Abstract: Combining the quantitative checking results of fluid inducing/triggering seismicity and simulation of pore pressure diffusion, major influence factors of the Three Gorges Reservoir-related seismicity have been studied. For the Three Gorges Reservoir, we separated the time period as two parts divided by Sep. 2008, namely earlier stage and later stage. During the earlier stage, the major influence factor of reservoir-related seismicity is the increment of pore pressure caused by fluid intrusion, which decreases the strength of crack or fault surface. In this stage, both the microseismic frequency and μ value, one of the ETAS model parameters which describing the ability of fluid inducing/triggering seismicity, increase fluctuantly and slowly. And there is no relationship with load and unload process by the reservoir water level. During the late stage, because the variation of pore pressure caused by the fluid intrusion tends to be zero, the stress change in the surface of crack or fault caused by the water load and unload process become the major influencing factor of microseismic activity, and the variation of the microseismic frequency and μ value show an obvious and positive correlation with the water level. The temporal changes of the focal depth support this viewpoint. Mechanical difference between reservoir induced seismicity (RIS) and reservoir triggering seismicity (RTS) is also discussed. We think that RIS is mainly due to the strength decrease of crack or fault surface on its own initiative, and RTS is mainly due to the stress variation in crack or fault surface caused by the water load and unload process. It has been inferred that both fluid ' induced' and ' triggered' are possible for small earthquakes, but it is impossible that an moderate or strong earthquake could be induced by reservoir.

Key words: Fluid intrusion, Load-unload process of reservoir, Strength of crack or fault, Stress change, Reservoir induced seismicity (RIS), Reservoir triggered seismicity (RTS), The Three Gorges Reservoir