1. Department of Earth Science, University of Bristol, Will's Memorial Building, Queen's Road, Bristol BS8 1RJ, UK
2. Present address: Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, Via della Faggiola 32, 56126 Pisa, Italy
3. Graduate School of Earth Science, University of Rhode Island, South Ferry Road, Narragansett, RI 02882, USA