1. Aso Volcanological Laboratory, Institute for Geothermal Sciences, Graduate School of Science; Kyoto University; Minamiaso Japan
2. Now at Hot Springs Research Institute of Kanagawa Prefecture; Odawara Japan
3. Disaster Prevention Research Institute; Kyoto University; Uji Japan
4. Division of Earth and Planetary Sciences, Graduate School of Science; Kyoto University; Kyoto Japan