Cracked seabed may cause trouble

Experts fear that section of continental shelf could give way, causing tsunami

May 07, 2000|By New York Times News Service

In waters off Virginia and North Carolina, scientists have found cracks in the seabed that might set off a tsunami, they say, sending waves as high as 20 feet speeding toward the mid-Atlantic states.

Neal W. Driscoll of the Woods Hole Oceanographic Institute, Jeffrey K. Weissel of Columbia University and John A. Goff of the University of Texas said in an article in a recent issue of the journal Geology that the recently discovered cracks could mean the continental shelf is unstable and could slide down like an avalanche, inducing the giant waves.

The scientists said they discovered the cracks about 60 miles from shore along a 25-mile section of the continental shelf off the Virginia and North Carolina coasts.

If the seabed there gave way, they said, the reaction could set off the giant waves.

"From a societal perspective, we need to evaluate the degree of tsunami hazard that might be posed by a major submarine landslide," the scientists wrote.

And they plan to find out if the cracks "are fossil features or are active and likely to produce a potentially disastrous, large submarine slide in the near future."

The scientists were preparing a 14-day expedition to gather more data to assess the risk.

"We don't want to sensationalize this," Driscoll said in an interview. "The risk associated with these cracks is finite. First, we have to determine if they're geologically active, then whether they're moving on human time scales."

The long fissures were found 325 to 650 feet beneath the waves and lie perched close to the edge of the outer continental shelf, where the seabed becomes gradually deeper and then quickly gives way to the abyss.

Never before, the scientists said, have cracks so large been seen, although hints of smaller ones have cropped up in undersea data.

If the danger of the cracks proves real, the scientists said, coastal areas of North Carolina, southern Virginia and the lower Chesapeake Bay would be at risk for wave heights similar to the surge of a Category 4 hurricane, which is characterized by top sustained winds of 131 to 155.

The problem is that a tsunami can come with no warning.

Such sudden events are not unknown.

In 1929, a submarine landslide associated with an earthquake on the Grand Banks set off a tsunami that killed 51 people along the coast of Newfoundland. The waves from that event were estimated to range in height from 3 to 40 feet.

Tsunamis are often larger than predicted from the force of earthquakes alone, suggesting that submarine landslides play an important role in their generation, the scientists said. In 1998, a tsunami in Papua New Guinea killed 2,000 people.

The scientists plan to make images of the cracks with sonar waves, and then take deep samples of the seabed to see if the cracks are fresh or covered with layers of sediment.

The latter finding would suggest that the cracks were geologically inactive and a low threat of producing giant waves.

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