The 21 May 2021 Mw 6.1 Yangbi Earthquake—A Unilateral Rupture Event with Conjugately Distributed Aftershocks

Abstract An Mw 6.1 earthquake occurred in Yangbi, Yunnan province, China, at 13:48:34 (UTC time) on 21 May 2021. It exhibited a typical foreshock–mainshock–aftershock sequence in the northwestern Yunnan rifting area. We used the first P-arrival time from regional broadband seismic stations to check the location of the mainshock’s hypocenter. The updated location of the epicenter is 99.88° E, 25.66° N. The focal mechanisms of nine earthquakes, including the mainshock and the largest foreshock and aftershock, were investigated using the general cut-and-paste method based on regional seismic waveforms to set up the fault geometry and to calculate the static coulomb stress change. The rupture process of the mainshock was constrained by 33 unclipped regional broadband seismic records from 15 stations and nine static displacements from three Global Navigation Satellite System stations. The results showed that the Yangbi earthquake was a unilateral rupture event propagating toward the southeast along-strike direction. The moment was 1.81×1018 N·m, and the corresponding magnitude was Mw 6.1. The peak slip value, the weighted rupture velocity, and the duration time of the mainshock were 0.95 m, 2.2 km/s, and 9.1 s, respectively. At 10 km southeast of the mainshock’s epicenter, the conjugately distributed aftershocks were observed, and the moment density of the mainshock along the strike direction decreased sharply (78% of the total moment released in the first 10 km), suggesting that the sharply decelerating rupture could be the reason for the conjugately distributed aftershocks. The changes in static coulomb stress due to the largest foreshock and mainshock were calculated. The results showed positive Coulomb stress on the major slip zone in the mainshock fault plane triggered by the largest foreshock. Moreover, the 0.01 MPa contour due to the mainshock radiated out of the bend zone of the Weixi–Qiaohou–Weishan fault.