be positive on the western Kunlun fault, the eastern Dari fault and Bayan-Har Mountain fault. However, the current observations and studies are quite insufficient on those fault segments, therefore, we need to focus on their faulting behavior and seismic risk in the future.

Afterslip and Creep in the Rate‐Dependent Framework: Joint Inversion of Borehole Strain and GNSS Displacements for the Mw 7.1 Ridgecrest Earthquake

AbstractThe elusive transition toward afterslip following an earthquake is challenging to capture with typical data resolution limits. A dense geodetic network recorded the Mw 7.1 Ridgecrest earthquake, including 16 Global Navigation Satellite System (GNSS) stations and 3 borehole strainmeters (BSM). The sub‐nanostrain precision and sub‐second sampling rate of BSMs bridges a gap between conventional seismologic and geodetic methods, exemplified by atypical postseismic shear strain reversals observed at nearfield (<2 km) station B921 that remain unexplained. We jointly invert GNSS displacements and BSM strains for coseismic and postseismic slip spanning hours to months over 7 independent periods. Cosiesmically, our model resolves the largest slip magnitudes of up to 6.6 m on the mainshock rupture plane, with similar patterns to other inferred slip distributions. The foreshock fault appears to slip coincidently with mainshock, revealing potential asperities activated during the preceding Mw 6.4 event. Postseismically, the best‐fitting models adhere to mechanical rate‐and‐state expectations of logarithmically decaying slip adjacent to the coseismic rupture terminus, and where deep rheologic conditions favor creep. Most spatial variation occurs in the early postseismic timeframe (<1–2 weeks), with evidence for regional rheologic control and static stress dependence. Triggered creep on the neighboring Garlock Fault unexpectedly persists for >178 days—further highlighting the importance of fault networks in postseismic stress redistribution, critical to assessing future hazard.

Deformed archaeological remains at Lilybaeum in Western Sicily (southern Italy): possible ground signatures of a missed large earthquake

AbstractArchaeoseismic analysis performed in Western Sicily points to deformed archeological remains at Lilybaeum, a Punic coastal city founded in 397 B.C. at the Island’s westernmost edge. Starting from the direct observation of deformed ruins, an interdisciplinary work strategy, which included field-structural analysis, drone-shot high-resolution aerial photogrammetry, and geophysical prospecting, was employed to investigate whether the identified deformations may represent the ground effects of a previously unknown large earthquake in the area. Among the unearthed remains, some mosaics and a stone-paved monumental avenue show evidence of tectonic deformation, being fractured, folded, and uplifted. The trend of folding and fracturing is consistent with the NNW–SSE oriented tectonic max stress axis to which Western Sicily is currently subjected. Displacement along a fracture deforming the Decumanus Maximus together with the finding of a domino-type directional collapse, enable us to interpret the observed deformation as the ground signature of a coseismic slip. The seismic rupture occurred along a previously unmapped deformation front that fits well within the seismotectonic context of Western Sicily. Measured offset, geophysical prospecting, and age-constraints all suggest the possibility that a highly-energetic earthquake nucleated in the area following a coseismic rupture along a NE–SW trending back-verging reverse fault towards the end of the fourth century CE. Since seismic catalogs do not provide evidence of such a large earthquake, this event might represent a missed entry in the historical seismic record. This finding provides constraints to redefine the seismic hazard of Western Sicily, a region where recurrence-time intervals for large earthquakes are still unknown.

