Probabilistic hazard assessment of ballistic bombs from paroxysms and major explosions at Stromboli.

Abstract

On the basis of a critical review of field data related to the dispersal area of ballistic bombs from major explosions and paroxysms occurred at Stromboli, we propose a first probabilistic hazard map of the areas exposed to future events of this kind. First, a conditional probability map, i.e. under the assumption of having a major explosion or a paroxysm, was calculated on the base of a Monte Carlo simulation varying circular sectors exposed to the bombs. The sectors are assumed to have an uncertain radius, direction, and angular amplitude. Due to the limited number of explosions that are well-mapped, and of possible under-recording of the bombs distribution in some parts of the island, we also performed a sensitivity analysis assuming uniform directions, or a round-angle exposed area. Then, we combined a temporal model of major explosions and paroxysms based on the available record with these estimates. The result was a map of the absolute annual probability of being exposed to ballistic bombs. Finally, we also produced a preliminary estimate of the spatial density of bombs over square meter in a few specific locations, based on literature review, field work, and satellite imaging. The spatio-temporal probability maps can also be used to provide first quantitative estimates of the risk rates taken by guides, volcanologists, tourists and people living on the island year-long. The total time that each person spends inside the areas exposed to the bombs, over a year, defines the annual risk taken by the individual. We show that, depending on the memory properties of the temporal models, the risk levels are not constant and they can increase for some weeks after any major explosive event, if compared with their mean statistics.