Saturday 19 December 2015

Magnitude 3.0 Earthquake in the Brittany region of northwest France.

The Centre Sismologique Euro-Méditerranéen recorded a Magnitude 3.0 Earthquake at a depth of 2 km  in the  Ille-et-Vilaine Department of the Brittany region of northwest France, at about 9.30 am local time (about 8.30 am GMT) on Thursday 17 December 2015. There are no reports of any damage or injuries associated with this event (and nor would they be expected with a quake this size), but people have reported feeling it locally.

The approximate location of the 17 December 2015  Ille-et-Vilaine Earthquake. Google Maps.

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 on land 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.

 (Top) Simplified diagram showing principle of glacial rebound. (Bottom) The extent of glaciation in Europe at the last glacial maximum. Wikipedia.

Witness accounts of Earthquakes can help geologists to understand these events, and the structures that cause them. The international non-profit organization Earthquake Report is interested in hearing from people who may have felt this event; if you felt this quake then you can report it to Earthquake Report here.
See also... 4.9 Earthquake in southern France.                                                          The Centre Sismologique Euro-Méditerranéen recorded a Magnitude 4.9 Earthquake at a depth of 5 km  in the  Hautes-Alpes Department of southeastern... 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... in the Auvergne Region, France, On Thursday 12 July 2012, at 3.22 am local time (1.22 am GMT) an Earthquake occurred in the Auvergne Region of France, roughly 19 k southwest of Clermont-Ferrand, according to the Centre Sismologique Euro-Méditerranéen, who reported the quake as measuring 3.0 on the Richter Scale and occurring at a depth of 2 km. A quake of this size is highly unlikely to have caused any damage or casualties, but may...
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