Rupture process of the 2023 MW 6.1 Jishishan (China) earthquake in a compressional stress regime and implications for seismic hazards

On 18 December 2023, an MW 6.1 earthquake struck Jishishan County, China, and occurred on a blind thrust fault at the northeastern edge of the Tibetan Plateau. We investigate the source characteristics of this earthquake sequence using both seismic and geodetic data. The focal mechanisms of ML ≥ 2.0 events in this earthquake sequence are determined using P wave first-motion polarity and S/P amplitude ratio data, suggesting predominant thrust motion with some strike-slip motion of the seismogenic fault. We perform kinematic finite-fault joint inversion for the mainshock using seismic and geodetic observations. This analysis reveals that the mainshock ruptured a major asperity and exhibited an asymmetric bilateral rupture pattern. The main rupture length and duration are approximately 15 km and 6.7 s, respectively. Stress field inversion results show a compressional stress regime in the source area. Through a comparative analysis of the spatial distributions of field-investigated intensity and peak ground velocity derived from strong ground motion simulations, we conclude that the severe casualties resulting from this moderate-sized earthquake were attributed not only to its night-time occurrence and the poor seismic resistance of some buildings, but also to key source rupture effects such as slip heterogeneity and rupture directivity.