1. Centro de Estudios Científicos, Valdivia, Chile and University of Bristol, School of Earth Sciences, Bristol BS8 1RJ, UK.(corresponding author).
2. University of Bristol, School of Earth Sciences, Bristol BS8 1RJ, UK..
3. University of Bristol, School of Earth Sciences, Bristol BS8 1RJ, UK and University of Oxford, Department of Earth Sciences, Oxford, UK..
4. NORSAR, Kjeller, N-2007, Norway..