Are the recent floods in Caen the cause of the earthquake felt this Monday?

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Around Caen (Calvados), these inhabitants were not surprised when they woke up. But from Fleury-sur-Orne to Douvres-la-Délivrande, and especially to Cambes-en-Plaine, the place of its epicenter, many felt the earthquake which happened at 9:34 a.m. this Monday, October 7, 2024. And some reported this shock on the dedicated site of the French central seismological office (BCSF), which analyzed and confirmed it for its part.

For Daniel Amorese, seismologist from the University of Caen, the earthquake measuring 2.7 on the Richter scale is “nothing to worry about”. The geophysicist returns to this more frequent event in Normandy.

At 2.7 on the Richter scale, the magnitude of this earthquake is low. Where could the seismic activity come from in this location?

The intensity of this earthquake is not so weak, as it was felt. But it remains very small. It occurred on an ancient fault, coming from the Armorican massif. It may be linked to a disturbance of forces on the earth’s crust. As it was measured at a depth of 4 km, it has the particularity of being superficial. This is only a hypothesis, but it may relate to hydro-seismicity. After an episode of intense rainy precipitation in the previous month [telles les inondations orageuses à Caen et Mondeville samedi 7 septembre]fully loaded water tables in a stressed environment can trigger a shock. It’s a popular theory at the moment. It has been tested in Japan or in the Ubaye region (Alpes-de-Haute-Provence).

How is seismic activity measured in Normandy? How many earthquakes have been recorded in the region over the past century?

Since 1241, only 23 destructive earthquakes have been felt in Normandy. There can be hundreds of tremors, but imperceptible, because they go down very low in magnitude. And around the world, millions. There may be a decade where there are quite a few earthquakes, and then fewer. There are no more sensors in Caen. But, since November 2018, we have been redeveloping a network of measurements from the European program European Plate Observing System (EPOS). The earthquake this Monday morning in Cambes-en-Plaine was measured by one of its stations in Saint-Lô (Manche). We are witnessing a revolution in seismology, since we use the networks of optical fiber (which everyone knows for the Internet and telephony), very sensitive to shocks.

Normandy is considered a low risk area. Could his situation change?

Normandy is well positioned on a very old system of destructive earthquakes. In recent years, there has been no particular increase in seismicity. For the pattern to change, it would take an earthquake of very high magnitude.

The small earthquake felt this Monday will not change anything. If there are other earthquakes in the coming days, this could be worrying.

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