Mount Etna is a volcano that forms some of the eastern part of Sicily. Its underwater southeastern flank is known to be slowly slipping. A detailed study of the process has shown the main reason is not what most geologists assumed, and the likelihood of a tsunami-unleashing catastrophic collapse is greater than previously recognized.
Tsunamis like the one that recently caused thousands of deaths in Sulawesi are most frequently caused by undersea earthquakes. However, the sudden collapse of mountainsides can trigger similar effects, even when they're already underwater. There is evidence, for example, that the Atlantic coast of North America may have suffered catastrophic tsunamis in the distant past from landslides off the Cape Verde Islands.
Slow, and apparently harmless, movements can indicate instability. So the fact the southeastern flank of Etna, within sight of a city of 300,000 people, has been shifting by some 3-5 centimeters (1-2 inches) a year for four decades is alarming.
This movement was thought to be a result of a build-up of pressure from magma within the volcano, something that was distorting the volcano's shape. However, a team led by Dr Morelia Urlaub of the GEOMAR Helmholtz Center for Ocean Research studied the movement of the seafloor during a period of accelerated slippage last year, and they found something different.
Between May 12 and 20, 2017, transponders Urlaub had placed on the seafloor off Etna recorded movement, with sites moving up to 3.9 centimeters (1.6 inches) relative to each other in the space of just over a week.
The gap between sensors changed most further away from Etna's summit, Urlaub reports in Science Advances, around what is known as Catania Canyon. Crucially, the movement was greatest far away from the magma chamber, where the pressure effects would be more significant. For that matter, there was no increase in magma observed around the same time.
If magma is not responsible, Urlaub and her co-authors think the explanation must lie in the gravitational pull from the continental margin, which has subsided into the sea and is pulling parts of the mountain down behind it. If magma played a role, it is subsidiary.
The authors point out that volcanic flanks in the Canary Islands have failed several times, but these preceded, rather than followed, explosive eruptions. The paper describes gravitational sliding as “the governing process”, causing collapses, and stimulating changes in magma that induced subsequent eruptions.
The most important outcome of this work is that a catastrophic collapse of Etna's underwater flank is more likely than previously realized. Gravitational sliding is likely to continue, and could induce shifts much larger and more sudden than the one Urlaub observed. Just as off Cape Verde this could induce a tsunami, but one much closer to heavily populated areas.