1. Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto 611‐0011, Japan, goto@catfish.dpri.kyoto-u.ac.jp
2. Graduate School of Engineering, Osaka University, 2‐1, Yamada‐oka, Suita, Osaka 565‐0871, Japan
3. Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 1‐1‐1, Higashi, Tsukuba, Ibaraki 305‐8567, Japan
4. Research Advancement and Management Organization, Kanto Gakuin University, 1‐50‐1, Mutsuura Higashi, Kanazawa‐ku, Yokohama 236‐8501, Kanagawa, Japan