Earthquakes within and Near the French Western Margins

These earthquakes are mainly recorded by the French short-period seismic network (CEA/LDG) and some other stations (British and Spanish stations). The location accuracy is rather poor in distance and depth with a magnitude detection threshold between ML = 2 and ML = 3. Nevertheless, the seismicity map we obtained complemented by some selected fault-plane solutions contribute to describe and confirm some tectonical aspects of that Western Europe margins region. According to the instrumental seismicity the Western Europe margins between Spain and Ireland appear to be weakly active. All earthquakes are located in the continental Plateau down to the slope foot (abyssal plains being totally aseismic) and most of them at depths between 10 and 26 km within the crust. For the last 25 years, the largest earthquake of the Armorican margin occurred 200km westward from La Rochelle (ML = 4.7 – 25.03.1982). Many hercynian transverse faults parallel to the South Armorican shear zone appear deep within the Armorican Plateau. The boarder fracture along the slope has also this same Armorican orientation from the Meriadzek zone down to the Cape Ferret where a change in orientation towards the Parentis basin is observed. On the other hand, this fracture is intersected with transverse faults approximately perpendicular to it which appear to be active. The North-Spanish margin is limited by a large East-West active fault, a trace of a fossil subduction zone associated with the Bay of Biscay and the Iberic Plate. Fault plane solutions indicate a relatively uniform compression axis of N 170 direction which is coherent with an African to Europe plate collision through the Pyrenean suture.