The last 5000 years of activity at Sete Cidades volcano (São Miguel Island, Azores): Implications for hazard assessment

Abstract

Sete Cidades is a central volcano with a summit caldera at the western end of São Miguel Island, Azores. Its stratigraphy comprises two main geological groups: the Inferior Group, the units of which date from more than 200 000 years ago through to 36 000 years before present, consisting of thick lava flows and subaerial volcaniclastic deposits that built the base of the central volcano; and the Superior Group which comprises all the activity from the last 36 000 years, including pumice and scoria fallout and PDC deposits with minor lava flows. The volcanostratigraphy is divided into six main formations — Risco, Ajuda, Bretanha, Lombas, Santa Bárbara and Lagoas, each defined by different activity phases in the volcano's evolution. The present caldera developed in three phases associated with massive paroxysmal eruptions which occurred approximately 36 000, 29 000 and 16 000 years before present. Since the last caldera forming event, eruptive activity has been predominantly explosive and a variety of deposits have been produced by different eruptive styles, associated with three specific settings: (1) pumice fall deposits, pyroclastic density currents and hydromagmatic events from eruptions inside the caldera; (2) lava domes, cinder cones and basaltic lava flows from eruptions on the volcano flanks; (3) tuff ring and tuff deposits from surtseyan eruptions offshore. The more recent history of the volcano is marked by a change in the intracaldera activity from dominantly magmatic behaviour to a hydromagmatic character, which happened about 5000 years ago. Since then, at least 15 basaltic subaerial and submarine eruptions have occurred on the volcano flanks and 17 trachytic (s.l.) explosive eruptions have occurred within the caldera, the most recent of which took place about 700 years ago. This eruptive frequency makes Sete Cidades probably the most active volcanic centre in the Azores. In this paper we present the stratigraphy and a description of the deposits which originate from the last 5000 years, in order to delineate Sete Cidades' recent eruptive history for the purpose of estimating the hazard associated with this volcano. An event tree has been designed to systematize possible future eruptive scenarios and to aid the assessment of relative probabilities of occurrence of different potential eruptive styles. The probabilities for all nodes on the event tree, together with their associated uncertainties, were obtained by expert elicitation, providing results that accord with the geologic record. These probability estimates indicate that (1) basaltic strombolian and submarine eruptions are the most likely to occur, (2) trachytic activity from the central volcano magmatic system is more probable in the intracaldera environment than on the flanks, and (3) explosive activity is more likely than effusive for the trachytic system. Worst-case scenario and maximum expected events are identified according to the eruptive scenarios and their probabilities of occurrence.