1. Graduate School of Science; Tohoku University; Sendai; 980-8578; Japan
2. Japan Agency for Marine-Earth Science and Technology; 3173-25, Showa-cho, Kanazawa-ku,; Yokohama; 236-0001; Japan
3. Seismological and Volcanological Department; Japan Meteorological Agency; 1-3-4, Otemachi, Chiyoda-ku,; Tokyo; 100-8122; Japan
4. Technical Department; Sendai District Meteorological Observatory; 1-3-15 Gorin, Miyagino-ku,; Sendai; 983-0842; Japan