The seismic mechanisms and seismogenic environment of the M_s 6.2 earthquake in Jishishan, Gansu, China: Evidence from relocation, focal mechanisms, and rupture processes

On December 18, 2023, an M_s 6.2 earthquake jolted Jishishan County in the Linxia Hui Autonomous Prefecture in Northwest China’s Gansu Province, causing substantial casualties and building collapses. The earthquake occurred in the Qilian Block on the northeastern border of the Qinghai‒Tibet Plateau, where faults are highly active and the geological structure is complex. In this study, we utilized methods such as relocation, focal mechanism solutions, and earthquake rupture processes to describe seismogenic faults. The results indicated that the majority of aftershocks occurred at a depth of 12 km. The centroid depth of the main shock and the depth of the maximum rupture point during the rupture process were also 12 km. Various geophysical methods exhibited a high degree of consistency in depth exploration. Aftershocks were distributed mainly to the west and north of the main shock and extended in the NNW direction, primarily through unilateral rupture. The main shock was a reverse thrust event with a small dextral strike-slip component. In this study, more regional data, such as previous GPS observations, field geological observations, and the distributions of the primary stress states in the region, were also incorporated. We inferred that the main shock was triggered by the main fault at the northern margin of the Lajishan Fault and that the movement of the main fault also activated some secondary faults. The compressive forces on both sides of the Lajishan Fault Zone led to the uplift of mountain areas, accompanied by some landslides, leading to this catastrophic earthquake event. In this article, the activity relationships among the 2022 M_s 6.9 Menyuan earthquake, the 2019 M_s 5.7 Xiahe earthquake, and the Jishishan earthquake under the action of regional stress are also discussed. This study provides additional evidence and new ideas for exploring the seismogenic process of the Lajishan Fault Zone and has implications for future in-depth research on underground activity in this region.