Earthquake under close monitoring in Sancy (Puy-de-Dôme)

It’s like a small detonation. “It’s not very violent, and it really doesn’t last long. It looks like an airplane breaking the sound barrier, but we feel it is coming from the ground,” describes Emmanuel Labasse, the mayor of Chambon-sur-Lac. Since March 2022, his city 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 around 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 by white dots above), an acceleration of the phenomenon is marked.Credit BCSF-RéNaSS.

The seismic swarm, a sector where the earthquakes are concentrated, stretches from the town of Chambon-sur-lac to the village 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’re not on the San Andreas fault either,” she relativizes. Before you doubt: “Should we be worried? »

Episodes from the past

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

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

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

Guillaume Boudoire (volcanologist at LMV)

“No development, neither in intensity nor in depth”

The phenomenon is impressive. Especially since scientists are not yet able to fully explain it. But the latter would like to be reassuring. In particular, they rule out one hypothesis: that of the outbreak. “In the collective imagination, a recurrence of earthquakes is sometimes associated with magmatic activity”, notes Lydie Gailler, also a volcanologist at LMV. In this context, there are two independent facts.

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

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

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

Lydie Gailler (volcanologist at LMV)

Using various tools, a wide range of parameters are investigated: potential soil deformation, water chemistry, the earth’s magnetic field, etc. Experts are looking 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.

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 GNSS station (for Global Navigation Satellite Systems) is set up a few hundred meters from the center of the seismic swarm.

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It registers its position using 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 on different scales.

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

Text: Fiona Farrell

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