1. Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Osservatorio Etneo, Italy
2. Conservatoire National des Arts et Métiers, Laboratoire Modélisation Mathématique et Numérique, Paris, France
3. McCormick School of Engineering and Applied Science, Northwestern University, Evanston, Illinois 60208, USA