关键词: 漾濞地震, 同震形变场, 断层滑动分布, 同震库仑应力, 地震危险性

Abstract: Due to the ongoing collision between Indian and Eurasian plates, the internal blocks of the Tibet plateau are experiencing eastward extrusion. Resulting from the blocking of the Sichuan Basin along the eastern boundary of the Bayanhar block, the plateau begins to rotate clockwise around the eastern syntaxis, and continues to move toward the IndoChina Peninsula. Such process forms the Hengduan Mountains with thousands of gullies in the Sichuan-Yunnan region, and generates major earthquakes across the entire Red River Fault, where infrastructures and residents are seriously threatened by the frequent earthquakes. InSAR observations feature a high spatial resolution and short intervals, ranging from several days to over a month, depending on the satellite revisit period.
On May 21, 2021, an earthquake struck the Yangbi city. This event provides a rare opportunity to look at the local tectonic and seismic risk in the north of the Red River Fault. We processed the Sentinel-1 SAR data with D-InSAR technology and generated the surface deformation caused by the Yangbi M_S6.4 earthquake occurring on May 21, 2021. Due to the abundant vegetation and moisture in Yunnan, significant atmospheric noise needs to be corrected for the derived InSAR displacement field. The results show a maximum deformation of~0.07m in line-of-sight for ascending track and~0.08m for descending track. The quality of interferogram on the ascending track is low, and only one of the quadrans can be distinguished, the rest of the interferogram is regarded as phase noise. However, the descending interferogram contains two deformation regions, with its long axis roughly along the NW-SE direction. The northeast part of interferogram moves towards the satellite, while the southwest part moves away from the satellite. The InSAR interferograms pattern shows a right-lateral strike-slip movement. Then, we combined coseismic displacement data obtained from the Global Navigation Satellite System(GNSS)and InSAR(both the ascending and descending)to invert the coseismic slip model of the Yangbi earthquake. The inversion test shows that our data cannot give strong constraints for the dip orientations, and the two slip models with opposite dip orientation can explain the observations within the noise level. No matter what the dip orientation is, the slip models show that the coseismic slip concentrated at depth of 2~10km, with a maximum slip of~0.8m, which corresponds to a moment magnitude of M_S6.4, and is consistent with body-wave-based focal mechanism. But the relocated aftershocks in 3 hours immediately after the mainshock reveal a SW-dipping fault plane 10km away to the west of Weixi-Qiaohou-Weishan Fault, we therefore conclude that the Yangbi earthquake ruptured a SW-dipping dextral fault, which is previously unknown. To analyze the effects of the Yangbi earthquake on the seismic risk of the regional dextral faults, we estimated the Coulomb stress change caused by our preferred slip model. The Coulomb stress at 7.5km depth is negative, indicating stress unloading, while the Coulomb stress at 15km depth is positive, indicating slightly loading, but still less than the empirical triggering threshold. The results indicate that Yangbi earthquake partially relieved the strain accumulated on the nearby faults, thus restraining the seismic risk of these faults.

Key words: Yangbi M_S6.4 earthquake, coseismic deformation field, fault slip distribution, coseismic Coulomb stress, seismic hazard