Thursday 23 June 2011

The Newton Abbot Earthquake.

The terms 'Newton Abbot' and 'Earthquake' do not usually go together, but at a quarter to two this afternoon (Thursday 23 June 2011) the Devon town was shaken by an Earthquake with a magnitude of 2.7 on the Richter Scale. People in the town and neighboring villages reported a bang similar to a sonic boom, followed by houses shaking. There are no reports of any significant damage or injuries.

Seismogram of the Newton Abbot Earthquake, from the British Geological Survey.

Normally a magnitude 2.7 Earthquake would pass without notice, detected only by seismometers (the UK is hit by about a dozen earthquakes of this magnitude every year), but this one was close to the surface, at a depth of 1.5 kilometers. For comparison the recent Earthquakes in New Zealand have been at depths of between 5 and 10 km and the devastating earthquake that hit Fukushima in Japan earlier this year was at a depth of 10 km. Earthquakes lose energy rapidly as they pass through the Earth, so they are most powerful close to their hypocenter (the centre of the earthquake, as opposed to the epicenter, the point on the ground above the hypocenter) so they have they are most dangerous to humans when this hypocenter is close to the surface - where we live.

The UK is not a notoriously earthquake-prone country, but we do have a few, and some bits of the country are more earthquake prone than others. The more earthquake prone areas are Cornwall, South-West England, the West Midlands, Wales, North-West England, Yorkshire, Western Scotland and around the southern English cities of Chichester and Dover. The last recorded fatality in a UK earthquake was in 1940 when an elderly lady in Wales fell down a flight of stairs during an earthquake.

The UK is far from an active plate margin, but it is not stationary, it is being pulled across the surface of the Earth by the movement of the plates. Several different plate motions effect the UK. Firstly the Atlantic sea-floor is spreading, with new material being created along the Mid-Atlantic Ridge, pushing North America to the west and Europe to the East. Then Africa, India and Australasia are pushing into Eurasia from the south, pushing the continent northwards; however Eurasia stretches more than half way round the northern hemisphere, so it cannot be pushed evenly north without something giving. Then the North Sea Basin is still expanding, pushing the UK westwards. It is also possible that the UK is still rebounding from the last Ice Age; in geological terms millions of tonnes of snow and ice were removed from the surface of the UK a mere eye-blink ago, so it is not surprising that the country is bobbing up and down a bit. This is of course most pronounced in Scotland; it is not clear if earthquakes in southern England can be attributed to this cause.

A map of Earthquakes in the UK, also from the British Geological Survey.