Two small earthquakes recorded in Wales this week have provided a reminder that, although earthquakes are relatively uncommon in Britain, North Wales has a surprisingly active seismic history.

The British Geological Survey recorded a magnitude 1.3 earthquake near Bontddu in Gwynedd at 1.30pm on Wednesday, 12 August, at a depth of around eight kilometres.

A second, smaller earthquake measuring magnitude 1.1 was recorded near Llangynidr in Powys earlier the same morning.

Earthquakes of this size are normally too small for most people to feel, but they are regularly detected by the UK's network of seismometers.

And North Wales has experienced considerably stronger events over the years.

One of the more sizeable earthquakes centred on Conwy happened on 14 February 2005.

The British Geological Survey recorded it at magnitude 3.3, with its focus around 10.7 kilometres beneath the surface.

It was the largest onshore earthquake recorded in Britain that year.

People across parts of the region reported being startled by the tremor. Reports received by the BGS included descriptions of a loud bang, windows shaking and entire houses appearing to move. Reports came from places including Conwy, Llandudno, Betws-y-Coed, Bethel and Abergele.

A magnitude 3.3 earthquake is still considered relatively moderate, but at a shallow enough depth it can certainly be felt – particularly when it produces the sudden bang or rumbling sound associated with some British earthquakes.

There have been other smaller earthquakes around the Conwy Valley too. In 2022, for example, a magnitude 1.0 earthquake was recorded around two kilometres west of Trefriw, at a depth of approximately seven kilometres. It was felt in parts of the Conwy Valley, with reports of a rumbling sound.

The most famous earthquake in the history of North Wales struck on 19 July 1984.

The LlÅ·n Peninsula earthquake, sometimes referred to as the Menai Strait earthquake, measured approximately 5.4 magnitude and was the largest onshore earthquake in the UK for more than a century.

The epicentre was in northern LlÅ·n, but the effects were felt over a huge area.

People across North Wales reported buildings shaking, while damage included cracked plaster and masonry and damage to chimneys. Rockfalls were also reported in parts of Gwynedd.

The earthquake was felt much further afield, including across large parts of England and into Ireland.

It originated at a depth of roughly 20–22 kilometres, making it a particularly interesting event for scientists studying the geology beneath North Wales.

The main earthquake was followed by a significant sequence of aftershocks, including events measuring around magnitude 4.

The 1984 earthquake wasn't a one-off. Historical records show that significant earthquakes have occurred in and around North Wales for hundreds of years.

A particularly notable event occurred in 1852, when an earthquake centred around the Caernarfon area was recorded at approximately magnitude 5.3 by modern estimates. The British Geological Survey's historical catalogue also records a series of associated tremors that day.

There were also notable earthquakes in the region during the late 19th and early 20th centuries, while smaller tremors have continued to be detected around Gwynedd, Anglesey and Conwy.

The LlÅ·n Peninsula is regarded as one of Wales' more seismically active areas. Research into UK earthquake hazards has found that seismic hazard is generally low across Britain but is somewhat higher in Wales and parts of north-central England because of the greater level of historical earthquake activity.

An earthquake happens when stress that has built up within the Earth's crust is suddenly released.

The Earth's crust isn't one completely solid, unbroken layer. It contains faults and fractures, and the rocks on either side can be subjected to enormous forces.

As those forces build, rocks can deform slightly or become locked against one another. Eventually, the stress can become greater than the friction holding the rocks together.

The rocks then suddenly move, releasing energy in the form of seismic waves.

That's what we experience as an earthquake.

Unlike places such as Japan, California or parts of Turkey, Britain isn't located on a major active plate boundary. That means earthquakes here are generally much smaller.

But Britain is still affected by stresses within the Earth's crust.

Some of those stresses are associated with the ongoing movement of tectonic plates, including the slow opening of the Atlantic Ocean and the northward movement of the African plate.

There is also a longer-term effect from the end of the last Ice Age. Huge amounts of ice once weighed down parts of Britain, and as that ice disappeared, the land began slowly readjusting – a process known as post-glacial rebound.

The reassuring news is that the overall earthquake hazard in the UK remains low.

The British Geological Survey monitors hundreds of small seismic events, most of which are never noticed by the public.

But the history of North Wales demonstrates that larger earthquakes are possible.

The 1984 LlÅ·n Peninsula earthquake remains a particularly important reminder of that.

And while nobody can predict exactly when the next significant earthquake will occur, every small tremor – including Wednesday's event near Bontddu – provides scientists with another piece of information about the forces and geological structures beneath our feet.

So if you hear a sudden rumble, feel the windows shake or experience what sounds like a heavy explosion with no obvious explanation, there is always the possibility that North Wales has given itself another little geological shake.