The variations of the seismic velocity ratio before some reservoir-induced seismic events are studied synthetically, These seismic events include the Xinfengjiang Reservoir impounding earthquakes(M_s=6.1,5.3,4.5,4.4,4.4,4.3),the Longyangxia Reservoir impounding small swarm (M_L=2.5)in China, and the induced seismic events(M_L=2.7,2.5,2.0,2.3)at the Monticello Reservoir in the USA. The main results are:
(1) The V_p/V_s anomaly may be observed before the reservoir-induced seismic events in a wide magnitude range from M=2.0 to M>6.0;(2) The relationship between the precursory time △T and magnitude M of reservoir induced earthquakes shows clustering about a linear increase in lg△T with M, but the slope of the average line lg△T=aM+b for reservoir induced seismic events is remarkably smaller than that for natural tectonic earthquakes;
(3) The V_p/V_s anomalous area of a reservoir-induced seismic event is much smaller than the area of a natural tectonic earthquake, with the same magnitude.
The results obtained may be used to distinguish the reservoir-induced seismic event from the natural tectonic earthquake. An actual example is given.