Characteristics and possible origins of the seismicity in northwestern France

Abstract : The macroseismic and instrumental observations accumulated by the Bureau Central Sismologique Français and other national agencies over the last 100 years show that the northwestern part of metropolitan France is affected by an apparently diffuse and moderate intraplate seismicity. Far from any plate boundary, well-documented inherited structures, such as the Armorican shear zone network, the Sillon Houiller, and the normal faults related to the Atlantic ocean margin, likely exert significant control on the regional seismicity pattern. However, in the absence of a clearly measurable strain field, processes other than far-field tectonic stress loading such as erosion, gravitational potential energy, and/or hydraulic loadings can co-exist, but their respective influence on the current seismicity is debated and remains to be fully addressed. Reliable detection/location of low-to-moderate magnitude events is one of the most important challenges in the near future to better understand the processes that control this intraplate seismicity. As shown here for a limited region, this issue can be achieved positively, thanks to the new Résif-Epos network, in conjunction with sophisticated algorithms for both earthquakes’ detection and discrimination.