1. Asst. Prof., Dept. of Engrg. Seismology, Int. Inst. of Earthquake Engrg. and Seismology, P.O. Box 19395/3913, Tehran, Iran. E-mail: mrgh @dena.iiees.ac.ir
2. Prof., Dept. of Civ. and Envir. Engrg., Saitama Univ., 255 Shimo-Okubo, Urawa, Saitama 338, Japan.