1. Department of Geology and Geophysics, Yale University, New Haven, Connecticut, U.S.A.
2. National Science Foundation, Alexandria, Virginia, U.S.A.
3. Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, U.S.A.
4. Now at Earthquake Science Center, U.S. Geological Survey, Menlo Park, California, U.S.A.
5. Now at Lincoln Laboratories, Massachusetts Institute of Technology, Lexington, Massachusetts, U.S.A.