GNGTS 2017 - 36° Convegno Nazionale

438 GNGTS 2017 S essione 2.2 The station T1364 shows a H/V peak centred at 0.6 Hz with an amplitude of 2.6 in the E- NE direction (~ N70) of the directivity plot in agreement with the polarization direction. This matches the prevalent geological structural features of the northwestern portion of the island. In the station IMTC we do not recognize properly an amplitude peak of the H/V, nevertheless at a 0.6 Hz there is a maximum of the ratio which assume a value of 2.1. This maximum is in the SW-NE direction (~ N150) of the directivity plot according to polarization direction and to the prevalent trend of the geological structures. Earthquake MD 4.0. During the earthquake the velocimeters of the permanent seismic stations network saturated and only the station IOCA, that is equipped alsowith an accelerometer, recorded entirely the earthquake (Fig. 3 top). We calculated the earthquake H/V ratios and compare it with the one obtained using the seismic noise (Fig. 3 bottom). We observe that both have a peak centred at 1.5 Hz, but the earthquake H/V peak reaches a value of 12, significantly higher than the noise H/V peak, that has an amplitude of 7. Furthermore at high frequencies, after 20 Hz, H/V earthquake ratio is more than three times higher than the seismic noise one. Conclusions. The H/V analyses performed with the noise data show that the analysed sites have two response behaviours, one almost flat in the entire frequency range and the other with a predominant amplitude peak well centred at a specific frequency. The results indicate that there is a good agreement between the H/V peak directivity and the polarization direction of the noise, which observing the geological map coincides with the main structural features of the island. We are confident that the sites that exhibit H/V amplification peaks are characterized by high seismic impedance contrast between the shallow reworked volcanic deposits and the underlying lavas. Further investigations are in progress. The comparison of the two ratios performed for the station IOCAhighlights that the noise H/ V ratio has strong similarities with the earthquake H/V, therefore we can assert that the seismic noise analysis identifies perfectly the resonant site frequency, even if with a lower amplitude value. References Acocella V., Funiciello R., Marotta E., Orsi G., de Vita S.;2004: The role of extensional structures on experimental calderas and resurgence. J. Volcanol. Geoth. Res., 129:199–217. Acocella V., Funiciello R.; 1999: The interaction between regional and local tectonics during resurgent doming: the case of the island of Ischia, Italy. J. Volcanol. Geoth. Res., 88, 109-123. Brown R.J., Orsi G., de Vita S.; 2008: New insights into Late Pleistocene explosive volcanic activity and caldera formation on Ischia (southern Italy). Bull. Volc., 70 (5): 583-603. doi: 10.1007/s00445–007–0155–0. de Vita S., Sansivero F., Orsi G., Marotta E., Piochi M.; 2010: Volcanological and structural evolution of the Ischia resurgent caldera (Italy) over the past 10 k.y. Stratigraphy and Geology of Volcanic Areas, Gianluca Groppelli, Lothar Viereck-Goette. de Vita S., Sansivero F., Orsi G., Marotta E.; 2006: Cyclical slope instability and volcanism related to volcano- tectonism in resurgent calderas: the Ischia island (Italy) case study. Eng. Geol., 86, 148-165. Della Seta M., Marotta E., Ordsi G., de Vita S., Sansivero F., Fredi P.; 2012: Slope instability induced by volcano- tectonics as an additional source of hazard in active volcanic areas: the case of Ischia island (Italy). Bull. Volc., 74: 79-106. doi: 10.1007/s00445-011-0501-0. Italian Journal of Engineering geology and Environment, 2, 15-28. DOI:10.4408/IJEGE.2015-02.O-04. Guidoboni E., Ferrari G., Mariotti D., Comastri A., Tarabusi, Valensise G.; 2007: CFTI4Med, Catalogue of strong Earthquakes in Italy (461 B.C.-1997) and Mediterranean Area (760 B.C.-1500), INGV-SGA, http://storing.ingv. it/cfti4med/ . Montalbetti J. R., Kanasevich E. R.; 1970: Enhancement of teleseismic body phase with a polarization filter, Geophys. J. R. Astron. Soc., 21, 119–129. Orsi G., de Vita S., Di Vito M., Isaia R., Nave R., Heiken G.; 2003: Facing volcanic and related hazards in the Neapolitan area. In: Heiken, G., Fakundiny, R, Sutter, J. (Eds) Earth Sciences in Cities, American Geophysical Union (Special Publication), Washington 121-170. Rovida A., Locati M., Camassi R., Lolli B., Gasperini P.; 2016: CPTI15, the 2015 version of the Parametric Catalogue of Italian Earthquakes. doi:10.6092 /INGV.IT -CPTI15. Vezzoli L.; 1988: Island of Ischia. CNR Quaderni de “La ricerca scientifica” 114 (10), 122 pp.