Slip Kinematics of the 2021 Yangbi Earthquake: Fore-Main-Aftershock Sequence Rupture along an Unknown Secondary Fault of the Weixi–Qiaohou Fault

Abstract

Abstract 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.