An unobserved lava fountain deciphered in real-time by high-precision borehole strainmeter and contribution to hazard evaluation: the Etna 21 May 2023 eruption

Abstract A powerful lava fountain took place on 21 May 2023 at Mt. Etna after a year of repose, preceded a few days earlier by a shallow seismic swarm. The main critical issue with the eruption was the difficulty to track this eruptive event with conventional remote sensing devices, such as webcams and satellite instruments, due to the bad weather conditions and especially the dense cloud cover. Despite the sophisticated monitoring available at the volcano, the eruption effectively remained “hidden”. It was here that the borehole dilatometers excelled: as with all recent lava fountains at Etna, these high-precision instruments detected significant strain changes and proved a valuable contribution to real-time monitoring of the event. Through recently implemented approaches, the analysis of the strain data allowed us to decipher the eruption: namely identify the timing of the events, evaluate the “size” of the fountain (i.e. its eruptive intensity), and also estimate the erupted volumes in near real-time. This provided a useful support during the Civil Protection emergency meeting at the Prefecture of Catania, where these results were presented and jointly discussed. Overall, the 21 May 2023 lava fountain was an important showcase, demonstrating the strategic contribution that real-time high-precision strain signals may have in defining and assessing the hazard associated with an eruptive event even in adverse weather conditions, when remote sensing systems may be unable to furnish direct information on the ongoing phenomenon.