1. Institute of Statistical Mathematics, 10-3 Midori-cho, Tachikawa, Tokyo 106-8659, Japan
2. Department of Statistical Science, Graduate University for Advanced Studies, Tokyo 190-8562, Japan
3. London Mathematical Laboratory, London WC2N 6DF, UK
4. Earthquake Research Institute, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan