Julian Bukits was one of only a handful of people in Edinburgh to have been woken up by the UK's biggest earthquake for more than 20 years.

Within seconds of it registering at 12.56am, an electronic pager had gone off, alerting the seismologist to the fact that more than 18 UK sensors had picked up the 5.2 tremor. Within minutes, he had checked the data on his home laptop and, by 1.20am, was in the offices of the British Geological Survey (BGS) with around eight colleagues, dealing with the flood of interest it provoked.

Others felt the impact of the quake more physically.

David Bates, a 19-year-old student from Wombwell, South Yorkshire, thought to have been the only casualty of the night, suffered a fractured pelvis when a chunk of masonry fell from a chimney stack into his room while he was watching television. His father, still clearly baffled, told reporters yesterday morning: "Of all the things that can happen - an earthquake. I could not believe it but when I think about it, it could have been worse."

Up and down the country, the phone lines of emergency services were jammed by startled residents who had felt their houses shaking, seen plaster fall off walls or heard windows smash. Though the epicentre of the quake was in Market Rasen, Lincolnshire, around 15 miles south west of Grimsby, reports spread as far as Newcastle, Yorkshire, London, Cumbria, the Midlands, Norfolk and parts of Wales.

A few even felt wobbles in southern Scotland, where Dumfries and Galloway Police confirmed they had received a call from a lady who had felt a "very slight tremor".

The Association of British Insurers said that the cost of damage to homes and property is likely to be in excess of £10m and, while most homeowners are expected to be covered, the association warned that a substantial number would not be.

As Mr Bukits and his colleagues quickly established, yesterday's earthquake was the largest since 1984 when a tremor measuring 5.4 on the Richter scale shook the Lleyn Peninsula of north Wales and was widely felt across England and Wales. By lunchtime yesterday, the BGS had received around 200 e-mails from residents who had felt its effects and expected to receive more than 400 by the evening.

To give an indication of the power unleashed by yesterday's tremor, the 32 kiloton explosion of the atomic bomb dropped on Nagasaki in Japan on August 9, 1945 would have registered 5.0 on the Richter scale had it been underground (in fact, its seismic effect was negligible as it detonated in the air).

And the effects of the biggest earthquakes far exceed any explosion mankind is capable of generating. While the 50 megatons produced by the Tsar Bomba, the largest thermonuclear weapon ever tested, would have registered 7.0, it pales beside the 2004 Indian Ocean earthquake, which registered 9.3 and caused a tsunami which led to the loss of nearly 300,000 lives.

The reason Britain is, thankfully, shielded from some of the most destructive powers the Earth's geology can unleash, Mr Bukits explained, is that it does not lie on or near any major fault lines which separate the tectonic plates that make up the Earth's crust.

"The UK, like the rest of Europe, is an interplate region. It is not on any tectonic fault lines, like the San Andreas fault, which lie between different tectonic plates. So we don't get big earthquakes registering seven or eight on the Richter scale.

"The reason we still get earthquakes - we measure, on average, around 200 a year - is that the UK is criss-crossed with geological faults. These are small faults within the Earth's crust which are always on the move. Mostly, they go unnoticed but we have one on this scale, on average, once every 25 to 30 years."

The pattern of earthquakes recorded by BGS instruments bears out this geological explanation. In Scotland, for instance, most major tremors have been felt along the country's three major fault lines: the Highland Boundary Fault, which runs from Loch Lomond to Stonehaven; the Great Glen Fault, which runs from Fort William to Inverness and beyond; and the Southern Upland Fault, which reaches from South Ayrshire to the East Lothian Coast.

However, the curious point about yesterday's tremor, Mr Bukits ventured, is that it occurred in an area of low seismic activity - so what caused it?

Among the theories - which ranked as unlikely but possible - forwarded yesterday by experts were that a collision of tectonic plates between Africa and Europe had triggered it.

Another, more likely, trigger is Britain's recovery from the ice age, 10,000 to 20,000 years ago. As the ice melted, the rock below it reacted like a coiled spring, releasing tension of ancient fault lines over the ensuing millennia.

According to Professor Robert Holdsworth, an expert in structural geology at Durham University, many of these fault lines are not mapped or known about.

He said: "Although the causes and controls of UK seismicity are still poorly understood, it is possible that this quake reflects the reactivation of an old fault zone that has lain dormant for tens or hundreds of millions of years. The UK crust is riddled with such old faults which form an important part of our geological heritage. Perhaps this one is just reminding us that it is still there."

Dr Dave Rothery, an Open University volcanologist and earth scientist said the tremor could have been caused by a sediment build-up under the North Sea.

He said: "If you get a build-up of sediment on the bed of the North Sea, that's going to weigh down the crust locally, and make it want to sink a bit. The movement may be gradual or it could be jerking suddenly, so you get a tremor."

Few seismologists believe they can accurately predict where and when another earthquake will occur - the next time Mr Bukits's buzzer wakes him will, it seems, be as impossible for him to predict as the rest of us.