Sunday, 31 December 2017

Magnitude 1.9 Earthquake off the coast of Devon.

The British Geological Survey recorded a Magnitude 1.9 Earthquakes off the south coast of Devon (about 40 km to the southeast of Dartmouth), slightly before 9.00 am GMT on Sunday 31 December 2017. Quakes of this size do not present any threat to human life or property, and there are no reports of this one having been felt by anybody.

The approximate location of the 31 December 2017 English Channel Earthquake. Google Maps.

Earthquakes become more common as you travel north and west in Great Britain, with the west coast of Scotland being the most quake-prone part of the island and the northwest of Wales being more prone  to quakes than the rest of Wales or most of England. However, while quakes in southern England are less frequent, they are often larger than events in the north, as tectonic pressures tend to build up for longer periods of time between events, so that when they occur more pressure is released.

The precise cause of Earthquakes in the UK 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.

Britain 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 north of the country was covered by a thick layer of glacial ice (this is believed to have been thickest on the west coast of Scotland), pushing the rocks of the British 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.

 
 (Top) Simplified diagram showing principle of glacial rebound. Wikipedia. (Bottom) Map showing the rate of glacial rebound in various parts of the UK. Note that some parts of England and Wales show negative values, these areas are being pushed down slightly by uplift in Scotland, as the entire landmass is quite rigid and acts a bit like a see-saw. Climate North East.

Glacial rebound seems an unlikely cause of Earthquakes beneath the English Channel to the south of Cornwall, 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.

Witness accounts of Earthquakes can help geologists to understand these events, and the structures that cause them. If you felt this quake, or were in the area but did not (which is also useful information) then you can report it to the British Geological Survey here.

See also...

http://sciencythoughts.blogspot.co.uk/2017/09/magnitude-19-earthquake-off-southwest.htmlhttp://sciencythoughts.blogspot.co.uk/2017/04/three-year-old-boy-rescued-from.html
http://sciencythoughts.blogspot.co.uk/2015/12/magnitude-30-earthquake-in-brittany.htmlhttp://sciencythoughts.blogspot.co.uk/2015/11/sinkhole-devours-garden-in-st-ives.html
http://sciencythoughts.blogspot.co.uk/2015/10/magnitude-10-earthquake-off-coast-of.htmlhttp://sciencythoughts.blogspot.co.uk/2015/07/magnitude-13-earthquake-off-east-coast.html
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