Waves of Death: Why the New Guinea Tsunami Carries Bad News for North America
Monastersky, Richard, Science News
Why the New Guinea tsunami carries bad news for North America
When a devastating tsunami hits anywhere in the world, Costas Synolakis gets on the phone and starts arranging flights to the afflicted region. A hydrodynamic engineer, Synolakis has specialized in studying these giant sea waves, and nature has provided plenty for him to study in recent years. Between 1992 and 1997, Pacific earthquakes spawned nine tsunamis that claimed more than 1,800 lives. Yet these events didn't prepare him for the scale of the disaster in Papua New Guinea, where three mountainous waves pounded the northern coastline on July 17 and carried away at least 2,500 people.
"We were in a state of shock," says Synolakis, a researcher at the University of Southern California in Los Angeles and co-leader of a science team that visited Papua New Guinea in early August. "It was really something we had not seen before. It was sort of a new threshold in terms of what a wave can do."
Debris hanging from the tops of palm trees indicated that the waves reached heights of 14 meters, taller than a four-story building and more than twice the estimate reported by news agencies immediately after the catastrophe. In surveys of past disasters, Synolakis and his team had never before witnessed evidence of so much water flowing over a shoreline. "This is about double the worst overland flow that we had seen before," he says.
All this from an otherwise ordinary earthquake. The jolt that preceded the tsunami measured 7.1 on the moment magnitude scale, meaning it was a strong but fairly common quake. Tremors of at least this size strike somewhere on the globe about every 3 weeks. The United States has suffered three in the past 7 years.
During their survey, Synolakis and his colleagues grew convinced of something they had suspected even before reaching Papua New Guinea. "Once we were there, we realized the earthquake itself could not have generated such a large wave." Their favored hypothesis now is that the earthquake triggered an underwater landslide that in turn generated the giant waves--a possibility that raises disturbing questions about the tsunami hazard elsewhere.
The coastline that stretches from California to the Pacific Northwest and southern Alaska has many similarities to the shores of Papua New Guinea, say tsunami researchers. The west coast of North America is laced with small coastal faults capable of producing moderate quakes and underwater slides in the steep-walled canyons offshore. "These [small faults] haven't been taken seriously as tsunami-generating sources before. We have to take a much closer look now," says Synolakis.
Of all natural disasters, tsunamis may rank as the most poorly understood by the general public. Media reports often describe them as tidal waves, although they bear no relation to tides. They are often portrayed as simply scaled-up versions of a breaking wave, similar to the famous painting by the Japanese artist Hokusai that appears on almost all tsunami web sites. But the truth lies far from that image.
Survivors of the Papua New Guinea disaster describe the tsunami as a wall of water barreling toward shore, according to the researchers. Unlike a normal wave with its prominent crest, the tsunami was more like a plateau of water, averaging 10 m high and extending 4 to 5 kilometers from front to back, says Synolakis. The largest wave swept over the shore at speeds of up to 20 km per hour for more than a minute, before draining away in preparation for the next. Imagine a mountain of water, appearing out of nowhere and plowing across the beach as if surging from a collapsed dam.
In their preliminary report to the National Science Foundation, which funded their postdisaster survey, Synolakis and his team say that the first of the three large waves arrived 5 to 10 minutes after the initial earthquake, with the others following several minutes later. …