Processes and deformation rates generating seismicity in metropolitan France and conterminous Western Europe

Abstract

Most of metropolitan France and conterminous Western Europe is currently located within the Eurasia intraplate domain, far from major plate boundaries (the Atlantic ridge and Nubia – Eurasia convergence zone). As in other intraplate regions, present-day deformation and seismicity rates are very slow, resulting in limited data and strong uncertainties on the ongoing seismotectonics and seismic hazards. In the last two decades, new geological, seismological and geodetic data and research have brought to light unexpected deformation patterns in metropolitan France, such as orogen-normal extension ca. 0.5 mm yr−1 in the Pyrenees and Western Alps that cannot be associated with their mountain-building history. Elsewhere, present-day deformation and seismicity data provide a partial picture that points to mostly extensive to strike-slip deformation regimes (except in the Western Alps foreland). A review of the numerous studies and observations shows that plate tectonics (plate motion, mantle convection) are not the sole, nor likely the primary driver of present-day deformation and seismicity and that additional processes must be considered, such as topography potential energy, erosion or glacial isostatic adjustment since the last glaciation. The exact role of each process probably varies from one region to another and remains to be characterized. In addition, structural inheritance (crust or mantle weakening from past tectonic events) can play a strong role in deformation localization and amplification up to factors of 5–20, which could explain some of the spatial variability in seismicity. On the basis of this review, we identify three research directions that should be developed to better characterize the seismicity, deformation rates and related processes in metropolitan France: macroseismic and historical seismicity, especially regarding moment magnitude estimations; geodetic deformation, including in regions of low seismicity where the ratio of seismic to aseismic deformation remains a key unknown; an integrated and consistent seismotectonic framework comprising numerical models, geological, seismological and geodetic data. The latter has the potential for significant improvements in the characterization of seismicity and seismic hazard in metropolitan France but also Western Europe.