Abstract: Dagangshan reservoir is located on the eastern boundary of the Sichuan—Yunnan rhombic block, the north—west Moxi fault and nearly N—S trending Daduhe fault through the reservoir area and natural background of seismic activity is relatively strong. In this paper, based on earthquake catalog from China Seismic Information, China Seismic Network Center and Dagangshan reservoir seismic network, we analyzed background seismicity near the reservoir area before impoundment according to seismic activity, focal mechanism and stress field. In larger study area, lots of historic earthquakes with magnitude larger 7.0 occurred, and the closest one with X degree intensity is 54 km away from the reservoir. Part of the reservoir area is Moxi 8 magnitude potential seismic zone and the dam is located in Liziping 7.5 magnitude potential seismic zone. The energy of modern earthquakes occurred in Longmenshan faults released in the last ten years. In the reservoir seismic monitoring area, earthquakes mainly distribute in two regions of the west side of the reservoir. One is near the boundary of Luding County and Kangding County, and the other one is on the west of Dagangshan reservoir and the present NW—SE belt. The seismicity parameter including a value, b value and maximum expectation magnitude (Mmax) indicate high seismic risk in the west side and northwest side of reservoir earthquake monitoring area. The tendency of seismic activity of reservoir seismic monitoring area and impoundment affected zone are consistent, which would impact judgment of induced seismicity in the future.

Key words: Natural background seismicity, Reservoir induced earthquake, Dagangshan reservoir, Moxi fault