1. U.S. Geological Survey, Hawaiian Volcano Observatory, Hilo, HI 96720, USA.
2. U.S. Geological Survey, Alaska Volcano Observatory, Anchorage, AK 99508, USA.
3. U.S. Geological Survey, Cascades Volcano Observatory, Vancouver, WA 98683, USA.
4. U.S. Geological Survey, California Volcano Observatory, Menlo Park, CA 94025, USA.
5. School of Integrated Arts and Sciences, Hiroshima University, Higashi Hiroshima, Hiroshima 739-8521, Japan.
6. Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa, 920-1192, Japan.