关键词: 漾濞地震, InSAR, 同震滑动分布, 库仑应力变化, 维西-乔后断裂, 红河断裂

Abstract:

A M_S6.4 earthquake occurred on May 21^th, 2021 at Yangbi, Yunnan. In this paper, high resolution InSAR coseismic deformation fields were obtained based on the ascending and descending track of Sentinel-1 SAR images. Based on the InSAR-derived deformation fields, the geometric model of the seismogenic fault was determined according to the aftershock relocation results. Then the fine coseismic slip distribution of the fault plane of Yangbi earthquake was inversed using a distributed sliding inversion method. Finally, the regional strain distribution and the Coulomb stress variation on the surrounding faults caused by coseismic dislocations and viscoelastic relaxation effect after earthquake were calculated, and the seismic risk of the seismogenic structure and the surrounding faults was discussed. The results show that the descending track co-seismic deformation field shows that the NE wall of the seismogenic fault moves close to the satellite, while the SW wall moves far away from the satellite, and the coseismic deformation is symmetrically distributed. The maximum LOS vectors were 8.6cm and 7.9cm, respectively, and the descending track profile showed a coseismic displacement up to 15cm. The fringes on the southwest side of the ascending track interferograms are relatively clear, showing movement close to the satellite, and the maximum LOS deformation magnitude is 5.7cm, while the interference fringes on the northeast side are not clear and the noise is obvious. The fault co-seismic dislocation is mainly of dextral strike-slip with a small amount of normal fault component. The coseismic slip mainly distributes at depths 2~10km, and the coseismic sliding rupture length is about 16km with the maximum slip of approximately 0.46m at a depth 6.5km. The average slip angle is 180° and the inverted magnitude is approximately M_W6.1. The causative fault did not rupture the surface. From the analysis of regional strain distribution and tectonic dynamic background, the Yangbi earthquake occurred in the region where the Sichuan-Yunnan rhomboid block is blocked in its process of SE movement by the South China block and deforms strongly. Combined with the analysis of the geometric occurrence and movement properties of faults, our study suggests that the causative fault of the Yangbi earthquake maybe is a branch of the Weixi-Qiaohou Fault or an unknown fault that is nearly parallel to it on the west side. This earthquake has a significant impact on the Coulomb stress of the Longpan-Qiaohou Fault, Chenghai Fault and Red River Fault in the southwestern Sichuan-Yunnan rhombic block. The Coulomb stress in the northern section of Red River Fault is the most significant. The cumulative Coulomb stress variations of the coseismic and 10 years after the earthquake show that the Coulomb stress variation has increased in the northwestern Yunnan tectonic area. This earthquake is another typical seismic event occurring in the southwest of the Sichuan-Yunnan block after the Lijiang M_S7.0 earthquake in 1996 and the Mojiang M_S5.9 earthquake in 2018. The risk of strong earthquakes in the regional extensional tectonic system in northwest Yunnan and in the north section of the Red River fault zone cannot be ignored.

Key words: Yangbi earthquake, InSAR, coseismic slip distribution, Coulomb stress change, Weixi-Qiaohou Fault, Red River Fault