Quantifying ballistic projectile hazards and risks due to paroxysms and major explosions at Stromboli (Italy)

Major explosions and paroxysms, respectively, have been the most powerful explosive phenomena at Stromboli in recent centuries. These two categories of explosions, although not sharply separable in terms of eruptive mechanisms and hazards, can produce ballistic projectiles affecting trails and observation sites in the summit area (both major explosions and paroxysms) as well as lower elevation areas of the volcano, down to the coast (paroxysms only). Time series analysis of reconstructed activity since the end of the XIX^th Century highlights that such unordinary explosions are strongly non-homogeneous in time and often show notable temporal clustering. We perform a critical review of the volcanic catalogs produced by the Italian volcanological observatories in the last ~40 years. In this review, we evaluate the effect of uncertainties on the characterization of such major explosions, in contrast to intense ‘ordinary’ Strombolian explosions that do not eject large ballistic projectiles outside the Craters Terrace and the upper portion of Sciara del Fuoco. Where sufficient information is available for major explosions, we devise an analytical summary and explore comparative mapping of field data related to the dispersal areas of ballistic projectiles, taking into account relevant uncertainties. Using Monte Carlo simulations, we propose preliminary probabilistic hazard maps for areas potentially exposed to future events of this kind, varying the radius and angle-size of the circular sectors affected. We also evaluate lateral hazard modulation in terms of the density variability of ballistic projectiles per square meter of ground, based on literature review and spatial statistics of newly collected UAV data from the ballistic deposits of the 3^rd July 2019 paroxysm on the slopes above Ginostra village. These new hazard maps, once combined with vulnerability and exposure data, allow preliminary quantitative estimates of individual risk exposure levels for guides, volcanologists, and tourists spending time in areas exposed to these unordinary events. Through a retrospective counterfactual analysis of the July 2019 eruption, we demonstrate how, in a future Strombolian paroxysm at another time of day, these risk rates might result in major casualty numbers.