3D VP, VS, and VP/VS Structures and Seismicity of the Lower Reaches of the Jinsha River, China

Abstract

Abstract The Jinsha River flows through the tectonically active Sichuan and Yunnan regions, which have frequent strong earthquake activities. Four large-scale hydropower dams have been built in the lower reaches of the river and had begun impounding water successively since October 2012, with the Baihetan (BHT) and Xiluodu (XLD) being the second and fourth largest hydropower stations in the world. Using the seismic data recorded by dense seismic stations in the lower reaches of the Jinsha River from January 2010 to December 2019, we obtain the 3D crustal VP, VS, and VP/VS models and seismic relocations by applying the VP/VS model consistency-constrained double-difference tomography method (tomoDDMC). After Xiangjiaba (XJB) and XLD started to store water, earthquakes were mainly clustered in two areas where the seismicity was weak before the impoundment, that is, (1) the western segment of the Mabian-Yanjin fault (MBYJF) zone on the west side of the river and the tail section of XJB, and (2) the southwest part of Leibo-Yongshan basin near the middle section of XLD reservoir. We find that the areas where earthquakes are concentrated have characteristics of high-velocity and low-VP/VS. These particular features may provide important information for predicting the potential earthquake risk areas. In addition, we found a well-resolved high-velocity and low-VP/VS zone sandwiched between two layers of low velocity and high VP/VS at depths ranging from 3 to 10 km beneath the Qiaojia area along the Xiaojiang fault (XJF), which suggests that the seismic activity in this area may increase with the storage of the BHT dam in April 2021.