1. Graduate Student, Dept. of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., Bldg. 48-114, Cambridge, MA 02139 (corresponding author).
2. Assistant Professor, Dept. of Earth Sciences, Univ. of New Hampshire, 214 James Hall, 56 College Rd., Durham, NH 03824.
3. Professor, Dept. of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., Bldg. 48-216D, Cambridge, MA 02139.