1. Department of Geology and Geophysics, University of California, Berkeley, CA 94720, and Agency for Defense Development, Yuseong Post Office Box 35, Taejeon 305-600, South Korea
2. Department of Geology and Geophysics, University of California, Berkeley, CA 94720
3. Agency for Defense Development, Yuseong Post Office Box 35, Taejeon 305-600, South Korea