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SEISMICITY IN FRANCE
Mainland France is a moderately seismic region. The frequency and magnitude of its earthquakes are much lower than in Italy or Greece. Destructive earthquakes are however possible with magnitudes that may exceed 6. The epicentres of the earthquakes are almost all located below the earth’s crust, at depths of less than 15km. The seismicity of the entire Alps-Pyrenees-North Africa zone is related to the slow convergence – averaging a few millimetres per year – of the African and Eurasian tectonic plates. As well as this large-scale tectonic convergence, there are also secondary phenomena, such as the isostatic rising of orogenic chains after erosion, and deep gravitational movements within sedimentary formations.
There have been no earthquakes in the last thousand years with a magnitude of more than 7 in mainland France or its immediate proximity, with the possible exception of the Basel earthquake in 1356. There have however, been several earthquakes of magnitude 6 in mainland France over the past thousand years, the latest of which occurred in Provence on June 11, 1909.
Many minor earthquakes (magnitude 1 or less) are detected every year in France. The average number of earthquakes increases by a factor of 10 when the magnitude threshold is reduced by 1. Estimates suggest that every century in France, there is approximately one earthquake of magnitude 6 or more, ten earthquakes of magnitude 5 or more, one hundred of magnitude 4 or more and more than one thousand earthquakes of magnitude 3 or more. Worldwide over the same period, there are more than 10,000 earthquakes of magnitude 6 or more. On average, about twenty earthquakes of magnitude 3.5 or more are detected every year in mainland France, while there are more than a thousand earthquakes of this magnitude throughout the Mediterranean basin.
Earthquakes of magnitude 3.5 are generally felt up to about thirty kilometres from the epicentre. The Rambervillers earthquake (February 2003), magnitude Mw 4.8, was felt more than 200km away. Information on the intensity of the earthquakes felt is important for the study of seismic ground movements, which is why the BCSF, via its website, "www.franceseisme.fr""www.franceseisme.fr", gathers witness reports after earthquakes using on-line questionnaires.
The main geographic regions that experience seismic activity are the Pyrenees, the Alps, the Massif Armoricain, the Massif Central, the Jura and the Vosges-Fossé Rhénan system. Provence, the Ligurean sea and Corisca also deserve a mention. The Paris basin and the Aquitaine basin are two almost aseismic areas.
Regional seismicity and tectonic contexts:
Evolution of seismicity in mainland France between 1997 and 2005:
http://www.franceseisme.fr/sismicite.html
The DOM-TOM overseas territories experience much stronger seismic activity than mainland France. Among the DOM-TOM territories, the Antilles has the highest level of seismic activity. The Lesser Antilles archipelago, formed by subduction of the North American plate under the Caribbean plate, has experienced many seismic catastrophes, particularly in 1690 in north west Guadeloupe, in 1831 east of Martinique and most importantly, the earthquake on February 8 1843 which destroyed Pointe-à-Pitre, causing the death of some 15,000 people. More recently, in 1985, an earthquake of magnitude 6 caused significant damage in Guadeloupe and Pointe-à-Pitre.
In general, the tectonic earthquakes in the Antilles can be classed into three types:
- Subduction earthquakes, caused by contact between the American plate (carrying the Atlantic ocean) and the Caribbean plate. These are inter-plate earthquakes with focal mechanisms, generally reverse faulting.
- Intra-plate earthquakes of intermediary depth, occurring within the downward moving American plate. These earthquakes often exhibit normal fault mechanisms. The earthquake on March 19 1953, south-west of Martinique, was probably an intra-plate rupture at intermediary depth, similar to the quake on November 29 2007.
- Crust intra-plate earthquakes, which generally occur at shallow depths of the Caribbean plate, like the November 21 2004 earthquake in Guadeloupe. They generally exhibit des normal fault or strike-slip fault mechanisms.
Reunion Island mainly suffers earthquake of volcanic origin.
The BCSF, via its "www.franceseisme.fr"website, gathers witness reports after an earthquake, using on-line questionnaires.
