The continental slopes off the Ischia island (Naples Bay): submarine gravity instability processes investigated by means of marine geological and geophysical data

The continental slope offshore the Campania region (Southern Tyrrhenian sea, Italy) represents a natural laboratory to study geological events and processes related to submarine gravity instabilities in deep waters, as a base to evaluate geological and environmental hazards triggered by earthquakes, volcanic eruptions and tectonic activity in correspondence to regional faults. The identification of submarine instabilities on continental slopes shows important implications in terms of applied research for the coastal zone knowledge and management, also in terms of geological and environmental hazard. This study is carried out by using marine geophysical data collected by the CNR-IAMC Institute of Naples, Italy, in particular Multibeam bathymetry and reflection seismics (Sparker Multitip seismic source). The Ischia island lies on a volcanic ridge showing a mainly E-W trending elongment. In the western offshore of the island a strong field of magnetic anomalies suggests the occurrence of a magmatic system, now inactive. Two main structural trends exist in the E-W trending volcanic ridge of the Ischia island: one E-W trending and another ENE-WSW trending, recognised especially in the western offshore of the island. At a regional scale the comparison between the distribution of magnetic sources and the morpho-structures indicate a poor correlation for the E-W trending morphostructures and a high correlation for the ENE-WSW (Bruno et al. , 2002; Passaro, 2005). Main work methodologies and steps include the acquisition and the processing of Multibeam bathymetry according to the IHO standards (IHO, 1998), the generation of a marine DEM (Digital Elevation Model), its morpho-structural geological interpretation and integrated interpretation of bathymetry and reflection seismics. A Digital Elevation Model (DEM) of the Ischia island based on Multibeam bathymetric survey and integrated to onshore topography is shown in fig. 1. The geological interpretation of the marine DEM of the island allows the identification of the main morpho-structural features of the sea bottom. Spectacular features characterize the continental slope off the south-western Ischia island, incised by a dense network of canyons and tributary channels, starting from a retreating shelf break, parallel to the coastline and located at varying depths. Large scars characterize the platform margin off the south-western Ischia island, in particular the scar of the southern flank of the island, corresponding onshore to the M.te Epomeo block and originating the Ischia Debris Avalanche (IDA; Chiocci and de Alteriis, 2006). Volcanic banks having irregular morphologies are identifiable in the south-western flanks of the island, as the "Banco di Capo Grosso", a complex morphostructural high located on the southern continental slope. The marine DEM shows the relic morphology of the "Banco di Ischia", a wide terraced volcanic bank located on the south-eastern flank of the island. Moreover, several volcanic highs are disposed along two main ridges. The first ridge, NE-SW trending, is about 10 kilometres and includes several morphological highs located in the south-western Ischia offshore at water depths ranging from - 800 m and - 100 m ("Banco Rittmann", "Banco G. Buchner", "Banco P. Buchner", "Banco di Forio"). The top of the Banco di Forio tuff cone occurs at water depths of about - 30 m. The second ridge, E-W trending, starts in the north-western Ischia offshore at water depths of about - 600 m (from the "Banco Pithecusa" and the "Banco Mazzella") and continues up to the coastal sectors of the island. The submerged sectors of the Ischia island are the site of submarine gravity instability processes, having catastrophic (instantaneous) characters (debris avalanches) or continuous cha