1. Institute for Computational and Mathematical Engineering, Stanford University, Stanford, CA 94305, USA.
2. Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA.
3. Geophysics Group, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
4. Department of Computational and Applied Mathematics, Rice University, Houston, TX 77005, USA.
5. Department of Geophysics, Stanford University, Stanford, CA 94305, USA.