An earthquake shook the Charente Maritime, Deux Sèvres and Vendée regions in the late afternoon of Friday June 16th at 6.38pm local time. It occurred around 25 km southwest of Niort and 14 km north/northwest of Surgères, near La Laigne. The magnitude is estimated at between 4.8 and 4.9 (1). An aftershock of magnitude 4 (1) was recorded at 4:27 a.m. the following day. Both hypocentres were located at shallow depths (3km for the first, 1km for the second). Numerous smaller aftershocks were recorded in the 24 hours following the main event.
The national post-seismic unit was activated.
An initial geological analysis determined that the epicenters/hypocenters of the main shock on June 16 at 6:38 pm and its main aftershock on June 17 at 4:27 am were located in a zone formed by Mesozoic-age sedimentary layers a few hundred meters thick, resting on Hercynian basement rocks visible in the Armoricain Massif. This zone corresponds to the Armorican margin of the Aquitaine basin. The closest fault to the earthquake in the database of faults potentially capable of producing an earthquake (BDFA) is located around 20 km to the northeast (Saint Maident l’Ecole fault). The most recent terrain affected by this fault is Pliocene in age (between 5.3 and 2.5 million years), which attests to activity contemporary with or subsequent to the deposition of this terrain. However, little is known about the neotectonic activity of these faults.
The epicentral area is also located to the south of the Marais Poitevin, one of the country’s largest wetlands, with potentially thicker and looser “alluvium” that may have locally amplified surface ground movement.
Thanks to the deployment of permanent instrumental networks over the last decade as part of Résif, GNSS and seismological stations (see map of stations) have been able to record the initial information essential to understanding the phenomena underway. Teams from the Résif community, in particular those from the Observatoire des sciences de l’Univers de Nantes, reacted immediately to install additional seismometers – sixteen accelerometers and eight vertical geophones (2) – on June 17, to gather as much data as possible on the aftershocks. They then set off for Grenoble to pick up 5 post-seismic stations and 20 light seismological stations (nodes) from the Sismob national park housed at OSUG. Strasbourg’s OSU (EOST), for its part, is preparing around 50 nodes from its own fleet for delivery on June 20, as is the CEA, with around 40 nodes to be deployed in the region.
Over the weekend, numerous scientists (CNRS, universities, CEA, IRSN) worked to better understand the processes involved. For seismologists, this means determining parameters linked to the seismic source, such as hypocentral location, magnitude, size and rupture mechanism, as well as elements characterizing ground motion, such as macroseismic intensity and maximum acceleration.
As part of the ISDeform National Observation Service (satellite imagery), preparations are being made for an analysis of ground deformation using InSAR (3) satellite radar interferometry. This will enable rapid measurements (interferograms) to be taken as soon as the first post-earthquake images from the Sentinel-1a satellite have been acquired over the epicentral zone and made available to scientists (from Wednesday June 21). Daily and high-frequency time series from GNSS networks are also being analyzed by the Renag to estimate the static displacement field associated with the earthquake, and to determine the relevance of further measurements in the area.
The Groupe d’intervention macrosismique coordinated by BCSF-Rénass will be on site. Eleven people (IRSN, Observatoires Midi Pyrénées, Grenoble and Strasbourg) and two observers (CEA and EDF) will be on hand from Tuesday 20th to assess macroseismic intensities in communes affected by significant damage. The study is based on the effects of the earthquake on buildings (according to their vulnerability), objects and people. These results (4) will be used by the interministerial commission in the procedure for recognizing the state of natural disaster.
Scientists are also much in demand by the media (for example, the person on call at BCSF-Rénass in Strasbourg answered some fifteen calls in the 48 hours following the first earthquake).
Data collection and analysis work continues throughout the week.
- 1- Moment magnitude (Mw). The local magnitude (Ml) has been estimated at 5.3 by BCSF-Rénass and 5.8 by CEA-LDG. This local magnitude (Ml) is used for warning purposes and is measured at high frequency. It is more sensitive to the nature of the rocks crossed by the waves. The moment magnitude (Mw) is measured at a lower frequency. It is more stable and a reference value for estimating the size of the rupture on the fault. Find out more (french)
- 2- Geophone: sensor for recording and/or measuring the velocity vector of seismic vibrations through the ground.
- 3- InSAR – Find out more (french)
- 4- EMS98 intensities – Find out more (french)
Find out more
- Analysis of the June 16 earthquake by B. Delouis, researcher at the Geoazur laboratory and member of the Cellule post-sismique (french – June 21)
- June 16 earthquake (BCSF-Rénass)
- June 16 earthquake (CEA)
- June 16 earthquake (IRSN) :
- Reply of June 17 at 4:27 a.m. (BCSF-Rénass)
- Macroseismic intensity of the June 16 earthquake (BCSF-Rénass)
- Database of faults potentially capable of producing an earthquake : BDFA