Thursday 4 July 2013

Magnitude 2.9 Earthquake in northern France.

A Magnitude 2.9 Earthquake at a depth of 10 km occurred beneath the commune of Miniac-Morvan, near St Malo in Brittany, northern France, slightly after 11.35 pm local time (slightly after 9.35 pm, GMT) on Wednesday 3 July 2013, according to the Laboratoire de Détection et de Géophysique. There are no reports of any damage or casualties, nor of anyone noticing the quake, though it was large enough that it is likely there were people who felt it.

Map showing the location of the 3 July 2013 quake (red star) and other historic quakes in the same region. Laboratoire de Détection et de Géophysique.

The precise cause of Earthquakes in France can be hard to determine; the country is not close to any obvious single cause of such activity such as a plate margin, but is subject to tectonic pressures from several different sources, with most quakes probably being the result of the interplay between these forces.

France is being pushed to the east by the expansion of the Atlantic Ocean and to the north by the impact of Africa into Europe from the south. It is also affected by lesser areas of tectonic spreading beneath the North Sea, Rhine Valley and Bay of Biscay. Finally the country is subject to glacial rebound; until about 10 000 years ago much of the mountainous and upland areas of the country were covered by a thick layer of glacial ice, pushing the rocks of the French lithosphere down into the underlying mantle. This ice is now gone, and the rocks are springing (slowly) back into their original position, causing the occasional Earthquake in the process. 

Glacial rebound seems an unlikely cause of Earthquakes in the Channel region, an area that was never glaciated, but this is not entirely the case. The northwest of Scotland is rising up faster than any other part of the UK, but the Earth's crust onland in the UK is fairly thick, and does not bend particularly freely, whereas the crust beneath the Channel is comparatively thin and more inclined to bend under stress. Thus uplift in Scotland can cause the entire landmass of Great Britain to pivot, causing movement in the rocks beneath the Channel, and affecting both the English and French coasts.

Witness accounts can help scientists to understand Earthquakes and the geological processes that cause them. If you felt this quake (or if you were in the area and did not, which is also useful information) then you can report it to the Bureau Central Sismologique Français here (note this page will default to the most recent quake, so it may be necessary to follow the 'Testify on another event' link.

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