Undersea cracks along the continental shelf of North America could spell big trouble for people living on the East Coast of the United States. The cracks indicate the sea floor along the continental shelf could slide like an avalanche, triggering a huge wave that would thunder towards the heavily populated eastern seaboard.
In a paper in the May 2000 issue of Geology, Peter Weissel of Columbia University’s Lamont-Doherty Earth Observatory, Neal Driscoll of the Woods Hole Oceanographic Institution in Massachusetts and John Goff of the University of Texas, say the cracks are an early warning sign that the sea floor is unstable.
Undersea cracks could cause landslides, triggering a tsunami like this.
Driscoll found the cracks when he made a map of the area using previous data that measured the depth of the ocean. When he noticed what looked like cracks in the ocean floor, he consulted Wiessel, an expert on landslides. “He (Driscoll) showed the map to me and he said ‘What do you think about these things?'” Weissel says. “And I looked, and he must have seen the colour drain out of my face because these things looked like the headwalls of recent big landslides.”
The cracks can make the sea floor unstable, leading to landslides, which could trigger a tsunami. “This is just gravity acting on the edge of the shelf, making it move towards the deep ocean,” Weissel says.
There is evidence that landslides occurred in the area around the cracks before, which are along a 40-kilometre section of the continental shelf, 140 kilometres off southern Virginia and North Carolina. Weissel says there was a massive undersea landslide just to the south of the cracks about 20,000 years ago, during the last ice age. “So we’re concerned about this area because we know there were big landslides here in the past.”
There is a chance that the cracks are just leftover remnants of past landslides, but Weissel and his colleagues don’t know for sure. “If they are active and presently slipping, we should be worried. If they’re not active, if they’re dead, then we can breathe a sigh of relief,” he says.
Tsunamis are created when an underwater earthquake, volcanic eruption or a landslide suddenly disturbs the ocean’s surface and the ocean floor shifts upwards. The water above the floor moves with it, creating a wave on the surface that can run for hundreds of kilometres in all directions. The higher water falls downwards into the lower water, spawning a massive wave several metres high, traveling at great speed. As the wave approaches land and the ocean depth decreases, the wave slows down, bunches up and rises. The tsunami surges inland, flooding coastal areas. Depending on how steep the shoreline is, it can travel hundreds of metres inland. When the tsunami recedes, it drags anything it has picked up on land back out to sea. In July 1998, a tsunami that struck the northern coast of Papua New Guinea killed 2,000 people.