Earthquake in the village of La Laigne in Charente-Maritime: initial analyses

Geological and seismotectonic context

The region affected by the earthquake lies on the northern edge of the Aquitaine basin, south of the southern Armorican massif. More precisely, it corresponds in part to the catchment area of Sèvre Niortaise and forms the southern edge of the Poitevin marshes. The epicentral zone is located approximately halfway between La Rochelle (Charente-Maritime) and Niort (Deux-Sèvres).

The subsoil consists of a sedimentary layer of marl and clay limestone around 150 to 200m thick, dating back to the Jurassic age (between 175 and 150 million years ago approximately), with slight folds facing SSW. This limestone rests on a bedrock of metamorphic rock (micaschists, gneiss) and volcano-sedimentary rock dating back to the Cambrian age (around 500 million years ago). This ancient bedrock outcrops around 30 km to the north, forming the relief of the southern Armorican Massif. These formations are covered locally by at least 2 to 10 metres of clay (mainly illite and kaolinite) of fluvio-marine origin, deposited 19,000 years ago when sea levels rose in the hollows of the Poitevin marshes. This layer of surface clay may have created a site effect, amplifying the seismic movement. This point needs to be verified and quantified by in-situ measurements.

The current structure of the southern Armorican massif and the northern edge of the Aquitaine Basin is the legacy of several successive geodynamic events:

• the Hercynian orogenesis that created the relief of the Armorican Massif (~290-420 Ma);
• the opening of the Bay of Biscay (~150-84 Ma);
• the convergence of the Iberian and Eurasian plates over the last 90 Ma, creating the Pyrenees and the Alps (50 Ma to the present day).

The bedrock of the Hercynian chain, forming the Armorican Massif that is now eroded, extending under the sedimentary overburden of the Aquitaine Basin. The same is true of the main faults, which can be traced from Brittany to the Limousin. The main faults run primarily NNW-SSE to NW-SE, such as the Cholet-Bressuire-Oradour-sur-Glane fault running NW-SE, which is 170 km long but segmented by other faults, and the Parthenay fault running NNW-SSE. Other fault segments run WNW-ESE to WE, identified on the Charente coast and offshore.

These Hercynian faults are pre-existing discontinuities that have been reactivated over the course of geological time by various geodynamic events (see the results of the Convergence and RGF Pyrenees projects). Some of these major faults are still active today, with a movement that is predominantly strike-slip (more locally reverse or normal), in response to the NS to NW-SE compressive regime caused by the convergence of the African plate towards the Eurasian plate. In the epicentral area, this is the case of the Surgères-Blanzay-Aulnay-Échelle fault mapped SE of the epicentre. This structure, which is probably strike-slip, has been mapped in (albeit uncertain) segments intersecting with the Jurassic formations from Mauzé-sur-le-Mignon, extending possibly to the bedrock in Corrèze.

The reactivation of these bedrock faults can sometimes propagate to the surface, intersecting recent overlying sedimentary deposits. At depth or near the surface, these reactivations can lead to earthquakes.

Although the geodynamic context is generally well known with respect to the plates, we do not have sufficient geological knowledge of the subsoil in Charentes Maritimes and neighbouring regions (land and sea) to be able to associate this earthquake with a known tectonic structure. In this region of historical seismic activity, BRGM initiated onshore and offshore studies in collaboration with several universities a few years ago in order to expand existing knowledge of the structural map. However, more targeted investigations are needed, along with data acquisition (structural, geophysical, geomorphological, etc.) on a local scale in order to understand the context of the La Laigne earthquake.