US tidal wave risk

Arrows show the cracks at the top of the continental shelf (Image: Geology)

Newly discovered cracks in the ocean floor off the East Coast of the US could cause a dangerous tidal wave in the future, US geologists believe.

The cracks are off the coasts of Virginia and North Carolina and if they gave way, a "potentially disastrous" underwater landslide would result.

This would throw a wave up to six metres (20 feet) high back on to the land.

The scientists are due to set sail for the area on Saturday to inspect the area more closely and make a further assessment of the risk.

In 1998, two strong earthquakes caused a series of tsunamis to hit the north coast of Papua New Guinea, killing 2,000 people. And the East Coast of North America suffered in 1929, when a 12m wave hit Newfoundland - 51 people died.

Dead or alive

"We need to understand whether these crack-like features are active or recently active, in which case we should be concerned," said one of the team, Jeffrey Weissel, of Lamont-Doherty Earth Observatory, New York.

A tsunami devastated Papua New Guinea

His colleague, Neal Driscoll, of the Woods Hole Oceanographic Institute, Massachusetts, added: "We believe the risk is slight but we just don't know and it's very difficult to predict when and where these tsunamis are going to occur."

Their research, published in the journal Geology, started with the study of sonar maps of the sea floor off the coasts of Virginia and North Carolina.

About 170km offshore, just along the top of the slope where shallow inshore waters give way to the deep ocean, they noticed a 40km long series of cracks. Furthermore, the down slope side of the cracks appeared to have already dropped by 50m.

On the rebound

If these cracks collapsed, then rock and sediment would rush down the slope.

The scientists predict that within about 30 minutes, the sea level at the coast would actually drop by several metres at first, as the water rushed out to fill the space left by the slumped rocks.

Then, anytime up to an hour later, the sea would crash back in, up to six metres higher than normal.

The size of the tsunami predicted is equivalent to the storm surge that comes with a category three or four hurricane. Hurricane Andrew, the category four storm that hit Florida in 1992 was the costliest natural disaster in US history, with over $25 billion in damage caused.

The extent of flooding resulting from any tsunami would depend on the tide level at the time it struck. The worst case scenario would be a tsunami coinciding with a high tide.