Seismic structure characteristics of the 18 December 2023 M6.2 Jishishan earthquake, Gansu Province

On 18 December 2023, at Beijing time, a M6.2 earthquake struck Jishishan in Gansu Province. A thorough analysis of the earthquake structure and characteristics was conducted by combining information on regional seismic tectonics, geology, seismic source mechanism, seismic intensity, and aftershock relocation. The earthquake was a reverse fault event trending north-northwest within the Xining-Lanzhou fault block. The earthquake-controlling tectonics of the earthquake is the Lajishan reverse fault zone, which is closest to the epicentre of the earthquake. The fault zone is situated at the intersection of the northwest-trending Riyueshan dextral strike-slip fault and the east-west-trending Western Qinling North Rim left-trending strike-slip fault. It has an overall northwest-to-north-northwest striking angle and comprises two branch fault zones with opposite trends in the southern and northern rims. The earthquake’s epicentre location, aftershock distribution, and intensity distribution data suggest that the specific fault responsible for this earthquake is the east branch fault of the southern section of the reverse fault zone at the northern rim of Lajishan. This is consistent with the characteristics of the upward disc effect of reverse fault-type earthquakes. Further detailed field investigations are required to determine the deformation of the earth’s surface. The Jishishan earthquake is thought to have been caused by reverse fault activity at the intersection of the Riyueshan strike-slip fault and the northern edge of the Western Qinling fault. This was triggered by the eastward lateral slip along the original left-hand strike-slip fault along the Xining-Lanzhou fault block on the northeastern margin of the Tibetan Plateau. This occurred under the northeast-directed extrusion tectonic stress field resulting from the ongoing land-land collision between the Indian and Eurasian plates. This earthquake suggests that the extrusion tectonic system at the eastern margin of the Tibetan Plateau remains the primary structure controlling strong seismic activity in China in recent years. Further attention should be paid to the risk of strong earthquakes within the fault block.