1. Power & Industrial Systems R & D Laboratory; Hitachi, Ltd.; 7-2-1, Omika-cho, Hitachi-shi,; Ibaraki-ken; 319-1221; Japan
2. Center for Northeast Asian Studies; Tohoku University; 41 Kawauchi, Aoba-ku, Sendai-shi,; Miyagi-ken; 980-8576; Japan
3. Faculty of Science; Shizuoka University; 836 Ohya, Shizuoka-shi,; Shizuoka-ken; 422-8529; Japan
4. Solid Earth Research Group; National Research Institute for Earth Science and Disaster Prevention; 3-1 Tennodai, Tsukuba-shi,; Ibaraki-ken; 305-0006; Japan