1. Department of Geophysics, Stanford University, 397 Panama Mall, Stanford, California 94305 U.S.A., kaiwenw@stanford.edu
2. Department of Earth Sciences, University of California Santa Barbara, Lagoon Road, Santa Barbara, California 93106 U.S.A.
3. U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 U.S.A.