The 2021 Yangbi earthquake sequence, focal mechanism, source rupture process, stress field, rupture complexity, Environmental technology. Sanitary engineering, TD1-1066, Environmental sciences, GE1-350, Risk in industry. Risk management, HD61

The 2021 Yangbi earthquake sequence occurred on the southwestern margin of the Chuandian block in China, which is a typical foreshock-mainshock-aftershock sequence. We investigate source properties of this earthquake sequence with seismic and geodetic data. Focal mechanisms of M ≥ 2.0 events in this earthquake sequence are determined using P wave first-motion polarity plus S/P amplitude ratio data from local short-period and broadband seismic stations. The determined focal mechanisms together with spatial distribution of the earthquake sequence indicate the overall northwest-southeast-trending seismogenic faults with dominant strike-slip motions. We conduct kinematic finite-fault inversion with strong-motion, local broadband seismic, and coseismic InSAR plus GNSS displacement data, finding that the mainshock ruptured a major asperity unilaterally with a rupture length and duration being approximately 15 km and 6.5 s, respectively. The released scalar seismic moment is 1.7 × 1018 Nm, corresponding to a moment magnitude MW 6.1. Besides, stress field inversion results manifest that source area is characterized by a strike-slip stress regime and the local stress field has changed before and after the mainshock. We suggest that rupture initiation, propagation, and arrest of the 2021 Yangbi earthquake sequence may be regulated by the immanent strength and stress heterogeneities of the seismogenic fault.