Three-dimensional VP, VS, and VP/VS imaging based on AI microseismic detection reveals the mechanism of induced earthquakes in the Xiluodu Reservoir Area, China

Abstract

The Xiluodu Reservoir (XLD), the third hydropower station developed downstream of the Jinsha River, is currently the fourth largest hydropower station worldwide. The reservoir has been impounded for 10 years, and numerous earthquakes have occurred, thus indicating an increased seismic risk. In existing studies, the deep tectonics and fine-scale structure in the reservoir area have not been sufficiently explained. Thus, the mechanism underlying reservoir-induced seismicity in this area remains unclear. In this study, we utilized a microseismic catalogue compiled based on the LOC-FLOW workflow (Li et al., 2023), alongside long-term observation data from a dense seismic array in the Jinsha River downstream, to obtain high-precision 3D VP, VS and VP/VS structures, as well as refined seismic relocations in the XLD Reservoir area. Additionally, we incorporated petrological, geomagnetic, gravity, crustal density, geodetic observation and numerical simulation data, which demonstrate that seismicity in the reservoir head area and near the Manao Fault is influenced by fluids. Moreover, the two M4 earthquakes that occurred east of the Lianfeng Fault are indicative of tectonic activity. Of these earthquakes, the Tianba earthquake cluster in the reservoir head area is located in a high-VP/VS zone, while a low-VS anomaly channel is observed here, which suggests that large amounts of fluid infiltrated along the fault, increasing the pore pressure and reducing the effective stress on the fault. The Wuji earthquake cluster in the head area is also located in a low-VP/VS zone, which indicates a low porosity and possibly undrained conditions. Thus, the earthquakes in this area are the result of coupled elastic stress-pore water pressure interactions.