METEOR cruise - HazELNUT - Hazards off Etna: Landslide Nucleation Under Tremor

21 Dec

In December 2021, researchers from GEOMAR, Kiel University and INGV revisited Europe's largest volcano with the research vessel METEOR. One goal of the cruise with the name HazELNUT - Hazards off Etna: Landslide Nucleation Under Tremor- was to further constrain the geological processes that control the instability of volcanic flanks in general and at this volcano in particular. Etna’s south-eastern flank slides seawards at rates of several centimetres per year and scientists from the PRE-COLLAPSE project want to understand if the volcano could collapse catastrophically.

At the foot of Etna, in sight of Sicily’s second largest city Catania, the geodesy team deployed instruments to measure the movement of the flank. Onshore, this has been done for decades with the help of satellites. The part of the flank that is underwater, however, has not been monitored until 2016. And this despite the fact that a huge part of the volcano is underwater and the flank reaches far into the Ionian Sea. "We want to understand the geometry and dynamics of the slowly sliding part of the volcano in order to infer if a sudden collapse could occur,” explains Morelia Urlaub, leader of the PRE-COLLAPSE project and Deputy Chief Scientist of the cruise. Satellite-based geodetic tools such as GPS use electromagnetic waves that do not penetrate the seawater. Sound waves however can. This is how the sonar-based underwater system works: A network of transponders is placed on the ocean floor. These geodetic stations are situated on both sides of a fault - the border of the slipping part of the volcanic flank. One side of the fault is the moving flank and the other side is the stable part of the volcano. The movement of one side of the fault in respect to the other side is represented by the distance changes between the transponders. These are calculated from measurements of sound speed and travel time of acoustic signals between the transponders.

Long-term measurements in a shoreline crossing observatory

"2016 was the first time that five geodesy stations were placed on the seabed in this way," says Morelia Urlaub about a research trip to Mount Etna with the research vessel POSEIDON. In 2018, with the Dutch research vessel PELAGIA, the researchers recovered the devices again to retrieve the data. They were in for a few surprises. The batteries were empty and the instruments were partly corroded.

The data, however, were very conclusive and were published in the journal Science Advances in 2018. The research team had documented rapid deformation of Etna’s offshore flank during the continuous time-series from April 2016 to July 2017. They combined offshore measurements with onshore ground deformation. Morelia Urlaub and her co-authors concluded that the submerged part of Etna’s south-eastern flank is sliding at a similar rate as the on-land part near the coast.

After these great results of the first deployment, the researchers wanted to extend the time-series even further. They revised the concept and again deployed six seafloor geodesy stations surrounded by a network of six Ocean Bottom Seismometers with the research vessel SONNE in 2020. And they were lucky as within the time-series lay again a special event: Etna’s extraordinary eruption phase in February 2021.

Back in December 2021, when they recovered the devices aboard the research vessel METEOR, all had worked well and they read out the data that look very promising. However, the geodesy stations did hide a surprise: "After almost fifteen months on the seafloor, the instruments were covered with a thick crust of carbonate and tephra,” reports Morelia Urlaub. The stuff that they only got off with the help of vinegar acid provided by the ship’s cook, was presumably the deposit of the last eruption phase in February 2021. “We did not observe anything like that in 2016, after the instruments had spent two years on the seafloor,” she remembers and concludes that the last eruption phase was certainly a special one.

So it came, that samples from the surfaces of the geodesy devices were the first that the geology team aboard METEOR collected. “The majority of all explosively erupted material is blown out towards the sea by the prevailing north-westerly winds,” explains Mirja Heinrich, postdoctoral researcher at GEOMAR. Therefore the geologists also recovered up to ten metre long sediment cores from the seafloor offshore Mount Etna. When opening the cores aboard of METEOR to extract poor water, the geology team already noticed many dark layers of volcanic material. Back in the GEOMAR Lab, Mirja Heinrich will geochemically analyse the tephra and correlate the layers amongst cores and to existing land-based data. As these cores reach further back in time than the 17th century and cover the previously unknown offshore area, her results could provide a more complete eruption record that will help to characterize Etna’s behaviour in the past and identify times of elevated activity.

How are the latest eruptions represented in the newly recovered records?

The researchers hope that the data will help to better constrain the casualty between earthquakes related to volcanic activity and small-to medium scale underwater landslides. The February 2021 paroxysms had their most dramatic events in the onshore summit region of Etna, which was intensely monitored and sampled. Felix Gross, Chief Scientist and initiator of the HazELNUT cruise wants to find out if Etna’s recent activity is linked to the movement of the flank. In the sediment cores, the geologists observed redeposited sediment that could have derived from small landsides. Could any of these also be correlated to the recent events? The researchers know that the volcanic and seismic activity within the last decade re-shaped the continental margin off Mount Etna. But has the volcano also undergone any changes prior, during, and in response to the recent events? To answer these questions, a closer inspection of the seabed morphology should provide a better insight. The geologists want to see if there have been any changes since the last mapping ten years ago that are related to the strong eruption phase in February 2021. Morelia Urlaub is sure: “There is a lot happening at the continental slope. With the newly obtained repeated bathymetry we hope to be able to also show it.” Therefore, the ocean floor was mapped again with a ship-based multibeam echosounder.

New data for the models and geodesy stations down for another couple of years

“We were able to deploy five geodesy stations on only one day at exactly the same position as they were before. All transponders stand well on the ocean floor and have sight contact with each other,” notes Morelia Urlaub with satisfaction and adds that this success was thanks to the well-trained geodesy team with Florian Petersen from the Centre for Ocean and Society at Kiel University and Christian Filbrandt, Geophysics Master student, the ship’s crew and the technical possibilities on board of the research vessel METEOR. The instruments will collect data and extend the time-series for another couple of years.

Although the crew was bothered by a rather windy phase, the excitement about the successful research missions and the views of many volcanoes prevailed. At Stromboli and Etna, they observed small eruptions at night, at Volcano they noticed gas escaping and at their final destination, La Palma, they were able to observe the impressive land changes of the island due to the current eruption.

Back to the GEOMAR offices, the PRE-COLLAPSE team will have a closer look at the obtained data. They hope to resolve fine-scaled small movements with the geodesy data. “The new data is an exciting new piece of the puzzle that we will inform our numerical models, from which we will draw conclusions about how the slow sliding of Etna’s Flank could be related to a fast slip event”, explains Morelia Urlaub. The numerical models aim to assess the stress changes in the Earth's crust associated with earthquakes, eruptions and slope processes at Etna's submarine slope and to understand the feedbacks between them.