Vp/Vs Ratio and Seismic Activity at Geothermal Fields in the North-Western Area of Kuju Volcano, Kyushu, Japan

Abstract

The north-western area of Kuju Volcano is one of the most active geothermal fields in Japan, where the seismic activity including seismic swarms is very active. The Vp/Vs ratio study for earthquakes occurred at this area from November 1995 to May 1998 was carried out. At the shallow part (above 2 km below sea level), Vp/Vs ratios are wide ranging 1.65 to 1.85. On the contrary, almost all of Vp/Vs ratios are limited at the values less than 1.73 at the deeper part (deeper than 2 km below sea level). The Vp/Vs ratio changes with time and increases before the seismic swarm. These characteristics are inferred to be affected by the presence of water and fractures in the geothermal field.