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Strain accumulation in foreland zones is important for understanding mountain‐building processes and seismic hazards. Material heterogeneity in this border zone, particularly the contrast between sedimentary cover and basement, affects strain accommodation and fault behavior. One manifestation of this effect is depth separation of Interferometric Synthetic Aperture Radar (InSAR)‐derived slip models and corresponding aftershock clusters; hypotheses for this vertical separation remain controversial. In this study, we investigated strain accumulation in the Kalpin fold‐and‐thrust belt (KFTB) of southwestern Tian Shan, China, using the integration of InSAR measurements, teleseismic body‐waves, near‐field strong motion data (SM), and relocated aftershocks of the 2020 Mw 6.0 Kalpin earthquake. The SM modeling and analysis of post‐seismic InSAR time series are performed for the first time for this earthquake. Our results confirm a vertical separation of the mainshock and aftershock cluster, with the former on a weak décollement and the latter on faults within the basement. InSAR time series analysis shows that post‐seismic deformation was dominated by afterslip on a splay fault directly above the ruptured décollement. Static stress transfer of co‐seismic rupture cannot explain these observations. We speculate that fluid flow and high pore pressure along pre‐existing fault planes may have reduced the fault strength and been involved in the evolution of this aftershock cluster. We conclude that compressive strain in the KFTB is accommodated by a mixture of thin‐ and thick‐skin faulting and seismic deformation across the entire crustal thickness. Specifically, we suggest that the observed shortening is partly accommodated by infrequent large earthquakes on the weak décollement. Plain Language Summary: In foreland areas, strain can be accommodated by earthquakes, aseismic slip, or both. The contrasting material properties of the sediment and basement certainly affects this process, for example, the depth separation of different fault behavior. However, it is unclear how the fault behavior varies in depth from basement to weak sediment throughout different stages of the seismic cycle. 2020 Mw 6.0 Kalpin earthquake occurred in Kalpin fold‐and‐thrust belt, southwestern Tian Shan provides some insight on this matter. Here, we combined Interferometric Synthetic Aperture Radar, aftershock relocation, teleseismic body‐waves, and strong motion data to investigate the strain release behavior during and immediately after the earthquake. We found that strain release behavior differs between sediment and basement during and immediately after the earthquake. The mainshock ruptured the weak décollement and the aftershocks were primarily rooted in the basement, while the transient afterslip was mainly distributed in the sediments. Two findings are worth noting: (a) the afterslip occurred on a fault, whose stress was partially unloaded during the earthquake, suggesting the fault strength was reduced, and (b) the earthquake rupture of the décollement highlights how destructive earthquakes can initiate on the weak décollement. Key Points: The moderate 2020 Mw 6.0 Kalpin earthquake was initiated on a weak décollement in southwest Tian ShanSeismological data and Interferometric Synthetic Aperture Radar postseismic time series analysis confirm a vertical separation of the mainshock and aftershocksCombination of thin‐skin and thick‐skin deformation processes throughout the different stages of the seismic cycle [ABSTRACT FROM AUTHOR]

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

Knowledge of the slip behavior of the Altyn Tagh fault (ATF) has significant implications for our understanding of the tectonic deformation of the Tibetan Plateau. In this study, we process Global Positioning System (GPS) data spanning 2009–2017 across the western ATF, merge the solution with recently published GPS velocities, and obtain a dense GPS velocity field for northern Tibet. We introduce an elastic block model and estimate the fault slip rate, interseismic fault coupling (ISC), and seismic moment accumulation rate along the ATF. The estimated left‐lateral strike‐slip rate of the ATF decreases eastward from 12.8 ± 0.4 mm/a to 0.1 ± 0.2 mm/a. Results