Aug. 24, 1998 The New Guinea Tsunami Catastrophe Suggests Rising Peril For Pacific Rim & Other Coastline Regions
The first scientific team to survey and report on last month's catastrophic tsunami at Sissano Lagoon in northwestern Papua New Guinea suggests that tsunamis, or tidal waves, may threaten more coastline regions of the world than previously thought, including much of the West Coast.
The team believes that the tsunami occurred when a 7.0 magnitude earthquake triggered a massive underwater landslide--creating a series of waves that swept across heavily populated shoreline strips at the lagoon's entrance.
"We used to think a magnitude 7.0 earthquake was too small to generate a tsunami," says Costas Synolakis, Ph.D., leader of the four-person team funded by the National Science Foundation to measure the tsunami's inundation heights and inland penetration distances. "Of the nine large tsunamis that have occurred in the past six years, only the New Guinea one resulted from an earthquake as small as magnitude 7.0."
"We probably have 10 earthquakes of that magnitude a year worldwide," says team member Emile Okal, Ph.D. "You can have a very large danger concentrated in a very small area because of the instability of submarine structures that we do not have very well mapped."
Like Dr. Okal, Dr. Synolakis laments the world's lack of knowledge about ocean-floor topography. "We have a better map of the surface of Venus than we do of our own ocean floor," he says.
The American team consisted of Synolakis, a professor of civil engineering at the University of Southern California's School of Engineering; Okal, a professor of geophysics at Northwestern University; Boyd C. Benson, a graduate student at the University of Washington; and José C. Borrero, an NSF fellow and graduate student from USC.
Also accompanying the team was Mr. Borrero's father, José L. Borrero, M.D., a physician from Altamonte Springs, Fla., who treated survivors of the tsunami. Randy Sherman, M.D., chief of the division of plastic and reconstructive surgery at the USC School of Medicine, assembled medical supplies for the team physician.
Earthquakes of magnitude 7.5 and greater trigger tsunamis that traverse thousands of miles of ocean to affect thousands of miles of coastline, Synolakis notes. In contrast, the New Guinea tsunami of July 17 started close to shore and damaged a relatively small, though heavily populated, area.
Many seismically active coastlines of the Pacific Rim are comparable to the New Guinea disaster site, where the ocean floor declines rapidly close to shore, falling away in precipitous chasms and steep canyons.
"The Cascadia Subduction Zone off the state of Washington threatens British Columbia, Washington, Oregon and the northern California coastlines," Synolakis says.
"Southern California, because of its population density and offshore topography, is also threatened. It would not take a large tsunami to cause a disaster here, where hundreds of thousands of people are often at the beach. If you're at the beach and feel an earthquake, you should move to higher ground as quickly as possible."
Synolakis relates some of the New Guinea survey findings:
"A 7.0 earthquake, with the epicenter significantly inland, occurred shortly after sundown and was followed 20 minutes later by a 5.9 aftershock. We think the initial quake triggered a landslide on the sea floor, probably on the order of four or five cubic kilometers. The wave arrived onshore 5 to 10 minutes after the initial quake.
"Two large tidal waves were followed by a third and smaller wave. Initially no more than two or three meters high, the waves gained height as they raced over steeply decreasing depths toward the shore.
"On touching land, the largest waves were 10 to 14 meters (32 to 46 feet) high at the center (the maximum height fell off significantly a short distance from each wave's center) and 20 kilometers (12 miles) long. The mass of water in each wave extended back from the crest as far as four kilometers (2.5 miles).
"The water surged over the peninsula at an estimated speed of 10 to 20 meters per second, or 22 to 44 miles per hour. This force induced by the tsunami current is 1,000 times greater than the force of a wind of the same speed."
Okal points out three reasons why the researchers have inferred that an underwater landslide was involved in this tsunami: "One, we can't explain the size of the tsunami otherwise. Two, we know submarine landslides have occurred in other areas, since they're conspicuous on the ocean floor in other subduction zones. And three, we found a landslide above sea level while we were there. A cliff at the western end of our land survey area showed a big landslide that was extremely fresh. People told us it went down during the earthquake of July 17."
Okal concedes it's a big jump to the conclusion that the same thing happened below the sea. "But certainly," he argues, "in terms of the accelerations and the intensity of motion, what took place under the water was probably the same as what took place next door above the water, and if the material had the same level of instability, one could imagine a submarine landslide."
To study the causation of future tsunamis more accurately, Okal says, scientists really need to map the sea floor with radar, sonar, etc., from an oceanographic ship to look for evidence of a fresh underwater landslide.
At the request of Papua New Guinea authorities, the American scientists appeared at a public meeting to allay survivors' fears about returning to their homes and to quash some wild rumors about the disaster's cause. Many survivors believed they were being punished for impiety and that more waves were imminent. One 15-year-old girl asked why eyewitnesses had described the tsunami as "an infernal mountain of water with fire sparkles flying" -- a description consistent with the severe burns observed among many of the dead.
Synolakis says the rapid rise in sea level occasioned by the tsunami may have enhanced the bioluminescence of dinoflagellates and other glowing organisms commonly found in the waters of the South Pacific. "The tsunami occurred about an hour after sundown," he explains. "In the darkness, the organic bioluminescence may well have caused the water to sparkle like fire."
He says the burns seen on bodies resulted from friction when people were dragged over hundreds of meters of sand, debris and trees, and from the sunburn as they lay decomposing after the disaster, giving the appearance of death by scalding.
Synolakis says that the death toll from the Papua New Guinea disaster will likely exceed 3,000 and be recorded as the deadliest tsunami of the century.
For further information and pictures of the Papua New Guinea tsunami, point your browser to http://www.usc.edu/dept/tsunamis/PNG/PNG1.html.
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