Sidao Ni, USTC University of Science and Technology of China, Hefei, China, Han Zhang, University of New Mexico, Albuquerque, United States, Yong Zhou, Institute of Geodesy & Geophysics, Hubei, China, Wenbo Wu, CalTech Seismological Laboratory, Pasadena, United States and Jessica C E Irving, Princeton University, Department of Geosciences, Princeton, NJ, United States