1. Bullard Laboratories, Department of Earth Sciences, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, UK
2. Department of Geosciences, Princeton University, Princeton, NJ 08544, USA
3. Program in Applied & Computational Mathematics, Princeton University, Princeton, NJ 08544, USA
4. Department of Earth and Planetary Sciences, Harvard University, 20Oxford Street, Cambridge, MA 02138, USA
5. Lamont Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA