[1] Ist Nazl Geofis & Vulcanol, Sez Bologna, I-40127 Bologna, Italy
[2] Univ Aquila, Ctr Excellence CETEMPS, I-67100 Laquila, Italy
[3] Sapienza Univ Rome, Dipartimento Ingn Informaz DIET, I-00184 Rome, Italy
[4] Univ Geneva, Dept Earth Sci, CH-1205 Geneva, Switzerland
[5] Univ Florence, Dept Earth Sci, I-50121 Florence, Italy
[6] Ist Nazl Geofis & Vulcanol, Osservatorio Etneo, I-95125 Catania, Italy