1. Earthquake Science Center, U.S. Geological Survey, Moffett Field, CA 94035, USA
2. Earthquake Science Center, U.S. Geological Survey , Pasadena, CA 91106, USA
3. Department of Statistical Data Science, Institute of Statistical Mathematics , Tachikawa, Tokyo 190-8562, Japan
4. Southern California Earthquake Center, University of Southern California , Los Angeles, CA 90089, USA