Kathleen F McKee, Carnegie Institution for Science Washington, Washington, DC, United States, Diana C. Roman, Carnegie Institution for Science, Department of Terrestrial Magnetism, Washington, DC, United States, David Fee, University of Alaska Fairbanks, Geophysical Institute, Fairbanks, AK, United States, Gregory P Waite, Michigan Technological University, Geological & Mining Engineering & Sciences, Houghton, MI, United States and Maurizio Ripepe, Universita di Firenze, Dipartimento di Scienze della Terra, Firenze, Italy