The Antilles concentrate most of the seismic risks in the country, while France and Corsica are less subject to these events.
While Morocco is barely recovering from a 6.8 magnitude earthquake and trying to help its victims, the country still fears possible aftershocks. If the area is characterized by significant seismic activity, it is the violence of this earthquake which surprises specialists, as seismologist Florent Brenguier explains to franceinfo. So, should France fear a similar event? The country is not immune to this hazard. In June, an earthquake measuring 5.3 on the Richter scale was measured in the West. Although only one minor injury had been reported, this earthquake had caused significant material damage, particularly in Deux-Sèvres and Charente-Maritime.
To assess this risk, the Ministry of Ecological Transition updated in 2011 the seismic hazard map, developed using a network of sensors and the history of earthquakes known since the 14th century. It is still the reference document today.
The Antilles, the most exposed area in France
This seismic zoning map establishes a risk scale comprising five levels, ranging from “very weak” has “strong“. “To establish this division, we are interested in the movement of the ground, that is to say whether it will move a lot or not. Consequently, the magnitude of the potential earthquake cannot be the only parameter that comes into account. We must also look at the possible depth of this earthquake. There are also areas where we will have very different reactions depending on the type of soil.” explains Christophe Larroque, geologist at the CNRS GéoAzur laboratory in Sophia Antipolis (Alpes-Maritimes).
The dark red areas represent the territories most likely to experience a very destructive earthquake. Martinique and Guadeloupe are the most affected by this type of event. “These islands are located on the boundary between the Caribbean tectonic plate and the North American tectonic plate. There, strong earthquakes can occur, because they are very large active faults”, advance the geologist.
On the French territory, on the other hand, the risk is more heterogeneous: if the South-East – and in particular the Alps – Alsace and the Pyrenees are affected by an average to moderate risk, most of the territory is only weakly or very weakly exposed to this hazard.
Mainland France is indeed located far from a plate boundary, that is to say the junction between two fragments of the solid envelope of the Earth. This does not prevent the presence of flaws, which are zones of rupture in the earth’s crust, within the plate itself, subject to the movements of other plates. “Forces accumulate in a privileged manner where there are breaks in the ground, often at the level of mountain ranges”then causing an earthquake, explains the Geological and Mining Research Bureau (BRGM).
Among the large French cities, if Paris or Rouen have very little chance of one day feeling the earth shake, Nice or Grenoble are exposed to an “average” risk. Strasbourg, Colmar and Biarritz are classified a notch higher, in a “moderate” hazard zone.
On French territory, an earthquake as powerful as that experienced by Morocco remains unlikely, but is not excluded, according to Christophe Larroque. “In 1887, an event of similar magnitude occurred in the Mediterranean Sea, about twenty kilometers from the French coast”explains the geologist.
Remember that even less powerful earthquakes can cause significant damage. In 1909, in Lambesc (Bouches-du-Rhône) a magnitude 6.2 earthquake killed 46 people and caused numerous damage in the villages around Aix-en-Provence. This is the earthquake the largest recorded in France since 1900.