Dynamics of Mount Etna’s submerged unstable flank

Mount Etna, located on the East coast of Sicily (Italy), is the largest active volcano in Europe. Its southeastern flank is unstable and slides into the Ionian Sea. Continuous GPS and InSAR monitoring show highest displacement rates at the coast reaching 2-3 centimetres per year. Large parts of the unstable flank lie under water, where the exact outline of the landslide and its dynamics has been unknown. Past research efforts concentrated on the description of active spreading and tectonics of the continental margin. Multibeam bathymetric and high resolution seismic data provide insights into the heterogeneous structure of the bulge associated to Etna’s flank. The data suggest that a right lateral oblique fault north of Catania Canyon marks the offshore southern boundary of the moving flank. Between 2016 and 2018 we installed a seafloor geodetic network across this fault. The network consisted of five transponders that continuously measured the acoustic range between each other. The data show clear evidence for sliding of the volcanic flank towards the Ionian offshore during an eight day long slow slip event in May 2017. Overall, the slip rate in the observation period (~4 cm in 20 months) is comparable to that at the Sicilian coast. Such large displacement cannot be explained by dike intrusions or volcanic activity near the crater. We therefore conclude that the bulk of Mount Etna’s present continuous deformation must be driven by gravity, while being further destabilised by magma dynamics.