Crustal electrical structure beneath the Garze fault in eastern margin of Tibetan Plateau and seismogenic tectonic

The Garze fault zone, located on the eastern margin of the Tibetan Plateau, is a sinistral strike-slip fault with significant Late Quaternary activity closely related to the occurrence of strong and large earthquakes. Together with the Xianshuihe fault zone, it forms the boundary between the SichuanYunnan Block and the Bayan Har Block. Here, we present a broadband magnetotelluric sounding profile collected across the Garze fault zone, spanning from the SichuanYunnan Block to the Bayan Har Block. We employed the impedance tensor method to qualitatively analyze the tectonic dimension and strike direction of the crustal structure, and the reliable electrical structure of the crust was then obtained using two-dimensional anisotropy and three-dimensional isotropic inversion methods. Our results reveal that the direction of the anisotropy of the upper crust beneath the SichuanYunnan Block and the Xianshuihe fault follows the strike of the Garze fault zone, while the direction of the anisotropy of the lower crust of the Garze fault is perpendicular to the strike of the fault zone, implying the coupling deformation of the upper and lower crust. We found significant differences in the electrical structure on both sides of the Garze fault zone, with the granite massif in the upper crust characterized by high resistivity. The low resistivity anomaly in the upper crust is mainly caused by the accumulation of saline fluid, while the low resistivity anomaly in the lower crust is primarily composed of partial melt and fluid. We conclude that the fluid in the crust migrates downward along the fault zone, increasing the pore pressure and reducing the normal stress of the Garze fault, which accelerates the process of earthquake nucleation. In tectonic environments where the upper and lower crusts are decoupled, the dragging effect of the lower crust results in easier stress concentration in the upper crust, leading to the occurrence of earthquakes. Our findings provide unique insights for the accurate assessment of seismic hazards in the Garze fault zone and beyond.