Monday 9 February 2015

Series of Earthquakes of the south coast of Cornwall.

The British Geological Survey recorded a series of small Earthquakes off the south coast of Cornwall (about 45 km to the southeast of Falmouth) on Saturday 7 February 2015. The had a Magnitude of 1.2 and occurred at about 6.12 am GMT, the second had a Magnitude of 1.7 and occurred at about 6.27 am GMT and the third a Magnitude of 1.3 and occurred at about 6.32 am. All were at a depth of about 3 km. Quakes of this size do not present any threat to human life or property, and this far offshore it is highly unlikely that they were felt by anyone.

The approximate location of the 7 February 2014 Earthquakes. 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 presures 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 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.

See also...

The British Geological Survey recorded a Magnitude 1.8 Earthquake at a depth of 8 km under the English Channel, roughly 60 km o the southeast of Falmouth in Cornwall, slightly before 11.15 am British Summertime...

Walkers between Bowleaze Cove and Redcliff Point, near Weymouth in Dorset, reported the appearance of a landslip on the cliffs on Thursday 14 March 2013, with a section of the cliff 50 m long sinking 2 m from the rest...

On Sunday 30 December 2012, slightly before 3.00 pm GMT, the British Geological Survey recorded a Magnitude 1.3 Earthquake 7 km beneath the Somerset town of Highbridge, roughly 15 km south of...

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