Swells, waves, rocks and rolls

 

Could Inis Mór have been separated from the mainland by a tsunami-like wave, and, given the recent earthquake, how likely is it that a major wave will hit Ireland, asks LORNA SIGGINS

WHEN THE Volvo Ocean race sets a course for the Irish west coast this weekend on its final leg from Lorient in Brittany, a good southwesterly could have it taking advantage of the Atlantic’s fetch. Waves that have travelled thousands of miles could sweep the fleet on a broad reach past the Aran islands and into the shelter of the bay.

What the weary round-world sailors may or may not appreciate is that these pristine waves are almost unique in Europe, with coastal topography ensuring that there is nothing to break the swell before it encounters Aran. And if some sailors have time to look through the binoculars, they might just spot a lone figure tapping the cliff edge with a silver hammer.

Prof Mike Williams, recently retired from NUI Galway (NUIG), believes that the Schmidt digital hammer, used by engineers to measure the strength of concrete, may prove invaluable in dating Aran’s lower carboniferous limestone rim, and calculating the frequency of significant collapses.

The recent magnitude-four earthquake some 60km west of Mayo – close to the Corrib gas field and Providence Resources’ prospects on the Slyne Basin – gives added significance to his research, as his own work has been central to a “storm wave versus tsunami” debate.

That debate has focused on the precipitous cliff-top location of Dún Aonghasa on Inis Mór, and specifically on whether the dún or cashel was originally built on an inland location, and was sliced as a result of tsunami-like waves.

During research into the curious frequency of enormous boulders or “megaclasts” tossed up on the Aran and north Clare cliff-tops, Williams delved into historical records dating as far back as the Annals of the Four Masters and forward to the “Night of the Big Wind” on January 8th, 1839.

One of the most significant events occurred in the previous century, in 1755, when an earthquake in Portugal’s Gulf of Cádiz rocked Lisbon, and generated a tsunami-type swell that rolled north towards Ireland and knocked away part of Galway city’s Spanish Arch.

However, Williams contends that the giant Aran boulders are more likely to have been cast up by storm waves than by an actual tsunami, as outlined in a 2010 paper published by the Irish Journal of Earth Sciences, where he analysed wave impact on vertical obstacles.

He tested the mathematical equation on Inis Mór’s Dún Duchathair or Black Fort, pointing out that “overtopping” by waves would be influenced by the ratio of the depth of water at the base of a cliff to the offshore wave height.

Large waves breaking on cliffs resulted in “overtopping of green or white water”, he says, creating a downward collapse of water mass and forming a landward-moving high-velocity “bore”. This “bore” then carried the megaclasts, some of up to 10 tonnes in weight, across the high platforms of cliffs.

His theory has been challenged by South African-based academic Jasper Knight of the University of the Witwatersrand’s school of geography, archaeology and environmental studies. Knight has suggested that a “tsunamigenic origin” for megaclasts occurring over a 3,000-year period could not be discounted, given the evidence of radiocarbon dating of shell material within the megaclast ridges.

However, Williams responded that he and colleagues have recorded megaclasts thrown up during a 170-year period when there was no evidence of tsunamis hitting the Irish and Scottish coasts. Tsunamis have been “virtually absent” from this part of the Earth since 1755’s Lisbon quake, he says, and the statistical likelihood of recurrence is about every 2,000 years.

The recent plate shift off Mayo was “not negligible” in scale, he says, but not completely unexpected. The perception that Ireland is “a-seismic” or an “earthquake-free zone” reminds him of the firm belief – reflected in school text books decades ago – that this island had and has no natural resources.

“There has been plenty of evidence of earthquakes in Ireland since the last ice advance, but we weren’t around to record them,” he notes. “Where there’s a fault, as in this case on the Slyne basin, there’s a chance it can be re-activated due to stress building up in the crust. These are tertiary faults, which are only a few million years old.”

Williams’s current project to calculate cliff age on the Aran Islands will involve testing the strength of similar material on land. Therefore he is also comparing readings from 4,000-year-old wedge tombs, stone used in medieval churches and 19th-century gravestones. The results will be fed into a mathematical model to show how rapidly the cliffs are eroding.

This erosion is already moving apace, with a 9,000 cubic-tonne collapse of material at the Black Fort on Inis Mór in August, 1986, and a massive slide just last year on Inis Meáin.

“The Atlantic’s fetch gives plenty of time to create an enormous animal,” he says, “an animal that doesn’t have to reach any great height as it indulges its appetite for carboniferous limestone rock.”

Coming soon? Earthquakes, slides and tsunamis

“DID you hear the earthquake?”north Mayo resident Caitlín Uí Sheighin asked her granddaughter Medhbh on June 6th. “Chonaic mé,” Medhbh replied in Irish, and when her grandmother quizzed her as to how she could have “seen” it, she explained how she had seen “na teddies ag damhsa ar an leaba”.

The seismic event, which cracked chimneys and silenced birds on the Erris coastline, didn’t cause unusual wave activity. Prof Mike Williams was out on the coastline that day, but not anticipating major swells, as he believes it would take an asteroid, an earthquake or an underwater landslide to trigger a repeat of Lisbon’s 1755 earthquake and a subsequent tsunami event.

There has been media speculation that a volcanic eruption on the Canary island of La Palma could cause a tsunami that might reach North America’s Atlantic coast, the Caribbean, the northern and western coasts of the South American continent, west Africa and the western coasts of Europe. However, Williams argues that this would take a collapse of La Palma’s entire volcanic mass; and that likelihood, he says, is “remote”.

There’s also been research on the stability of the Rockall Trough in recent years, with the first comprehensive survey using sidescan sonar in 1996. It showed evidence of enormous sediment slides caused by “slope failure”.

Australian geoscientist Prof Edward Bryant of the University of Wollongong suggested in a book published a decade ago that there were seven major submarine slides off the west coast of Ireland between 30,000 and 8,000 years ago.

In Tsunami: The Underrated Hazard, Bryant suggested that an earthquake measuring as little as three to four on the Richter scale could trigger a slide big enough to generate a tsunami that would reach the Donegal west coast in two hours, causing devastation.

Prof Williams points out that the last recorded tsunami due to submarine slope failure to affect the islands of Britain and Ireland was about 7,000 years ago in the North Sea when one of the Storrega slides (Norway slope) generated a tsunami on the Scottish coast.“Slides in this situation are most likely to occur during periods of low sea level, whereas high sea levels tend to stabilise sediment due to the hydrostatic load of the water column.”

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