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Keywords:

  • focal mechanisms;
  • France;
  • intraplate stress field;
  • relocation of earthquakes;
  • seismotectonics

SUMMARY

Refinement of the seismicity distribution (4574 events) in western and central France, has been done by synthesis of seismological bulletins. Earthquakes have then been relocated by joint hypocentre and velocity structure inversion. The new hypocentre distribution indicates that the seismicity of those regions is much less diffuse than previously thought, mainly with regard to the depth distribution. The hypocentre improvement allows us to compute 44 new focal mechanisms and to revise bibliographic focal mechanism solutions. Then, the regional stress field was determined from 119 available focal mechanisms. It is characterized by a regionally significant strike-slip regime with NW-trending σ1. However, the refinement in location and increasing available focal mechanism solutions allow us to show that this strike-slip regime is overprinted by local extensional perturbations in three distinct areas. In the Massif Central, the Sillon Houiller, a ancient vertical shear zone, appears to be acting as a passive boundary between a western unit and an eastern unit that is uplifted by the ascension of a hot mantle plume at the base of the lithosphere. Extension is unexpectedly observed in the southern Armorican Massif (SAM), as well as in northwestern Massif Central. One can observe a good relation between the observed perturbed zones and lateral variation of Pn anisotropy in the mantle. This correlation and the scale of these areas are arguments in favour of a lithospheric process as the origin for the stress characteristics. We believe these extensional perturbations could be related to the anticlockwise rotation of the Iberian microplate and/or incipient subduction in the Bay of Biscay.