1. School of Geography, Environment & Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6040, New Zealand
2. GNS Science, P.O. Box 30-368, Lower Hutt 5040, New Zealand
3. Department of Earth and Environmental Sciences, University of Michigan, 1100 North University Avenue, Ann Arbor, Michigan 48109-1005, USA
4. Institute for Geophysics, University of Texas at Austin, 10601 Exploration Way, Austin, Texas 78758, USA