GNGTS 2017 - 36° Convegno Nazionale

194 GNGTS 2017 S essione 1.3 However, apart from such specific investigations, a wider geophysical imaging of the island is still not available. Such knowledge could represent a crucial task to improve the understanding of volcano dynamic, to support the modeling of its physical state and to assess the related volcanic hazard. To this aim, the electrical resistivity mapping represent a well suited tool, being this physical parameter a reliable marker to the presence of anomalous structures buried in similar settings. This is confirmed by the relevant results obtained during the many applications of electrical and electromagnetic geophysical methods carried on in the last decade on both the volcanic and tectonic structures of the Campanian district (Troiano et al. , 2008, 2009, 2014; Di Giuseppe et al. , 2015). Here, the new findings of a magnetotelluric survey recently realized in the central-western sector of the island are presented. Through this survey, the electrical resistivity distribution has been reconstruct in correspondence of two separated profiles (deployed in the N-S and WSW-WNE directions, respectively). In such a way two resistivity sections, about 5km and 3km long respectively, have been obtained and interpreted in order to highlight the main geological features of the crust (0-3km in depth), such as its thermal state, fluid circulations and structural discontinuities along both the collapsed and uplifted areas. Discussion of results. Two main hot aquifers, whose existence was previously supposed by geochemical studies (Chiodini et al. , 2004; Di Napoli et al. , 2011), are definitely localized in the south-western sector and are dislocated at different depth, depending of the vertical tectonic which affected the resurgent block of Mt. Epomeo since at least 33Ka. The top of the deeper aquifers, feeding the main south-western fumaroles fields, are located at a depth of about 1000m to 1500m respectively, and deepen towards the sea. A quite good correlation, also according to literature data (Schön, 2015), has been observed between the known geological information (stratigraphy inferred from drilling data) and the variation of resistivity anomaly (shallower and altered tuff formations: 130 Ωm-1000 Ωm; deeper MEGT and tuffs:1000 Ωm-2000 Ωm; lavas: >2000 Ωm). The rocks resistivity at Ischia is reduced by the presence of saline fluids and high temperature. For this reason all the above resistivity values fall close to the lower limit of typical range of resistivity for volcanic rocks (Schön, 2015). The most important feature of the resistivity imaging is the high anomaly located above the resurgent structure of Mt. Epomeo, particularly evident along the N-S profile. Taking into account the previous geophysical, geological and geochemical studies of the island (Nunziata and Rapolla, 1987; Civetta et al. , 1991; Paoletti et al. , 2009; Sbrana et al. , 2009) and also considering the value of this resistivity anomaly, and its location and shape, we interpret it as the apical part of a crystalline (and very low permeable) igneous body intruded below the central part of the island (slightly dislocated towards south-west). The apex reached a depth of about 1km b.s.l. and the bulk body is bounded by abrupt resistivity drop corresponding to the faults nearby the resurgent block of Mt. Epomeo and to the change of lithology (from crystalline rocks to tuffs). Accordingly, we think that this body can be related to the laccolith of Ischia (Sbrana et al. , 2009; Carlino, 2012), which produced the bending and fracturing of the above crust and the partial further magma intrusion during the final stage of the resurgence. The finding of such a shallowmagma body is important in terms of volcanic risk assessment in the island. The volcanism of the island seems to be strictly correlated to the resurgence process that in turn is dependent on the dynamic of the laccolith. A renewal of resurgence can be associated to the reactivation of this very shallow magma body, with high potential volcanic risk for the island. On the other side, the present subsidence of Mt. Epomeo reflects a gradually depressurization of the magmatic and/or hydrothermal system beneath the Mount Epomeo (Sepe et al. , 2007; De Martino et al. , 2011) or a viscoelastic response of the brittle layer to the lithostatic loading of the resurgent structure (Castaldo et al. , 2017). At this process can possibly be associated the release of seismic energy on the norther sector of the island. Further geophysical investigations of this area are crucial to improve the knowledge of the crustal geology of the island, to assess its thermal state and rheology, and to evaluate the volume of magma located at shallow depth.