1. Department of Physics and Astronomy, University of Florence, Sesto Fiorentino 50019, Italy
2. INFN - Firenze, Via Sansone 1, Sesto Fiorentino 50019, Italy
3. Department of Physics, University of Naples, Via Cinthia, Napoli 80126, Italy
4. INFN - Napoli, Via Cinthia, Napoli 80126, Italy
5. INGV (Istituto Nazionale di Geofisica e Vulcanologia), Osservatorio Vesuviano, Napoli 80100, Italy