1. School of Earth and Space Sciences, Peking University, No. 5, Yiheyuan road, Haidian District, Beijing 100871, China, zyxiong@pku.edu.cn, zsy@pku.edu.cn
2. The Institute of Statistical Mathematics, Research Organization of Information and Systems, 10‐3, Midori‐cho, Tachikawa, Tokyo 190‐8562, Japan, zhuangjc@ism.ac.jp