Our own careers and personal trajectories have been affected as well as those of our colleagues. The Teil earthquake turned the attention and activity of the research community towards seismic hazard and seismicity in intraplate zones and particularly in France. A few fundamental scientific questions must now be addressed.
Identification of other unknown but potentially active faults
We had to adapt our usual reconnaissance strategy: the source of the Teil earthquake was only 5 km long, hitherto unknown to be active, and located in an area a priori unfavorable to morphological analyses. Following our ambitious and successful field campaign, it is now clear that the fault in question was active in the Quaternary, probably as a repeated reactivation of an originally Oligocene age structure.8 (Fig.2). It is worrying to think that other flaws not yet studied, could, in fact, be just as active. There is potentially a lot of work to be done in the longer term. We need to build a clear picture of seismic risk. The overriding lesson we learned this year is that paleoseismological investigation, namely trenching, is a powerful tool for documenting past faulting activity, even in unexpected areas.
Potential surface failure threats and their mitigation
The Teil earthquake focused attention on surface faults and the acute risks they pose to infrastructure: it occurred not only a few kilometers from a nuclear power station, but also in one of the most industrialized regions of France. Methodologies for assessing the risk of superficial faults on infrastructures have been improving for several years in an international framework.9,10,11. This hazard has become a concern for sensitive installations12, 13, and has only just burst onto the French scene. If other sensitive sites are threatened by earthquake-related surface failures, we need to identify and mitigate those risks.
Causes of Intraplate Seismicity
Plate tectonics classically leads to the hypothesis that the cyclic accumulation of stresses on faults leading to earthquakes is essentially controlled by distant tectonics, that is to say in the case of France, the convergence of Africa and Eurasia, the counter-clockwise rotation of the Adria microplate and the extension into the Atlantic Ocean. It is still a commonly accepted model for active areas, including in the French Alps14. However, alternatives have been proposed for intraplate zones that involve different internal and external geodynamic processes at regional and local scales, such as climatic or even anthropogenic factors.15,16,17,18. Geological and paleoseismological data, the only ones that can provide long-term constraints on the timing of tectonic deformation, will be essential to get a clearer picture of the driving forces responsible.