1. Department of Earth & Planetary Sciences, Graduate School of Environmental Studies Nagoya University Nagoya 464‐8602 Japan
2. Earth Science Course, School of Natural System, College of Science and Engineering Kanazawa University Kanazawa 920‐1192 Japan
3. Institute of Geology and Geoinformation Geological Survey of Japan AIST Tsukuba 305‐8567 Japan
4. Institute of Socio‐Arts and Sciences University of Tokushima Tokushima 770‐8502 Japan
5. Department of Earth Sciences Faculty of Science Okayama University Tsushima‐naka 3‐1‐1 Okayama 700‐8530 Japan