Low‐Viscosity Zones Beneath the Coso Volcanic Field Revealed by Postseismic Deformations Following the 2019 Ridgecrest Earthquake

AbstractThe rheology of materials near fault zones controls the deformation of the fault, thus playing an important role in the rupture propagation of large earthquakes. Here, we model the postseismic deformation in the first 4 years following the 2019 Ridgecrest earthquake, and probe the laterally variable lower‐crustal viscosity and fault afterslip, using a combined model incorporating afterslip and viscoelastic relaxation. Modeling results reveal that laterally variable low‐viscosity zones (1017∼1018 Pa·s) beneath the Coso Volcanic Field are required to explain the observations well. These low‐viscosity materials may reduce the width of the brittle part of the fault, thus affecting the northwestward propagation of the 2019 Ridgecrest earthquake rupture and the spatial pattern of nearby seismicity. Numerical simulations reveal that the postseismic viscoelastic relaxation of the event will cause contractional strain in the upper‐crustal magma reservoirs, which may counteract the extensional strain caused by the coseismic rupture.

Coseismic Slip and Downdip Afterslip Associated with the 2021 Maduo Earthquake Revealed by Sentinel-1 A/B Data

On 22 May 2021, an earthquake (98.3° E and 34.59° N) struck Maduo town in Qinghai province, occurring along a relatively obscure secondary fault within the block. We utilized 105 archived Sentinel-1A/B acquisitions to investigate the coseismic deformation and the evolution of postseismic displacements in both the temporal and spatial domains, as well as the associated dynamic mechanisms of the 2021 Maduo earthquake. The interference fringes and coseismic deformation revealed that the primary feature of this event was the rupture along a left-lateral strike-slip fault. The released seismic moment was close to 1.88 × 1020 N·m, which is equivalent to an Mw 7.45 event. Simultaneously, the maximum coseismic slip reached approximately 4 m along the fault plane. The evolution of postseismic displacements in both the temporal and spatial domains over 450 days following the mainshock was further analyzed to explore the underlying physical mechanisms. Generally, the patterns of coseismic slip and afterslip were similar, although the postseismic displacements decayed rapidly over time. The modeled afterslip downdip of the coseismic rupture (at depths of 15–40 km) effectively explains the postseismic deformation, with a released moment estimated at 4.57 × 1019 N·m (corresponding to Mw 7.04). Additionally, we found that regions with high coseismic slip tend to exhibit weak seismicity, and that afterslip and aftershocks are likely driven by each other. Finally, we estimated the Coulomb Failure Stress changes (ΔCFS) triggered by both coseismic rupture and aseismic slip resulting from this event. The co- and postseismic ΔCFS show similar patterns, but the magnitude of the postseismic ΔCFS is much lower (≤0.01 MPa). We found that ΔCFS notably increased on the Yushu segment of the Garze-Yushu-Xianshuihe Fault (GYXF), as well as the Maqin–Maqu and Tuosuo Lake sections of the East Kunlun Fault (EKF). Therefore, we infer that these fault segments may have a higher potential seismic risk and should be carefully monitored in the future.

Forearc faults in northern Cascadia do not accommodate elastic strain driven by the megathrust seismic cycle

We employ numerical models to explore the connection between subduction zone coupling or megathrust rupture and upper plate faults in the northern Cascadia forearc. Active forearc faults north of the Olympic Peninsula exhibit similar characteristics: west-northwest strike, oblique right-lateral slip senses, and low slip rates (<1 mm/yr), but a potential to generate large (M ~ 7) earthquakes. Previous hypotheses suggest that stress in the upper plate due to interseismic coupling or coseismic rupture along the subduction zone interface could drive permanent forearc strain. To test these hypotheses, we used a 3D boundary element method model to predict slip on the LRDM if interseismic coupling or coeseismic rupture cause deformation. Our model predicts reverse left-lateral slip if the strain results solely from subduction zone coupling, or normal right-lateral slip if these faults accommodate strain during a megathrust rupture. These results contradict the observed fault kinematics. Additionally, if we use our model to mimic strain partitioning, where only the strain from the strike-slip component of subduction zone coupling is accommodated in the forearc, our results are also inconsistent observed fault kinematics. These models challenge the hypothesis that subduction zone coupling or coseismic rupture are the primary driver of permanent forearc deformation in northern Cascadia.

