The Brainstorm Long Read: earthquakes happen in Ireland all the time, but we just don't feel them
"The unearthly silence was unreal, no bird sounds, nothing, it was an eerie feeling, as if the end of the world had come and all life had ceased."
In June 2012, a magnitude 4.0 earthquake off the west coast of Ireland caused strong tremors to be felt by people in Mayo, Galway and Sligo with North Mayo resident Pap Murphy describing his "end of the world" feeling to RTÉ at the time.
Earlier this year, Donegal was struck by an earthquake twice in the space of three weeks. The first, at 11.58pm on April 7th, was of magnitude 2.4 with an epicentre in Donegal Bay, not far off the coast. The second, at 9.18pm on April 29th, was magnitude 2.1 and hit inland. The Irish National Seismic Network (INSN) received over a hundred "felt" reports following the first quake, which means thousands probably experienced the tremors.
From RTÉ Archives, Teresa Mannion reports for RTÉ News on the earthquake which occurred off the coast of Mayo in June 2012
The Mayo earthquake was Ireland's second-largest, after a 1984 event where a magnitude 5.4 earthquake occurred in the Irish Sea. But while earthquakes of that size are rare in Ireland, the reality is that smaller earthquakes happen "all the time", says Chris Bean, senior professor of geophysics and Director of the School of Cosmic Physics at the Dublin Institute for Advanced Studies (DIAS).
So the good news is that no, Ireland is not about to be hit by a massive earthquake or a tsunami. Ireland, Bean says, is "not in a particularly tectonically active area of the world.
"We will have earthquakes of that size again, eventually, but the question is will they happen in a human lifetime or not? Geological processes are very slow relative to the human biological timescale. It could be tens, it could be hundreds of years before we will see something like that again. But the processes that led to that event are still in play, they're still ongoing".
From RTÉ Six One News, Southern California hit by 7.1 magnitude earthquake, the strongest in two decades
Just like the film on top of your porridge forms when you’ve got it simmering on a low heat, the earth’s mantle, below the 30km deep crust, is convecting. "It’s like a very, very slow stressing of the material,’ Bean says.
The Irish National Seismic Network (INSN) records our earthquakes and is operated by DIAS with Geological Survey Ireland (GSI). INSN has six permanent seismic stations dotted around the country and Bean explains that for the number of stations we have, the detection threshold for earthquakes in Ireland is about magnitude 1.0. These wouldn’t be felt by people.
Earthquakes are measured on the Richter scale, a logarithmic scale that goes up in factors of ten, meaning a magnitude 2 earthquake is ten times more powerful than a magnitude 1.0 and so on. Earthquake scientists use equipment called seismometers to measure the events and although Ireland is a relatively stable place, it was Irish scientists Robert Mallet who created the first seismometer in 1846.
From RTÉ Radio 1's Ray D'Arcy Show, UCD environmental scientist Dr. Fergus Mc Auliffe and the National Seismic Network's Tom Blake discuss the science of sound waves and earthquakes
The three "hotspots" for quakes in Ireland are Donegal, Wexford and the Irish Sea, but the biggest earthquakes we have in Ireland are actually offshore in the Atlantic in Irish territory. While earthquakes can’t be predicted, scientists can use the production rate of earthquakes in a given area to estimate the likelihood of getting another earthquake of a given size, "but it doesn’t tell you when it will come," Bean stresses.
However, there's a good reason why Donegal sees so much activity, says Susan Hegarty from the School of History and Geography at Dublin City University. "Ireland’s oldest rocks occur off of Donegal in Inishtrahull and their bed is 1.6 billion years old, almost half the age of the earth. They’re really, really old.
"About 500 million years ago Ireland was split in two. The area around Donegal and Mayo was on one plate and Wexford was on another… When these two plates came together the rocks were already ancient and they cracked and they formed differently to the rocks in the south, so you’ve got a huge fault going through the Barnesmore area."
From RTÉ Radio Morning Ireland, Stephen Murphy reports on why Co Donegal experiences a higher proportion of earthquakes
Hegarty says the drive from Donegal Town up into Letterkenny or Ballybofey takes you through a very large valley called the Barnesmore Gap and that gap is a "very major fault line" created about 500 million years ago.
"Between 100,000 and up until about 20,000 years ago, Ireland was covered by a very large ice sheet and has been covered by ice sheets for the past two million years or so on and off… If the ice disappears, the earth's crust rebounds and pushes back up. In geological terms that’s very recent. Ireland is still rebounding, the actual island itself is still springing up, trying to recover from the weight of the ice that has been pushing it down. Those faults that were there, that were in place millions of years ago all of a sudden are reactivating."
Though the two events in Donegal were "relatively big" for Ireland, it still wasn’t unusual, Chris Bean from DIAS says. "The magnitude 2.4 earthquake in Donegal was about twice as big as the other one that occurred later, the magnitude 2.1. So what we think happened in Donegal is that the magnitude 2.4 event actually triggered the other event. It doesn’t have to happen immediately because it’s a very complex system.
From RTÉ Radio News At One, Dr Martin Möllhoff from the Dublin Institute for Advanced Studies on the two earthquakes in Donegal in April 2019
"The probability of getting two greater than magnitude 2 events right beside each other in Donegal that are independent of each other are very, very slim, because normally you have to wait for years to get one of these events. So what are the chances you are going to get two of them very close together that have nothing to do with each other? It’s like winning the lotto twice."
In 1755, a powerful earthquake in Lisbon caused a tsunami in Cork. While "you can never say never", Bean believes that the likelihood of seeing that again "in our lifetime or that of our children or grandchild" is "extremely low. If you go out tens of thousands of years? Then it’s almost certainly going to happen sometime in the future."
Bean stresses that we're not going to get a big tsunami off the coast of Ireland from an earthquake. However, sediments on the shelf edge 10 to 50 kilometres out in the Atlantic could slump if an earthquake happened out there, something which could generate "more local tsunamis." It’s more likely that it would break telecommunications cables, he says.
Ireland is one of the quietest places on the planet, but there’s still this activity
DIAS is hoping to establish a new seismometer 3km under water in the Atlantic to be able to detect and measure the activity out there. "It shows us that even in a place where we think nothing is going on, the world is a dynamic place," says Bean. "Ireland is one of the quietest places on the planet and even at that, in a place where we think there’s almost nothing going on, there’s still this activity. I think it adds a bit of spice to the place.
"In some places, people are very aware of the dynamism of the planet on which we live. Here in Ireland, we tend to be blind to that. We know that it’s happening in other places, but we kind of miss the sense of what it’s really telling us about how the planet works. The important thing is to show that even in quiet places, there is a lot of dynamism in the planet and it has its plusses, it makes the place interesting, but it also brings its problems."
So while many might have been surprised in the past to feel the earth move beneath their feet on Irish soil, it’s just one part of a bigger picture. "The fact that you have small ones does demonstrate that every so often you will get big ones," Bean explains. "Because there is a relationship between the number of big ones versus small ones. Once we can see these small ones on the network, we know that every so often we’re going to get a bigger one, because that’s just the way it is."
The views expressed here are those of the author and do not represent or reflect the views of RTÉ