1. System Safety Laboratory, Department of Social and Environmental Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashihiroshima, Hiroshima 739-8527, Japan. Tel.: +81 824 24 7814; Fax: +81 824 22 7194; E-mail: {yoshi, furuki, shintaku, fujimoto}@naoe.hiroshima-u.ac.jp