[1] Exxon ExxonMobil Res & Engn Co, Annandale, NJ USA
[2] Columbia Univ, Dept Earth & Environm Sci, Palisades, NY USA
[3] Carnegie Inst Washington, Dept Terr Magnetism, Washington, DC 20015 USA
[4] Univ Alaska Fairbanks, Inst Geophys, Fairbanks, AK 99775 USA
[5] Alaska Volcano Observ, Alaska Div Geol & Geophys Surveys, Fairbanks, AK USA