1. Research Institute of Geology and Geoinformation, Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan (e-mail: )
2. Graduate School of Science and Technology, Niigata University, 8050 Ikarashi 2-nocho, Niigata 950-2181, Japan
3. Faculty of Science, Niigata University, 8050 Ikarashi 2-nocho, Niigata 950-2181, Japan