Analysis of earthquake sequence and seismogenic structure of the 2023 M_S6.2 Jishishan earthquake，Gansu Province，China

On December 18th, 2023 at 23:59, an earthquake of M_S6.2 occurred in Jishishan County, Linxia Prefecture, Gansu Province, with the epicenter located near the southern segment of the Lajishan fault system in the northeastern Qaidam-Qilian block. In this paper, we conducted the relocation, focal mechanism and stress inversion results to analyze the seismic structure and regional seismic risk. 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 relocated earthquakes were revealed and the epicenter of the mainshock was determined to be at （35.748°E, 102.812°N）, with a depth of 16.4 km. Most aftershocks located at depths of 6−15 km spreading along NW-SE direction near the mainshock epicenter, while some aftershocks gradually shift to a near NS direction to the north. Depth profile of aftershocks further suggests 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 M_S6.2 mainshock was obtained. The parameters of two nodal planes are as follows: strike 302°, dip 58°, rake 45° for nodal plane I and strike 184°, dip 53°, rake 138° for nodal plane Ⅱ. Consistent with the direction of aftershock distribution near the mainshock, the plane Ⅰ is considered as the fault plane responsible for this earthquake. Stress field inversion results indicated that the stress in the epicenter belongs to thrust-type and the azimuth of σ₁ axis orients WSW-ENE. Combining regional gravity anomaly and previous geological data, we suggest that the seismogenic fault is a deeply buried back thrust fault in the east Jishishan fault system and named it as “Dahejia fault”. Historical earthquake records in the Qaidam-Qilian block show that the strong earthquakes gradually migrate eastward in recent two decades. This earthquake further enhances the seismic hazards of the western Qinling fault to the east of Jishishan.