1. Seismological Laboratory, Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Boulevard, MC 252‐21, Pasadena, California 91125 U.S.A., zefengli@gps.caltech.edu
2. Institut de Physique du Globe de Strasbourg, Universite de Strasbourg, EOST/CNRS UMR 7516, F‐67084 Strasbourg Cedex, France