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Another small earthquake in Morbihan

October 24, 2022 at 9:08 p.m. – Modified: October 24, 2022 at 9:12 p.m. by Dolorès CHARLES

Morbihan earthquake (October 2022)

3.7 on the open Richter scale: the earth shook last night (Sunday October 23) in Brittany. An earthquake felt in Morbihan with an epicenter in Landévant, 17 km from Lorient. A stronger shock than the last, perceptible even in Finistère, arousing many reactions on social networks.

Sunday evening’s earthquake in Morbihan adds to a rather long list of small earthquakes in the West in recent years. Earthquakes with a magnitude between 2 and 5, which do not cause damage, but which are felt by the population. These earthquakes occur particularly along two major fault zones: the first goes from Quimper to Angers, the second from Quimper to Vendée, passing through Nantes (editor’s note, this is called the South Armorican shear).

What activates these vulnerabilities?

Why is the earth shaking? The answer of Julie Perrot, seismologist at the University of Western Brittanyjoined by Yann Launay: “because there are a few tectonic constraints with Africa pushing Europe; a very long time ago there was an ice cap which withdrew 10,000 years ago and which means that there are small readjustments. Brittany rises a little after the melting of the ice and finally there is the oceanic ridge which is more than 3,000 kilometers from us, but which can also have an effect on the region… It is a subject of research on intra-plate seismicity, it is still things that we don’t know very well. There are plenty of other hypotheses, which would be gravitational reasons that could explain why the faults move upstream. We’re trying to understand it all better!”

Julie Perrot, seismologist at the University of Western Brittany

Credit: Yann Launay

Why not one day, a more violent earthquake?

Is a destructive earthquake, with an intensity greater than 6, possible in southern Brittany in the coming years? For Julie Perrot, a priori “nope. The earthquake that took place in 2002 was of magnitude 5, it was an earthquake whose fault was one kilometer by one kilometer. Today, we have a 3.7 which means that the fault is a few hundred meters, 500m not so much more. These are very small faults that move to have a magnitude of 7 or 8, you need faults that move over several tens of kilometres… We are not in a regional context, where a priori this kind of earthquake can take place, and I say a priori, because I cannot say that it cannot take place, but historical seismicity, in any case, does not say so.

Julie Perrot, seismologist at the University of Western Brittany

Credit: Yann Launay

The limited threat

For those who are worried about the consequences for buildings of even slight but repeated earthquakes, the Building Federation wants to be reassuring: the threat is very limited. Aude Le Vaillant, General Secretary of the Federation in Morbihanas evidenced by the zoning decided by the government, which dictates the construction standards:

There have been five seismic category zones in France since 2010, this was modified on that date and we in Morbihan are in zone two, in a low seismic risk zone. Concretely, in zone two, individual houses, for example, do not have to integrate seismic parameters. But for example, a school or a hospital must integrate these seismic standards. If we stay in category or zone two, it is because the legislator and the technicians and engineers believe that there is no risk.

Aude Le Vaillant, of the Federation in Morbihan

Credit: Yann Launay

To monitor seismicity in France in real time, go to the website: renass.unistra.fr

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