We analyzed the temporal variation in the travel times of both the P and S waves (Vp and Vs) for 14 months at Toyohashi (central Japan) with a continuously operating vibration source that could produce both P and S waves efficiently. The seismic waves produced by the source, which is named ACROSS (accurately-controlled routinely-operated signal system), were recorded by three nearby seismic stations, and the travel time variation at each station was calculated using the transfer function calculated from the recorded data. We observed the seasonal variations in the Vp and Vs for all the stations—which can be interpreted using the change in the fluid saturation and crack density of subsurface rocks—are consistent with the variation in the ground water level. The short-term responses to rainfall are observed at the nearest station; the interpretation of the changes in crack density and saturation is inconsistent with the ground water observation partly due the initial response to rainfall. This can be interpreted as an air-water mixture within pores or cracks on a fine scale.
