The Le Teil earthquake of magnitude Mw 4.9 that occurred in the Rhône valley is a historically unprecedented event in mainland France. It is the first time that a surface rupture associated with an earthquake has been observed and characterised “live” in France, and it is the first time that an earthquake associated with the reactivation of an ancient fault that was not listed as potentially active has been observed (the La Rouvière fault is part of the Cevennes fault cluster bordering the Massif Central to the south-east over a length of 120 km; the fault is inherited from the Oligocene extension phase during which the Gulf of Lion opened up ~20 million years ago).
This unprecedented earthquake raises many questions, particularly in terms of seismic hazard: had the Rouvière fault already ruptured in the past? Could other faults, particularly at the NE end of the Cevennes cluster, also produce this type of event?
To answer these questions, a collaborative research programme supported by the Institut National des Sciences de l’Univers of the CNRS (project “FREMTEIL 2021-2023” for Faults, Ruptures and Strong Movements: What consequences for the seismic hazard in the TEIL region, coord. J-F Ritz) was launched with the support of many other partners. Within the framework of this project, numerous palaeoseismological investigations (trenches dug across faults) are being carried out in order to analyse the stratigraphy of Quaternary deposits (deposits with an age of less than 2 million years), and to characterise the existence of surface ruptures prior to the Teil earthquake. This project is also part of the research carried out as part of the FACT (Failles Actives France) axis of the Résif-Epos consortium.
The Major Hazards Institute has published a video presenting this project and the issues involved in the palaeoseismological investigations carried out over the last two years.
To know more
Séisme du Teil #1 La faille © Institut des risques majeurs