Near-real time recognition of dike arrest at Mt. Etna from reverse focal mechanisms

During eruptive crises, one of the key elements in emergency management is assessing whether and how magma is propagating, especially in cases of potentially dangerous lateral intrusions. A crucial issue is predicting in near-real time whether dyke propagation is likely to arrest or continue toward the lower flanks, where towns and villages are commonly located.

Magma ascent typically generates an extensional stress field around the dike propagation path, associated with earthquakes displaying normal focal mechanisms. The occurrence of events with reverse focal mechanisms, indicative of compressional regime, is rare in such settings. However, analysis of several eruptive episodes at Mt. Etna, from the 1989 crisis through the 2002 eruption, up to the eruptions of 2008 and 2018, reveals a consistent picture: the terminal portion of lateral intrusions that do not reach the surface is systematically characterized by the appearance of reverse focal mechanisms, which are absent during the initial propagation phases. According to the study, the appearance of reverse focal mechanisms is linked to a change in the stress field, likely associated with the magma's cooling and solidification processes, which favour compressive conditions. What emerges is a simple yet extremely effective indicator: reverse focal mechanisms are not an anomaly, but a key signal that allows us to recognize the potential arrest of a dike in near-real time, providing valuable constraints for operational decisions-making during eruptive crises.