Britain's latest earthquake was felt in an arc around the Irish Sea. But what makes the Llyn peninsula so prone to tremors? And when will the next one be?
The Llyn peninsula in north-west Wales is known locally as the "Dragon's Tail". At 4.15am on Wednesday morning, the proverbial Welsh dragon flexed its tail as a 3.8 magnitude earthquake struck the area. The tremor was felt as far away as Merseyside to the east, north to the Isle of Man and over to Galway in Ireland to the west. The epicentre was under the Irish Sea, eight miles from the town of Abersoch, Gwynedd, where the quake woke people up.
Little damage was reported, but the Llyn peninsula is developing something of a reputation. On 19 July 1984, it was the location for the largest recorded onshore quake in the UK when a 5.4 magnitude tremor caused structural damage to buildings and a handful of minor injuries. Two smaller quakes – measuring magnitudes of 4.0 and 4.3 – rocked the area up to a month later. The British Geological Survey's statistics show that in the last 40 years, the only other place in the UK to record three significant quakes is Kintail in the north-west Highlands (although towns from Folkestone through Melton Mowbray to Longtown in Cumbria have all reported quakes of similar magnitude in that period).
Robert Holdsworth, a structural geologist at Durham University, says the Llyn peninsula earthquakes are an enigma to researchers: "This area seems to be a hotspot for tremors. It's part of a wider band of activity that spreads from north Wales, up through north-west England into Scotland. We don't really understand what's causing them, unlike, say, the San Andreas fault line in California, which lies on a plate boundary. And we don't understand why they localise where they do."
The UK, which is positioned "mid-plate", as opposed to straddling a volatile plate boundary, experiences between 20 and 30 tremors each year strong enough to be felt by people, says Holdsworth, although quakes of magnitude 5 or larger are very rare.
"We know there are two driving forces in the UK," he says. "First, there is the 'ridge push' caused by plate-tectonic activity from the mid-Atlantic ridge under the ocean. Second, there is the fact that 20,000 years ago this area was covered in ice 1km thick. Even though it melted thousands of years ago, the release of that weight is still causing so-called 'bounce back' in the crust. Imagine the end of a diving board springing back when you jump off, but in very slow motion."
It is still impossible to predict earthquakes, stresses Holdsworth, with researchers preferring to talk in probabilities. He says a magnitude 4 quake will occur roughly every two years in the UK, whereas a magnitude 5 event will occur every decade or two. And the next "big one"? It isn't time to panic yet. A magnitude 6 earthquake is powerful enough to fell chimney stacks and knock over heavy furniture. Holdsworth says that, statistically, the UK could experience one within the next 10,000 years.