FOCUS – UNESCO considers the occurrence of a tsunami in the Mediterranean to be very likely within the next 30 years.
Can a tsunami hit France? If the question may seem fictional, the risk is very real according to Unesco, which considers the occurrence of such a phenomenon in the Mediterranean in the next 30 years very likely. This wave, caused by the rapid movement of a volume of water, can be caused by an earthquake, a volcanic eruption or even underwater movements.
Although rare, several tsunamis have already occurred in Europe. To study the risks, the tsunami warning center CENALT (Commissariat for Atomic Energy and Alternative Energies) analyzed historical precedents. What forms could they take today? What damage could they cause? For Tsunami Awareness Day this Saturday, November 5, we interviewed Hélène Hébert, national coordinator of the center, who deciphers the different scenarios that could concern France.
The risk of tsunami in the Mediterranean Sea
Earthquakes are the main cause of tsunamis. The Nice region is particularly exposed to it, several events of this type have already been reported. In 1887, for example, in winter, during the Nice carnival, a tsunami reached the coast around 6 a.m. and caused the death of 8 people in the Alpes-Maritimes, and injured 55 others. It was especially deadly in Liguria, Italy, where 635 people died.
Contrary to what one might imagine, the waves did not reach 15 meters high. “They measured rather one or two meters“, rectifies Hélène Hébert. “A sea flow of 50 cm of water can be particularly devastating: it can arrive at a speed of 40 km/h and then cause fragile people, the elderly or even children to fall and carry them away.“, adds Hélène Hébert, who adds that this scenario is particularly worrying. “The flood of several meters will last several hours, and will have an economic impact and will eventually lead to human losses.»
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The Algerian margin, south of the western Mediterranean,is also subject to very significant seismic zones“, continues Hélène Hébert. We remember the strong earthquake of 1980 in Algeria, or that of 2003. An earthquake in Boumerdès, on the Algerian coast, had caused waves which had traveled the Mediterranean: two to three meters in height could be observed in the Balearic Islands. In France, they had reached a meter in height. Hundreds of boats had been damaged in the Balearic Islands.
The risk of tsunami in the Atlantic Ocean
If the Mediterranean is the most affected by the risk of tsunami, the Atlantic Ocean is not left out. Here too areastsunamigenicare listed. History reminds us of this with the tsunami that devastated the city of Lisbon in the 18th century.
On November 1, 1755, a very strong earthquake destroyed the Portuguese city. A few minutes or even a few hours later, a tsunami followed, with waves five to fifteen meters high. The port on the Tagus and the city center were also submerged. “Portugal, Morocco, the bay of Cadiz in Spain were at the forefront, and were also affected by fairly high waves, almost what we saw in Japan in 2011. The French West Indies also observed waves of three to four meterscommented Hélène Hébert. Between the earthquake, the tsunami and the fires, 100,000 people were victims of this earthquake and its indirect consequences in Lisbon.
This tsunami is one of the most significant natural disasters in the Atlantic. The French coasts were more or less protected due to the configuration of the Iberian Peninsula. “But we think, with numerical simulations, that this tsunami would have potential effects on certain littorals of the Atlantic coast “, warns Hélène Hébert. “It happened three centuries ago and can therefore happen again tomorrow, as in a century or two centuries. It is estimated that such tsunamis are possible in this region approximately every 400 – 500 years».
Rock collapses
Besides earthquakes, tsunamis can be caused by other factors, such as rock collapses and avalanches. This is what happened on October 16, 1979, following a landslide on the Nice airport site: a tsunami flooded several districts of Antibes and Nice, with a wave of almost four meters high in La Salis, causing material damage and around ten deaths. “There, we have an underwater collapse which generates an undulation of the sea above. It is not possible to anticipate this kind of phenomenon. When it collapses, major destructive effects are caused right in front of the source“, explains Hélène Hébert.
Least likely scenarios: volcanoes and meteorites.
Other events can cause tsunamis, but the probability is lower. Volcanoes, in particular:We have an example recently in Tonga, with the eruption of Hunga Tonga in 2022. The nearest islands received waves reaching fifteen meters in height; Oceania, America and Asia were also affected by these waves.»
«You have to understand that when a volcano explodes, the whole building is pulverized.It can both generate waves because it collapses under the sea, but also because the explosion is felt in the atmosphere, which generates waves. The coupling between the atmosphere and the water layer is called a meteotsunami.».
Last scenario, of a tiny probability: the tsunami caused by a meteorite, as it was 66 million years ago in the Gulf of Mexico. The meteorite formed a wave 150 meters high which may have contributed to the extinction of the dinosaurs. It turns out that yes, in these cases, we can have tsunami-type phenomena, more complex, and cataclysmic.“. But it is extremely rare, concludes Hélène Hébert.