now at: ExxonMobil Exploration Company, 22 Benmar Rd., Houston, TX 77060, United States ; Department of Geophysics, Stanford University, MC 2215, Stanford, CA 94305, United States
now at: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025, United States ; Department of Geophysics, Stanford University, MC 2215, Stanford, CA 94305, United States
Department of Geophysics, Stanford University, MC 2215, Stanford, CA 94305, United States
Department of Geophysics, Stanford University, MC 2215, Stanford, CA 94305, United States