On December 18th, 2023 at 23:59, an earthquake MS6.2 occurred in Jishishan County, Linxia Prefecture, Gansu Province, with the epicenter located near the southern segment of the Lajishan fault system in the eastern Qidam-Qilian block. In this paper, we conducted the relocation, focal mechanism and stress inversion methods to analyze the seismic structure and regional seismic hazard, . Using digital waveform data from the Seismic Networks of Gansu and Qinghai provinces, the double-difference location algorithm was conducted on the earthquake sequence within 10 days after the earthquake.
A total of 605 high-precision relocation of earthquakes was revealed and the epicenter of mainshock was determined to be at 35.748°E, 102.812°N, with a depth of 16.4 km. Most of aftershocks are located at depths of 6～15 km with spreading along NW-SE direction near the mainshock epicenter, while some aftershocks gradually turn along aboutNS direction northward. Depth profile of aftershocks further suggest that the seismogenic fault plane dips to NE. By applying the FocMech-Flow automatic inversion of focal mechanism, the best double-couple solution for the M6.2 mainshock was obtained. The parameters of two nodal planes are as follows, strike 302°, dip 58°, rake 45° for nodal plane I and strike184°, dip 53°, rake of 138° for nodal plane II. Stress field inversion results indicated that the stress in the epicenter belongs to thrust-type and the azimuth of σ1 axis orients WSW-ENE. Consistent with the direction of aftershock distribution near the mainshock, the plane I is considered as the fault plane responsible for this earthquake. Combining previous geological data, we suggest that the seismogenic fault is a deeply buried back thrust fault segment of the East Jishishan Fault and named it as “Dahejia Fault”. Historical earthquake records in the Qidam-Qilian block show that the strong earthquakes gradually migrate eastward in recent two decades. Therefore, this earthquake further enhances the seismic hazards of the West Qiling fault to the east of Jishishan.