1. Department of Earth and Planetary Science and Berkeley Seismological Laboratory, 307 McCone Hall, University of California, Berkeley, Berkeley, CA 94720, USA.
2. Mail Stop 300-233, Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA.
3. Department of Geology, University of California, Davis, Davis, CA 95616, USA.