Why do earthquakes happen all the time in Britain but not in Ireland?

[ad_1]

The village of Tian in Staffordshire, England was hit by a 3.3-magnitude earthquake on June 28, 2023. The tremors caused windows and doors to rattle in the surrounding area.

Earthquakes of this type are not uncommon in Britain (island including England, Scotland and Wales). In fact, hundreds of earthquakes rock Britain every year.

Most of these earthquakes are small in size and do not cause any damage. However, there are occasional earthquakes in Britain that are potentially devastating. Scientists estimate that the likely largest earthquake in Britain is of magnitude 6.5 – surpassing the magnitude of the 6.3-magnitude earthquake that struck Christchurch, New Zealand in 2011 and killed 185 people.

The largest earthquake so far recorded in Britain occurred in 1931 near Dogger Bank, 97 km off the east coast of England. It measured 6.1 on the Richter scale, and damaged buildings along the east coast.

Most earthquakes in Britain are concentrated within a north-south band on the western side of the island. However, neighboring Ireland is almost completely free of seismic activity – a phenomenon that has puzzled scientists for hundreds of years.

Research by my colleagues and I has provided a possible explanation for the scanty seismic activity in Ireland. We found that the lithosphere – the hard outer layer of the Earth that makes up its tectonic plates – is thicker and cooler under Ireland than it is under Britain. This makes the tectonic plate under Ireland less prone to deformation – a process that can trigger earthquakes.

Missing Ireland Earthquakes

Even before earthquakes were recorded by seismographs as they are today, reports of earthquakes were documented in many towns and monasteries throughout Britain and Ireland. In the mid-19th century, Robert Mallet, an Irish scientist who is credited with coining the term “seismology,” created maps of earthquakes based on these reports. Note that Britain has moderate seismicity (a term for earthquake activity), while Ireland has low seismicity.

In 1884, Irish seismologist Joseph O’Reilly published the first seismic map of Britain and Ireland, stressing that Great Britain “was more prone to earthquakes than Ireland.”

Understanding the reasons behind this unequal distribution remains important today, particularly in relation to how it affected Britain’s growing population. Between 2011 and 2021, the UK population increased by 6%, to a total of 67 million people.

O’Reilly’s seismic map of Britain and Ireland. O’Reilly (1884), Internal Earthquakes CC BY-NC-ND

Most earthquakes occur at plate boundaries where tectonic plates converge, diverge, or slide across each other. More than 80% of the world’s largest earthquakes occur around the Pacific Ocean – an area known as the Pacific “Ring of Fire”.

Earthquakes that occur in the inner part of the plates are less common and are usually smaller in magnitude. But there are some notable exceptions. Between 1811 and 1812, the New Madrid seismic region in the central United States experienced a series of powerful earthquakes, ranging in magnitude from 7 to 8.

Geologically, Britain and Ireland are similar. It was formed in the same continental collision about 400 million years ago and is made up of parts of the same continents. The two islands are also equidistant from plate boundaries and tectonic stress (pressure or tension exerted by other plates or the underlying mantle) is similar across them.

So why is the distribution of earthquakes so different in Britain and Ireland?

through thick and thin

Seismic tomography, a technology that uses seismic waves from distant earthquakes to create 3D images of the Earth’s interior, provides valuable insights. Research I co-authored in 2021 discovered previously unknown differences in the structure of the tectonic plate that Britain and Ireland sit on.

The tectonic plates are cold and rigid compared to the hot mantle slowly creeping beneath. Thicker panels are cooler, mechanically stronger, and less prone to deformation. Conversely, thin sheets are warmer, weaker, and more prone to deformation.

In our recent research, we found that the plate thickness under Britain and Ireland ranges from 75 km to 120 km. Ireland has a relatively dense lithosphere (about 95-115km under most of the island), and as a result, there are very few earthquakes. Southeast England and eastern Scotland have a similarly thick lithosphere.

By contrast, western Britain has a thinner lithosphere (about 75-85 km thick) and experiences regular earthquakes. Most Irish earthquakes occur in the north of the island, which is the only place whose lithosphere is thinner, warmer and weaker.

Left: earthquakes in Ireland and Great Britain. Right: differences in lithosphere thickness. Raffaele Bonadio, author provided

This discovery solves a long-standing mystery. Moderate variations in plate thickness, which occur far from plate boundaries, can influence patterns of seismic activity within those regions.

This breakthrough opens new avenues of research for seismologists. In Britain and Ireland, scientists can now focus on filling in the remaining gaps in the coverage of seismic stations (which monitor ground motion at specific locations) and building a lithosphere model to see why earthquakes are concentrated where they are.

Catalogs of earthquakes in other regions of the world often do not go back so far as in Britain and Ireland. Seismic hazards in these areas can be more uncertain. Modeling the thickness and strength of tectonic plates gives scientists the tools to study the puzzling distribution of earthquakes and improve their ability to predict.

[ad_2]