Scientists fear mega-tsunami threat

Sat, 03 Oct, 2015 - 01:00

John von Radowitz

The event happened 73,000 years ago in the Cape Verde Islands off west Africa, long before there were any coastal cities that might have been flattened by the deluge.

However, experts fear a similar giant collapse could present a threat today, especially around volcanic islands.

Lead scientist Dr Ricardo Ramalho, from Columbia University in New York, said: “Our point is that flank collapses can happen extremely fast and catastrophically, and therefore are capable of triggering giant tsunamis.

“They probably don’t happen very often. But we need to take this into account when we think about the hazard potential of these kinds of volcanic features.”

The ancient collapse occurred at Fogo, one of the world’s largest and most active island volcanoes that today towers over 2,700m above sea level.

An estimated 40 cubic miles of rock fell into the ocean in one go, resulting in an 250m-high tsunami wave that engulfed an island almost 50km away.

By comparison, the largest known recent tsunamis, which devastated Indian ocean coasts in 2004 and eastern Japan in 2011, attained maximum heights of around 30m. These were triggered by undersea earthquakes rather than volcanic collapses.

Clues left by the mega- tsunami included boulders the size of delivery vans that had been carried up to 600m inland and nearly 200m above sea level on Santiago Island, 55km from Fogo.

The boulders, weighing up to 770 tonnes, matched marine-type rocks ringing the island’s shores and were quite unlike the volcanic terrain in which they were found.

By calculating the energy needed to hurl the boulders so far, scientists were able to estimate the wave’s size. The findings are reported in the Science Advances journal.