show a heterogeneous distribution of ISC along the fault, with the fault locking depth varying from 5 to 20 km. The seismic moment accumulation rate is 2.16–2.37 × 1018 N m/km along three segments between Sulamu Tagh and Aksay bends, where the accumulated seismic moment over the last five centuries could be balanced by earthquakes with magnitudes of Mw 7.7, Mw 7.6, and Mw 7.8, respectively. We calculate the ratio of the Indo‐Eurasia convergence accommodated by the ATF and find that 10.3% of the convergence is accommodated along the fault by lateral extrusion; quantitative analysis of the strain rate, however, shows that a large part of the northern Tibet is not well described by elastic block rotation. Consequently, we suggest that a hybrid kinematic model that includes both block‐like and continuous deformation is needed to better delineate the crustal deformation of the Tibetan Plateau. Key Points: Altyn Tagh fault slip rate and coupling inverted using new, dense interseismic GPS velocitiesThe three fault segments between Sulamu Tagh and Aksay bends have the potential to rupture in Mw 7.7, Mw 7.6, and Mw 7.8 earthquakesA hybrid kinematic model is needed to delineate the crustal deformation of the Tibetan Plateau [ABSTRACT FROM AUTHOR]

We use Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data to quantify the interseismic coupling along the Xianshuihe‐Xiaojiang fault system (XXFS). Our results confirm 7–11 mm/a of left‐lateral strike‐slip motion along the XXFS. South of the Shimian County, high interseismic coupling is found down to 20 km‐depth along the Anninghe‐Zemuhe fault. A second 130 km‐long section, highly coupled down to 10–15 km‐depth, is identified along the northern Xiaojiang fault. North of the Kangding County, the Xianshuihe fault appears to be predominantly creeping at depth. Previously identified long‐lasting surface creep following the 1973 M 7.6 Luhuo earthquake has now ended, possibly marking the initiation of a new period of stress accumulation and progressive relocking of the fault. Farther south along the fault, we identify a ∼30 km‐long section (∼30.2°–30.4°N) creeping at ∼7 mm/yr. Furthermore, creep accelerated by ∼2 mm/yr during the 2008–2014 period, possibly as a result of the static Coulomb failure stress increment induced by the 2008 Mw 7.9 Wenchuan earthquake, that occurred ∼200 km northeast of it. The 2014 Mw 5.9 Kangding earthquake occurred along the creeping section at the end of the period of accelerated creep. Finally, taking advantage of the long historical records of past large earthquakes available since 1327, we quantitatively compare the moment budget along the XXFS. Results highlight that along the Anninghe, Zemuhe, and Xiaojiang faults, moment deficit for Mw > 7 earthquakes has now accumulated. Plain Language Summary: More probable location and size of future large earthquakes can be anticipated if we accurately know the present‐day distribution of interseismic fault coupling. Knowing date of the latest large earthquake, one can infer slip deficit and estimate the energy available for forthcoming seismic ruptures. Here, we focus on the Xianshuihe‐Anninghe‐Zemuhe‐Xiaojiang fault system in Tibet, which is the largest strike‐slip fault crossing southwestern China. Historical records attest numerous earthquakes during the last few centuries, but the fault has remained relatively silent in the past few decades. We use GPS data spanning nearly 20 years to investigate the present‐day interseismic fault coupling along the fault system. We find the Anninghe, Zemuhe, and Xiaojiang faults have accumulated enough energy for several Mw > 7 earthquakes. However, our results show that the northern segment called the Xianshuihe fault is not locked, suggesting a relatively low energy accumulation rate and seismic potential, despite a long history of frequent earthquakes. Finally, we identify a new ∼30 km‐long creeping section on the Xianshuihe fault, whose creep rate show a transient behavior, possibly induced by the effect a ∼200 km remote earthquake that occurred in 2008. Key Points: Highly heterogeneous interseismic coupling along the Xianshuihe‐Xiaojiang fault system, with the northern Xianshuihe fault predominantly creeping at depthSurface creep observed during 4 decades after the 1973 M 7.6 Luhuo earthquake has endedA new ∼30 km‐long transient creeping segment on the Xianshuihe fault, with creep rate increasing after the 2008 Mw 7.9 Wenchuan earthquake [ABSTRACT FROM AUTHOR]

