The Case of Ischia Underwater Debris Avalanche (Italy, Tyrrhenian Sea) and Its High Mobility

Ischia volcanic island is one of the few cases in the Mediterranean where a clear relationship between constructional volcanism, volcano-tectonic uplift and cyclic multi-scale gravity failures is evident. One of these collapses involved the island’s southern flank and led to the emplacement of a submarine debris avalanche spreading over an area of 150 km2 with an estimated volume of 1.5 km3. C14 stratigraphy suggests that it may have occurred during pre-historical times or even during the Greek colonisation of the island (around 7th century BC). This very large mass movement i.e. the “Ischia debris avalanche” is still apparent on the seafloor with a mega-blocks field detectable until 40 km from the island along the lower continental slope. Its high mobility is proved by the very low H/L ratio when compared to other submarine debris avalanches. Here we report the preliminary results of a back-analysis in which we modeled a land-sea collapse based on some simplified assumptions. The implemented code used in this study assumes a fluid-solid mixture with given rheological behaviors and was already used for simulate the mobility of some terrestrial debris flows in volcanic areas.