When half a Spanish island collapses, tsunamis will devastate the coastline of countries all around the Atlantic 鈥� all because tsunamis can turn corners.
Last year, Simon Day of University College London and his colleagues reported that a flank of a volcano on the island of La Palma in the Canary Islands was unstable (New 杏吧原创, 7th October 2000, p 26).
If the flank collapses, which Day expects to happen sometime in the next few thousand years, the resulting landslide will dump a trillion tonnes of rock into the Atlantic within minutes. Day predicted that this would probably send a huge tsunami raging towards America鈥檚 East Coast.
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Now he has teamed up with Steven Ward, a wave expert at the University of California, Berkeley, to work out the tsunami鈥檚 size and spread. Ward has developed a model of waves triggered by underwater earthquakes, ranging from a mere eight centimetres high for a magnitude-6.5 earthquake, to tsunamis several metres high for a real whopper.
Caving in
When the La Palma volcano caves in, Ward says it will trigger a series of around ten waves, spaced about a hundred kilometres apart. As they reach the shallow water near the North American coast, they will build up to about 50 metres high, enough to travel several kilometres inland. 鈥淭here鈥檚 a significantly broad danger zone,鈥� says Day.
Although the volcano鈥檚 unstable flank points directly towards North America, it is not just North Americans who should be worried. Day originally estimated that the collapse would create a shockwave travelling in a straight line across the Atlantic, directly towards America鈥檚 East Coast. This would happen if the speed of the landslide was faster than the speed of the waves in deep water.
But the model shows that the landslide will actually move at around 100 metres per second, about two-thirds as fast as the waves in the water. This means the tsunamis will spread out in an arc.
Shallower water near La Palma would then slow the waves down, forcing them to curl around towards northern Africa and northern Europe, even behind La Palma on the Spanish coast.
Rare events
Gary McMurty, who works on landslides at the University of Hawaii, says there is no point losing sleep just yet. 鈥淭hese events are very rare and shouldn鈥檛 worry anyone who has a lifetime of less than a hundred years,鈥� he says.
But even if beach bums can rest easy, Day expects the result to shake up geologists. Now they know that tsunamis can spread out and turn corners, he hopes researchers will be more flexible in matching inland deposits of shells, coral and sand to ancient landslides on distant continents.
鈥淣orthern Brazil is going to be a good place to look for past evidence of collapses at the Canaries,鈥� says Day. 鈥淲e wouldn鈥檛 have thought of that.鈥�
More at: Geophysical Research Letters (vol 28, p 3397)