Offshore debris avalanches at Somma-Vesuvius volcano (Italy): Implications for hazard evaluation.

Somma-Vesuvius is an active volcano with a breached crater close to the continental shelf of Naples Bay, Italy. The interpretation of high-resolution seismic profiles offshore of Somma-Vesuvius documents interlayered volcanic and marine units in the Late Quaternary succession. In particular, the detailed seismo-stratigraphic analysis permitted, for the first time, the identification and mapping of two thick debris avalanche deposits located on the continental shelf. With a volume of 2.9 km3, the older debris avalanche, linked to the 18 ka-old Pomici di Base plinian eruption, was emplaced in a subaerial environment and then overlain by the sedimentary deposits of the lowstand prograding wedge. The younger debris avalanche, linked to the 3.4 ka-old Avellino plinian eruption, has a volume of approximately 1 km3 and is interlayered within the marine sediments deposited during the sea level highstand. The identification of two debris avalanches originating from Somma-Vesuvius has two important implications: i) the occurrence of two sector collapses in the history of Somma-Vesuvius volcano; ii) a potential volcano slope failure and consequent tsunamis produced by the entry of a debris avalanche into Naples Bay should be considered in the volcanic hazard evaluation of this densely populated area.