Ramp-flat Seismogenic Structure of the 2020 Yutian (Xinjiang， China) MW 6.3 Earthquake Revealed by InSAR Observations

Objectives:On 25 June 2020, a shallow MW 6.3 normal-faulting earthquake struck the southwestern segment of Altyn Tagh fault in the northwest margin of Tibetan Plateau, which is characterized by left-lateral strike-slip motion. This earthquake offers a rare opportunity to understand the seismogenic structure and seismogenesis in the region. Methods:In the paper, we first apply the ascending and descending SAR images of the Sentinel-1A satellite to extract the coseismic and the first three months postseismic surface deformation related to the earthquake. Then, we utilize the coseismic and postseismic observations to invert for the detailed coseismic slip and postseismic afterslip of the earthquake, respectively. Results and conclusions:Coseismic inversion result reveals that the earthquake nucleated on a moderate-angle (a dip of 56°) normal fault buried at a depth of 3 to 11 km while further postseismic analysis uncovers that afterslip propagates in the shallow fault structure with a dip of 76°. The shallow postseismic afterslip and deep coseismic slip of the earthquake finely image a ramp-flat seismogenic fault structure. According to a joint analysis of the regional geology geomorphology, active fault distribution and kinematics, historical earthquakes and the inversions, we conclude that the occurrence of the Yutian earthquake may be attributed to the release of extensional stress in a stepover zone controlled by the bounding strike-slip faults.