1. Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Road, Northwest, Washington, DC 20015, USA.
2. Southern Arizona Seismic Observatory, Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA.
3. Carnegie Institution of Washington, Geophysical Laboratory, 5251 Broad Branch Road, Northwest, Washington, DC 20015, USA.