1. Department of Physics and Earth Sciences, Faculty of Science; University of the Ryukyus, Senbaru 1, Nishihara; Okinawa 903-0213 Japan
2. Tropical Biosphere Research Center; University of the Ryukyus, Senbaru 1, Nishihara; Okinawa 903-0213 Japan
3. Department of Earth and Planetary Sciences, Graduate School of Environmental Studies; Nagoya University; Nagoya 464-8601 Japan