1. Department of Geophysics, Stanford University, Stanford, CA 94305.
2. U.S. Geological Survey, Menlo Park, CA 94025.
3. Department of Geology and Geophysical Sciences, Princeton University, Princeton, NJ 08544.
4. U.S. Geological Survey, Pasadena, CA 91107.
5. Lamont-Doherty Geological Observatory, Columbia University, Palisades, NY 10964.
6. Woodward-Clyde Consultants, San Francisco, CA 94102.