1. Northwestern Institute on Complex Systems, Northwestern University, 2145 Sheridan Road, Room F374, Evanston, Illinois 60208 U.S.A., kchao@northwestern.edu
2. Also at Center for Optimization and Statistical Learning, Northwestern University, Evanston, Illinois 60208 U.S.A. and Department of Earth and Planetary Sciences, Northwestern University, Evanston, Illinois 60208 U.S.A.
3. Seismological Laboratory, California Institute of Technology, 1200 E California Boulevard MC 252‐21, South Mudd Building, RM 266, Pasadena, California 91125 U.S.A.