For the scientific community, this was a real shock. She became aware that tsunamis did not only affect the Pacific Ocean. Zero risk does not exist, even in the Mediterranean or the Atlantic. France could therefore also be affected. This scenario is taken seriously by the authorities.
On January 19, 2024, a sound signal accompanied by an alert message indicating the imminence of a tsunami was sent to all geolocated phones on the Mediterranean coast. Fortunately, it was only a simple exercise piloted from the national tsunami warning center (CENALT)located near Paris. This center is, in a way, a control tower with a giant screen on the wall and a world map displaying small colored triangles.
These triangles symbolize the 400 seismic sensors distributed across the globe and monitored by the teams of Pascal Roudil, sismologist and head of the national tsunami warning center at the CEA : “QWhen we have stations that start flashing red, that means there is a little activity happening right now. This activity can be people tapping their feet next to the seismometers, it can be a truck on the road that is not far away or it can be a phenomenon that generates acoustic waves inside the ground like a earthquake.” These seismometers are, moreover, monitored 7 days a week, day and night.
“In a real case, a magnitude 6.8 earthquake off the coast of Nice, all the stations that would be affected would start flashing red.”
In this specific case, “the whole Côte d'Azur” would be threatened, “but this would concern Italy, Spain, Algeria, Tunisia, Morocco and there we would put ourselves in a crisis situation“, warns the seismologist. Between the triggering of the earthquake and the arrival of the waves, residents have only fifteen minutes to take shelter. Since the creation of this alert center in 2012, around a hundred notable events were reported but without any real risk of tsunami.
This type of event has already occurred in the Mediterranean in the 16th and 19th centuries, although it was not on the scale of the Asian tsunamis. “We do not expect tsunamis that are more than 2 or 3 meters high, unlike in the Pacific or Indian Ocean where there were waves that were more than 30 meters high.“, reassures Pascal Roudil. However, it is not necessary to have such impressive waves to suffer damage.
“To do damage, you only need about 50 cm. One meter is more than enough to lift boats and propel them along the seafront.”
at franceinfo
It is for all these reasons that exercises are organized by coastal municipalities in order to raise awareness among residents.
Scientists are developing new technologies to better understand earthquakes. For several years, some seismologists have been using the fiber optic network to probe the bowels of the Earth, for example with the operator Altitude Infra, which is deploying fiber in Haute-Savoie. “In this example, we have a cable of one centimeter in diameter with 48 glass fibers inside for connecting subscribers and providing them with high speed. One of the fibers was made available for research on seismic study“, explains project manager Sébastien Arlant.
Seismologist Olivier Coutant carried out the experiment, on this fiber buried near Annecy, using a very special machine, the DAS, an acoustic detection system: “it is exactly a laser which sends 'pulses' at an extremely high rate and we measure the propagation time of the light in the fiber. We deduce possible deformations of the fiber which can lengthen or shorten when it is subjected to seismic movements of the ground. This technology is ideal for studying what we call microseismicity.
“Large major earthquakes don't happen like that, overnight, without warning. We have always had an interest in looking at small seismicity to discover faults before they really break.”
Olivier Coutantat franceinfo
Although optical fiber does not replace seismographs, it has the advantage of being everywhere, in inaccessible places and particularly in the deep seabed, with several thousand kilometers deployed across the globe.
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