Probabilistic Hazard From Pyroclastic Density Currents in the Neapolitan Area (Southern Italy)

The metropolitan area of Napoli ( ∼ 3 M inhabitants) in southern Italy is located in between
two explosive active volcanoes: Somma-Vesuvius and Campi Flegrei. Pyroclastic density currents (PDCs)
from these volcanoes may reach the city center, as witnessed by the Late Quaternary stratigraphic record.
Here we compute a novel multivolcano Probabilistic Volcanic Hazard Assessment of PDCs, in the next
50 years, by combining the probability of PDC invasion from each volcano (assuming that they erupt
independently) over the city of Napoli and its surroundings. We model PDC invasion with the energy cone
model accounting for flows of very different (but realistic) mobility and use the Bayesian Event Tree for
Volcanic Hazard to incorporate other volcano-specific information such as the probability of eruption
or the spatial variability in vent opening probability. Worthy of note, the method provides a complete
description of Probabilistic Volcanic Hazard Assessment, that is, it yields percentile maps displaying the
epistemic uncertainty associated with our best (aleatory) hazard estimation. Since the probability density
functions of the model parameters of the energy cone are unknown, we propose an ensemble of different
hazard assessments based on different assumptions on such probability density functions. The ensemble
merges two plausible distributions for the collapse height, reflecting a source of epistemic (specifically,
parametric) uncertainty. We also apply a novel quantification for a spatially varying epistemic uncertainty
associated to PDC simulations.