Flank instability at Mount Etna: new insights from seafloor deformation monitoring

Coastal and ocean island volcanoes are renowned for having unstable flanks, which expresses as slow seawards flank sliding observable by geodetic techniques and/or catastrophic sector collapses. A large section of these unstable flanks is often below sea level, where information on the volcanotectonic structure and, in particular, ground deformation are limited. Consequently, kinematic models that attempt to explain measured onshore ground deformation associated to flank instability are poorly constrained in the offshore area. This is also the case for Mount Etna’s unstable south-eastern flank that slides seawards at rates of 2-3 cm/yr. Displacements associated to flank movement, observed onshore by geodetic and remote sensing techniques, show maximum values at the coast and kinematic models consistently predict even larger movements seawards of the coast. Our seafloor geodetic measurements between 2016 and 2018 confirmed that offshore flank slip is equal or slightly larger compared to onshore slip. The main displacement was released during one slow slip event. Here, we present new data from a second deployment of the seafloor geodetic network in the same location with the same direct-path acoustic ranging technique and a modified network design. The measurements allow reconstructing relative seafloor displacement within the network at sub-centimetre precision, from September 2020 until November 2021. The preliminary results indicate a possible eastward sliding of the flank, although the overall slip of <1 cm is close to the limit of resolution. Flank slip is continuous over the observation period. With our seafloor geodetic network, we are able to record different styles of fault slip and deformation rates. Ongoing long-term monitoring will show how these styles of deformation interact, and which type of flank movement is dominant in the offshore sector.