1. Postdoctoral Scholar, Pacific Earthquake Engineering Research Center, Dept. of Civil and Environmental Engineering, Univ. of California Berkeley, Berkeley, CA 94720-1792 (corresponding author).
2. Professor, Chair of Structural Dynamics and Earthquake Engineering, Dept. of Civil, Environmental and Geomatic Engineering, Swiss Federal Institute of Technology (ETH) Zurich, 8093 Zurich, Switzerland.