1. 1Department of Earth Sciences, University of Oregon, Eugene, Oregon, U.S.A.
2. 2Now at, Nanohmics, Austin, Texas, U.S.A.
3. 3U.S. Geological Survey, Earthquake Science Center, Moffett Field, California, U.S.A.
4. 4Now at, Agrograph, Madison, Wisconsin, U.S.A.