1. Research Institute of Geology and Geoinformation, Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan (e-mail: )
2. Faculty of Science, Niigata University, 8050 Ikarashi 2-no-cho, Nishi-ku, Niigata 950-2181, Japan
3. RC GEAR, 4-20-14 Nakamura-kita, Nerima-ku, Tokyo 176-0023, Japan