Assessing future vent opening locations at the Somma-Vesuvio volcanic complex: 2. Probability maps of the caldera for a future Plinian/sub-Plinian event with uncertainty quantification

In this study, we combine reconstructions of volcanological data sets and inputs from a
structured expert judgment to produce a first long-term probability map for vent opening location for the
next Plinian or sub-Plinian eruption of Somma-Vesuvio. In the past, the volcano has exhibited significant
spatial variability in vent location; this can exert a significant control on where hazards materialize
(particularly of pyroclastic density currents). The new vent opening probability mapping has been
performed through (i) development of spatial probability density maps with Gaussian kernel functions for
different data sets and (ii) weighted linear combination of these spatial density maps. The epistemic
uncertainties affecting these data sets were quantified explicitly with expert judgments and implemented
following a doubly stochastic approach. Various elicitation pooling metrics and subgroupings of experts and
target questions were tested to evaluate the robustness of outcomes. Our findings indicate that (a)
Somma-Vesuvio vent opening probabilities are distributed inside the whole caldera, with a peak
corresponding to the area of the present crater, but with more than 50% probability that the next vent could
open elsewhere within the caldera; (b) there is a mean probability of about 30% that the next vent will open
west of the present edifice; (c) there is a mean probability of about 9.5% that the next medium-large
eruption will enlarge the present Somma-Vesuvio caldera, and (d) there is a nonnegligible probability (mean
value of 6–10%) that the next Plinian or sub-Plinian eruption will have its initial vent opening outside the
present Somma-Vesuvio caldera.