[1] Univ Liverpool, Sch Environm Sci, 4 Brownlow St, Liverpool L69 3GP, Merseyside, England
[2] Ist Nazl Geofis & Vulcanol, Sez Pisa, Via Cesare Battisti 53, I-56125 Pisa, Italy
[3] Ist Nazl Geofis & Vulcanol, Sez Catania Osservatorio Etneo, Piazza Roma, I-95125 Catania, Italy
[4] Univ Catania, Dipartimento Sci Biol Geol & Ambientali, Piazza Univ 2, I-95131 Catania, Italy