1. Faculty of Education, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan (e-mail: )
2. Geological Survey of Japan, AIST, Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan
3. Department of Earth and Environmental Sciences, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata 990-8560, Japan
4. Department of Earth and Planetary Science, Graduate School of Science, the University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan