Probabilistic hazard assessment of pyroclastic avalanches at Mt. Etna volcano through numerical modeling

One of the most hazardous phenomena which characterizes the summit activity at Mt. Etna (Italy) is represented by pyroclastic avalanches, gravity-driven flows of pyroclastic material at high particle concentration, characterized by modest volumes (usually lower than a few millions of cubic metres) and small thickness-to-length ratio. The frequency of pyroclastic avalanches at Mt. Etna has increased during the recent 2020-2022 volcanic activity, where a series of intense paroxysmal eruptions took place at the South East Crater (SEC). The accumulation of proximal deposits generated by the explosive activity led to the growth of SEC, which posed favorable conditions in triggering partial collapses of unstable flanks of the crater. Pyroclastic avalanches propagated mainly eastward and southward of the SEC, up to distances of about 2 km from the source.Numerical modeling of pyroclastic avalanche propagation and emplacement constitutes a powerful tool for hazard assessment, despite several difficulties in simulating the rheology of the polydisperse granular mixture. In this work, pyroclastic avalanches are simulated using the open-source code IMEX-SfloW2D. Depth-averaged equations are implemented in the code to model the granular flow as an incompressible, single-phase granular fluid, described by the Voellmy–Salm rheology. Comparison of numerical results with the well-documented pyroclastic avalanches occurred on 11 February 2014 and during the 10 February 2022 paroxysmal eruption (one of the most intense of the 2020-2022 series) allowed us to investigate the variability of the avalanche dynamics with its volume, the influence of the three-dimensional volcano morphology, and to statistically calibrate the unknown rheological parameters (i.e., the dry-friction coefficient µ and viscous-turbulent friction coefficient ξ). Finally, we provide a preliminary quantification of new potential collapse scenarios, to assess pyroclastic avalanche probabilistic hazard on the summit area, one of the most preferential tourists destination at Mt. Etna.