Volcanic hazard and risk assessment from pyroclastic flows at Ischia island (Southern Italy).

The areas exposed to pyroclastic flow invasion in case of explosive eruptions at Ischia island (Italy) have been evaluated, together with the related volcanic hazard and risk. Ischia island lacks a central vent so, for the evaluation of volcanic hazard, it is necessary to forecast where a future explosive event might occur. At this purpose, we have evaluated the Vent Opening Probability by taking into account all the relevant geological information and processing them through spatial analysis. The whole Ischia island has been subdivided into 358 cells, in which we take into account the contribution of volcanological, geophysical and geochemical data to define the probability of opening of the vents. As a result, the highest spatial probability is found on the eastern sector of the island. Two series of simulations, by using the “energy cone” model [Malin, M.C., Sheridan, M.F., 1982. Computer-assisted mapping of pyroclastic surges. Science 217, 637–640] and assuming column collapse heights of 100 m, for eruptions of Volcanic Explosivity Index (VEI)=3, and of 300 m for eruptions of VEI=4, with Heim coefficient equal to 0.1, were performed. Through the overlay of the energy cone to the Digital Elevation Model of Ischia, it has been possible to automatically detect the invaded areas for each hypothesized vent. The intersection of this areas made it possible to define the different spatial probabilities of invasion by pyroclastic flow (PyF) for each point of the Ischia territory. Two different types of hazard maps have been derived: an equi-probability hazard map, sensitive to the distance from the vents and the presence of topographic obstacles, and a weighted hazard map, in which the probability of vent opening has also been taken into account. For both types of eruptions, the highest spatial probability of invasion are the so-called “graben of Ischia”, and the coastal plain between Monte Vico and Ischia Porto. Finally, by intersecting the exposed value map with the weighted hazard maps, the classification of the Ischia territory into three risk classes has been performed. The villages located in the Ischia graben and in the northern sector of the island have resulted to be exposed to the highest risk.