On June 16, 2023, an earthquake hit western France. Many tremors were felt several hundred kilometers around the phenomenon. According to France Séisme, the earthquake was recorded at 6:38 p.m. Its epicenter would be between La Rochelle, in Charente-Maritime and Niort, in Deux-Sèvres. Seismologists recorded a magnitude greater than 5.3 on the Richter scale.
Since this earthquake, local authorities and firefighters in the region have been mobilized to assess the damage. On June 26, the Reinsurance Fund estimated between 200 and 350 million euros in damage, according to figures collected by Ouest-France. This figure can be reassessed. But what consequences could this earthquake have? Does it herald more frequent earthquakes in the region? Yann Klinger, research director at the CNRS and head of the tectonic group at the Institut de Physique Globe de Paris (IPGP), sheds some light.
Planet. According to your institute, the magnitude of the earthquake in Charente-Maritime was assessed between 4.8 and 4.9. Can this be considered a significant magnitude?
Yann Kingler. Yes, that is one of the fairly large magnitudes. Nevertheless, it is not exceptional but it is one of the biggest tremors among the earthquakes of the last ten years.
Planet.How would you rate the intensity of the earthquake?
Yann Klinger. When an earthquake occurs, there are waves in the ground. It is these waves that make it possible to locate the earthquake. We can also evaluate the energy released by the earthquake by looking at the amplitude of these waves, then we measure this energy which is transformed into magnitude. In short, we use the waves emitted by the earthquake.
Planet. According to the report of the Bureau of Geological and Mining Research (BGRM), 5,000 buildings were affected. How far can the damage of an earthquake of such magnitude extend?
Yann Klinger. What is interesting with this earthquake is that it was felt in a fairly large area. There are quite a few testimonials from people who have even felt it as far away as Paris, Lyon, Le Havre. From this point of view, it was an earthquake that was strongly felt.
But for damage to buildings, it is much more difficult to give a definitive answer because it depends on several parameters. For example, the depth at which the earthquake occurred plays a role. In this case, it looks quite superficial. If we brought the source closer, the vibration would be more sensitive with waves that have difficulty attenuating.
Then there are the site effects: we have the source of the vibrations, but depending on where your buildings are built, for example if they are built on very hard rock, very stable or if they are built on alluvium in a plain, it is not at all the same. Therefore, the effect on the building is going to be very different. So there is not a canonical distance that we can give like that.
Planet. Could it cause other natural disasters?
Yann Klinger. There were aftershocks. This contributes to the normal activity of an earthquake. We know that an earthquake is always followed by its string of aftershocks. From a scientific point of view, this is not surprising or exceptional. But of course from the point of view of the population, it is still worrying. After a big earthquake, which is not common in France, you then have smaller ones, but despite all this reinforces the level of concern. And of course the structures have been weakened and having lower magnitude aftershocks can cause damage to already damaged structures.
Planet.Is this event a harbinger of a higher earthquake frequency in France?
Yann Klinger. Not particularly. This earthquake is a little stronger than the average seismicity that can be observed in France each year. But it is not exceptional. We have earthquakes of comparable magnitude fairly regularly, every few years, let’s say. Particularly in western France, we have a weak but constant seismological activity over the years.
Planet. Can you anticipate them?
Yann Klinger. No, we cannot anticipate earthquakes. We know the areas that are conducive to the occurrence of earthquakes, which allows a certain degree of preparation. But we must keep in mind that we cannot predict them.
Planet.Which regions are most at risk? For what reasons ?
Yann Klinger. The whole south-east of France is concerned. This zone corresponds to the entire front of the Alps, the Jura where there was a fairly regular micro-seismicity such as the earthquake at Teil, in Ardèche, two years ago or that of Annecy in 1996. So you have regular earthquakes . The front of the Pyrenees is also concerned.
In short, wherever you have landforms, you have tectonics, because it’s intertwined. If you have tectonics, even slow or a little old, you have seismological activity. For Brittany, it is a fairly flat area, but there were mountains so there are still scars from this mountain range which has disappeared but which is active. It is simply a legacy of ancient tectonics.
Planet.Are there regions which are not seismic in France and which could become so?
Yann Klinger. If we speak from a natural seismicity point of view, these are not things that change very quickly. The engine of natural seismicity is above all plate tectonics. We are talking about the movement of continental and oceanic crusts over hundreds of thousands of years, even millions of years. The time scales are therefore not ours.
With regard to metropolitan France, and even Western Europe, we know the areas that concentrate the maximum seismicity so necessarily the areas that contain little are not likely to change.
What you have to keep in mind is that our observation window is still very short. Earthquakes have not been measured in Europe for very long. Taking the widest window, therefore that of instrumental earthquakes (recorded with an instrument), it only begins at the beginning of the 20th century. The largest earthquake recorded in France is of magnitude 6 which took place in 1909 in the south of France. But we cannot exclude that we have stronger earthquakes occurring. But the time interval between these earthquakes can be several thousand years so we lack hindsight. You can’t discount the idea that it’s not going to happen.