Prime stime mediante simulazioni numeriche delle velocita' di lancio di ejecta allo stromboli e valutazioni sul rischio vulcanico indotto
The typical strombolian activity of Stromboli volcano ranges from a nearly continuous venting of ash and gases (normal strombolian activity) to moderate, up to violent explosive pulses (violent strombolian activity). Moreover, both regimes are sometimes punctuated without warning by phases of scaled up, distinct and more energetic huge bursts separated by time intervals ranging from minutes to hours (paroxysmal activity). As this eruptive style causes the blasting from the summit vents of hot fragments of new lava and blocks with different velocities, densities and impact sites, the main volcanic risk at Stromboli is related to the hazard due to ballistic showers of these ejecta on the built-up areas of the island (Stromboli and Ginostra villages). For that reason i) a land use map of the island has been produced and ii) a number of numerical simulations of ballistic trajectories for to constrain the velocity ranges of the three different explosive regimes has been performed with the “Eject!” software (L.G. Mastin, 2001) by using the average density of dense blocks (2800 kg/m3) and juvenile bombs (1970 kg/m3) from a set of 44 ejecta that we collected on the flanks of the volcano. The reliable ranges of initial velocity at the takeoff point (summit vents) that we obtained are: 40 m/s ÷ 60 m/s for normal strombolian activity, 70 m/s ÷ 100 m/s for violent strombolian activity and 120 m/s ÷ 200 m/s for paroxysmal strombolian activity. Without wind, the cross-correlation of the land use - velocity data indicates that both for the villages of Stromboli and Ginostra an ejecta impact probability is realistic only in paroxysmal activity periods, while it’s definitively absent during normal and violent strombolian regimes. Following tail-winds = 25 m/s simulations (wind velocity sometimes observed at the Stromboli island), the resulting hazard increases with respect to the tail-winds = 0 m/s simulations for both Stromboli and Ginostra during the paroxysmal regime, and become appreciable for the Ginostra village even during violent strombolian activity.