Insurance Day
Scott Farley
October 18 2000
IT looked like a scene from Hollywood‘s latest summer blockbuster. But the image of a giant tsunami engulfing Manhattan island published in last week‘s Insurance Day did not come from the over-active imagination of an eager scriptwriter.
It was, in fact, an artist‘s impression of a very real threat. One that has been established through years of authoritative research from scientists at the Benfield Greig Hazard Research Centre at University College, London.
Their work has established that much of La Palma, the most volcanically active of the Canary Islands, is now inherently unstable.
Another eruption could cause half the island to simply fall away and crash into the ocean. The resulting impact would generate a giant wave big enough to sweep westwards across the Atlantic and devastate the entire eastern seaboard of the United States.
Such theories of enormous tsunamis being generated by island landslides first emerged from studies of the seabed around Hawaii in the Pacific. Here great slabs of rock were identified which, it was suggested, had slipped below the surface after detaching themselves from the main islands as a result of volcanic activity.
Meanwhile, in the Canaries Dr Simon Day, of the Benfield Greig Centre, began mapping La Palma in 1994.
His studies of the summit area gradually uncovered dozens of volcanic vents formed over a period of hundreds of thousands of years by various volcanic eruptions.
All the evidence pointed to one startling conclusion - that the western half of the volcano is gradually separating itself from the eastern half.
Another important realisation was the influence of water within the rock. When this is heated by magma rising to the surface it expands and adds to the pressure on the island to fragment.
This particular effect may well prove significantly more crucial in years to come, speculates Dr Day. He believes the process of global warming could contribute to the heating of water and make the set of conditions necessary for a landslide more likely.
“There is an interesting possibility that the collapse may become much more likely over the next century or so because of global warming,” he warned.
So what happens when the fracture does take place? Well, quite simply, 500bn tonnes of rock plunge straight into the ocean.
In an attempt to determine the nature and impact of such an event Dr Day co-operated with researchers from the Swiss Federal Institute of Technology in Zurich.
Here a laboratory model was constructed of La Palma island in a wave tank. The collapse was then reconstructed and recorded by a high-speed camera.
The results were incredible. They showed that as the rock crashed into the ocean it would in real life generate a wave up to 650 m tall.
“The models still have a lot of uncertainty attached to them, but the wave height is estimated at around 650 m and that is in my view using a number of conservative assumptions,” explained Dr Day.
“Other methods have predicted anything from 1 km to 3 km.
“As the wave spreads out it looses its strength, but it will still be several tens of metres high as it strikes the United States.”
But it would not be just the eastern seaboard of America to suffer. The wave would spread out in a circular pattern like the ripples generated by dropping a stone into a pond.
They would reach as far as Florida and the Caribbean as well as bending back to hit Western Europe.
The big question of course is how probable is this nightmare scenario? It is one which is answered simply by Professor Bill McGuire, also from the Benfield Greig Centre.
“There‘s no if about it,” he stated. “It will happen, it‘s just a question of when.”
But when exactly? Here at least there is some comfort. While the landslide will be caused by volcanic activity it will not necessarily happen the very next time the island erupts.
Indeed, it may take many more eruptions before the split finally occurs.
And eruptions do not happen on a regular basis.
The last one was in 1971 and those in the future may only strike at intervals of anything from a few decades to a couple of centuries.
The probability of a giant tsunami being generated in any one century, therefore, is estimated by Dr Day at 5%.
When it does finally happen, however, there may be little opportunity for New York and the rest of the eastern coast of the United States to head for the hills.
“No volcano ever erupts without some warning,” said Professor McGuire.
“The difficult thing is determining whether that particular eruption will be the one to cause the landslide or not.
“When it does go it will go in a matter of minutes and I wouldn‘t like to be around measuring things at that point.”New York (right) and much else could be devastated by a tsunami emanating from the eruption of La Palma, the most volcanic of the Canary islands, whose main port is Las Palmas (below). A volcanic eruption (left) last came on La Palma in 1971 - and those in future may come at any time “The difficult thing is determining whether that particular eruption will be the one to cause the landslide or not.