1. 1Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, U.S.A.
2. 2Woods Hole Coastal and Marine Science Center, U.S. Geological Survey, Woods Hole, Massachusetts, U.S.A.
3. 3Lamont–Doherty Earth Observatory, Columbia University, Palisades, New York, U.S.A.
4. 4Institute for Geophysics, University of Texas, Austin, Texas, U.S.A.
5. 5Now at, Department of Geosciences, Auburn University, Auburn, Alabama, U.S.A.