Characteristics of seismicity before and after impoundment of Baihetan dam in the lower reaches of Jinsha River

Baihetan Hydropower Station is the world's second largest alpine -gorge-type high-dam and large-capacity reservoir after the Three Gorges Reservoir. The height of the dam is 289 meters, the storage capacity is as high as 20. 627 billion cubic meters, and the maximum designed water level is 825 meters. The dam is located on the eastern boundary of the Sichuan-Yunnan diamond shaped block. There are several large scale active tectonic fault zones since the late Quaternary in the reservoir area, such as the Sikai-Jiaojihe fault, the Zemuhe fault and the Xiaojiang fault, and the regional tectonic background is very complex. Since 2016, aiming to monitor the seismic activity and its changes before and after the impoundment of Baihetan reservoir during the whole process, a dense reservoir seismic monitoring network has been set up. The dam began to store water on April 6, 2021, the water level in front of the dam rose from 658. 44 m to 816. 61 m on September 30, 2021, with a total rising of 158. 17 m. Based on the relocation results obtained by applying tomoDDMC method to observed data recorded by the seismic array that has been deployed around the reservoir area, we find that with the impoundment, the seismic activity in the reservoir area has been changed obviously, and the microseismic activity in the reservoir area along Jinsha River and Heishui River tributary has increased significantly, which is closely related to the change of water level. As of December 31, 2021, 7,401 earthquakes have been recorded in the reservoir area, including 12 earthquakes above M(L)3, the largest being the M(L)3. 9 earthquake that occurred in Qiluogou Town, Ningnan County, Sichuan Province on December 21, 2021. The monthly frequency of earthquakes in the reservoir area increased from less than 80 to more than 500, and reached 1200 in October 2021. After the impoundment, earthquakes cluster in a Y-shaped area that centered on Hulukou, Qiaojia County. That is towards the dam along the Jinsha River, to the NW Ningnan direction along the Heishui River tributaries, to the south along both the northern section of the Xiaojiang Fault and the Jinsha River respectively. The focal depth predominately distributed less than 10 km, and the earthquakes around Hulukou town, the first active place, occurred at shallower depth. The b value of seismic activity in the reservoir area is mostly greater than 1. 0. We found that with the impoundment of Baihetan dam, there are seismic activity occurred in the northern section of the Xiaojiang fault zone, and the monitoring and research should be strengthened during the later storing stage.