Abstract: Hunanzhen reservoir with a dam 129m high and a volume of 2?10⁹m³ is located in Quxian County,Zhejiang Province.A series of tremors have occurred since its impoundment in 1979.A dense seismic network was installed in the epicentral area during high water season of 1982 and 1983.Hypocenters of 1029 were located and focal mechanism for 372 events was individually determined.The observations show that almost all of the events are clustered along the immediate banks upstream 15km from the dam and have quite shallow depth only of hundreds of meters.Seismic activities bursted with rapid fluctuation of lake level only in hours to days'delay.Focal mechanisms demonstrate that most are of thrust fault and next the normal.Analysis of the data suggests that the seismicity is caused by the slips along the preexisting fractures due to the water seeping under the effect of local stress in the banks or the gravitation of rock mass.Future seismic activity is expected to be continued for a time but to be limited in magnitude.