1. Department of Earth and Environmental Engineering, Columbia University, New York, NY 10027
2. Department of Structural Engineering, University of California, San Diego, CA 92093
3. SV Laboratory, International Center for Applied Mechanics, School of Aerospace, Xi'an Jiaotong University, Xi'an 710049, China;
4. Department of Earth and Environmental Engineering, Columbia University, New York, NY 10027 e-mail: