1. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
2. Laboratory of Seismology and Physics of Earth’s Interior, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China
3. Department of and Geoscience, University of Wisconsin-Madison, Madison, WI 53706, USA
4. Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, USA