Another Life: An Irish tsunami? It’s not as unlikely as it sounds

When I look at the Marine Institute's Real Map of Ireland I still wonder at our nerve in casting a boundary around such a great wedge of Atlantic seabed. But we mapped our share, 10 times the size of the State, before anybody else, under the UN's Convention on the Law of the Sea.

The far corner of the wedge stops where the “natural prolongation” of the Irish continental shelf peters out at the great Iceland basin, east of the Mid-Atlantic Ridge. Halfway between here and there is the mouth of the Rockall Trough, the great canyon, bigger than the Irish Sea, that cuts northeast towards Scotland. Here the edge of the continental shelf takes on a drama imaginable even from the map’s contours.

The near rim of the trough, along the Porcupine Bank, is awesomely precipitous, scalloped with channels and canyons that pour debris down to a floor as much as 2,000m below. Sonar surveys have pointed up the risks of setting up any future oil-gas exploration equipment too near the foot of the bottom slopes.

The far western wall of the trough, across a wide basin scoured by powerful currents, is that of the Rockall Bank, facing the west coast of Ireland. The stability of its slopes relates, perhaps academically, to a quite different order of risk

In 2002 I discussed here a new book called Tsunami: The Underrated Hazard, by Prof Edward Bryant, head of the school of geosciences at the University of Wollongong, in Australia. If I then dealt at all lightly with some of its content I have to note that its second edition followed the great Indian Ocean tsunami of 2004 and that a third edition came last year after the catastrophic Japanese tsunami of 2011.

Some of the biggest tsunamis have occurred nearer home. They arose from three underwater landslides at the edge of Norway’s continental shelf, the most recent about 8,000 years ago.

The Storegga slides, as they are known, collapsed 290km of coastal shelf with a total volume of about 3,500 cubic kilometres of debris. The last one swept far up the Firth of Forth, in Scotland, and, washing over the Dogger land bridge, helped separate Mesolithic Britons from their European brothers. There seems to be no evidence that the wave swept around to Ireland.

According to Bryant, however, there were also at least seven major submarine slides off the west of Ireland over the same time span. Among hypothetical scenarios in an epilogue to his book is a tsunami, triggered by a small and unnoticed offshore earthquake, that sweeps in to inundate the coast of Donegal.

“At 4.58am on that Sunday morning in April,” he supposes, “the shelf slope finally succumbed to the enormous pressures that had been building up over the last 5,000 years of higher sea-levels. . .”

He imagines graphically how the water is funnelled between Donegal’s headlands, rearing from 8m along the open coast to more than 15m as it rushes inland towards Letterkenny. University students living in Portstewart and Portrush “succumbed to the waves”, and, as for the countryside around Gweedore and Donegal, “the number of dead will never be known . . .”

Bryant’s disquieting fantasy did little to disturb the tenor of Donegal’s everyday life. But scientists here and in Britain were already well aware of the Rockall Bank Slide Complex, thought to have been created 15,000 years ago. Among images recorded in the mapping of Ireland’s submarine territory was a great slump of debris from the Rockall Bank covering 11,000sq km.

Last July a 13-strong team of international scientists, led by Dr Aggeliki Georgiopoulou of University College Dublin and Dr Veerle Huvenne of the UK National Oceanography Centre, sailed aboard the RV Celtic Explorer to the southeastern corner of the Rockall Bank, 500km from the Irish coast.

Here they used a multibeam echo sounder, mounted on the front of a remotely operated vehicle, to sweep and map the vertical cliffs of the bank, and a new kind of gravity corer to retrieve a score of metre-long samples of rock. The events of the expedition make an engaging team blog at marine.ie.

A major surprise was that neighbouring scarps in the landslide differed from each other geologically, in appearance and in their rich aquatic organisms. Seismic data from the original sea-floor map had suggested at least three episodes of collapse, totalling up to 765 cubic kilometres of debris. And in a paper published a year ago Dr Georgiopoulou (“Aggie” to her team) concluded: “We believe that where contour-swept slopes are steep, the resulting intensification of near-bottom velocities [water currents] prevents healing slope processes, and, as a result, steep slope gradients remain steep and thus unstable.”

Last summer’s expedition will have checked even more of this out.

It could even help us decide where, among the many worries on offer, we should fit the chances of an Irish tsunami.