Seismic hazard and seismotectonics in France

Most of metropolitan France territory lies in a zone with little crustal strain (baselines within France change by no more than 2mm/ry as shown from geodetic measurements [Vigny et al, 2002]. This is consistent with the very low slip rates, of the order of a few 0.1mm/yr at most, on the few recognized active faults [e.g., Schlupp et al, 2001, and reference therein]. Nevertheless, background seismicity in France is not negligible (Figure 1) and has produced a few historical earthquakes with local magnitudes above 6. The mechanisms governing the spatial distribution of these earthquakes and their frequency remain poorly understood. Most probably the seismicity of southern France reflects primarily deformation induced by the ongoing collision between Africa and Europe (the convergence rate being of the order of 5mm/yr at most). Elsewhere the mechanism governing the seismicity distribution is more enigmatic and might involve fluids circulations in crystalline massifs [Audin et al, 2002], or other time dependent source of stress such as erosion and sedimentation. It might be noted that the seismicity rate is higher within the exhumed crystalline basement (Massifs Aroricains, Massif central, Alps) and lower within the Paris and Aquitaine basins.

In this context where the active faults and their characteristics are poorly known, a probabilistic approach to seismic hazard seems most appropriate  [Marin et al, 2004]. The attenuation law was determined from local seismic records and peak ground acceleration is computed considering the local surface geology based on a 1/1,000,000 geological map. The seismotectonic zonation consists of areas of diffuse seismicity characterized by a truncated frequency-magnitude distribution. This methodology allows to produce maps of maximum peak horizontal ground accelerations expected for various return periods (Figure 2).

Figure 1: Seismicity of France between 1962-2000 recorded by CEA/DASE/LDG

Figure 2: Expected Peak Ground Acceleration for a 1975-year return period.

Audin, L., J. P. Avouac, et al. (2002). "Fluid-driven seismicity in a stable tectonic context: The Remiremont fault zone, Vosges, France." Geophysical Research Letters 29(6): art. no.-1091.
Marin, S., J. P. Avouac, et al. (2004). "A Probabilistic approach to seismic hazard in metropolitan France." Bulletin of the Seismological Society of America 94(6): 2137-2163. [PDF]

Schlupp, A., J.P. Avouac and G. Clauzon (2001). "Post-Messinian activity of the Nimes fault." Bull. Soc. Geol. Fr. 172: 697-711.