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Mainland France is part of a plate interior with a strong structural heritage, undergoing a low rate of deformation, where destructive earthquakes can nevertheless occur. In this paper, we emphasize that the knowledge of active faults is still largely fragmentary, and that significant efforts are needed to generate robust data, in particular on the numerous faults, that still lack any study. This is the aim of the “Failles ACTives France” (FACT) axis launched in the framework of the Transverse Seismicity Action (ATS) of the Resif-Epos consortium. We present some recent investigations carried out along suspected active faults in mountainous areas, their forelands and remote lowlands, which implement new approaches and new tools, and allow characterizing their Quaternary activity.

On November 11, 2019, a M$_{\mathrm{w}}$ 4.9 earthquake hit the region close to Montelimar (lower Rhône Valley, France), on the eastern margin of the Massif Central close to the external part of the Alps. Occuring in a moderate seismicity area, this earthquake is remarkable for its very shallow focal depth (between 1 and 3 km), its magnitude, and the moderate to large damages it produced in several villages. InSAR interferograms indicated a shallow rupture about 4 km long reaching the surface and the reactivation of the ancient NE–SW La Rouvière normal fault in reverse faulting in agreement with the present-day E–W compressional tectonics. The peculiarity of this earthquake together with a poor coverage of the epicentral region by permanent seismological and geodetic stations triggered the mobilisation of the French post-seismic unit and the broad French scientific community from various institutions, with the deployment of geophysical instruments (seismological and geodesic stations), geological field surveys, and field evaluation of the intensity of the earthquake. Within 7 days after the mainshock, 47 seismological stations were deployed in the epicentral area to improve the Le Teil aftershocks locations relative to the French permanent seismological network (RESIF), monitor the temporal and spatial evolution of microearthquakes close to the fault plane and temporal evolution of the seismic response of 3 damaged historical buildings, and to study suspected site effects and their influence in the distribution of seismic damage. This seismological dataset, completed by data owned by different institutions, was integrated in a homogeneous archive and distributed through FDSN web services by the RESIF data center. This dataset, together with observations of surface rupture evidences, geologic, geodetic and satellite data, will help to unravel the causes and rupture mechanism of this earthquake, and contribute to account in seismic hazard assessment for earthquakes along the major regional Cévenne fault system in a context of present-day compressional tectonics.

The SISFRANCE database aims to collect and interpret archives relating the historical seismicity in metropolitan France. The database presently contains some 12,500 documents that allow to define 108,832 macroseismic data points, for a total of 6427 events, of which 5743 can be qualified as real earthquakes. Although half of these events are and will probably remain poorly known, completing our knowledge of historical earthquakes remains a fundamental element for the definition of seismic hazard and the understanding of seismicity in France and low to moderate seismicity regions in general. The first aim of this paper is to explain the state of the art of the database and remind end users how the limits introduced by the necessary interpretation of archival data are translated into quality factors that should be carefully considered when using such data for seismotectonic and seismic hazard purposes.Originally built as part of the necessary datasets to define the seismic hazard for nuclear facilities, the SISFRANCE database is the result of 40 years of work within a consortium bringing together the French civil nuclear operator (EDF), the French geological survey (BRGM), and the French institute of nuclear safety (IRSN). SISFRANCE is also the heir to archival research carried out since the second half of the 19th century, thus constituting the richest collection of data related to historical seismicity in France. The second aim of this paper is to explain to end users the current state of archival research within the SISFRANCE consortium, present avenues for future research strategies, and list potential improvements of the database structure.Archival research within the SISFRANCE consortium, no longer carried out by a dedicated historian since 2018, is presently being pursued by engaging individual researchers or academics, focusing research on specific archives and regions, as well as by developing data-mining techniques to exploit the increasing wealth of accessible numerical archives. Given the new impetus of French governmental research agencies that encourage transdisciplinarity research, SISFRANCE is also pursuing the idea of further engaging the academic community of historians and seismologists, presently greatly underrepresented in the field of historical seismicity in spite of the importance of this data, fundamental for any seismotectonic and seismic hazard study in France.