Something stirred deep below the surface of a small village in the Upper Swansea Valley on Saturday afternoon which quite literally shook the world.

According to the British Geological Survey (BGS), at 2.31pm on Saturday tectonic movements in the Earth’s crust, 7km below Cwmllynfell, took place resulting in an earthquake which measured 4.4 on the Richter scale.

But why did it happen, and will there be more to come? The BGS has the answers.

The facts about the quake

A spokesman said the earthquake was the biggest in mainland Britain for 10 years and shockwaves were felt as far away as Cornwall and Blackpool.

“This is the largest earthquake on mainland Britain in almost 10 years, since the magnitude 5.2 ML Market Rasen earthquake on 27 February 2008. The earthquake occurred in a part of south Wales that has been struck by a number of other significant earthquakes in the last few hundred years,” he said.

“A magnitude 5.2 ML earthquake in 1906 was one of the most damaging British earthquakes of the 20th Century, with damage to chimneys and walls reported from Kidwelly to Cardiff. Earthquakes with magnitudes of 5.2 and 5.1 occurred near Swansea in 1727 and 1775 respectively.

“BGS received reports of the earthquake being felt from several thousand people. The maximum observed intensity was 5 EMS, which corresponds to strong shaking. The earthquake was felt over much of Wales and southwest Britain.

“An earthquake of this size occurs somewhere on mainland Britain roughly every 4 years. In 2014, a magnitude 4.2 earthquake occurred in the Bristol Channel and was widely felt in south Wales.”

Cwmllynfell was the epicentre of the earthquake

What actually is the epicentre?

Clive Phillips, a 78-year-old member of Cwmllynfell Community Council, did not feel a thing despite living at the epicentre of the quake.

As for what that is - it’s the point on the Earth’s surface directly above where the quake is recorded as occurring - also known as a hypocenter or focus.

The BGS says measurements from three different seismic stations are needed to locate the epicentre. A circle is drawn around each station with a radius equal to its distance from the earthquake. Where the circles intersect is the epicentre of the quake.

Mr Phillips said: “I’m told there’s a fault line running beneath Railway Terrace but it’s not a straight line, it’s zig-zagged. I was in the kitchen but I didn’t hear or feel a thing. My daughter lives next door and her doors were shaking, her dishes were shaking and her dog was going ballistic; it was ill afterwards.

“Apparently we had a tremor in the village in 2008 but I didn’t feel anything then either. I get more bothered by the RAF jets flying overhead on exercise.”

Cwmllynfell where the quake struck

However, Paul Randle, from Saron near Ammanford, and his family were one of many to feel the earthquake.

He said: “We were enjoying a relaxed day at home, I was in our lounge, my wife was upstairs and daughter was in the bath when the house shook, furniture was shaking and mirrors and pictures on the wall were moving around. The noise and thunder like sound was frightening.

“Initially I thought a truck or car had hit the house or that a wall had collapsed.

“I jumped up and made sure my wife and daughter were ok but obviously shook up and scared and went outside to see what damage had occurred when it dawned on me that it had to be an earthquake.

Why did it happen here?

Dr Roger Musson, of the British Geological Survey and one of the world’s leading seismologists, said that Swansea was something of a hot spot for earthquakes in Britain but added that there was no need for alarm.

He said: “The reason for all the earthquakes in the British Isles, the large ones at least, is that the crusts that make up the British mainland are being stressed from the north west.

“It seems that the stress is particularly concentrated in the Swansea area.”

Will it happen again? It already has, apparently...

While some may be alarmed, we will need to wait a long time for the next earthquake.

Dr Musson said: “In British terms we get earthquakes like this every few years. In terms of Swansea we get them on this scale every 75 years although it has been 112 since the last one, in 1906, which was 5.2 on the Richter scale and caused quite a lot of damage.

“The interesting thing with the earthquakes that we have experienced in Swansea is that they don’t tend to have any after-shakes. This time we have only recorded two and they have been tiny and not felt by anyone.”

It was completely natural

Dr Musson also dispelled concerns that man had played a role in the earthquake.

He said: “Open cast mining and fracking would not have had any effect at all. This earthquake happened 7km down from the surface.”

His assessment of the situation was echoed by Dr Geraint Owen a professor of geography at Swansea University .

Dr Geraint Owen
Dr Geraint Owen

He said: “A major fault runs through Cwmllynfell, affecting the coal measures and other carboniferous rocks. It can be traced from the Swansea Valley in the south, near Pontardawe, northwards through Cwmllynfell to the summit of the Black Mountain, where the A4069 road follows a valley eroded along the line of the fault. This Cwmllynfell Fault is probably what shifted on Saturday, generating the earthquake felt across much of Wales.”

Adding that there was no need for concern he said: “South Wales is one of the most seismically active parts of Britain. A larger earthquake (magnitude 5.2) affected Swansea in 1906, the same year as the well-known - and much more damaging - San Francisco earthquake, and there were others in 1868, 1832, 1775 and 1727. More recently, people in the Swansea area have felt earthquakes generated by faults in the Bristol Channel (2014, magnitude 4.1), the West Midlands (2002, magnitude 4.7), Welsh Borders (1990, magnitude 5.1) and North Wales (1984, magnitude 5.4).

"All of these have been minor earthquakes, causing little or no damage. Because of our position well away from tectonic plate boundaries, it is exceedingly unlikely that the Swansea area will ever experience a large, damaging earthquake.”

He also rejected claims that the earthquake might have been man-made, saying: “The fracturing of rocks that caused Saturday’s earthquake occurred 7km beneath the ground.

"Shock waves spread out from the point of fracturing, and it is these shock waves that were felt and measured. 7km is much deeper than any mining activity and deeper than wells are drilled for fracking. It is not likely that human activity played any part in generating Saturday’s earthquake.”

What causes an earthquake?

Earthquakes occur because the Earth’s crust is moving. The build-up of stress causes rocks to crack and move along faults. The biggest faults are the edges of the Earth’s tectonic plates. It is along these plate boundaries that major, damaging earthquakes occur (magnitude 6, 7, 8 or 9), such as in California (San Andreas Fault), Japan or Turkey. Away from plate boundaries, earthquakes occur less frequently and are usually much smaller; these are called intraplate earthquakes.

Faults are common in most rocks. Faults in the rocks of south Wales were active hundreds of millions of years ago, when movements occurred regularly along them, probably associated with large earthquakes at that time. It is along these faults that intraplate earthquakes can occur.