Long-Term Probabilistic Volcanic Hazard Assessment for Tephra Fallout from Neapolitan Volcanoes on Southern Italy

Somma-Vesuvius, Campi Flegrei, and Ischia are three active volcanoes in the Neapolitan Area (Italy). In this work, we evaluated the combined tephra fallout hazard posed by the three volcanoes on a large-size (~600 km × 700 km) and high-resolution (~3 km) domain. In order to explore the effect of the intrinsic variability in eruption and wind conditions on tephra hazard we used a probabilistic approach (probabilistic volcanic hazard assessment, PVHA). For Somma-Vesuvius and Campi Flegrei, we grouped the possible eruptive size classes into small-, medium- and large-scale eruptions, while for Ischia we considered only the large-scale scenario, as it is the only one that can affect the mainland. We created a synthetic dataset of ground loads by performing 1500 tephra dispersion simulations for each eruption size class of each volcano (for a total of 10,500 simulations) using the numerical model Fall3D. For each simulation, we randomly sampled the eruptive parameters from suitable probability density functions and meteorological conditions from the ERA5 reanalysis dataset on the period 1990–2020. The hazard evaluation has been performed using a Bayesian Event Tree (BET) approach accounting for the results of the simulations, the variability in vent location and the mean annual rates of eruption for each volcano and eruption size class. In this way we obtained a set of hazard maps for Southern Italy showing the threshold tephra load that would be exceeded with selected mean annual rates within a 50-year exposure time. We found that greater tephra load thresholds are exceeded in the south–south-eastern regions.