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Monday, 23 September, 2002, 08:56 GMT 09:56 UK
Q&A: UK's small-scale tremors
Graphic, BBC
As large parts of England and Wales are hit by an earthquake measuring 4.8 on the Richter scale, BBC News Online looks at the history and causes of tremors in the UK.

What happened and where?

Seismographs started to pick up Monday's earth tremor, centred on the English Midlands, at 2353 and 14.7 seconds GMT (0053 BST). Experts at the British Geological Survey say the epicentre was located at 52.52 North/2.14 West - which is close to Dudley.

The focus of the tremor, the actual location beneath the surface where the energy is released, was 9.7 kilometres down. So, even those sitting directly above the quake were still some 10 km away from the action.

Just how big is 4.8?

Although reasonably big in British terms, this is slight compared with the seven-plus quakes witnessed elsewhere in the world. The Richter scale is a logarithmic measurement: a six is ten times more powerful than a five; a seven is a hundred times more powerful, and so on.

Also, the scale is a measure of the energy involved, not of the damaged caused. This will depend on many things: the number and quality of buildings close by, the nature of the soils on which they are built, etc.

How often do such tremors occur in the UK?

There are 2-300 quakes in Britain every year. Most are so small, no-one notices them. Something on the Dudley size is seen once every 10 years or so.

The largest tremor within 100 km of Monday's event in recent times was a 5.1 at Bishops Castle on the Welsh Borders in 1990.

The largest recorded earthquake to be experienced in the UK occurred in 1931 and measured 6.1. The epicentre was Dogger Bank in the North Sea and so had little impact on the mainland.

The largest earthquake recorded on the UK mainland was in 1984 when the Llyn Peninsula in north Wales suffered a quake of 5.4 magnitude.

What causes the earthquakes in the UK?

The largest earthquakes in the world occur close to plate margins, areas of the Earth's upper layers which are being rammed together or pulled apart. Tremendous energy is released when these rocks grind past each other.

The UK lies well away from the world's tectonic hotspots but they still play a role in the country's relatively small tremors.

Researchers studying Monday's quake will centre their attention on a enormous block of rock known as the Midlands Microcraton. This is an ancient, Precambrian (older than 590 millions years) feature that runs up through Birmingham towards the Potteries.

It is composed of harder rocks than those either side of it. Although the details are not well understood, it seems likely that, in response to tectonic pressures originating in the Atlantic (where the surface of the Earth is being pulled apart), those softer rocks on either side are disturbed.

There are a number active faults that line the Midlands Microcraton and it is almost certainly one of these faults that has moved to cause the tremors felt in Dudley.

Will there be more shocks in the area?

As with many of the devastating earthquakes that occur in India, Turkey and Japan, there are quite likely to be aftershocks. Some could approach the scale of the Dudley event, but most will likely prove so small they will only show up on special measuring equipment.

 WATCH/LISTEN
 ON THIS STORY
The BBC's John Sudworth
"It's Britains biggest quake in ten years"
Insp. Bob Wilson of Merseyside Police
"We have had around 30 calls from the Merseyside area regarding the tremor"
Dr Chris Browitt
"It is not as unusual as many people might think"

Talking PointTALKING POINT
Earthquake
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See also:

23 Sep 02 | UK
23 Sep 02 | England
23 Sep 02 | UK
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