Etna

towering above the City of Catania on the Sicilian east coast (Italy) is the largest and most active volcano in Europe. Etna’s record of historical volcanism, dates back to 1500 BC. Historical lava flows cover much of the surface of this massive ~3329 m high complex basaltic stratovolcano.

Etna is considered, after Kilauea, the second most productive volcano on earth with several major eruptions per year. Volcanic activity at Etna is characterized by frequent effusive eruptions with large lava flows accompanied by persistent Strombolian activity and lava fountains with occasional sub-Plinian eruptions.

In the last three years, volcanic strombolian and seismic activity have been impressive at Christmas 2018 and in February 2021. During a violent eruptive episode in October 2021, parts of the SE crater collapsed into dangerous pyroclastic flows.

Etna is located in a complex tectonic setting due to the convergence of the African and European plates. The basement geometry promotes lateral spreading and flank instability. About 10.000 years ago, the structure of the volcano was radically modified by slope failures at the eastern flank that entailed the wide depression of the Valle del Bove.

Etna’s southeastern flank slides seawards at 2-3 cm per year

Over the last decades, extensive geodetic surveys focusing on the onshore flanks of Mount Etna have revealed instability of its eastern flank, which continuously moves seawards with displacement rates of up to 50 millimetres per year.

The unstable flank is bound by fault systems. Their displacement is a good indicator for the acceleration of the flank movement. One side of the fault, moves downhill while the other is more or less stable. This movement of the flank and the deformation of the volcano are monitored for example by Satellite-based ground deformation observations - GPS or Interferometric Synthetic Aperture Radar measurements (InSAR) - which show that the highest displacement rates are at the coast.

The Flank movement does not stop at the shoreline

Etnas eastern foot extends offshore, along the coast of Sicily, and reaches well into the Ionian Sea to a major tectonic lineament, the Malta-Hyblean escarpement.

Electromagnetic signals, as they are used by satellites however, do not penetrate water. Previously it was difficult to measure movements and deformation of the ground underwater. Now, Morelia Urlaub and her research team are using a sonar-based alternative. The marine geodetic measurements can show that the flank instability extends far into the sea.

Numerous hypotheses have been proposed to explain flank sliding at Mount Etna. Basically two processes are capable of triggering flank instability: horizontal pushing of ascending magmatic intrusions or gravitational pull.

Shoreline crossing observatory

During a research cruise with the research vessel RV Poseidon in April 2016 (POS496) an acoustic geodetic network consisting of five transponders was deployed at the seafloor offshore the East Coast of Sicily. The target was a seafloor structure that marks the boundary between the downwards sliding southeastern flank of the volcano and its stable surroundings. These seafloor geodetic transponders measured acoustic distances across the fault, absolute pressure and tilt between 2016 and 2018.

The network recorded distance changes between the transponders, which is most likely caused by downwards movement of the unstable sector. Over a period of eight days the southeastern flank slid down by at least 4 cm.

In their study that was published in 2018 in SCIENCE ADVANCES Morelia Urlaub and her colleagues documented rapid deformation of Etna’s offshore flank and combined the offshore measurements with onshore ground deformation. These data define the dynamics of the entire volcanic flank.

The seafloor geodetic measurements suggested that the submerged part of Etna’s southeastern flank is sliding at a similar rate as the on-land part near the coast. The observed large deformation of 4 cm away from the magmatic system can only be explained by a gravitational effect that is further destabilized by magma dynamics.

During a research cruise with the research vessel RV Alkor in January 2020 (AL532), Morelia Urlaub and her team mapped the fault system that represents the boundary between the stable and unstable volcano flanks at high resolution. They used a multibeam echosounder carried by the AUV Abyss of GEOMAR and collected hydroacoustic data with a ship-based multibeam echosounder.

Additional parts of the seafloor off Mount Etna were mapped during a cruise with RV Sonne in August-October 2020 (SO277). Also six seafloor geodesy stations and six ocean bottom seismometers were deployed, aiming for a long-term observation period of 1-3 years.

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