GNGTS 2018 - 37° Convegno Nazionale

56 GNGTS 2018 S essione 1.1 (ISIDe, 2016), in proximity of the maximum damage intensities observed. The calculated magnitudes of M L 3.6, Mw 3.8 and M D 4.0 seemed to be at odds with the high macroseismic intensity (hereafter reported as IEMS – the Intensity of the European Macroseismic Scale). The disagreement between the observed maximum macroseismic intensity and seismic energy seems to be related to the particular seismotectonics and geological situation of the volcanic island. This situation recalls what happened in 28 July 1883, when Ischia was struck by an earthquake that caused more than 2300 deaths and severe damage in the northern part of the island, near the small town of Casamicciola (Mercalli, 1884; Cubellis and Luongo, 1998; Carlino et al. , 2010). In this paper we analyze the 2017 Ischia earthquake, trying to account for the disagreement between the high maximum macroseismic intensity and the relatively low magnitude, and to clarify the contradictory information on the source parameters calculated. To this end, we relocate the hypocenter by a single-station method using an ad hoc velocity model and calculate moment magnitude and source mechanism by inversion of the full moment tensor. Geological Framework. Ischia island is a volcano, active over the last 150 ky (Vezzoli, 1988; Della Seta et al. , 2012). The widespread presence of volcanic rocks, epiclastic deposits, and subordinate terrigenous sediments reflects the complex sequence of alternating constructive and destructive phases of the volcanic edifice (Della Seta et al. , 2012). The main recent volcanotectonic event was the resurgence of the caldera started after the explosive eruption (55 ky B.P.) and deposition of the Mt. Epomeo Green Tuff (Acocella and Funiciello, 1999; Carlino, 2012). A maximum uplift of 900 m (Orsi et al. , 1991; Della Seta et al. , 2012) of the caldera floor is testified by the presence of marine sediments outcropping in the inner part of the island. The resurgent block has a polygonal shape and is bordered in its northern part by high-angle E-W and NE-SW-trending faults (Acocella and Funiciello, 1999; Molin et al., 2003). The uplift of the resurgent block seems to be connected to the intrusion of a magmatic body at shallow depths (2 km) below the surface and seems to be responsible for the gravimetric and geothermic anomalies observed in this area (Carlino, 2012; Capuano et al. , 2015). Cubellis and Luongo (1998) report that, due to the high geothermal gradient, the seismogenic volume (brittle regime) is confined in the upper 2–2.5 km of the crust. Hypocentral determination. The origin time and coordinates reported by the ISIDe (2016) for the 21 August 2017 seismic event are: 18:57:51.260 (UTC); Latitude 40.739° N; Longitude 13.903° E; and depth 1.7 km. Any attempt to relocate the hypocenter by using arrival times, did not provide consistent and stable solution, due to the uncertainty about the local velocity model, as well as the lower coverage of the seismic network in case of off-shore earthquakes for the lack of ocean-bottom seismometers, leading often to large azimuthal gaps in the location process. Therefore, we tried to improve the hypocenter location by analyzing in detail the seismogram from station IOCA, concerning travel times (S-minus-P travel time difference), particle motions and azimuthal provenance of spectral energy. To relocate the event with one 6-channel station (IOCA), we estimate the direction of seismic energy from the azimuthal distribution of spectral Fig. 1 - (a) Three-component accelerometric traces and the particle motion of the P wave in the (b) horizontal (north- east) and (c) vertical-radial (35°N) plane.