GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.2 - POSTER GNGTS 2023 formation of a polyphase caldera. Uplift of the very thick sequence (600-700 m thick) of the so-called Monte Epomeo resurgent block started probably shortly after the caldera's formation (60-50 ka). The sequence of tuffs forming Mt. Epomeo shows a pervasive hydrothermal high-T alteration. Post-resurgence volcanism is very active, with eruptive centres mainly located on the edges of the normal faults bordering the resurgent block. The Holocene volcanism appears mainly on the island's northwest, north and east sides. This intense volcanic phase is accompanied by a further uplift of the resurgent block of Mount Epomeo, inducing its sector collapse in the western (Forio), northern (Casamicciola) and southern (Maronti) sides and the formation of debris avalanches extending in the coastal and marine areas.  Numerous structural studies of Ischia volcano have highlighted that the most relevant tectonic features (seismogenic faults and fault scarps) are located in the northern region of the island (Acocella and Funiciello, 1999; Sbrana et al., 2009; Carlino et al., 2010; Sbrana and Toccaceli, 2011;De Novellis et al., 2018; Nappi et al., 2018). The main EW, NW–SE, and NE–SW fault systems developed during the early stages of resurgence. Subsidence has occurred, at least in the northern region of the island, possibly since Roman times (Buchner, 1986) and has been recorded by InSAR, GPS, and levelling data for the last ~30 years at a rate of ~1 cm/yr (Manzo et al., 2006; De Martino et al., 2011). The island is also characterized by the circulation of hydrothermal fluids linked to its volcano-tectonic setting, which favours the raising of fluids through pathways linked to high permeability zones such as at fractures and faults in welded tuffs and lavas and within the pores of unconsolidated pyroclastic deposits (Celico et al. 1999; Carlino et al. 2014). Seismicity at Ischia has occurred recently in the northern region of the island near the town of Casamicciola, at a very shallow depth, as indicated by historical records and recent seismological data (Carlino et al., 2010; De Novellis et al., 2018).  Geoelectrical survey ERT prospections consisted of surveys performed along seven profiles, ranging from 350 m to 710 m in length (Fig.1). The geoelectrical measurements were realized using a 120-channels Syscal Pro georesistivitymeter produced by Iris Instruments © . A dipole-dipole configuration with a 10 m electrodic distance was adopted, which provided a relative depth of investigation, ranging from 90 m to 140 m below ground level (b.g.l.). ERT data inversion was performed by Res2Dinv software, produced by Geotomo © (Loke and Barker, 1996; Loke et al., 2003) and based on a smoothness-constrained least-squares method which allows obtaining two-dimensional resistivity sections through finite differences computations, also considering the topographic corrections. The root means square error (RMS) was considered to evaluate the reliability of the obtained resistivity sections, which provides the percentage difference between measured and calculated data. RMS errors of less than 10% were generally attained. Discussion and conclusions