Relocation of the M_s6. 4 Yangbi earthquake sequence on May 21, 2021 in Yunnan Province and its seismogenic structure analysis

In this paper, we relocated the n6. 4 Yangbi earthquake sequence in Yunnan province by the "multi-stage" method and the phase arrival data from the Yunnan regional seismic network. The focal mechanism solutions of 13 M-s >> 4. 0 events were calculated by the CAP program. We found that the sequence is about 3 similar to 10 km away from the nearest known Weixi-Qiaohou fault, and the long-axis of the aftershock events is about 22 km striking NW-SE, with its dominant focal depth is 4 similar to 8 km, reflecting the Yangbi earthquake occurred in the shallow part of the upper Crust. Earthquakes along the northern segment of the aftershock area are relatively shallow and dense, while along the southern segment earthquakes are deeper and more dispersed and deflected to the south. Different short-axis cross-sections reveal that the seismogenic structure is dipping towards SW with a gentler angle for the SE segment and steeper angle for the NW segment. We identified two small-scale NE-striking conjugate faults (about 5 km long) in the northern part of the sequence. The foreshocks on May 18 propagated gradually from the center towards NW where the mainshock is located. Since the foreshock sequences were dense and triggered by each other, we thus concluded a cascade mode of the foreshock occurrence. The main shock was located in the NW end of the aftershock intensive area, which indicated that it had the characteristics of SE-trending unilateral rupture. The focal mechanism solutions of most M-s >> -4. 0 events have NW-SE striking steep nodal planes, consistent with the long axis of the aftershock distribution. The stress inversion results revealed that the seismogenic structure is affected by the NNW-SSE striking nearly horizontal principal compressive stress, which is consistent with the fault occurrence and dislocation type of the adjacent Weixi-Qiaohou fault. It can be preliminarily determined that the seismogenic structure is a parallel secondary structure of Weixi-Qiaohou fault. The formation of this structure may be related to the SE escape of the active Sichuan-Yunnan block and the clockwise rotation of Southwest Yunnan block.