Submerged hummocky topographies and relations with lanslides. Northwestern flank of Ischia island, southern Italy

High resolution sidescan sonar and multibeam surveys carried out offshore northwestern sector of the Ischia volcanic island revealed a complex seafloor topography controlled by hummocky features. This peculiar morphology results from irregular mounds and ridges, often organized as composite structures, characterizing landslide deposits of catastrophic events. In the study area submerged hummocky topography well correlate with the extensive slope instability affecting the northwestern border of the Mt. Epomeo, the main volcanic structure of the island, corresponding to a resurgent dome uplifted at least of 800 m in the last 33 Ka. Mostly of these landslide accumulations are related to historical events, but their stratigraphic positions within the post-resurgence volcanic units testify an older slope instability, probably coeval and recurrent with respect to resurgence. Erosional scarps and sedimentary structures characterizing the northern offshore suggest the occurrence of recent depositional events, while in the western offshore hummocky deposits result partially covered by prograding depositional prism. In both cases the morphologic features point to a recurrence of significant landslide events with catastrophic implications. Historical seismicity has been recognized as a main triggering factor for slope instabilities along the northern border of Mt. Epomeo. Macroseismic analyses suggest a restricted epicentral area coincident with such a border and controlled by a principal seismo-tectonic structure ENE-WSW oriented. This situation well conforms with the recent individuation of the submerged hummocky topography found offshore Lacco Ameno village, hypothesized on the base of bathy-morphologic analysis.