GNGTS 2017 - 36° Convegno Nazionale

434 GNGTS 2017 S essione 2.2 Slejko D, Carulli G.B., Garcia J. Santulin M.; 2011: The contribution of “silent” faults to the seismic hazard of the northern Adriatic Sea. Journal of geodynamics, 51, 166-178. Wilson, R.C., and Keefer, D.K.; 1985: Predicting areal limits of earthquake-induced landsliding, in J.I. Ziony, editor, Evaluating earthquake hazards in the Los Angeles region-an earth-science perspective. U.S. Geological Survey Professional Paper 1360, p.317-345. Wieczorek, G.F., R.C. Wilson, R.C., and E.L. Harp; 1985: Map Showing Slope Instability during Earthquakes in San Mateo County, California. U.S. Geological Survey Miscellaneous Investigations Series Map I-1257-E. Youd T.L., and Perkins D.M.; 1978: Mapping liquefaction-induced ground failure potential. �� ������������ J. Geotechnical Engineering Division, American Society of Civil Engineers, 104, 4, 433-446. Preliminary analysis of site effects in the Ischia Island: new insights from M D 4.0 earthquake of 21 August 2017 and seismic noise data L. Nardone, S. Petrosino, D. Galluzzo, R. Esposito, A. Tramelli, P. Cusano, E. Marotta, M. Di Vito, S. de Vita, A. Carandente, M. Orazi, A. Caputo, A. Bobbio, F. Bianco Istituto Nazionale di Geofisica e Vulcanologia, Sez. di Napoli, Osservatorio Vesuviano, Italy Introduction. On August 21, 2017, at 18:57 UTC, an earthquake of MD 4.0 occurred in Casamicciola, district of Ischia island. The damage caused by the earthquake was massive, with two victims and several buildings collapsed, and circumscribed to the areas of uptown Casamicciola, particularly in the Piazza Maio-La Rita area, and in a small area, called Fango, in Lacco Ameno. Medium and minor damages occurred in Piazza Bagni, in the area around the town hall of Casamicciola and in the Sentinella area (Zuccaro et al. , 2017). Even assuming the poor quality constructions and/or not in compliance with the anti-seismic regulations, such a level of damage has induced the scientific community to analyse the effects of local site amplifications, that usually are not negligible in volcanic areas (Galluzzo et al., 2009; Tramelli et al., 2010; Nardone and Maresca, 2011). As a matter of fact the seismic station IOCA (Fig.1), located very close to the high damage areas, recorded a peak ground acceleration (PGA) of 2.6 m/s 2 (red circle in Fig.3 top). The island of Ischia is the emerged portion of a large volcanic active complex, part of the Phlegraean Volcanic District, which also includes the island of Procida and the Campi Flegrei caldera (Orsi et al., 2003). Ischia is composed of volcanic rocks, epiclastic deposits and subordinate terrigenous sediments, reflecting a complex history of alternating constructive and destructive phases. The volcanic history of the island has been dominated by the large caldera-forming Mt. Epomeo Green Tuff (Brown et al., 2008) eruption occurred ca. 55 ka (Vezzoli, 1988). The caldera depression was the site of marine sedimentation, with generation of a sequence of tuffite, sandstone and siltstone (Fig. 1). The main morphostructural feature is the Mt. Epomeo resurgent block, delimited by two main trending fault systems (Acocella et al., 2004), that has an asymmetrical profile, with the northwestern flank considerably steeper than the southeastern one as a result of an asymmetrical uplift. The surface expression of the volcano-tectonic features includes the north-northwest and southwest-facing steep flanks of Mt. Epomeo that are the gravitationally least stable slopes of the island. Mass movement deposits are widespread on the island and were generated by rock falls, slides, toppling, debris flows and debris avalanches (de Vita et al., 2006). In the past the island was affected by 12 earthquakes (Rovida et al., 2016) with magnitude between ~3 and 4.3, the strongest occurred on July 28 1883. Common feature of all these earthquakes, largely concentrated in the north wester sector in the area between Casamicciola