1. George W. Housner Research Fellow, Dept. of Civil Engineering, Mail Code 104-44, California Institute of Technology, Pasadena, CA 91125 (Corresponding author). E-mail: jyching@caltech.edu
2. Professor, Dept. of Applied Mechanics and Civil Engineering, Mail Code 104-44, California Institute of Technology, Pasadena, CA 91125. E-mail: jimbeck@caltech.edu