1. Department of Earth Sciences, University of Western Ontario, London, Ontario N6A5B7, Canada
2. School of Earth and Space Sciences, Peking University, Beijing 100871, China
3. Department of Earth & Space Sciences and Institute for Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, California 90095, USA
4. Department of Physics, New Mexico State University, Las Cruces, New Mexico 88003, USA
5. Department of Earth and Planetary Science, University of California, Berkeley, Berkeley, California 84720, USA
6. Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA