Quantifying volcanic hazard at Campi Flegrei caldera (Italy) with uncertainty assessment: 2. Pyroclastic density current invasion maps

Campi Flegrei (CF) is an example of an active caldera containing densely populated settlements
at very high risk of pyroclastic density currents (PDCs). We present here an innovative method for assessing
background spatial PDC hazard in a caldera setting with probabilistic invasion maps conditional on the
occurrence of an explosive event. The method encompasses the probabilistic assessment of potential vent
opening positions, derived in the companion paper, combined with inferences about the spatial density
distribution of PDC invasion areas from a simplified flow model, informed by reconstruction of deposits from
eruptions in the last 15 ka. The flow model describes the PDC kinematics and accounts for main effects of
topography on flow propagation. Structured expert elicitation is used to incorporate certain sources of
epistemic uncertainty, and a Monte Carlo approach is adopted to produce a set of probabilistic hazard maps for
the whole CF area. Our findings show that, in case of eruption, almost the entire caldera is exposed to invasion
with a mean probability of at least 5%, with peaks greater than 50% in some central areas. Some areas outside
the caldera are also exposed to this danger, with mean probabilities of invasion of the order of 5–10%. Our
analysis suggests that these probability estimates have location-specific uncertainties which can be substantial.
The results prove to be robust with respect to alternative elicitation models and allow the influence on hazard
mapping of different sources of uncertainty, and of theoretical and numerical assumptions, to be quantified.