The earthquake hit near Montélimar (Drôme, Auvergne-Rhône-Alpes) on November 11, and notably caused considerable damage in the commune of Teil (Ardèche), requiring thousands of people to be evacuated overnight.
Now, French researchers from an interdisciplinary “post-seismic” team - including from quake specialists GéoAzur, ISTerre and radioactive safety institute the IRSN - are attempting to pinpoint the cause, with human quarrying one of the possible lines of enquiry.
According to reports, the scientific documents include the specific question: “Could the presence of an active quarry located above the apparent faultline have contributed to starting the earthquake?”
Said quarry is limestone, and located between the communes of Teil and Viviers, a few kilometres from Montélimar. Owned by the industrial construction group Lafarge, it has been in use since 1833. In 2018, the company requested and obtained permission to extend activity at the quarry.
The team is exploring this possible avenue due to several unusual elements - including the fact that the quake was found to be very shallow; at just “one or two kilometres” below the surface.
Jean-Robert Grasso, quake expert and member of the ISTerre laboratory at the University of Grenoble-Alpes, said: “This is very surprising for a country such as France, where earthquakes are usually at five to 20km deep. A shallow depth is a peculiarity that we [often] find when quakes have been caused by human activity.”
Another factor is the lack of aftershocks. An early report said: “After the main shock of a magnitude of 5, we would expect far more aftershocks. This is also why we are questioning the link with the quake’s origin and its very surface-level character.”
The qualities of the quake appear similar to other comparable incidents around the world, researchers said, including shocks caused by quarries, oil fields, and mines.
Mr Grasso said: “Some well-known examples happened near quarries in the state of New York (USA) in the 1970s, [and] in Pennsylvania in the 1990s. There were also cases around oil fields in California in the 1990s, and in Uzbekistan in the 1980s.”
But the researcher added: “It is surprising to see quakes linked to human activity on mainland France, at a magnitude of 4 to 5, which is very close to the maximum levels of nearby natural quakes. But in the case of the shock in Teil, this is just one line of enquiry amongst others.”
Sign up to our free weekly e-newsletter
Subscribe to access all our online articles and receive our printed monthly newspaper The Connexion at your home. News analysis, features and practical help for English-speakers in France