1. Professor, Dept. of Civil and Environmental Engineering, Seoul National Univ., 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Korea (corresponding author).
2. Graduate Student, Dept. of Civil and Environmental Engineering, Seoul National Univ., 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Korea.
3. Associate Professor, Disaster Prevention Research Institute, Kyoto Univ., Gokasho, Uji, Kyoto 611, Japan.
4. Professor, Disaster Prevention Research Institute, Kyoto Univ., Gokasho, Uji, Kyoto 611, Japan.