[1] Dept. Earth Syst. Sci. and Technol., Kyushu University, Kasuga, Fukuoka 816, Japan
[2] Department of Electrical Engineering, Doshisha University, Kyotanabe, Kyoto 610-03, Japan
[3] Res. Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka 816, Japan