On a bright, sunny summers morning a loud rumbling, that grew louder and louder as each second passed, work up thousands of people in Gwynedd.

Many took to the streets, some still in their nightclothes, fearing one of the nuclear power stations at Trawsfynydd or Wylfa had blown up.

It was just before 8am of July 19, 1984 when the realisation struck that North Wales was at not at war, was not facing nuclear meltdown but at the epicentre of a huge earthquake, at least by northern European standards.

Measuring 5.4 on the Richter scale it last for 12 seconds and the British Geological Survey (BGS) later declared it the largest onshore earthquake in the UK in the 20th century.

Yr Eifl
Yr Eifl

Centred on Llithfaen it was felt over an area of around 250,000 square kilometres.

A BGS spokesman said at the time: “Most of the UK would have felt it to a degree, right up to Edinburgh. They happen very rarely.”

Gwynedd named as a top UK earthquake hotspot

After the rumbling stopped electricity supplies were cut off to large parts of Gwynedd. Elsewhere animals stampeded in shock and dry stone walls collapsed.

The gable end of a house near Aberdaron collapsed and the outer walls of another in the area started bulging outwards. A crack appeared in the spire of a chapel in Pwllheli and chimney stacks were toppled over a wide area.

Information gathered from 7,000 BGS questionnaires issued in the Gwynedd area revealed many people had held onto furniture to steady themselves, pictures and mirrors were knocked against walls, heavy furniture moved and water splashed out of fish tanks.

Yr Eifl on the Llyn Peninsula
Yr Eifl on the Llyn Peninsula

One parent thought his 15-month old son, sleeping in his room, had woken up and was shaking the cot against the wall. He shouted at him to stop not realising what was happening until later.

Seismologists rushed to Gwynedd to experience the expected aftershocks which continued for several months. The experts argued amongst themselves if these shocks were aftershocks or separate earthquakes. One measured 4.3 on the Richter scale.

Earthquake hits North Wales amid reports of 'loud bang' heard in parts of Gwynedd

Devised by American geologist Charles Francis Richter, the scale measures the magnitude of earthquakes on a logarithmic scale, so that a quake of magnitude 5 is 10 times more powerful than one of magnitude 4.

North west Wales is a hotspot of seismological activity which has long puzzled experts as the region is not situated on a major junction of tectonic plates.

Donald Glyn Pritchard
Donald Glyn Pritchard

Amateur geologist Donald Pritchard, former headmaster at Ysgol Penysarn near Amlwch, expounded his own theory as to what caused this tremor, and others in the past.

“I believe that it was caused by a geological fault known as the Berw Fault, which runs almost parallel with the Menai Strait, from Pentraeth through Pentre Berw and Llangaffo to the coast.

“It’s a geological feature that has been known about for a long time, although perhaps it’s not as famous as the San Andreas Fault! But Greenly described it at length in his book Geology of Anglesey published early this century.

“It’s where two different rock strata meet, 17.3km beneath the surface, and is responsible for the rocky outcrops which can be seen on the surface all along its line,” he said.

Only one earthquake more powerful than the Llithfaen one has ever been scientifically recorded in the UK, one of magnitude 6.1 that struck off the east coast of Essex in 1931.

Earthquake damage in Bodrum, Turkey

Earthquakes usually cause little damage in northern Europe, but ones of similar magnitude have been known to wreak havoc in other parts of the world.

According to the United Nations there were 1,564 earthquaks of magnitude 5 or higher across the world last year.

The strongest magnitude tremor was an earthquake measured at 8.2 on the Richter scale that struck parts of western Mexico last September and which caused widespread damage and 98 fatalities. A second earthquake, at 7.1, struck the country’s capital city two weeks later killing 370 people.

But the deadliest was one at 7.3 which struck the Iran/Iraq border region on November 12 killing 630.

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