Characteristics of seismic activity and stress field changes in the adjacent area before and after the impoundment of Baihetan Reservoir

The reservoir-induced earthquake is an important content in the study of water conservancy and hydropower projects and regional tectonic stability. The Baihetan Reservoir is located in the lower reaches of the Jinsha River, where several large faults such as the Xiaojiang fault, the Zemuhe fault and the Daliangshan fault intersect. It belongs to a high seismic intensity area. On April 6, 2021, the Baihetan Reservoir began to store water. In this paper, 11477 seismic events in the vicinity of the reservoir were determined by using the seismic records of Qiaojia Array in the middle and northern section of the Xiaojiang Fault Zone from January 2020 to December 2021. Using Generalized Polarity and Amplitude Technique, 654 focal mechanism solutions of earthquakes with M-L >= 1. 5 were obtained. The analysis of the distribution characteristics of seismic activity and focal mechanism solution before and after the impoundment of the reservoir shows that, overall, the seismicity in the study area is closely related to the water level of the reservoir. When the water level rises, the seismic activity increases, and when the water level remains unchanged or falls, the seismic activity decreases. After the impoundment of the Baihetan Reservoir for about 6 months, the number of earthquakes basically returns to the average level before impoundment. In the main fault zone area along the water line, the seismic activity after impoundment increased significantly with the number and magnitude increasing, and the depth decreasing. The seismic type before impoundment is mainly strike-slip, and after impoundment the normal fault earthquake dominates. Outside the main fault zone and far away from the water line, seismic activity is inhibited after impounding. Based on the focal mechanism solutions, the spatiotemporal distribution of the stress field in the study area was obtained using the regional stress field damping inversion method. The results show that the direction of the main tensile stress axis is stable with little fluctuation over time, while the direction of the main compressive stress axis fluctuates greatly over time. Especially in the intersection area of the main fault zones near the dam, the stress field before impoundment is strike-slip type, and after impoundment becomes normal fault type. These results reveal more detailed characteristics of reservoir-induced seismic activity and stress field changes, which can provide important reference for further risk assessment of reservoir-induced earthquakes.