On 21 May 2021, an earthquake sequence occurred at the Weixi–Qiaohou fault that is considered as a boundary of the Chuandian block in Southwest China, providing a significant opportunity to understand the seismogenic tectonics of the fault and its secondary faults. We used Interferometric Synthetic Aperture Radar and Global Navigation Satellite System observations to obtain surface displacements and slip kinematics of the Mw 6.1 mainshock. The Mw 6.1 event ruptured along a previously unknown secondary fault of the Weixi–Qiaohou fault; this newly identified southwest-dipping fault is dominated by right-lateral strike slip with a small normal component. Fault slip was concentrated between depths of 3 and 8 km, with a maximum slip of ∼0.8 m. Given that there is minor slip and lack of foreshocks and aftershocks rupture in 0–3 km depth, a systemic shallow slip deficit is observed in the depth. In addition, stress change simulations suggest that the Mw 5.4 foreshock promoted the Mw 6.1 event. The increased coulomb stress change region extends approximately 16 km (from 25.65° N to 25.52° N) along the Weixi–Qiaohou fault triggered by the Mw 6.1 mainshock. Combined with modeling results of the Yangbi earthquake sequence and magnetotelluric model across the Weixi–Qiaohou fault, the fore-main-aftershock sequence initiate in depths of 8–16 km (corresponding to relatively low resistivity) and further promote larger slip upward along depths of 3–8 km (relatively high resistivity). Finally, aftershocks rupture and release stress below slip depths of the mainshock.

Summary Strain partitioning between strike-slip faults in mountains and thrust faults in the foreland is a typical mountain building process to accommodate oblique plate convergence. Studying the geometry and movement of such strain-partitioned fault systems is key to understanding the mountain building process and related seismic hazards. The 2022 Mw 6.6 Menyuan earthquake is the largest strike-slip earthquake to have ruptured the northern Qilian Shan fault system in the modern geodetic era. We combined satellite and field observations to determine the fault geometry and coseismic slip distribution in the Menyuan earthquake, and link the distribution of coseismic slip with the pattern of interseismic strain accumulation within the northern Qilian Shan from our geodetic slip model. We find that the Menyuan earthquake ruptured a 25 km-long section of the left-lateral Longlongling Fault (LLLF) between the surface and 7 km depth. The maximum slip was 4 m at 3–4 km depth. Damage to a high-speed railway tunnel recorded a fault offset of 2.7 m at a depth of 200 m compared to 2.5–3.0 m on the surface, suggesting that dispersion of the rupture through unconsolidated shallow sediments was limited, at least at the tunnel site. We also determined the pattern of interseismic deformation prior to the earthquake using Interferometric Synthetic Aperture Radar (InSAR) and Global Navigation Satellite System (GNSS) data. We found the interseismic geodetic data can be explained by the oblique movement of a low-angle décollement beneath the Qilian Shan rather than a strain partitioning fault system. We suggested that the strike-slip faults and foreland thrusts are separated by a creeping décollement, which would act as a barrier to stop the cascading rupture of the strike-slip and thrust fault.

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On 23 March 2021, an Mw 5.2 earthquake struck Baicheng city at the southern foreland of the Tian Shan mountains and produced unexpected severe damages. Based on Interferometric Synthetic Aperture Radar observations, geological investigations, petroleum seismic-reflection profiles, and available seismological results, we found that (1) the earthquake rupture was sinistral-slip dominated and mostly concentrated at ≤2 km depth, suggesting an extremely shallow earthquake; (2) the earthquake produced a 4.1-km-long surface rupture zone, along which the lateral slip ranged from 0 to 135 cm and showed a significant short-wavelength variation; and (3) the causative fault is poorly developed (i.e., has low slip accumulation and slip rate) and cuts through a shallow-buried salt dome. These findings, along with those of previous studies, document some unique characteristics of moderate earthquakes at extremely shallow depths (≤3 km) relative to those at greater depths. First, shallow earthquakes occur on faults that are poorly developed and bracketed by rocks being able to behave as velocity weakening at shallow depth (including crystalline basement rocks, carbonites, volcanic rocks, or evaporites). Second, the surface rupture produced by shallow earthquakes has abnormally large length and slip, and possibly includes an abrupt slip variation. These characteristics highlight potential severe damages of the shallow moderate earthquake but enhance the challenge of identifying the earthquake causative fault (the earthquake source) prior to the event using traditional methods (e.g., geomorphic mapping, geophysical imaging surveys, and paleoseismic trenching).

Quantifying surface deformation due to earthquake-related surface rupturing is a critical research focus. Localized offsets on the primary fault can be obtained via field measurements of dislocated landforms. However, effectively quantifying distributed deformation, which can extend for tens to hundreds of meters around the fault zone, has only become possible with the development of remote sensing technology and optical pixel correlation techniques. In this study, we correlated pre- and post-earthquake GaoFen (GF)-2 and -7 images that were ortho-rectified by a digital elevation model generated from GF-7 stereo images to obtain surface horizontal deformation of the 2022 Mw 6.6 Menyuan earthquake. The surface rupture had a total length of 28 km along two segments separated by a stepover; in this study, we focused on the northern segment (23.5 km), which was distributed along the Lenglongling fault (LLLF). The total surface offset measured by our study had the maximum value of 4.0 m and a mean value of 1.9 m. The mean offset measured by field observations captured just 50% of the mean offset from optical pixel correlation. Overall, 57% of off-fault deformation (OFD) occurred on the LLLF, which is a mature fault, owing to soft near-surface materials. Comparison of the surface offset measured by pixel correlation data in our study and near ground slip from joint inversion of Interferometric Synthetic Aperture Radar and pixel correlation data suggests that OFD played a significant role in accommodating the shallow slip. The results of this study offer new insight into the characteristics of surface deformation.