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Earthquakes under close surveillance in Sancy (Puy-de-Dôme)

It’s like a little detonation. “It’s not very violent and it really doesn’t last long. It looks like a plane breaking the sound barrier, but we feel that it comes from the earth, ”describes Emmanuel Labasse, the mayor of Chambon-sur-Lac. Since March 2022, his town has been the scene of an astonishing natural phenomenon. More than 250 micro-earthquakes were recorded in a few months in a very small area. Their magnitude never exceeds 2.5 on the Richter scale. This is about the limit for an earthquake to be perceived. “I must have felt only two or three. »

The seismic swarm is located in an area of ​​six by three kilometers around Lake Chambon. The first event is recorded in November 2021. In March 2022 (represented with white dots above), an acceleration of the phenomenon is marked.Credit BCSF-RéNaSS.

The seismic swarm, a sector where the earthquakes are concentrated, extends from the town of Chambon-sur-lac, to the hamlet of Beaune-le-Froid, in Murol. In this small mountain village where Saint-Nectaire cheese is king, the phenomenon feeds the discussions. “Yes, we hear about it,” reports Isabelle, owner of a cheese dairy. Some of his neighbors witnessed it. Not her. “We are not on the San Andreas fault either,” she relativizes. Before doubting: “Should we be worried? »

Episodes from the past

In real time, everyone can access public data from the National Seismic Monitoring Network (RéNaSS) on the Internet. “A lot of people have connected recently,” says Guillaume Boudoire, volcanologist at the Magmas and Volcanoes laboratory (LMV*). And for good reason. These repeated earthquakes are not usual. Normally, the area is fairly quiet. “One or two events a year,” says the specialist.

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However, it hovers like a slight impression of deja-vu. In the past, the area has experienced a few episodes of one-off earthquakes. “Between 1979 and 1984, in 2008 and 2013”, lists Guillaume Boudoire. Each time, ten to a hundred earthquakes had been recorded in a few days or weeks. Nothing to do with the extent of the current surveys.

“The event that has been going on since March is remarkable for its duration and the number of earthquakes. »

Guillaume Boudoire (volcanologist at LMV)

“No evolution, neither in intensity nor in depth”

The phenomenon is impressive. Especially since scientists are not yet able to fully explain it. But the latter want to be reassuring. In particular, they exclude one hypothesis: that of the eruption. “In the collective imagination, a repetition of earthquakes is sometimes associated with magmatic activity”, observes Lydie Gailler, also volcanologist at the LMV. In this context, these are two independent facts.

A magnitude 4 earthquake recorded in Combrailles on the border between Puy-de-Dôme and Allier (May 2022)

In the field, the LMV and the Clermont-Ferrand Earth Physics Observatory (OPGC) are joining forces. They deploy resources to closely monitor these exceptional events.

“The important thing is to monitor earthquakes in real time in order to better characterize them. »

Lydie Gailler (volcanologist at LMV)

Using different tools, a multitude of parameters are scrutinized: potential soil deformation, water chemistry, the Earth’s magnetic field, etc. Experts are on the lookout for the slightest change. If the earthquakes continue every day, no change is observed: “neither in intensity nor in depth. Research shows that tremors always occur between two and four kilometers below our feet. “This stability is a fundamental point”, emphasizes Guillaume Boudoire.

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In addition to the permanent national system, three new seismic stations have been installed locally. “This makes it possible to be as close as possible and to collect precise measurements”, explains geologist Thierry Souriot, from the same laboratory. A few hundred meters from the center of the seismic swarm, a GNSS station (for Global navigation satellite systems) is set up.

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It records its position by satellites, “within five millimeters”, every thirty seconds. “If it moves, we can say in which direction and follow the movement over several months”, predicts the expert. Another resource used: a drone, equipped with a magnetic sensor. It is used to study the intensity of the magnetic field and its variations at different scales.

Cross-checked with each other, all these elements make it possible to draw lines of understanding. In the light of current responses, the favored hypothesis today is that of the reactivation of old faults. But the research work is complex. “There are as many hypotheses as there are people working on the phenomenon. »

The Magmas and Volcanoes Laboratory is a joint research unit of Clermont Auvergne University (UCA), the National Center for Scientific Research (CNRS), and the Research Institute for Development (IRD).

Texte : Fiona Farrell

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