1. Department of Geological Sciences; Brown University; Providence Rhode Island USA
2. Istituto Nazionale di Geofisica e Vulcanologia; Roma Italy
3. Now at Department of Earth Sciences; University College London; London UK
4. Now at Department of Earth and Environmental Sciences; University of Pennsylvania; Philadelphia Pennsylvania USA
5. Department of Geological Sciences; Padova University; Padova Italy
6. School of Engineering and Applied Sciences; Harvard University; Cambridge Massachusetts USA