VP / Vs variations before reservoir-induced seismic events

The variations of the seismic velocity ratio V P /V S 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: 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; The relationship between the precursory time Δ Τ and magnitude M of reservoir-induced earthquakes shows clustering about a linear increase in lg Δ Τ with M, but the slope of the average line lg Δ Τ = aM + b for reservoir-induced seismic events is remarkably smaller than that for natural tectonic earthquakes; 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.