1. Department of Environmental Systems, Rissho University , Kumagaya 360-0194 , Japan
2. Department of Geology, Shinshu University , Matsumoto 390-8621 , Japan
3. The Hokkaido University Museum, Hokkaido University , Sapporo , 060-0810 , Japan
4. Department of Science and Mathematics Education, University of Fukui , Fukui 910-8507 , Japan
5. Department of Geosciences, Shizuoka University , Shizuoka , 422-8529 , Japan
6. Institute of Tephrochronology for Nature and History Co., Ltd , Maebashi 371-0803 , Japan