Monitoring Underwater Landslides Using Seafloor Geodesy: Evidence for Active Sliding of Mount Etna's Submerged Flank

Mount Etna, located on the East coast of Sicily (Italy), is affected by sliding of its southeastern flank into the Ionian Sea. Continuous GPS and InSAR monitoring show highest displacement rates at the coast reaching up to 3 centimetres per year. Large parts of the unstable flank lie under water, where the exact outline of the landslide and its dynamics are completely unknown. In 2016 we installed a network of transponders that measure the acoustic range between each other at the submerged part of the volcano's flank. The instruments monitor length changes across the fault that marks the boundary between the stable and the unstable sectors for a period of three years. In July 2017 we downloaded 14 months of data via an acoustic link. The data show clear evidence for sliding of the volcanic flank towards the Ionian offshore at a rate comparable to that on land. The observation suggests that the origin of the landslide is the unstable continental margin rather than voclanic processes. This is the first time to observe active displacement of an underwater landslide.