Three Dimensional Seismic Velocity Structure in the Otake-Hatchobaru Geothermal Area in Central Kyushu, Japan

Abstract

We studied the three-dimensional seismic velocity structure in and around the Otake-Hatchobaru geothermal area in central Kyushu, Japan, using the tomographic method of Zhao et al. (1994). A low Vs with high VpNs region, suggesting a cooling magma body beneath the volcanoes around the geothermal area, is located between 4.0 km and 7.0 km depth. Above an anomalous low Vs region at 2.5 km depth, a high Vs with low VpNs region exists indicating the granitic basement rock. Most of seismic sources relating to the movement of geothermal fluids were found at shallower depths (x2.5 km) and at the boundary between the low and high Vs regions. We suggest fluid movements in Otake-Hatchobaru geothermal area are associated with the high Vs region at 2.5 km depth and the low Vs region deeper than 4.0 km..