This study presents and quantitatively analyzes a new dataset detailing areas affected by ballistic projectiles generated by major explosions and paroxysms at Stromboli. The dataset includes a total of 67 events over ≈150 years, based on an extensive review of historical, observational, and monitoring data. By quantifying distances, directions, and areas affected by ballistic fallout, we provide the necessary data, together with their uncertainty, to produce probabilistic maps of ballistic hazard as presented in a companion study. Our main findings include: (1) 23%-37% of major explosions do not exceed 500 m in their maximum ballistic distance, while 17%-42% of paroxysms remain under 1,500 m; however, 12%-14% of major explosions can exceed 1,000 m, and 29% of paroxysms extend over 2,000 m; (2) sector-based analysis of ballistic distance distribution produces probabilities up to three times lower compared to those based on the maximum distances; (3) directional analysis of ballistic dispersal shows a predominant direction towards the East half-plane (87%) for major explosions and towards the North half-plane (64%) for paroxysms; (4) the average affected area was 6.9e+04 m² for major explosions and 3.6e+05 m² for paroxysms with a mean amplitude of ≈90 degrees for both categories. Notably, major explosions and paroxysms show a continuous distribution of maximum ballistic distance and area affected, suggesting the absence of a net separation between these two categories with respect to this phenomenon. Results highlight the limited influence of uncertainty in reconstructing the dispersal areas and stress the importance of volcanological monitoring of Stromboli’s explosive phenomena

This study presents first probability hazard maps of the areas potentially affected by ballistic fallout from major explosions and paroxysms at Stromboli (Italy), based on mathematical analyses of the extensive historical and recent records of its explosive activity. The novel approach develops and integrates three statistical models that describe ballistic fallout patterns under different assumptions and considering the associated uncertainty. Model 1 mirrors the areas observed to be affected in the past, with major explosions displaying NE and SE asymmetries, while paroxysms show NE and mostly WSW predominant dispersal. To address likely data under-sampling and morphological and dynamics changes, Models 2 and 3 assume independency between ballistic distance and dispersal direction. By combining these models, robust and conservative ballistic fallout hazard maps are produced for major explosions and paroxysms, considered as separated categories, as well as for the two categories combined together by assuming a relative proportion. The new combined maps allow highlighting the most exposed areas of the island and quantifying the probability to be affected in case of event. For instance, the NE trails at 600 m are affected with ≈25% probability and the viewpoints at 400 and 290 m a.s.l. on Labronzo trail with ≈8% and 5% probability, respectively. Remarkably, the entire village of Ginostra is affected with ≈3% probability. Notably, ballistic fallout probability contours are moderately influenced by mapping uncertainties and by the assumed proportion between major explosions and paroxysms. These findings open the way to individual and societal risk assessments for this phenomenon at Stromboli.

The study focuses on the estimation and modeling of the temporal rates of major explosions and paroxysms at Stromboli volcano (also named small-scale and large-scale paroxysms respectively). The analysis was further motivated by the paroxysm of July 3rd 2019, which raised, once again, the attention of the scientific community and civil protection authorities on the volcanic hazards of Stromboli. In fact, at the present state of knowledge, major explosions and paroxysms cannot be forecasted based on monitoring data, and a full probabilistic assessment based on past eruption data would be quite useful for scientific and civil protection purposes. In the study we perform a time series analysis either considering the last ~150 years of reconstructed activity and the most recent 35 years. We included the estimation of event rates and rate changes in time. Results clearly highlight that the activity is non-homogeneus in time, with a significant low of activity between about 1960 and 1990. Maximum values of event rates were computed during the first half of last century, for both major explosions and paroxysms, whereas the rate of paroxysms is significantly lower in the last decades with respect to maximum rates. We also accomplish a statistical analysis of the inter-event times, enabling us to determine if the data can be modeled as a Poisson process or not, e.g. if it shows time dependent distributions, recurring cycles, or temporal clusters. The uncertainty quantification on the current and future rates is mainly related to the choice of the modeling assumptions. The study represents a crucial progress towards quantitative hazard and risk assessments at Stromboli, which is particularly relevant for the thousands of people (e.g. tourists, guides and volcanologists) that regularly climb the volcano every year.