Rapid report of source parameters of 2023 M6.2 Jishishan, Gansu earthquake sequence

The M6.2 earthquake in Jishishan, Gansu Province, on December 18, 2023, caused extraordinary earthquake disasters. It was located in the northern part of the north−south seismic zone, which is a key area for earthquake monitoring in China. The newly built dense strong motion stations in this area provide unprecedented conditions for high-precision earthquake relocation, especially the earthquake focal depth. This paper uses the newly built strong motion and traditional broadband seismic networks to relocate the source locations of the M3.0 and above aftershocks and to invert their focal mechanisms. The horizontal error of earthquake location is estimated to be 0.5−1 km, and the vertical error is 1−2 km. The focal depth range of aftershocks is 9.6−14.6 km, distributed in a 12-km-long strip with SSE direction. Aftershocks in the south are more concentrated horizontally and vertically, while aftershocks in the north are more scattered. The focal mechanisms of the main shock and aftershocks are relatively consistent, and the P-axis orientation is consistent with the regional strain direction. There is a seismic blank area of M3.0 and above, about 3−5 km between the main shock and aftershocks. It is suggested that the energy released by the main shock rupture is concentrated in this area. Based on the earthquake location and focal mechanism of the main shock, it is inferred that the Northern Lajishan fault zone is the seismogenic structure of the main shock, and the main shock did not occur on the main fault, but on a secondary fault. The initial rupture depth and centroid depth of the main shock were 12.8 and 14.0 km, respectively. The source rupture depth may not be the main reason for the severe earthquake disaster.