8.9 Earthquake hits Japan

What concerns me is that I am aware there is a major shelf off the coast of Japan and if the aftershooks cause it to collaspe it would generate waves a mile high.

April 23, 2002

A microphone in the Pacific Ocean near Wake Island recorded a 45-second, low-frequency roar, too low to be heard by human ears. It was the sound of nearly a cubic mile of sediment giving way along an ocean bottom slope 2,200 miles away off Papua New Guinea.

That recently examined recording is the latest evidence that an underwater landslide, not an earthquake, churned up the 30-foot-high tsunami that crashed onto coastal villages of Papua New Guinea on July 17, 1998, killing more than 2,100 people.

Once thought rare, landslide-generated tsunamis have caught the attention of geologists, who now look with concern at other continental shelves that could collapse with equal disaster. Three-dimensional maps of the bottom of Monterey Bay off California, for example, show several sections that have given way - and others that have cracked and may collapse in the future.

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Small landslides - or ones that slip slowly - do not cause tsunamis. Cataclysmic landslides, like the partial collapse of a midocean volcano, generate giant waves that scour thousands of miles of coastline around an entire ocean basin, but they occur very rarely, once every few hundred thousand years.

But moderate-size underwater landslides like the one off Papua New Guinea may pose an uneasily plausible risk in some places, occurring once every few hundred years.

"It is a reasonably significant hazard," said Dr. Emile A. Okal, a professor of geological sciences at Northwestern University in Evanston, Ill.

Almost immediately after it happened, scientists realized the Papua New Guinea tsunami was unusual. An offshore earthquake of magnitude 7.0 preceded the waves, but earthquakes that size strike that area every year or two; only the 1998 one was accompanied by a tsunami. The deadly devastation was also confined to a 15-mile stretch of the coast; villages only a few miles east or west escaped almost unscathed.

That led to speculation that the earthquake had shaken loose a landslide that in turn caused the tsunami. Surveys of the ocean bottom found freshly collapsed sediment that slid nearly a mile down a 25-degree slope.

Other scientists argued that a vertical thrust of the sea floor during the earthquake directly caused the tsunami, but that the amphitheater-shaped depression around the epicenter focused the waves onto the small section of the shoreline.

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The theory that underwater landslides can set off tsunamis dates back more than a century. In recent decades, tsunami researchers shifted their attention to offshore earthquakes, still thought to be the cause of most tsunamis.

But after Papua New Guinea, scientists thought they might have underestimated the dangers of landslides. In 2000, scientists at Pennsylvania State University warned of unstable, waterlogged sediments under the seabed off New Jersey. The weight of rocks above could potentially blow the sediments out the side of the continental slope like a stepped-on water balloon, causing a landslide and a tsunami.

Scientists also see potential collapses in places like the mouth of the St. Lawrence River where sediment from the river piles up. In 1929, a 7.2 earthquake toppled part of the sediment pile, causing a tsunami.

Underwater landslides have also occurred off the coast of California. In Monterey Bay, "you see large numbers of bites taken out of the canyon essentially," said Dr. Steven N. Ward, a research geophysicist at the University of California at Santa Cruz. "Some look very fresh. Some look very old. Some look like they haven't happened yet."

The canyon is cracked in some places, Dr. Ward said, and even a small earthquake near a crack could set off a landslide. Most of the slides in Monterey Bay are small - only about a fortieth the volume of the Papua New Guinea landslide - but because they occur very close to shore, they could still create 15- to 20-foot-high waves that strike a small portion of the coast. "Ten miles up or down the coast, you won't see it," he said. "It's big, but it's fairly local."

Fissures off Va. Coast May Presage Tsunami, Experts Say

Wednesday, May 3, 2000

Researchers setting out to sea from the Virginia coast hope to find answers to a new scientific riddle: Could tsunamis, the large, destructive waves that have terrorized Japan and the rest of the Pacific for centuries, pose a threat to the mid-Atlantic coast as well?

According to the latest research, they might. Scientists have discovered a 25-mile series of cracks along the edge of the continental shelf, about 100 miles east of Virginia Beach and the mouth of the Chesapeake Bay.

If the fissures are moving, they could be the beginning of the type of underwater landslides that trigger tsunamis, according to a report just published in the May issue of Geology magazine.

Scientists estimate that a landslide from the fissures could send a wave 20 feet high hurtling up the region's beaches. Such a wave hitting southern Virginia would be similar to the force of a severe hurricane. In 1972, Hurricane Agnes caused the worst flooding the Washington area had seen in decades; it washed out roads and bridges and killed 118 people from Florida to New York.

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Yet even on North America's Atlantic coast, tsunamis are not unprecedented. In 1929, a wave measured at up to 40 feet slammed into the southern coast of Newfoundland, killing 51 people. The landslide in that case was about the same size as the one that researchers fear could happen off the Virginia coast.

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Driscoll, Weissel and colleague John A. Goff, of the University of Texas, plan to set out to sea Saturday for two weeks to get a closer look at the potential landslide area. The researchers will use special sonar equipment to determine how recently the cracks opened and, perhaps, predict how soon a tsunami could occur.

The Geology article stresses that "it is unclear whether these cracks are fossil features or are active and therefore likely to produce a potentially large submarine landslide in the near future." Driscoll and Weissel said that if their examination shows that the cracks are recent, they will install monitoring equipment to see whether they are moving.

Driscoll said the cracks were not the only thing that caught researchers' attention: The fissures are just north of an Ice Age landslide that occurred about 18,000 years ago--which is not all that long in geologic terms. "If it was just the cracks themselves, that might be different," Driscoll said. "But this has happened before in this area."