1. Department of Mathematics and Physics, Universitá della Campania “Luigi Vanvitelli” , Caserta 81100 , Italy
2. INGV-Osservatorio Vesuviano , via Diocleziano, 328 - 80124 Napoli , Italy
3. The Institute of Statistical Mathematics, Research Organization of Information and Systems , 10-3 Midori-cho, Tachikawa Tokyo 190-8562 , Japan