Deep crustal structure and seismogenic environment of the 2023 Jishishan earthquake in the Southeastern Lajishan tectonic belt revealed by three-dimensional magnetotelluric imaging

In 2023, an Ms. 6.2 earthquake struck the southeastern segment of the Lajishan fault system on the northeastern margin of the Tibetan Plateau, inducing extensive secondary disasters and resulting in casualties. The dip polarity of the seismogenic fault and deep seismogenic mechanism of this event remain subjects of ongoing debate. Clarifying the deep structural controls of the earthquake acquiring two magnetotelluric (MT) datasets along profiles traversing and surrounding the epicentral area, followed by constructing a three-dimensional resistivity model using 3D inversion.The results reveal that the southeastern Lajishan fault system is imaged as a low-resistivity boundary zone in the middle–upper crust and delinates steep, southwest-dipping imbricate structures verging northeast at depth. The Northern Margin fault of the West Qinling orogen constitutes a principal crustal electrical boundary, with its southern side featuring a prominent mid–lower crustal conductor exhibiting higher melt fractions than those beneath the Linxia Basin to the north. Comparison with the EKLA-S profile to the south indicates that crustal flow originating from the Bayan Har block is obstructed beneath the East Kunlun fault by an overlying resistive body. This implies that the northeastward extension of the low-resistivity layer beneath the West Qinling orogen does not follow a typical continuous crustal-flow pattern. High-precision hypocenter relocation further indicates that the mainshock nucleated within the “ductile–rigid” transition zone between an upper-crustal resistive block and an underlying conductive layer. Considering these findings, we propose that the Jishishan earthquake was generated through the following process: under regional northeast-directed compression, the northeastward migration of the weakened mid–lower crust beneath the West Qinling was impeded by the rigid upper crust of the Lajishan belt, leading to intense stress accumulation within the transition zone and ultimately triggering reverse–strike-slip rupture along a steep, southwest-dipping fault. The presence of a strong upper-crustal resistive block also facilitated efficient upward energy transmission into shallow unconsolidated sediments, significantly amplifying secondary hazards. This study provides crucial electrical constraints on the seismogenic fault dip of the Jishishan earthquake and illuminates the deep seismogenic process of intracontinental reverse-faulting earthquakes along the northeastern Tibetan Plateau, with important implications for regional seismic hazard assessment.