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

Abstract

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.

Towards Quantitative Volcanic Risk of Pyroclastic Density Currents: Probabilistic Hazard Curves and Maps Around Somma‐Vesuvius (Italy)

“…For dense‐PDC volume ( V 0 ) and bed friction angle ( ϕ bed ), however, we explore and quantify the aleatory uncertainty that may be expected from column‐collapse, dense PDCs at Somma‐Vesuvius (see Table ; Cioni et al, ; Gurioli et al, , and references therein). To this end, we build probability density functions (PDFs) for these two model parameters, in particular, considering three different eruption sizes (e.g., Sandri et al, , ; Tierz, Sandri, Costa, Sulpizio, et al, ; Tierz, Sandri, Costa, Zaccarelli, et al, ).…”

“…This allows computation of statistical moments (e.g., the mean) or full distributions for these model outputs. Monte Carlo methods are still a popular choice to achieve this goal but they suffer from slow convergence rates, so precise uncertainty assessment requires many simulations of the PDC model, something that is only achievable if very simple models are used (e.g., Bevilacqua et al, ; Neri et al, ; Sandri et al, ; Tierz, Sandri, Costa, Sulpizio, et al, ). An alternative approach to Monte Carlo, when the PDC model is more computationally expensive, is PCQ (e.g., Dalbey et al, ), a nonintrusive polynomial chaos expansion method.…”

“…Probability maps display the probability of exceedance of a given threshold of the output variable over the hazard domain. Such maps can also simply display the probability of arrival of the hazardous phenomenon (e.g., Bevilacqua et al, ; Neri et al, ; Sandri et al, ; Tierz, Sandri, Costa, Sulpizio, et al, ; Tierz, Sandri, Costa, Zaccarelli, et al, ). They are derived by cutting each hazard curve vertically, at a given threshold of the intensity measure.…”

“…However, they are becoming more customary over recent years. The majority of these hazard assessments have quantified the probability of PDC invasion around the target volcano without considering other hazard intensity measures such as the flow depth or speed (e.g., Bevilacqua et al, ; Neri et al, ; Sandri et al, , , ; Tierz, Sandri, Costa, Sulpizio, et al, ; Tierz, Sandri, Costa, Zaccarelli, et al, ). Some studies that have quantified uncertainty in relation with intensity measures have mostly focused on dividing the PDC model parameter space into regions that do or do not lead to a catastrophe (defined as a given flow depth being overcome) occurring at selected locations (e.g., Bayarri et al, ; Spiller et al, ).…”

“…These risk analyses have been entirely based on single‐scenario (or single‐eruption) hazard assessments. However, modern hazard analyses should give quantitative answers to the following questions: (i) how likely the given scenario is (e.g., Marzocchi et al, ) and (ii) the likelihood of different locations around the volcano being invaded by PDCs (e.g., Sandri et al, ; Tierz, Sandri, Costa, Sulpizio, et al, ; Tierz, Sandri, Costa, Zaccarelli, et al, ). Such quantitative hazard assessments can then be coupled with vulnerability and exposure analyses to quantify volcanic risk in an accurate and complete manner (e.g., Marzocchi & Woo, ).…”