The La Laigne earthquake shook the Charente Maritime region on June 16. Although the magnitude of the quake was relatively modest (Mw 5), considerable damage was caused in communities close to the epicenter. The scientific community quickly mobilized to better understand the event, deploying geophysical instrumentation in the field.

During the more superficial Le Teil earthquake in Ardèche on November 11, 2019 (Mw 4.9), rapid mapping of surface deformation using radar interferometry (InSAR) enabled the fault responsible for the rupture to be identified, and the slip to the surface to be analyzed in detail. Initial satellite images of deformation in the La Laigne area are dominated by atmospheric disturbances and show no surface rupture or clear deformation, whereas theoretical models predict a few centimetres of horizontal and vertical displacement in the epicentral zone. The permanent GNSS station networks that criss-cross the country (Rénag, RGP, Centipède, Teria, Orphéon, etc.) have no stations within 12 km of the epicenter. Initial processing by the academic community shows no significant displacement at these distances.

IGN’s networks of historical markers, whose position and altitude are measured during regular campaigns, are very dense in the area and may have been distorted by the earthquake. Following coordination between teams from IGN’s geodesy and metrology department and Rénag’s scientific teams, two measurement missions took place over the summer. On the one hand, IGN teams re-measured the vertical position (levelling) of a set of markers located in the heart of the village of La Laigne, and on the other, three teams of LiENSs scientists re-measured the position of some fifteen markers of the so-called detail network using real-time kinematic positioning (RTK) with a view to extracting the displacement associated with the earthquake. It remains difficult to measure centimetric displacements from old geodetic networks, given their imprecision. Levelling measurements should make it possible to reveal them; other measurements should take place shortly. Levelling measurements should make it possible to reveal them; other measurements should take place shortly.

The La Laigne earthquake is an interesting case study for testing this kind of approach in a context of low deformation and moderate seismic activity. The collaboration established between the academic community and IGN teams will enable us to react quickly to future seismic crises and adapt pre-seismic measurement plans.

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Carte de la répartition des stations permanentes GNSS des réseaux académiques et privés opérationnelles au moment du séisme. Les cercles représentent les isodistances par rapport aux épicentres calculés du séisme.

Map showing the distribution of permanent GNSS stations from academic and private networks operational at the time of the earthquake. The circles represent isodistances from the calculated epicenters of the earthquake.

Mesure du triplet de nivellement de La Laigne par les équipe du SGM de l’IGN à l’été 2023.

Measurement of the La Laigne levelling triplet by IGN’s SGM teams in summer 2023. Credits (pdf)

Mesure RTK de la position d’une borne du réseau RDF de l’IGN dans la zone de La Laigne par les équipes du LiENS en juillet 2023. Crédit Médéric Gravelle

RTK measurement of the position of an IGN RDF station in the La Laigne area by LiENS teams in July 2023. Credit Médéric Gravelle