Deep seismogenic tectonics of Yangbi MS6.4 on 21 May 2021 in the SE margin of the Tibetan plateau from earthquake sequence relocation, stress field and seismic anisotropy

The MS6.4 earthquake occurred in Yangbi County, Yunnan Province, China, on 21 May, 2021. The earthquake is located on the west side of the Red River fault, the western boundary of the Sichuan-Yunnan block in the SE margin of the Tibetan Plateau. The Yangbi MS6.4 earthquake has aroused great concern due to its unique geographical location and frequent strong earthquakes in Yunnan. In this study, a total of 4695 local earthquakes of the Yangbi MS6.4 earthquake sequence are relocated using the double-difference location method, including the mainshock source location of 99.879 degrees E, 25.687 degrees N, and depth of 9.67 km. The mainshock has a moment magnitude of 6.1, and one of the fault-plane solutions is 137 degrees, 83 degrees and -163 degrees in strike, dip, rake respectively. Combined with the relocation and focal mechanism solutions results, it is suggested that the seismogenic fault of the Yangbi MS6.4 earthquake is a NW-SE trending fault on the southwest side of the Weixi-Qiaohou fault (WQF), with high dip of SW, which is a previously-unknown blind fault. The regional 2D stress field shows that the principal compressive stress around the Yangbi earthquake is NNW-SSE and nearly N-S direction, which appears to be deflected counterclockwise by a small angle on its east side and changed to N-S and NNE-SSW direction on its west side, while both the north and south sides of the Yangbi earthquake show directional coherence. Strong differential deformation distorts in the upper crust to the northeast of the seismic source as reflected by tectonic stress field and the upper crust seismic anisotropy, indicating strong influence of Lijiang-Xiaojinhe fault (LXF). The stress field in the source area is jointly controlled by southeastward movement of southeastern margin of Tibetan Plateau, the northeastward movement of Indian plate and the differential movement between the different tectonic blocks.