1. Department of Geology, Portland State University, Portland, Oregon, U.S.A.
2. Department of Earth Sciences, University of Oregon, Eugene, Oregon, U.S.A.
3. Nevada Seismological Laboratory, University of Nevada, Reno, Reno, Nevada, U.S.A.
4. Now at U.S. Geological Survey Earthquake Science Center, Pasadena, California, U.S.A.
5. California Geological Survey, San Mateo, California, U.S.A.
6. Department of Geography, University of Oregon, Eugene, Oregon, U.S.A.
7. Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, California, U.S.A.
8. Ocean Sciences Department, University of California, Santa Cruz, Santa Cruz, California, U.S.A.