We investigated the spatial distribution of aseismic creep on the Laohushan‐Haiyuan fault using Global Positioning System (GPS) data (1999–2017) and Interferometric Synthetic Aperture Radar (InSAR) data (2003–2010). Comparisons among GPS, InSAR line‐of‐sight (LOS) rates, and leveling show that neither leveling nor GPS vertical velocities can fit the vertical signal mapped into the LOS, implying either complicated vertical crustal deformation in northeastern Tibet and/or complex error structures in the InSAR data. Thus, we combined horizontal GPS with high‐pass filtered InSAR data to produce a continuous LOS rate map crossing the fault. Our geodetic data reveal three creep sections along the fault. Both the restored LOS data and decomposed ascending and descending InSAR data highlight the fact that vertical motion can cause an overestimation of creep rate; we obtained a refined creep rate of 2.5 ± 0.4 mm/a on the Laohushan fault. We further identified a 10 km‐long, ∼3–5 mm/a creep section (∼104.2°E−104.3°E) and a 43 km‐long, ∼1–3 mm/a creep section (∼105.3°E−105.7°E) on the western and eastern Haiyuan fault respectively. Both are located on fault sections that ruptured during the 1920 M∼8 earthquake, suggesting that the 1920 earthquake was able to cross pre‐existing creep sections or that the fault shows heterogeneous relocking after large earthquakes, with creep lasting decades on some parts of the rupture. Fault coupling shows a highly variable rate of slip deficit accumulation along strike, suggesting that coupling might significantly evolve during the period between two large earthquakes. Plain Language Summary: Ample observations document slip on a fault plane is seismic and/or aseismic; the former situation refers to a fault slips suddenly and produce earthquakes, the latter means a fault slips gradually without generating seismic energy. Studying the fault‐creep activity contributes to better understand the seismic behavior of crustal faults. We focus on the Haiyuan fault system, which is located on the northeastern margin of Tibet. Several large earthquakes occurred along the fault system in the past, including the 1920 M∼8.0 Haiyuan and the 1927 M ∼8–8.3 Gulang earthquakes. Using Interferometric Synthetic Aperture Radar (InSAR) and Global Positioning System (GPS) data, we identified three creep sections along the fault. Our results show vertical deformation that has mapped into the InSAR line‐of‐sight (LOS) could leads to overestimation of the creep rate. Besides, our findings imply either large earthquakes can rupture creep regions or they can generate long‐lasting afterslip/creep. Key Points: Of three creep segments on the Laohushan‐Haiyuan fault, two are located along fault sections that ruptured during the 1920 M∼8 earthquakeVertical motion leads to overestimation of the creep rate on the Laohushan fault; our refined creep rate is 2.5 ± 0.4 mm/aOur findings support that large earthquakes can rupture pre‐existing creep regions or that they can generate long‐lasting afterslip or creep [ABSTRACT FROM AUTHOR]

Large‐scale geodetic measurements of crustal deformation in the north‐central Tibetan Plateau are crucial for improved understanding of earthquake‐cycle processes and long‐term seismic hazard assessment. We use GPS velocities and Interferometric Synthetic Aperture Radar (InSAR) observations (2015–2020) on eight descending and eight ascending Sentinel‐1 tracks to map surface motions and their gradients for an area of over ∼2,000 km × 350 km around the Kunlun‐Manyi fault system. The derived line‐of‐sight (LOS) and 3D velocity fields demonstrate ongoing postseismic transients along the ruptured segments of the 1997 Manyi and 2001 Kokoxili earthquakes, interseismic deformation along locked fault segments, and strain accumulation on a large subsidiary fault (i.e., the Kunlun Pass fault). We use elastic dislocation and analytical postseismic deformation models constrained by our dense InSAR measurements to quantify the interseismic and postseismic contributions, which reveal the along‐fault distribution of fault slip rate and locking depth along the entire length of the fault. The results