Paleoseismological evidence for segmentation of the Main Himalayan Thrust in the Darjeeling-Sikkim Himalaya

Whether the Main Himalayan Thrust can host a single surface-rupturing event in the Himalaya with a rupture length of > 700 km remains controversial. Previous paleoseismological studies in the Darjeeling-Sikkim Himalaya (DSH) suggested medieval surface-rupturing earthquakes, correlating them with the eleventh–thirteenth century events from Nepal and Bhutan and extending the coseismic rupture length > 700 km. Conversely, there is no rupture evidence of the 1714 Bhutan and 1934 Bihar–Nepal earthquakes in the DSH, resulting in a discrepancy in the rupture extent of the great earthquakes. Consequently, we conducted a paleoseismological investigation across a ~ 10 m-high fault scarp on the Himalayan Frontal Thrust at Chenga village, DSH, revealing a surface-faulting event during 1313–395 BCE. We suggest that the DSH is a 150 km-long independent segment bounded by a transverse ridge and fault and has a recurrence interval of ~ 949–1963 years, which is significantly larger than Nepal (~ 700–900 years) and Bhutan Himalaya (~ 339–761 years).

The 2022 Har Lake earthquake sequence highlights a complex fault system in the western Qilian Shan, northeastern Tibetan Plateau

SUMMARY The 2022 Har Lake earthquake sequence, which began in 2022 January and lasted for ∼70 d, jolted the Har Lake area, which is located in the western Qilian Shan, northeastern Tibetan Plateau. Two Mw > 5.5 earthquakes occurred during the earthquake sequence, among which the March 25 Mw 5.8 event is considered the largest event recorded in the area. However, determining the seismogenic faults of the earthquake sequence, as well as the detailed rupture features, is difficult due to the lack of geological data and near-field seismological observations. In this study, we use Sentinel-1 synthetic aperture radar (SAR) data to obtain the coseismic deformation field, identify possible ruptured faults and associated fault geometries, and further estimate detailed coseismic slip models of the two Mw > 5.5 earthquakes. The results show that the January 23 Mw 5.6 earthquake (Earthquake A) occurred on a N15° W-trending dextral-slip fault with a dip angle of ∼61°. For the March 25 Mw 5.8 earthquake (Earthquake B), the interferometric synthetic aperture radar (InSAR) data can be described by either an ∼N–S-trending dextral-slip fault or an ∼E–W-trending sinistral-slip fault. The ∼N–S-trending fault better describes the aftershock distribution, while the ∼E–W-trending model is more consistent with the regional geological setting. We suggest that the complex coseismic ruptures in the multiple-fault system are driven by widespread NE–SW-trending compression in the western Qilian Shan. This study demonstrates the importance of integrating geodetic and seismological observations to capture the full complexity of moderate earthquakes and further suggests potential seismic hazards in the Har Lake area.

Double-Vergent Plate Boundary Faults and Triggered Coseismic Rupture of the 2022 Chihshang Doublet Earthquake Occurred in Eastern Taiwan

Abstract The earthquake doublet of Mw 6.5 and 6.9 occurred along the west-dipping Central Range fault (CRF) adjacent to the east-dipping creep segment of the Longitudinal Valley fault in eastern Taiwan on 17–18 September 2022. The faulting model derived from the Interferometric Synthetic Aperture Radar and Global Positioning System observation suggests that the west-dipping CRF rupture is responsible for the two mainshocks. Meanwhile, the two major earthquakes resulted in ∼100 km of fault slip along the double-convergent plate boundary faults near the Longitudinal Valley in eastern Taiwan. The 2022 Chihshang earthquake sequence filled the seismic gap of the CRF located between the 2006 ML 6.1 Taitung earthquake and the 2013 ML 6.1 Ruisui earthquake. Finally, the 2022 Chihshang earthquake sequence increased the Coulomb failure stress in the southernmost segment of the CRF, which may cause a higher seismic potential in the future.