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- Assessment of volcanic hazards from explosive eruptions on islands: the case of São Miguel (Azores)Publication . Aguiar, Simone Correia; Pacheco, José Manuel Rodrigues; Pimentel, Adriano Henrique Gonçalves; Sandri, LauraABSTRACT: Volcanic islands face unique challenges when impacted by explosive eruptions due to their geographic isolation, small land area, typical rough topography, and proximity of populated areas and infrastructure to volcanic centres. The stratigraphic record of island volcanoes is often incomplete or poorly preserved, generating large uncertainties regarding their past eruptive behaviour, limiting eruption forecasting. São Miguel Island in the Azores Archipelago is one such case, hosting three active central volcanoes (Sete Cidades, Fogo, and Furnas) that have produced a wide variety of eruptions in the last millennia, ranging from highly explosive trachytic events to basaltic flank eruptions. This work presents, for the first time in the Azores, a long-term Probabilistic Volcanic Hazard Assessment (PVHA) using São Miguel Island as a case study. The development of this type of analysis requires the integration of different sources of information, including the probability of future eruptions, the probability of opening new vents, and probabilistic numerical simulations of tephra fallout for different eruption scenarios, which are combined into Bayesian Event Trees to generate probability maps, hazard maps, and hazard curves. Temporal analysis was performed first to estimate inter-event times (IET), i.e., the time intervals between successive eruptive events, using different statistical distributions, and to compute the probabilities of an eruption in the next 50 years for each central volcano. This was achieved by implementing an innovative methodology that focuses on the generation of synthetic catalogues by randomly assigning stratigraphic-consistent ages and accounting for age uncertainties of past events. Catalogues of basaltic (s.l.) and trachytic (s.l.) events (BT catalogues), and catalogues of trachytic (s.l.) events (T catalogues) were generated for each of the three central volcanoes. The results revealed that Sete Cidades data are non-stationary, suggesting an incomplete eruptive record or changes in eruption frequency over time. The mean IET were shorter for Sete Cidades Volcano (154 yr and 205 yr for BT and T catalogues, respectively) than for Fogo (546 yr for BT catalogues and 909 yr for T catalogues) and Furnas volcanoes (318 yr for T catalogues). The mean probabilities of an eruption within the next 50 years are higher for Sete Cidades (21% for BT and 17% for T catalogues), followed by Furnas (13% for T catalogues), and Fogo (8% for BT and 5% for T catalogues). Spatial analysis was performed separately for each central volcano to identify areas with a higher probability of hosting future eruptive centres. This was accomplished by implementing the kernel method and using the location of past vents, either central vents or eruptive fissures. Different datasets were considered for this analysis: basaltic (s.l.) and trachytic (s.l.) centres together, including fissures and central vents (BT datasets), trachytic (s.l.) vents of explosive and effusive events (T datasets), and only explosive trachytic (s.l.) vents (ET datasets). Different kernel functions (Gaussian, Cauchy, Exponential, and Uniform) were explored and the best degree of clustering of eruptive centres (given by a smoothing parameter h) was determined for each dataset of each volcano. Vent opening probability maps (susceptibility maps) were computed by combining the pair (kernel function and smoothing parameter) that best described the empirical cumulative distribution function of the distance between eruptive centres. The results show that for the Sete Cidades BT dataset, the most likely locations to host future vents are located on the southeast flank (30%), the caldera (17%), and on the southwest flank (8.7%). However, when considering future trachytic (s.l.) vents, higher probabilities were obtained for the caldera (51%) and the southwest sector (28%). Explosive trachytic (s.l.) eruptions are more likely to occur only inside the caldera (83%). On what concerns Fogo Volcano, the most likely locations to host future eruptive centres (BT dataset) include the west-northwest flank (29%), the caldera (9.6%), and the southwest flank (6%). Regarding future trachytic (s.l.) events, the most probable locations to host new vents are the west-northwest flank (25%), the caldera (17%), and the north-northeast flank (15%). Explosive trachytic (s.l.) eruptions are more likely to occur inside the caldera (29%) and on the north flank (9.3%). For Furnas Volcano, the most probable locations to host future eruptive centres (BT dataset) are inside the caldera complex (34%), on the northern flank (31%), and on the eastern flank (15%). Regarding the opening of future trachytic (s.l.) vents, the most likely locations are the interior of the caldera complex (63%) and on the north flank (21%). In the case of explosive trachytic (s.l.) eruptions, future vents are highly probable inside the caldera complex (91%). A long-term PVHA framework, rooted on Bayesian inference, was applied to each central volcano, focusing on explosive trachytic (s.l.) eruptions to evaluate the impact of tephra load on São Miguel Island over the next 50 years (absolute probabilities) and conditional to the occurrence of a future VEI 4 eruption (conditional probabilities). The computed absolute probability maps indicate a higher probability of exceeding lower tephra load thresholds (e.g. 1 and 10 kg/m2) in the next 50 years compared to higher accumulations of tephra (e.g. 400 kg/m2), with Sete Cidades and Furnas volcanoes being the main contributors to exceeding these tephra loads. The obtained hazard maps indicate that tephra loads between 200 kg/m2 and 300 kg/m2 and between 50 kg/m2 and 200 kg/m2 (thicknesses of 20 to 30 cm and 5 to 20 cm) are expected at locations close to Sete Cidades and Furnas, respectively, suggesting that these volcanoes represent a greater threat to local populations over the next 50 years. The hazard curves computed for critical infrastructures such as the Ponta Delgada airport, Divino Espírito Santo hospital, and Pico Vermelho geothermal powerplant reveal that Sete Cidades Volcano represents the greatest threat to the airport and the hospital, while Fogo Volcano has the greatest impact on the geothermal powerplant within the next 50 years. Combining the results of the PVHA for each volcano revealed that the area between Relva and Caloura parishes, in the southern part of São Miguel, and the area of Nordeste parish, are less vulnerable to tephra fallout in the next 50 years.