The 1984 Llŷn Peninsula earthquake struck the

Llŷn Peninsula The Llŷn Peninsula ( or , ) is a peninsula in Gwynedd, Wales, with an area of about , and a population of at least 20,000. It extends into the Irish Sea, and its southern coast is the northern boundary of the Tremadog Bay inlet of Cardigan Ba ...

Gwynedd Gwynedd () is a county in the north-west of Wales. It borders Anglesey across the Menai Strait to the north, Conwy, Denbighshire, and Powys to the east, Ceredigion over the Dyfi estuary to the south, and the Irish Sea to the west. The ci ...

, northwest

Wales Wales ( ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by the Irish Sea to the north and west, England to the England–Wales border, east, the Bristol Channel to the south, and the Celtic ...

on Thursday 19 July 1984 at 06:56

UTC Coordinated Universal Time (UTC) is the primary time standard globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for civil time and time zones. UTC facilitates international communica ...

(07:56 BST). Measuring 5.4 on the

Richter scale The Richter scale (), also called the Richter magnitude scale, Richter's magnitude scale, and the Gutenberg–Richter scale, is a measure of the strength of earthquakes, developed by Charles Richter in collaboration with Beno Gutenberg, and pr ...

, it is the largest onshore earthquake to occur in the UK since instrumental measurements began. The effects were felt throughout Wales, most of England, and parts of Ireland and Scotland. There were many reports of minor damage to chimneys and masonry throughout Wales and England, the biggest concentration of damage being in

Liverpool Liverpool is a port City status in the United Kingdom, city and metropolitan borough in Merseyside, England. It is situated on the eastern side of the River Mersey, Mersey Estuary, near the Irish Sea, north-west of London. With a population ...

, which is located around 65 miles northeast of the

epicentre The epicenter (), epicentre, or epicentrum in seismology is the point on the Earth's surface directly above a hypocenter or focus, the point where an earthquake or an underground explosion originates. Determination The primary purpose of a s ...

. Minor injuries were also reported in the areas surrounding the epicentral area, and rockslides occurred at

Tremadog Tremadog (formerly Tremadoc) is a village in the Community (Wales), community of Porthmadog, in Gwynedd, north west Wales; about north of Porthmadog town centre. It was a urban planning, planned settlement, founded by William Madocks, who boug ...

in Gwynedd. The 19 July earthquake was followed by many

aftershock In seismology, an aftershock is a smaller earthquake that follows a larger earthquake, in Epicenter, the same area of the Mainshock, main shock, caused as the displaced Crust (geology), crust adjusts to the effects of the main shock. Large earthq ...

s in the following months, the largest measuring 4.3 on the Richter scale, which itself was felt as far away as

Dublin Dublin is the capital and largest city of Republic of Ireland, Ireland. Situated on Dublin Bay at the mouth of the River Liffey, it is in the Provinces of Ireland, province of Leinster, and is bordered on the south by the Dublin Mountains, pa ...

, in Ireland.

Earthquake characteristics

The earthquake's epicentre was located on the Llŷn Peninsula at coordinates 52.978°N, 4.427°W (near

grid reference A projected coordinate systemalso called a projected coordinate reference system, planar coordinate system, or grid reference systemis a type of spatial reference system that represents locations on Earth using Cartesian coordinate system, Car ...

SH371451). The focus, or

hypocentre A hypocenter or hypocentre (), also called ground zero or surface zero, is the point on the Earth's surface directly below a nuclear explosion, meteor air burst, or other mid-air explosion. In seismology, the hypocenter of an earthquake is its p ...

(the point within the Earth where the earthquake originated), was unusually deep at roughly 18 km beneath the surface.

Body-wave magnitude Seismic magnitude scales are used to describe the overall strength or "size" of an earthquake. These are distinguished from seismic intensity scales that categorize the intensity or severity of ground shaking (quaking) caused by an earthquake at ...

measurements from European stations gave a value of 5.2, while the United States

National Earthquake Information Center The National Earthquake Information Center (abbreviated NEIC) is part of the United States Geological Survey (USGS) located on the campus of the Colorado School of Mines in Golden, Colorado. The NEIC has three main missions: * First, the NEIC de ...

reported magnitudes of 5.0 Mb (body-wave) and 4.7 MS ( surface-wave). The earthquake was notable for its significant depth, occurring in what

seismologist Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes (or generally, quakes) and the generation and propagation of elastic ...

s describe as the lower crust. This deep origin indicates an unusually deep transition between the

brittle A material is brittle if, when subjected to stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. ...

upper crust (where rocks fracture) and the

ductile Ductility refers to the ability of a material to sustain significant plastic deformation before fracture. Plastic deformation is the permanent distortion of a material under applied stress, as opposed to elastic deformation, which is reversi ...

lower crust (where rocks deform plastically without breaking). This depth is possibly related to reduced heat flow in the region. The

focal mechanism The focal mechanism of an earthquake describes the Fault (geology)#Slip.2C heave.2C throw, deformation in the Hypocenter, source region that generates the seismic waves. In the case of a Fault (geology), fault-related event, it refers to the ori ...

