P-wave velocity structure beneath reservoirs and surrounding areas in the lower Jinsha River

The lower reaches of the Jinsha River are rich in hydropower resources because of the high mountains, deep valleys, and swift currents in this area. This region also features complex tectonic structures and frequent earthquakes. After the impoundment of the reservoirs, seismic activity increased significantly. Therefore, it is necessary to study the P-wave velocity structure and earthquake locations in the lower reaches of the Jinsha River and surrounds, thus providing seismological support for subsequent earthquake prevention and disaster reduction work in reservoir areas. In this study, we selected the data of 7,670 seismic events recorded by the seismic networks in Sichuan, Yunnan, and Chongqing and the temporary seismic arrays deployed nearby. We then applied the double-difference tomography method to this data, to obtain the P-wave velocity structure and earthquake locations in the lower reaches of the Jinsha River and surrounds. The results showed that the Jinsha River basin has a complex lateral P-wave velocity structure. Seismic events are mainly distributed in the transition zones between high- and low-velocity anomalies, and seismic events are particularly intense in the Xiluodu and Baihetan reservoir areas. Vertical cross-sections through the Xiangjiaba and Xiluodu reservoir areas revealed an apparent high-velocity anomaly at approximately 6 km depth; this high-velocity anomaly plays a role in stress accumulation, with few earthquakes distributed inside the high-velocity body. After the impoundment of the Baihetan reservoir, the number of earthquakes in the reservoir area increased significantly. The seismic events in the reservoir area north of 27° N were related to the enhanced activity of nearby faults after impoundment; the earthquakes in the reservoir area south of 27° N were probably induced by additional loads (or regional stress changes), and the multiple microseismic events may have been caused by rock rupture near the main faults under high pore pressure.