[1] Osservatorio Etneo, Ist Nazl Geofis & Vulcanol, Catania, Italy
[2] Univ Geneva, Dept Earth Sci, Geneva, Switzerland
[3] Ist Nazl Geofis & Vulcanol, Sez Bologna, Bologna, Italy
[4] Ctr Excellence CETEMPS, Laquila, Italy
[5] Sapienza Univ Rome, Dept Informat Engn Elect & Telecommun, Rome, Italy
[6] Univ Bari Aldo Moro, Dept Earth & Geoenvironm Sci, Bari, Italy