1. Berkeley Seismology Laboratory, University of California, Berkeley, 289 McCone Hall, Berkeley, California 94720 U.S.A., kongqk@berkeley.edu
2. Berkeley Seismology Laboratory, University of California, Berkeley, 279 McCone Hall, Berkeley, California 94720 U.S.A.
3. Civil and Mechanical Engineering, Caltech, MC: 104‐44, Pasadena, California 91125 U.S.A.
4. 109 Powell‐Booth Computing Center, Caltech, MC: 158‐79, Pasadena, California 91125 U.S.A.