1. Institute of Theoretical Geophysics, Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Centre for Mathematical Sciences, Wilberforce Road, Cambridge CB3 0WA, UK
2. Faculty of Science, University of Bristol, Bristol BS8 1UH, UK
3. School of Mathematics and Statistics, University of New South Wales, Sydney, New South Wales 2052, Australia
4. BP Institute, Bullard Laboratories, Bullard Laboratories, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, UK
5. Department of Earth Sciences, Bullard Laboratories, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, UK