1. Earth Sciences Department, University of California, Santa Cruz, CA 95064, USA.
2. Institute of Geophysics and Planetary Physics, University of California, Santa Cruz, CA 95064, USA.
3. Seismological Laboratory, California Institute of Technology, MS 252-21, Pasadena, CA 91125, USA.
4. Department of Geosciences, The Pennsylvania State University, 440 Deike Building, University Park, PA 16802, USA.
5. Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA.