Saturday, 22 February 2014

Bristol Earthquake

You have probably all heard about the magnitude 4.1 earthquake that hit the Bristol Channel on Thursday at 1:21pm. It was felt by people in South Wales, Somerset and Devon (unfortunately not as far as Plymouth!), but did not cause any significant damage.

Earthquakes in the UK are more common than you might think - we have around 20-30 each year that are felt by people, and hundreds of smaller ones that are felt by sensitive instruments. The largest British earthquake occurred near the Dogger Bank, 60 miles offshore in the the North Sea, and it had a magnitude of 6.1. As it was so far out to sea, the damage was reduced, but it was still powerful enough to cause minor damage to some east coast buildings. The most damaging British earthquake occurred in Colchester in 1884 with a magnitude of 4.6, damaging 1,200 buildings. The picture below shows a map earthquakes from 1932 to 1970 with a magnitude of 3 and above (yellow) and earthquakes from 19070 to present with a magnitude of 2 and above (red).

UK seismicity (courtesy of the BGS)
The UK experiences a magnitude 2 earthquake roughly once every two years, and a magnitude 5 earthquake roughly once every 10-20 years. Most earthquakes occur on the western side of the British mainland, and are nearly absent from eastern Scotland and north east England. These earthquakes are due to the many faults in Britain. The actual driving forces are unclear, however it is likely that they include regional compression due to the motion of the Earth's plates and uplift due to the melting of ice sheets that used to cover Britain.

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