(which describes the orientation of the fault and the direction of slip) indicated a predominantly strike-slip movement—where blocks of crust slide horizontally past each other—with a component of normal faulting. The earthquake did not correlate with movement on any of the major faults visible at the surface in the area, including the Menai Straits fault system. Analysis revealed a northwest-southeast oriented maximum compressional stress direction, which agrees with measurements made in other parts of Britain. This provided valuable information about the regional stress patterns affecting the British Isles.

Aftershocks

Following the main shock, a dense network of 14

seismograph A seismometer is an instrument that responds to ground displacement and shaking such as caused by quakes, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to form a seismograph. The out ...

stations was installed in Northwest Wales to study the

sequence. In the first four weeks, aftershocks with magnitudes greater than 1.0 ML occurred at an average rate of about 20 per week. The two largest aftershocks, on 29 July (magnitude 4.0) and 18 August 1984 (magnitude 4.3), were felt as far away as

and

Merseyside Merseyside ( ) is a ceremonial counties of England, ceremonial and metropolitan county in North West England. It borders Lancashire to the north, Greater Manchester to the east, Cheshire to the south, the Wales, Welsh county of Flintshire across ...

. The aftershocks clustered at depths between 21 and 24 km, forming a group about 3 km wide with no clear linear pattern. The aftershock mechanisms showed varied types of faulting, including

thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that ...

, normal, strike-slip, and oblique movements.

Effects and damage

The maximum intensity of the earthquake reached VI on the MSK scale (similar to the

Modified Mercalli scale The Modified Mercalli intensity scale (MM, MMI, or MCS) measures the effects of an earthquake at a given location. This is in contrast with the seismic magnitude usually reported for an earthquake. Magnitude scales measure the inherent force or ...

), affecting most of the Llŷn Peninsula over an area of roughly 800 km². At this intensity, widespread cracks appeared in plaster, and some chimneys and weak masonry collapsed. The earthquake produced notably loud audible effects, indicating significant high-frequency energy. Liverpool experienced anomalously high intensities (V-VI), likely due to the vulnerability of some buildings and possible soil amplification effects on made-up ground and alluvium. The earthquake was felt across a total area of approximately 250,000 km², with the mean radius of the area where it was clearly felt (intensity III) extending to 280 km from the epicentre. The extensive macroseismic data (information about how people experienced the earthquake) collected from this event has been valuable for calibrating historical earthquake records where instrumental data are limited or absent.

Historical context and significance

The 1984 earthquake was not an isolated event for the region. Northwest Wales had previously experienced significant earthquakes in 1852, 1903, and 1940, with smaller events felt in Bangor and

Anglesey Anglesey ( ; ) is an island off the north-west coast of Wales. It forms the bulk of the Principal areas of Wales, county known as the Isle of Anglesey, which also includes Holy Island, Anglesey, Holy Island () and some islets and Skerry, sker ...

in 1967 and 1969. In the broader context of British seismicity, the 1984 Llŷn Peninsula earthquake was comparable to the 1896 Hereford earthquake, and its felt area exceeded that of the more damaging 1884 Colchester event. The earthquake provided a rare opportunity to obtain accurate

focal parameter A conic section, conic or a quadratic curve is a curve obtained from a cone's surface intersecting a plane. The three types of conic section are the hyperbola, the parabola, and the ellipse; the circle is a special case of the ellipse, thoug ...

s for a major British earthquake, helping scientists better understand the deep structural geology and regional stress regimes in Britain. The depth of the earthquake suggested unusual crustal conditions in North Wales, with the brittle-ductile transition occurring much deeper than typically expected, possibly due to lower heat flow in the area.

References

External links

Lleyn Penin, NW Wales
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British Geological Survey The British Geological Survey (BGS) is a partly publicly funded body which aims to advance Earth science, geoscientific knowledge of the United Kingdom landmass and its continental shelf by means of systematic surveying, monitoring and research. ...