Statistical investigation of induced seismicity associated with the impoundment of the Xiangjiaba Reservoir, Southwestern China

The Xiangjiaba (XJB) hydropower station is the most downstream dam among the cascade hydropower projects in the lower reach of the Jinsha River. Reservoir-induced seismicity (RIS) has been detected in the XJB region since the initiation of the reservoir impoundment in October 2012. This study investigates the correlations between the spatiotemporal evolution of seismicity and the impoundment process using regional permanent network data in the XJB area. Various statistical parameters, including seismic b-values, are estimated to identify the typical stages of earthquake occurrence from January 2009 to May 2022, showing a temporal correlation with reservoir water levels. The epidemic-type aftershock sequence (ETAS) model reveals that the proportion of the seismicity conforming to Omori’s law is higher than that before impoundment, indicating a tectonic influence on RIS. Notably, major seismic clusters recognized via the double-difference location method demonstrate a spatiotemporal migration corresponding to the impoundment. Focal solutions for \(M3.0\) $M3.0$ earthquakes are obtained utilizing the full moment tensor inversion, presenting a dominantly strike-slip mechanism inconsistent with surrounding faults. The distribution of their relocated aftershocks implies an unmapped 5-km-long fault with striking S30 \(^{\circ }\) ${}^{\circ }$ E-S10 \(^{\circ }\) ${}^{\circ }$ E and dipping \(\sim 80^{\circ }\) $\sim {80}^{\circ }$ to the west. Considering geological context, we propose that shallow karst structures probably facilitate hydraulic channel development and effective stress reduction for the inferred fault, thereby inducing earthquakes due to fault dislocation. This study strengthens the understanding of RIS occurrence with artificial impoundment process and signifies that seismicity in the tail area of the XJB reservoir still warrants attention.