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The southeastern flank of Etna volcano slides into the Ionian Sea at rates of centimeters per year. The prevailing understanding is that pressurization of the magmatic system, and not gravitational forces, controls flank movement, although this has also been proposed. So far, it has not been possible to separate between these processes, because no data on offshore deformation were available until we conducted the first long-term seafloor displacement monitoring campaign from April 2016 until July 2017. Unprecedented seafloor geodetic data reveal a >4-cm slip along the offshore extension of a fault related to flank kinematics during one 8-day-long event in May 2017, while displacement on land peaked at ~4 cm at the coast. As deformation increases away from the magmatic system, the bulk of Mount Etna’s present continuous deformation must be driven by gravity while being further destabilized by magma dynamics. We cannot exclude flank movement to evolve into catastrophic collapse, implying that Etna’s flank movement poses a much greater hazard than previously thought. The hazard of flank collapse might be underestimated at other coastal and ocean island volcanoes, where the dynamics of submerged flanks are unknown.
Catastrophic collapses of ocean island volcanoes or those built at the shoreline pose the largest threat as the sudden displacement of large amounts of material in water can trigger tsunamis with extreme effects (4, 5).
Notably, the observed length change in the network of ~4 cm provides a minimum estimate of the true slip along the fault during the May 2017 event.
A slip of 4 cm corresponds to a moment magnitude release equivalent to a Mw of 4.3 to 5.3 earthquake (26). Since the initiation of instrumental seismic recording at Etna in the 1980s, no earthquake with a magnitude larger than 4 has been observed in the area (27). Hence, the main style of deformation of the offshore volcanic flank is episodic and aseismic sliding rather than seismic rupture.
In the case of Mount Etna, our shoreline-crossing deformation analysis implies a greater hazard for flank collapse than previously assumed, as deep-seated gravitational sliding can potentially lead to catastrophic collapse (2, 3, 16).