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Last Updated: Wednesday, 27 February 2008, 11:43 GMT
Q&A: UK's small-scale tremors
Map of Market Rasen (BBC)
The magnitude 5.2 earthquake that struck Lincolnshire has been the biggest tremor to hit the UK in the past 25 years. But the UK, like the rest of northern Europe, is relatively quiet seismically.

What happened and where?

Seismographs indicate that Wednesday's earth tremor was centred approximately 4km north of Market Rasen in Lincolnshire. Experts at the British Geological Survey say it occurred at 0057 GMT at 53.42 degrees North and 0.35 degrees West.

The focus of the tremor - the actual location beneath the surface where the energy is released - was approximately 15km down. So even those sitting directly above the quake were still a little way away from the action.

Just how big is magnitude 5.2?

Since 1 January 2008, there have worldwide been more than three earthquakes per day with magnitudes greater than 5.0.

So although big in British terms, this is slight compared with the nine-plus magnitude quake that rocked Sumatra in 2004 and triggered the Boxing Day tsunami.

The magnitude scales quoted by seismologists are not linear. Each step equates to a 30-fold increase in the release of energy - so magnitude nine is actually 800,000 times bigger than a magnitude five quake.

Also, the numbers do not give any indication of the damage 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.

For this reason, people living on either side of the street can have very different experiences. When someone says: "But I felt nothing", it may be explained simply by the construction used on their house.

How often do such tremors occur in the UK?

There are 200-300 quakes in Britain every year. Most are so small, no-one notices them. Something of Market Rasen's size is seen once every 30 years or so, although they are more common in offshore areas.

This was the largest earthquake in the UK since the magnitude 5.4 Lleyn Peninsula earthquake in 1984, which was widely felt across England and Wales.

The largest recorded earthquake 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.

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 that are being rammed together or pulled apart. Tremendous energy is released when these rocks break or grind past each other.

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

Many of the quakes in the UK are clustered around an enormous block of rock known as the Midlands Microcraton. This is an ancient, Precambrian (older than 590 million 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 of active faults that line the Midlands Microcraton and many of the tremors experienced on the western side of the English Midlands, up to the Pennines, are a result of rocks jostling in this area.

Britain also feels the effects of earthquakes in the North Sea. Here, the crust is much thinner and it is also marked by large numbers of faults.

Remember, the earth beneath our feet is never completely static. The crust is constantly on the move. For example, much of the UK is still rebounding after the melting of the great block of ice that covered the country thousands of years ago.

And even on a daily basis, the crust will move up and down by many millimetres as the tides roll around the island nation.

But, specifically, what can we say about Wednesday's quake?

What is interesting about this event is that there has been no big quake in Lincolnshire since 1755. Further back in history, though, there are quite a few recorded tremors, particularly in the 12th Century. This suggests some old fault may have been reactivated. This will now become a focus for seismic research.

The British Geological Survey will be keen to hear from anyone who experienced the quake. It will be useful to know how strong the shaking was at different distances, measured in terms of the effects that people noticed - rattling windows, falling ornaments and so on.

Knowing the distance at which strong effects might occur in an earthquake is important for safety planning. Engineers at nuclear power stations, for example, need to understand how their plant may be affected by tremors.

If you felt the quake, the BGS is urging you to visit its website and to fill in one of its online forms.

Will there be more shocks in the area?

As with many of the devastating earthquakes that occur in Indonesia, Turkey and Japan, big UK tremors are quite likely to be followed by aftershocks. In the UK, however, these tend to be very much smaller than the main event.

London's small but relentless dip
12 Jul 07 |  Science/Nature

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