1. 1Institute of Theoretical and Applied Geophysics, Peking University, Beijing, China
2. 2Hebei Hongshan Geophysical National Observation and Research Station, Peking University, Beijing, China
3. 3Now at, School of Earth Sciences and Engineering, Nanjing University, Nanjing, China
4. 4U.S. Geological Survey, Albuquerque Seismological Laboratory, Albuquerque, New Mexico, U.S.A.