Earthquake Research Institute, The University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113- 0032, Japan ; Now at Department of Natural History Sciences, Hokkaido University, N10 W8, Kita-ku, Sapporo, 060-0810, Japan
National Geophysical Research Institute, Hydelabad, Hydelabad, 500 007, India