The Relationship between Seismicity and Fluid Existing in the Crust Inferred from Vp/Vs Ratio

The relationship between seismicity and fluid existing in the crust has been discussed by several experiments since the middle of the 1960's. It is considered in these experiments that earthquakes occur at regions when and where pore pressure is high, since pore pressure has been thought to decrease the frictional strength. In this paper, the relationship between seismicity and fluid (degree of saturation) existing in the crust was investigated by analyzing the data sets of the seismic activity and tomography obtained by the dense networks installed in the Western Nagano prefecture. The distributions of degree of saturation and crack density were inferred from the three-dimensional velocity tomography obtained from the 1986 Joint Seismological Research in Western Nagano Prefecture, by using the theory of new self-consistent scheme (NSC) or differential scheme (DS). Comparing these distributions with the hypocentral distribution in the same region, we found that earthquakes occur at the regions where the degree of saturation is low, namely, at little saturated rocks. Further, it appears that the blocks of low degree of saturation are surrounded by more saturated blocks. This result suggests that earthquakes possibly occur when fluid begins to flow into cracks which are little saturated.