One year ago, the Teil earthquake of November 11, 2019, of magnitude Mw 4.9, shook the Ardèche and heavily damaged the epicentral region with an intensity EMS98 estimated at VIII, causing very large damage estimated at several tens of millions of euros.
After the first media frenzy, notably after the publication of an expert report commissioned by the CNRS, scientists were able to begin a detailed analysis of this earthquake with characteristics that were unprecedented in the French seismotectonic landscape, with a surface depth of about 1 km, very few aftershocks, ground accelerations that exceeded the value of g, and a surface rupture of 5 km along which the ground moved an average of 10 cm in shortening.
Analyses have shown that the surface rupture is associated with the reactivation of the Rouvière fault – a fault inherited from the Oligocene extensional phase that was not previously mapped as active. An important issue raised in an article published in Communication Earth & Environnement (Nature) is the risk of surface rupture on other faults of the Cevennes network, as well as on other fault networks in metropolitan France that have not been mapped as active, such as the Rouvière fault. Another important question also raised by this study is whether the La Rouvière fault was reactivated for the first time since the Oligocene or whether it had already ruptured the surface during previous paleoseismic events, without these ruptures having been detected at the surface due to the rarity of this type of event and erosion.
To answer these questions, paleoseismological investigations, supported in particular by funding from the CNRS-Insu-Tellus program, have been launched since June 2020 on the entire northeast Cévennes network. About fifteen trenches have already been made in the northern section of the Rouvière fault (the one that ruptured during the Teil earthquake) and preliminary results suggest that it would have effectively broken the surface at least once before 2019 in a period of time from the end of the Pleistocene to the historical period. The data collected is still being analyzed and will further refine these preliminary results.
As for the analysis of the strong movements, geophysical investigation campaigns were conducted in the fall of 2020. These investigations will help to understand the spatial distribution of damage in the villages of Teil and St. Thomé. They will also allow to refine the numerical predictions of the strong movement during the Teil earthquake and the damage suffered in some historical buildings instrumented after the earthquake. These near-surface imagery will also be used to refine the 3D geological model currently under construction by the ENS of Lyon.
Read the full article in Resif Newsletter n°18 (french)
One of the trenches dug on the northern section of the La Rouvière fault. The vertical white band could be a trace of previous seismic activity © J.F. Ritz, Geosciences Montpellier. To know more about it.