Abstract: The Hunanzhen reservoir with a dam 129m high and a volume of 2.06 billion m³ is located in Quxian County, Zhejiang province. The seismic activity started along the bank 15km up stream from the dam, 5 months after impounding of this reservoir. The earthquake of magnitude 2.8(Ms)occurred on October 7, 1979 and caused a light damage in the central area. A mobile seismograph was installed in the epicentral area and from March 14 to May 26, 1980 1483 tremors were recorded. Most of them are ultramicroear-thquakes and have a regular distribution in space with extremely shallow depth. The seismic activities vary with the water level sensitively and lag in a short time. Based on the geological environment and macroscopic characteristics of seismic records it can be suggested that the seismicity is likely to be caused by the gravitational slip of the unstable rock masses on the banks beneath the reservoir studied.