1. Seismological Laboratory, California Institute of Technology, 1200 E. California Boulevard MC 252‐21, Pasadena, California 91125 U.S.A., yucq@sustech.edu.cn
2. Now at Department of Earth and Space Sciences, Southern University of Science and Technology, 1088 Xueyuan Road, Nanshan District, Shenzhen, Guangdong 518055, China.
3. Earthquake Science Center, U.S. Geological Survey, 525 South Wilson Street, Pasadena, California 91106 U.S.A.