The Mysterious Case of the Missing Earthquakes in Ireland

Ireland and Britain should, from a seismological point of view, be equally boring. The two islands lie thousands of kilometers from the nearest plate boundary and are not volcanic hot spots. But although the ground rarely subsides in Ireland, neighboring Britain experiences many earthquakes, both weak and moderate.

New research has suggested that the lithosphere — Earth’s outer, rocky shell, which includes the crust and the solid upper mantle — is thicker and cooler under Ireland than it is under Britain. The cold, thick lithosphere is mechanically stronger than the thin, warm lithosphere, which may explain the puzzling rarity of earthquakes on the Emerald Isle. The new findings, published in Geophysical Journal International, suggest that the thickness of the lithosphere could support patterns of seismic activity elsewhere far beyond plate boundaries.

Missing earthquakes in Ireland

About 90% of all earthquakes – including almost all earthquakes – start at the boundaries between tectonic plates. The interior or continental plates tend to be seismically calm, although they are not dead. The New Madrid seismic zone in the Mississippi River Valley, for example, triggered a series of earthquakes of magnitude 7.3-7.5 in late 1811 and early 1812, despite being located in the middle of the North American plate.

“We have a very good framework for understanding seismic activity at plate margins,” said geologist Mike Sandiford of the University of Melbourne, who was not involved in the new study. “The question is, what controls earthquakes in continental interiors away from plate margins?”

The rarity of internal earthquakes has made it difficult for scientists to understand them. And it is often not clear why there are more earthquakes in some places deeper in the continents than in others.

Such is the case in the British Isles, where the dramatic seismic contrast between Britain and Ireland has been a mystery for centuries.

Sergey Lebedev, a University of Cambridge seismologist and lead author of the new study, described the situation by quoting British Geological Survey seismologist Roger MW Mawson: “Seeking to understand how seismic activity is distributed [in Britain and Ireland] related to the geological structure is long and fruitless.” Lebedev added that the main question is why there are so few earthquakes in Ireland.

Defects are not the answer, Lebedev said, because many of Britain’s great defects also pierce Ireland. Nor the type of crust found on the two islands. Britain and Ireland share a geological history, as both were pieced together from parts of an ancient continent called Laurentia and scraps of crust called Avalonia during the Genesis of Caledonia – one of the mountain-building events that held together the supercontinent Pangea.

“crustal masses that are made [up the islands] From Britain to Ireland, Lebedev said, “they have nothing to do with the distribution of earthquakes.”

seismic survey

Until recently, missing earthquakes in Ireland were a mystery without evidence.

Until recently, missing earthquakes in Ireland were a mystery without evidence. In 2008, Lebedev said, there was only one broadband station monitoring apparently uninteresting seismology in Ireland. But that has since changed dramatically.

The Irish National Seismological Network now operates six permanent broadband seismometers and is expanding its network. Between 2010 and 2012, Lebedev and his colleagues deployed 20 additional temporary seismometers that are in continuous operation through 2021.

With this augmented Irish seismic network and data from seismometers in Britain, Lebedev and colleagues previously used surface wave tomography to map the thickness of the lithosphere in Britain and Ireland. This technique takes advantage of the fact that some seismic waves travel faster through the cooler lithosphere, which also tends to be thicker. With it, scientists can map subterranean structures in 3D. “It’s similar to a medical CT scan where doctors shoot different types of rays through patients and see what’s inside,” Lebedev said.

Earthquakes in Ireland and Great Britain according to the British Geological Survey catalog, superimposed on a map representing the thickness of the lithosphere. Blue hues over much of Ireland and southeastern Great Britain indicate a thicker, cooler, stronger lithosphere. Gray lines are major defects. Credit: Raffaele Bonadio

Then, in the new study, the research team created a detailed map of seismic activity and compared it to a map of lithosphere thickness in Britain and Ireland. This comparison revealed a striking correlation: earthquakes were rare where the lithosphere was cool and thus likely to be dense and strong.

“Ireland is low-seismic, and we see that it has a thick, cold lithosphere,” Lebedev said, while the British lithosphere is much warmer and thinner. He added: “Most of the Irish earthquakes are in the north of the island in County Donegal – more than 90% of them, and they happen to be in places where the lithosphere is thin.”

deep structure

This isn’t the first time scientists have linked lithosphere thickness to interseismic earthquakes in general. For example, geophysicist Walter Mooney of the US Geological Survey showed in 2012 that large intraplate earthquakes are rare exactly where surface waves travel rapidly at depths of 175 kilometers — an indication of a “dense rock root,” he said. .

However, it was long assumed that the lithosphere under Ireland and Britain was equally thick — there was no evidence to the contrary, Lebedev said. “So we have a 150-year-old mystery that has not yet been solved.”

More mysteries remain – such as why the Irish lithosphere is thicker than the British lithosphere. Lebedev said his team hopes to answer that question soon. Lebedev also said he would like to see if surface wave tomography could reveal similar correlations between lithosphere thickness and earthquakes elsewhere, outside of Britain.

“While lithosphere thickness tends to be associated with cooler temperatures, it doesn’t necessarily have to be.”

However, “while lithosphere thickness tends to be associated with cooler temperatures, it doesn’t necessarily have to be,” Sandiford said. This uncertainty can complicate simple generalizations linking lithosphere thickness to earthquakes like those suggested in the new study, Sandiford said.

There are even places, like Western Australia, that are seismically active but have a thick lithosphere. So, Sandiford added, “There are other factors at play. But they seem, as a first case, to make a reasonable assessment of Ireland and the British Isles.”

Many questions still surround intracontinental earthquakes, Mooney said, and the new finding is a good reminder that answers may be buried deeper than scientists expect. Traditionally, he said, seismologists focus on the top 15 kilometers or so of the Earth’s crust because most earthquakes at plate boundaries are shallow.

Scientists assume “they don’t need to think about the lithosphere because it’s lower than their field of interest,” Mooney said. “The deeper structure has a big impact on shallow activity.”

—Elise Cutts (@elisecutts), science writer

Citation: Cutts, E (2023), The Mysterious Case of Missing Earthquakes in Ireland, Eos, 104, https://doi.org/10.1029/2023EO230240. Posted on June 23, 2023. Text © 2023. Authors. CC BY-NC-ND 3.0 Unless otherwise noted, images are subject to copyright. Any reuse without the express permission of the copyright owner is prohibited.

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