Rapid report of the December 18, 2023 MS 6.2 Jishishan earthquake, Gansu, China

Abstract

On December 18, 2023, the Jishishan area in Gansu Province was jolted by a M_S 6.2 earthquake, which is the most powerful seismic event that occurred throughout the year in China. The earthquake occurred along the NW-trending Lajishan fault (LJSF), a large tectonic transformation zone. After this event, China Earthquake Networks Center (CENC) has timely published several reports about seismic sources for emergency responses. The earthquake early warning system issued the first alert 4.9 ​s after the earthquake occurrence, providing prompt notification that effectively mitigated panics, injuries, and deaths of residents. The near real-time focal mechanism solution indicates that this earthquake is associated with a thrust fault. The distribution of aftershocks, the rupture process, and the recorded amplitudes from seismic monitoring and GNSS stations, all suggest that the mainshock rupture predominately propagates to the northwest direction. The duration of the rupture process is ∼12 ​s, and the largest slip is located at approximately 6.3 ​km to the NNW from the epicenter, with a peak slip of 0.12 ​m at ∼8 ​km depth. Seismic station N0028 recorded the highest instrumental intensity, which is 9.4 on the Mercalli scale. The estimated intensity map shows a seismic intensity reaching up to IX near the rupture area, consistent with field survey results. The aftershocks (up to December 22, 2023) are mostly distributed in the northwest direction within ∼20 ​km of the epicenter. This earthquake caused serious casualties and house collapses, which requires further investigations into the impact of this earthquake.