With its 5,800 kilometers of coastline, France is vulnerable to the risk of tsunamis. This is the message that the Tsunami Warning Center (Cenalt) intends to deliver on the occasion of its ten years of operation. Installed in Bruyères-le-Châtel (Essonne), in a building of the Commissariat for Atomic Energy and Alternative Energies (CEA) housing an analysis room and another for data processing, this structure piloted by the Ministry of ecology, sustainable development and energy, and that of the interior, brings together the experts responsible for alerting the public authorities to the arrival of a tidal wave on French beaches.
Tsunamis in mainland France? The idea is not ridiculous, assures Hélène Hébert, researcher at the CEA and national coordinator of the Cenalt project, which also counts among its partners the National Center for Scientific Research (CNRS) and the Hydrographic and Oceanographic Service of the Navy ( SHOM). “These ocean waves are created by the sudden movement of a large volume of water, following the rupture of a seismic fault over hundreds of kilometers, she describes. Capable of propagating at speeds of the order of 500 to 1,000 kilometers per hour when they circulate offshore, they slow down and gain in intensity as they approach the coasts, where they cause variations in the sea level, comparable to successions of rapid and strong tides. These sweep away everything in their path, flooding beaches, moving pontoons and ships in ports and creating dangerous whirlpools that can dig into the ground. »
If the “subduction zones” placed at the junction between the plates are the most likely to produce large-scale phenomena, such as the huge waves that hit Japan on March 11, 2011, resulting in the Fukushima nuclear accident, simple submarine or coastal earthquakes of magnitude 6.5 can generate tsunamis, provided they occur at shallow depth on suitably oriented faults. Result, indicates Pascal Roudil, of the CEA, rare are the regions of the globe not concerned by the risk: “The Mediterranean and the North-East Atlantic themselves have experienced at least a hundred events of this type since the beginning of the 20th century.e century, including Bourmedès, in Algeria, on May 21, 2003. Pascal Roudil also quotes “the Lisbon earthquake on 1is November 1755, when parts of the Spanish and Moroccan coastline were submerged in 5 to 10 meters of water. Even Brittany was affected at the time. »
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