Why does Donegal get a disproportionate number of Irish earthquakes?

Two weekends ago, an earthquake in Donegal registered as a curiosity in the news cycle. The earthquake was in Donegal Bay, it measured 2.4 on the Richter scale and was felt primarily around Killybegs. How odd. Wait, actually, how normal.

Earthquakes along and off the coast of Ireland are not freak occurrences. That said, the Donegal location is significant, as this part of the country is a (relative) hotbed of tremors, and seismic activity has been noticed officially and unofficially, and written about in The Irish Times by reporters, scientists and in letters to editors for at least 130 years.

On March 10th, 1973, an Irish Times reporter wrote about microtremors in the Inishowen area, which had been recorded at a seismic investigation station set up by the geophysical section of the School of Cosmic Physics, and a report was published by the Dublin Institute of Advanced Studies. “These tremors have been of several minutes’ duration and it is thought that this is the first time that such a phenomenon has been observed in hard rock areas,” the news article said.

In August 2017 an earthquake was logged at Fanad peninsula in Donegal, and in December 2014 another was logged at Fanad Head. A large amount of seismic activity occurred in 2012, with earthquakes logged off the northwest coast of Donegal, another at Ramelton, another at Rooskey in Donegal, one at Lough Swilly and another in Buncrana.

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There were two earthquakes a day apart in January 2010 at Burnfoot in Donegal, and one in the same month at Bridge End. The latter earthquake, “was felt in Bridgend and Buncrana in Inishowen and westwards across Lough Swilly in the Fanad peninsula,” a report by Linda McGrory and Paddy Clancy detailed in The Irish Times, the article quoting shop assistant Teresa Gillespie, who was with her parents in their home in Desertegney, 9km from Buncrana: “It was a big rattle and it lasted a couple of minutes. It started in one place and seemed to roll along Lough Swilly.”

In 1999 there were earthquakes in Milford and Kerrykeel; in 1994 there was one at Clonmany. The latter quake was reported on by Alison O’Connor in the newspaper: “Last Monday a remarkable number of Donegal residents reported hearing lorries crashing outside their front door. In fact, the noise was an earthquake measuring 2.1 on the Richter scale that was felt by hundreds of families in the area.” Her article also contained the fact that earthquakes in Donegal are usually deep, up to 15km underground.

There were two earthquakes a day apart in May around Milford and Dunfanaghy in 1986, and another in Dunfanaghy in January 1984. Only Wexford can contest Donegal as the earthquake capital of Ireland, with about a dozen earthquakes logged there over the past 40 years.

Why do a disproportionate number of Irish earthquakes happen in Donegal? They tend to be connected to the Great Glen fault line, which runs from the Great Glen in Scotland, and becomes the Leannan Fault, travelling down through Lough Foyle and Donegal Bay and into Clew Bay.

Going further back into Donegal’s earthquake history is difficult give the limited documentation of seismic activity. But a letter to The Irish Times on October 9th, 1884, indicates that tremors in the northwest have been happening for a while. “Sir - On my return home here from Dublin yesterday I was able to collect, in a short time, abundant evidence of the fact that this district was visited on Thursday night last, at about 10 o’clock, by a slight shock of earthquake,” a reader wrote. “Almost everyone in the neighbourhood heard the peculiar rumbling noise, and many felt the shaking of the ground, among them one man who is familiar with such occurrences in tropical regions. Yours, etc., FW Egan, HM Geological Survey. Killygorden, County Donegal.”