1. Division of Geological and Planetary Sciences; California Institute of Technology; Pasadena CA USA
2. Now at Geophysical Institute; University of Alaska Fairbanks; Fairbanks AK USA
3. Walter Burke Institute for Theoretical Physics; California Institute of Technology; Pasadena CA USA
4. NASA Jet Propulsion Laboratory; California Institute of Technology; Pasadena CA USA
5. Earth Observatory of Singapore; Nanyang Technological University; Singapore