The seismic tectonics of the MS6.2 Jishishan Earthquake in Gansu Province on the northeastern margin of the Tibetan Plateau were analyzed based on the earthquake relocation, focal mechanism solution, and rupture process

On December 18, 2023, an Ms6.2 earthquake struck Jishishan County, Linxia Prefecture, Gansu Province, China. The earthquake occurred in the boundary area of Qilian orogenic belt and Bayanqala block in the northeast margin of Tibetan Plateau. Near the epicenter, the Laji Mountain fault, the Riyueshan fault and the north margin of the West Qinling fault developed. In this study, 598 aftershocks of the Ms6.2 earthquake sequence in Jishishan were relocated using the double difference positioning method. The main earthquake location was 102.826°E, 35.743°N, and the focal depth was 12.3 km. The main moment magnitude is 6.1, and the strike, dip and slip Angle of the fault plane are 162°, 44° and 122°, respectively. Combined with the results of relocation and focal mechanism solution, it is believed that the seismic fault of this earthquake is a hidden NNW-SSE trending NE-dip fault between the northern and southern margin faults of Laji Mountain. The fault located at the main earthquake mainly leans to the east, the aftershock extends to the northwest, and the fault changes from east dip to vertical dip to west dip to the north, with a transitional zone in between. The transformation form presents a "fan" shape, which intuitively shows the complexity of the tectonic environment in this area. From the source rupture inversion, the rupture is mainly an elliptical area with a long axis of about 10 km and a short axis of about 6 km in the northwest direction of the source, with a maximum error momentum of about 40 cm, located at a depth of about 14.5 km underground. From the perspective of the seismic rupture direction, the rupture mainly propagates along the direction of striking NWW, and the rupture maximum value appears at 2.2 seconds. The rupture velocity is about 1.5 km/s, and the rupture is mainly unilateral. The extreme earthquake area of strong ground motion estimated by finite fault model is up to Ⅸ degrees, which is consistent with the seismic field investigation results.