Mount Etna, Europe’s most active volcano (also one of the most active volcanoes in the world) is sliding into the Ionian Sea at rates of centimeters per year. If part of it falls into the sea, it could create horrible mega-tsunamis that would devastate the eastern Mediterranean shores.

A team led by Morelia Urlaub of GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany, deployed several underwater transponders at a water depth of ~1200 meters (~3940 feet) around Mount Etna’s southeastern flank, which they suspect is the most mobile section of the stratovolcano (see notes 1). They conducted the first long-term seafloor displacement monitoring campaign from April 2016 until July 2017.

View of Mount Etna from Reggio Calabria, Italy
View of Mount Etna from Reggio Calabria, Italy (10 February 2017). Photo By Jacopo Werther, CC BY-SA 4.0, Link

On October 10, 2018, the team published their results in a study titled “Gravitational collapse of Mount Etna’s southeastern flank” on Science Advances. The biggest movement has occurred during May 2017, the southeastern flank of Mount Etna has slipped around 4 centimeters. According to the researchers, a slip of 4 cm corresponds to a moment magnitude release equivalent to an Mw of 4.3 to 5.3 earthquake.

But, no earthquake with a magnitude larger than 4 has been observed in the area since the initiation of instrumental seismic recording at Mount Etna in the 1980s. So, the reason of the sliding is episodic and aseismic, rather than seismic rupture.

In other words, gravity is the primary force causing this flank of the stratovolcano to move.

In the “discussion” section of the study, researchers say “Reasons for instability of Mount Etna’s southeastern flank have been related either to the volcano’s magmatic plumbing system or to gravitational forces. … Inflation of the volcanic edifice caused by uprising magma is expected to cause the highest displacements near the volcanic center, which is inconsistent with our data. In contrast, our geodetic measurements demonstrated that flank movement increases away from the summit toward the coast and into the Ionian Sea, while no increase in magma activity was noticed simultaneous to the May 2017 offshore event, implying that magma dynamics cannot be solely responsible for the observed deformation pattern.”

If a major flank collapse occurs, and part of Mount Etna falls into the Ionian sea (see notes 2), it could create mega-tsunamis that would devastate the eastern Mediterranean shores.

And that has happened before: Around 8,000 years ago, the eastern flank of the mountain experienced a catastrophic collapse, generating an enormous landslide in an event similar to that seen in the 1980 eruption of Mount St. Helens. The landslide caused a huge mega-tsunami, which left its mark in several places in the eastern Mediterranean. It may have been the reason the settlement of Atlit Yam (Israel), now below sea level, was suddenly abandoned around that time.

Still, there is no sign of an imminent collapse. But we must keep an eye on it since we don’t have data on any similar incident (see notes 3). So, there isn’t any way to tell when a major flank collapse might occur.

What is a mega-tsunami?

mega-tsunami is a very large wave created by a large, sudden displacement of material into a body of water.

They have quite different features from tsunamis. Most ordinary tsunamis are caused by underwater tectonic activity: an earthquake occurs, the seafloor rises and falls, causing a massive amount of water to be displaced. Tsunamis have a small amplitude (wave height) offshore, and a very long wavelength (often hundreds of kilometers long, whereas normal ocean waves have a wavelength of only 30 or 40 meters), which is why they generally pass unnoticed at sea, forming only a slight swell usually about 30 centimeters (12 in) above the normal sea surface.

As the tsunami approaches the coast and the waters become shallow, wave shoaling compresses the wave and its speed decreases below 80 kilometers per hour (50 mph). Its wavelength diminishes to less than 20 kilometers (12 mi) and its amplitude grows enormously. The water piles up to a wave height of up to about 10 meters/33 feet.

By contrast, megatsunamis can occur in locations where there is a very large amount of material that suddenly falls into the water (landslide), or anywhere in water (for meteor impact), or maybe caused by volcanic activity, and can have extremely high initial wave heights of hundreds and possibly thousands of meters, far beyond any ordinary tsunami, as the water is “splashed” upwards and outwards by the impact or displacement. As a result, two heights are sometimes quoted for megatsunamis – the height of the wave itself (in water), and the height to which it surges when it reaches land, which depending upon the locale, can be several times larger.

Mount Etna

Mount Etna
Mount Etna

Mount Etna is an active stratovolcano on the east coast of Sicily, Italy. It is Europe’s most active volcano, one of the world’s most active volcanoes and is in an almost constant state of activity.

Due to its history of recent activity and nearby population, Mount Etna has been designated a Decade Volcano by the United Nations. There are 16 Decade Volcanoes in the world identified by the International Association of Volcanology and Chemistry of the Earth’s Interior (IAVCEI). Because of their history of large, destructive eruptions and proximity to populated areas, Decade Volcanoes considered being worthy of a particular study.

In June 2013, Mount Etna was added to the list of UNESCO World Heritage Sites.


  1. A stratovolcano, also known as a composite volcano, is a conical volcano built up by many layers (strata) of hardened lava, tephra, pumice, and ash. Unlike shield volcanoes, which usually composed almost entirely of fluid lava flows, stratovolcanoes are characterized by a steep profile with a summit crater and periodic intervals of explosive eruptions and effusive eruptions.
  2. The Ionian Sea is an elongated bay of the Mediterranean Sea, south of the Adriatic Sea. It is bounded by Southern Italy including Calabria, Sicily, and the Salento peninsula to the west, southern Albania to the north, and the west coast of Greece.
  3. In the modern ages, a few mega-tsunamis have been recorded. Some of them are Lituya Bay (Alaska) mega-tsunami (July 9, 1958, caused by a giant landslide which triggered by an earthquake, generated a wave with an initial amplitude of up to 520 meters / 1,710 feet); Vajont Dam (Italy) mega-tsunami (October 9, 1963, caused by a landslide) and Mount St. Helens mega-tsunami (May 18, 1980). There are many prehistoric and historical records as well, for example, approximately 66 million years ago, the asteroid which created the Chicxulub crater in Yucatán (scientists believe that this impact is the main cause of the extinction of numerous animal and plant groups, including non-avian dinosaurs) caused some of the largest megatsunamis in Earth’s history: the waves should be as high as 5 kilometers (3.1 mi) and would have reached all the way to Texas and Florida.


M. Özgür Nevres
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