GNGTS 2013 - Atti del 32° Convegno Nazionale

seismic profiles. Detailed piston coring and tephrostratigraphy suggested that the volcano- tectonic collapse originating the avalanche occurred during prehistorical times (Chiocci and de Alteriis, 2006). A stratigraphic framework for the last 23 ky marine record in the southern Ischia offshore has been recently constructed based on AMS 14 C dating and tephrostratigraphic analysis (de Alteriis et al. , 2010). Data and methods. The Multibeam bathymetry here presented was recorded from September to November 2001, using a Reson Seabat 8111 Multibeam system, which works properly in the 50-600 m depth range, onboard the Thetis R/V (Aiello et al. , 2012a). The Multibeam system, interfaced with a Differential Global Positioning System (DGPS) was composed of a ping source of 100 kHz, 150° for the whole opening of the transmitted pulse and a 101 beams receiver, with a beam opening of 1.5°. Sound velocity profiles (CTD) were regularly recorded and applied every 8 hours. The data were processed using the PDS2000 software (Reson-Thales), according to the IHO standards (IHO, 1998), with a real time acquisition control and partial beam exclusion filtering. Offline swath editing and despiking were also carried out. The DTM generation and rendering of the whole dataset were reorganized in a MXN matrix having a grid cell of 20x20m. The meaning of morphometric indicators is explained, necessary for a correct comprehen- sion of the data presented in this paper (Figs. 1 and 2). The elevation versus slope plot was used as a morphometric tool for an objective description of the DTM. The elevation distribution provides a standard tool to highlight the relative elevation distribution of topography and ba- thymetry, thus allowing to describe or compare different DEM’s (Passaro et al. , 2010, 2011). Seismic acquisition was carried out using a multielectrode sparker system (SAM96 model). Its technical characteristics include shorter pulse lengths for an equivalent energy discharge, as well as an increase in peak pressure. The seismic sections were recorded graphically on continuous paper sheets with a vertical recording scale of 0.25 s. The best vertical resolution was approximately 1 m for the sparker data. The seismic grid surrounding the Ischia island facilitated the stratigraphic correlations between the seismic sections and revealed structural and stratigraphic variations along the seismic lines. The morpho-depositional environments of Ischia island. Marine geological survey and morpho-bathymetric analyses carried out in the frame of the CARG research project (Ispra, 2010; Geological map n. 464 “Isola d’Ischia”) have evidenced in the Ischia island such a geomorphological characters allowing to distinguish different physiographic and depositional environments. A strong control of the lithostratigraphic and structural setting on the sea bottom evolution and on the present-day or inherited morphological processes can be singled out. A first control on the physiography, structure and evolution of the sea bottoms is furnished by the coastal types which characterize the Ischia littorals, characterized by both coastal cliffs (high relief coasts) and beaches (low relief coasts). The morpho-depositional environment of low relief coasts individuates the sectors of littoral environment mainly characterized by the occurrence of coastal plains joining onshore through reliefs having an average to low energy, more or less close to the coast. In this way, coastal cliffs do not realize and the submerged beach and adjacent continental shelf may be considered as the physical prolongation along a homogeneous and continuous profile. In some cases the continental shelf is characterized by the occurrence of detritic deposits having a chaotic internal structure and a hummocky topography, articulating on the sea bottom both with a longitudinal and transversal trending (Ispra, 2010). The morpho-depositional environment of high relief coasts includes the sectors of littoral characterized by slopes directly joining the sea with high steepness, not depending from the lithology of the units forming the slopes themselves. This condition, mainly morphometric, of the sector of the emerged coastal slope (emerged coastal cliff) is connected to a sector of submerged slope (submerged coastal cliff), developing with similar characteristics up to 20-30 m of water depth. 227 GNGTS 2013 S essione 1.3