We would like to thank Mauricio Sacchi, Jeffrey Shragge, Ian Moore, David Halliday, and three anonymous reviewers for constructive suggestions. The paper was prepared in the Madagascar open-source platform (Fomel et al., 2013), and all the examples are reproducible. This work is partially supported by the Natural Science Fund of China(grant NOs.:91330108, 41374121, and 61327013), the fundamental Research Funds for the Central Universities (grant NO.: HIT.BRETIV.201314), and the Program for New Century Excellent Talents in University (grant NO.: NCET-11-0804), and the Texas Consortium for Computational Seismology (TCCS).