Effect of interaction between fluid and fault zone on triggering earthquakes in the shallow crust

Reservoir induced seismicity provides a suitable method for studying the roles of fluid in inducing earthquakes. The fault structure plays a predominant role in the occurrences of earthquakes, and the influences of fluid also cannot be disregarded. In this study, we investigate the active Fairy Mount fault in the Three Gorges Reservoir. Since water impoundment in 2003, more than 4000 detectable earthquakes have occurred along the fault. The vast majority of these earthquakes are associated with the fault and water impoundment. To explore the effects of water-fault interactions on induced earthquakes, a permeability structure of the fault zone is established by a series of geological experiments. Fault rocks, including unconsolidated breccias and fault gouges, collected from a presentative outcrop are employed for detailed microstructural and mineralogical analyses. The results reveal a complex internal fault structure and widespread fluid-rock interactions. The hydrogeological property of the fault exhibits a typical conduit/barrier permeability structure. Highly permeable damage zones act as fluid conduits for the infiltration of reservoir water to the subsurface, while the low permeable fault core renders the fault core as a potential fluids storage area to weaken the fault in the shallow crust. In sum, both the pore pressure changes due to water infiltration and the long-term chemical effect of water on the fault plane promote instability of the fault and induce earthquakes.