In Essonne, a center to watch for tsunamis

The nearest sea is 150 kilometers away. However, it is in Bruyères-le-Châtel (Essonne), in the south of the Paris region, that the Cenalt, the tsunami warning center, is located. In premises belonging to the Commissariat for Atomic Energy and Alternative Energies, seven people take turns, their eyes glued to the screens, 24 hours a day, seven days a week. case of a deadly wave on the French coasts.

The “tsunami risk” in mainland France is not omnipresent, barely once or twice a century, with waves up to five meters. But the drama of 2011 in Japan caused cold sweats and since 2012, surveillance has been put in place. « Tsunami risks in the Mediterranean are more recurrent, but with small waves, while in the Atlantic the danger is extremely rare but greater, as shown by the Lisbon tsunami in 1755 and its tens of thousands of dead », describes Hélène Hébert, national coordinator of Cenalt.

Two phenomena can cause tsunamis: earthquakes, most frequently, and underwater volcanism, very rarely. In the event of an earthquake, the formation or not of a tsunami then depends on three factors: the magnitude (more than 7), the focal mechanism (the way in which the earthquake is triggered), and the depth. Unlike other natural disasters, this tsunami risk will not increase with climate change at work. Hélène Hébert, on the other hand, recalls that « Climate change is causing sea level rise, which must be taken into account ».

Different alert centers for overseas

At Cenalt, more than 300 seismic stations are monitored all over the world. Because even if the center deals mainly with the metropolis, it registers and issues bulletins for the whole globe. The overseas territories depend on warning centers managed by other countries, the United States for risks in the Pacific and Australia for the Indian Ocean. With this idea that closer centers detect earthquakes earlier and therefore alert more quickly.

“Five minutes maximum after the rupture of the seismic fault, we can say if there is a tsunami and of what magnitude, and in less than fifteen minutes, the message reaches the populations concerned”, trace the coordinator. The inhabitants must take shelter by going as far inland as possible and climbing as high as possible.

An instruction not always easy to coordinate with those to be respected in the event of an earthquake, where it is necessary to move away as much as possible from the buildings. If the earth is only shaking, it is better to run in the street. But if the tremor is followed by a wave, it is better to stay on the 5th floor. « The messages transmitted via the FR-Alert system on mobile phones specify the attitudes to adopt », reassures Yves Hocdé, of the General Directorate of Civil Security.

Deploy signage for residents

Some coastal towns, such as Antibes and Cannes, have also begun to deploy signs to mark evacuation routes and refuge areas. But on the whole, the municipalities lack support to prepare and carry out exercises. Especially since even a wave of 60 centimeters can be dangerous, because of the currents and the waste carried along.

« Natural disasters are inevitable, so we must prepare for them beforehand, inform populations and reduce the vulnerability of infrastructures wherever possible », insists Mendy Bengoubou-Valérius, from the General Directorate for Risk Prevention.

As these phenomena are, fortunately, very rare, the populations are poorly aware of them. « We can clearly see, on the contrary, that the multiplication of floods has generated reflexes among the inhabitants affected », continues the national referent. Specialists believe that after ten years, the memory of a natural disaster fades.


The Mediterranean, the highest risk area

Nearly 80% of tsunamis are caused by an earthquake. For France, the greatest risk lies in an earthquake at the level of the Atlas, which would cause a wave in the western Mediterranean.

The other tsunamis are due to underwater volcanism, and researchers are scanning pockets of magma around Stromboli in the Tyrrhenian Sea.

It is almost certain that a wave of tsunami of more than one meter will take place in the Mediterranean in the next thirty years, according to Unesco. This wave could occur in the eastern part, towards Turkey, where the earth shakes regularly.


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