indicate that the slip rate of the Kunlun fault systematically decreases toward the west to the west of the Taiyang Lake fault. We also find that the interseismic strain accumulation rate along the main trace of the Kunlun fault is to first‐order spatially constant (0.1–0.15 microstrain/yr), except around its restraining bend. As the historically unruptured segments, especially the eastern segments (from 95°E to 101°E) of the Kunlun fault, have relatively fast slip rates and high locking depths (>15 km), they have the potential to generate large and damaging earthquakes in this region. Plain Language Summary: On the north‐central Tibetan plateau, the ∼2,000‐km‐long Kunlun fault, characterized by rapid long‐term fault slip rates (8–12 mm/yr), and its subsidiary faults represent substantial earthquake hazards. However, the spatial variations of deformation and strain accumulation are still poorly understood due to the limited geodetic (i.e., GPS, InSAR) observations on the high plateau. Here, we use the spatially and temporally dense InSAR observations obtained from Sentinel‐1 images to map the large‐scale crustal deformation around the Kunlun fault, during 2015–2020. The derived deformation map covers an area of over ∼2,000 km × 350 km. The results illustrate postseismic deformation along the historically ruptured segments of the 1997 Manyi and 2001 Kokoxili earthquakes and interseismic strain accumulation along locked segments, indicating their potential to generate future earthquakes. Models of interseismic and postseismic deformation reveal the variations of fault slip rate and locking depth along the entire length of the Kunlun fault. We infer that the historically unruptured segments of the Kunlun fault between 95°E and 101°E, with relatively fast slip rates and high locking depths (>15 km), are capable of generating large earthquakes in the future. Key Points: We generate high‐resolution and large‐scale 3D velocity maps for the north‐central Tibetan Plateau from Sentinel‐1 InSAR observationsGeodetic velocity field indicates relatively high strain rate around the restraining bend (∼98°E) of the Kunlun faultLong‐lived postseismic transients following the Manyi and Kokoxili earthquakes are revealed in InSAR velocities during 2015–2020 [ABSTRACT FROM AUTHOR]

We model the time‐dependent postseismic displacements following the 2001 Mw7.8 Kokoxili earthquake, including both GPS (2001–2002 for near‐field and 2001–2010 for far‐field) and descending‐track InSAR line‐of‐sight time series (2003–2010) to study three postseismic deformation processes. The predicted deformation patterns and magnitude from poroelastic rebound are inconsistent with the geodetic observations. Far‐field postseismic deformation (>200 km from rupture) is primarily induced by upper mantle viscoelastic relaxation beneath Tibet and the Qaidam Basin and places a lower bound on transient and steady‐state viscosities on the order of 1019–1020 Pas. Shallow stress‐driven afterslip (<20 km) on the Kunlun Pass fault localizes deformation in the near‐fault area and helps explain the GPS‐measured displacement gradient across the fault. Deep stress‐driven afterslip in the downdip region of the coseismic rupture (>20 km) has an amplitude of ∼1 m during the first 3 years. Our results indicate that the consideration of deep afterslip increases our estimates of transient viscosity of the lower crust beneath Tibet and the Qaidam basin for this earthquake by as much as a factor of three. Our combined model incorporating viscoelastic relaxation and afterslip suggests that the effective transient and steady‐state viscosities in the Tibetan lower crust are 5 × 1018 Pas and 4 × 1019 Pas, respectively (transient viscosity = 2 × 1018 Pas without afterslip considered), while the effective transient and steady‐state viscosities below the Qaidam Basin area are 1 × 1019 Pas and 6 × 1019 Pas (transient viscosity = 4 × 1018 Pas without afterslip considered). Key Points: Remarkably asymmetric postseismic deformation following the 2001 Kokoxili earthquake reveals contrasting rheology across the Kunlun faultDeep afterslip localized in the shear zone increases the estimate of lower‐crustal transient viscosityShallow afterslip helps explain GPS‐measured displacement gradient across the Kunlun Pass fault [ABSTRACT FROM AUTHOR]