Upper crustal structure of the Xinfengjiang reservoir from ambient noise double beamforming tomography and its implications for induced seismicity.

The Xinfengjiang Reservoir in Guangdong, China, triggered one of the largest reservoir-induced earthquakes in the world, with a magnitude of 6.1 in 1961 March. Frequent local seismic events have been recorded in the vicinity of the reservoir, posing a potential threat to the dam infrastructure and surrounding regions. In this study, we obtain a high-resolution S -wave velocity model of the upper crustal structure underlying the Xinfengjiang Reservoir, using double beamforming tomography method from newly deployed dense array across the reservoir. Our results reveal that several high-velocity structures are distributed beneath the Xinfengjiang Reservoir. These structures are robust, facilitating a greater accumulation of stress before fault slip. Earthquakes beneath the reservoir are primarily concentrated at the boundaries of the high-velocity bodies, indicating their controlling role on the location of seismic events. Low-velocity zones, acting as pathways for fluid migration, extend from the surface to the depths of seismic sources, thereby inducing earthquakes by elevating pore pressure within fault and fracture systems. Due to significant differences in the fluid diffusion coefficient, the delay times between the onset of earthquakes and the peak water levels vary considerably with the different crustal structures. [ABSTRACT FROM AUTHOR]