The seismogenic structure of the 2021 Yunnan Yangbi M(S)6. 4 earthquake sequence and the difference between the Eryuan earthquake in 2013, Yangbi earthquake in 2017 and 2021

An M(s)6. 4 earthquake occurred on May 21st, 2021 in Yunnan Yangbi country. In order to analyze the seismogenic fault and structure deeply, and discuss the difference of seismogenic structure and background between the 2013 Eryuan, 2017 and 2021 Yangbi earthquakes. The earthquake sequence is relocated by double-difference method with the observation report from the China Earthquake Networks Center (CENC). And the tectonic stress field is calculated with nine focal mechanism solutions from the Global Centroid Moment Tensor (GCMT) catalogue and the United States Geological Survey (USGS) in seismic source region. The initial conclusions are as follow: (1) The Yangbi earthquake sequence is distributed in the direction from NW to SE. More aftershocks are located in the part of SE segment than that in NW segment of earthquake cluster. Few aftershocks occurred around the earthquakes with magnitude larger than 5.0 because of heterogeneity release of crustal stress. Initial rupture depth of four events with magnitude larger than 5.0 are deeper than their centroid depth, their rupture process of seismogenic faults are beginning from fault bottom to fault upper. (2) Two unknown faults F2 and F3 are the seismogenic faults of the Yangbi earthquake sequence, which are lying down southwestern of Weixi-Qiaohou-Weishan fault. Both of them are similar with nearly vertical dip, tending to SW direction, trending from NW to SE direction and sliding in right-lateral strike-slip. The length of F2 is about 30 km, another one (F3) is shorter with the length of nearly 11 km and intersecting with F2 fault in the middle of earthquake cluster. (3) The tectonic stress field is strike-slip regime in seismic source region. The principal compressive stress axis is tended in SSE direction (174.57 degrees) with lower plunge (18.79 degrees). And the principal extensional stress axis is tended in SWW direction (-93.65 degrees) with nearly horizontal plunge (5.21 degrees). The seismogenic structure in seismic source region is controlled by right-lateral strike-slip boundary between Sichuan-Yunnan Block and Southern Yunnan Block. (4) All the three earthquakes occurred under the background of strike-slip regime in southwestern boundary of Sichuan-Yunnan Block with right-lateral strike-slip movement. The seismogenic structure of the 2013 Eryuan earthquake is controlled mostly by local structure with vertical movement. The 2017 Yangbi earthquake is controlled only by Sichuan-Yunnan Block boundary with right-lateral strike-slip movement. The 2021 Yangbi earthquake sequence is controlled mostly by right-lateral strike-slip movement on the boundary of Sichuan-Yunnan Block and little vertical movement from local structure crust.