1. Department of Earth and Atmospheric Sciences, Cornell University, 112 Hollister Drive, Ithaca, NY 14853-1504, USA.
2. Department of Earth and Planetary Sciences, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA.
3. Geography and Spatial Sciences, University of Tasmania, Hobart, Tasmania 7001, Australia.
4. Department of Geophysics, Colorado School of Mines, Golden, CO 80401, USA.