1. 1Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, U.S.A.
2. 2Illinois State Geological Survey, Champaign, Illinois, U.S.A.
3. 3Sandia National Laboratories, Albuquerque, New Mexico, U.S.A.
4. 4Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, U.S.A.