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Tsunamis
POSTED 30 JULY 1998

Killer tsunamis in the Pacific
On July 17, three huge waves smashed across the northeast shore of Papua New Guinea, a Pacific island nation with a mountainous spine and hundreds of indigenous tribes. As they crossed the beach, the waves were up to 30 feet tall.

victim of tsunami
A young tsunami survivor is loaded on a rescue plane as he calls for his mother at the destroyed village of Sissano, Papua New Guinea on July 19. AP/Brian Cassey

By now, an estimated 1,600 people are dead: crushed, maimed or pulled to sea and drowned as the mighty waves withdrew. At least seven villages have been obliterated, and, when the missing are accounted for, the final death toll could exceed 5,000. Many of the dead were children, who had no chance of battling the phenomenally powerful waves.

map of Australia, Papua New Guinea and Indonesia As stunned villagers try to rebuild their lives, The Why Files wants to know what causes tsunamis, and how they travel across the ocean. As coastal populations continue rising around the world, we wonder about the history of tsunami assaults on humans. Are there ways to reduce the vulnerability of people to this cataclysmic force of nature?

What are tsunamis and what causes them?
Tsunamis -- sometimes incorrectly called tidal waves -- are extremely powerful waves caused by large undersea disturbances. (The name tsunami derives from Japanese for "harbor wave," reflecting the fact that harbors can concentrate the energy of a tsunami. True tidal waves, also known as tide waves, are long-period waves associated with the tide-producing forces of the moon and the sun and which are identified with the rising and falling of the tide.) Although landslides and volcanoes cause some tsunamis, probably 95 percent result from earthquakes -- usually under the ocean floor but occasionally near shore. Want an update on the cause? Because the volume of water is essentially constant, up or down movement of the sea floor will raise or lower the water above it, causing a wave. Similarly, you can make a (somewhat smaller) wave by throwing a stone in a pond. (Granted, earthquakes disturb the bottom of the water while stones disturb the top, but it's the best analogy we could concoct.)

illustration of how a tsunami works
Vertical movement of the ocean floor will raise or lower the water above it, causing waves that move outward. Almost undetectable in the deep ocean, tsunamis rise to a mighty crest as the ocean shallows.

Just as a larger stone, thrown with more force, makes a larger wave, the size of the tsunami is related to the area that moves on the ocean bottom, and how far it moves. Obviously, more movement of a larger piece of ocean floor makes a larger wave.

The intensity at which a tsunami strikes land is also related to the distance between the land and the center of the earthquake. It's here, at the epicenter, that the tsunami originates. As the waves spread from the epicenter in the typical arc-shaped pattern, their energy also spreads out, making tsunamis usually most dangerous to those closest the epicenter.

Spread out, but still powerful
One factor that distinguishes tsunamis from small, familiar waves is their extremely long wavelength. On the open ocean, the peaks of neighboring waves in a tsunami may be 300 kilometers -- you read that right: 300 kilometers! -- apart.

Even in the deepest ocean, that wavelength makes tsunamis what scientists call "shallow water waves." The speed of a shallow-water wave depends on the water depth, and in deep water, tsunamis can move at 500 to 600 miles per hour. That gives them the ability to keep pace with a Boeing 747, yet even after crossing the entire Pacific Ocean, the waves retain huge amounts of energy.

But you wouldn't see a tsunami from the cockpit of a 747. A killer tsunami may be only 2 feet tall in mid-ocean -- far too small to be noticed from an airplane or even a ship.

illustration of a tsunami wave
Although waves are most obvious at the water surface, the motion actually goes much deeper. That's why the ocean bottom can affect a wave's shape, height and force.

How can all that destructive kinetic energy hide in waves that we can barely see? Because waves are far more than what you can see on the surface -- they also include the nearly circular movement of water below the surface. Because long-wavelength waves extend far deeper into the water than waves with peaks closer together, there's a massive amount of water in motion beneath the surface. That's where these practically invisible waves store an enormous amount of energy.

As we've indicated, boats in deep water ride over the worst tsunamis without even noticing them. It's only when they reach shallow water and "run aground" that these waves become intense and dangerous. Like all shallow water waves, tsunamis slow when the lower part of the wave -- where the water is moving in a circle -- encounters the bottom as the ocean floor slopes up toward land.

But while the front of the wave slows, the wave behind is still moving rapidly, causing a giant pile-up at the front. Then the kinetic energy that was spread through the entire depth of the ocean becomes concentrated in a towering wave at the surface. (More on the physics of tsunamis.)

It is these surface waves -- which can be 10 meters high or taller as they cross the beach -- that cause the utter destruction of tsunamis. Usually, a series of waves, often as much as an hour apart, come crashing in -- killing those who return to help victims of the previous waves.

Adding it up
How did these factors affect the New Guinea disaster? The first earthquake -- centered on the shoreline -- measured 7 on the Richter scale. (Any earthquake in the Pacific that's at least magnitude 7 will trigger a tsunami watch or warning; more on that later.) A second, magnitude 5.7 quake then struck a few miles from shore.

still image of how Papua New Guinea tsunami spread
Federal investigators have assembled this preliminary image of how the Papua New Guinea tsunami spread out from its epicenter. Click image to download Quicktime Link will open in new window. animation [500k]. NOAA.

The two earthquakes were so close to the shore that the waves had little time to spread out and lose intensity before hitting. This proximity also reduced the escape time. And the water over the moving ocean floor was apparently deep enough to transfer a lot of energy from the earthquake to the water.

Responding to preliminary data from the disaster, George Curtis, a tsunami researcher at the University of Hawaii, said, "It was an awful combination from the point of view of having a tsunami. The focus [depth of the earthquake in the earth's crust] was not too deep, but the epicenter was in the right depth of ocean, and yet close to land. There was no space for it to dissipate, and people didn't have time to run."

Although Papua New Guinea has no tsunami warning system, there would not have been time to issue a warning in any case. And with the tsunami so close to shore, even prudent villagers who fled to higher ground immediately after feeling the earthquake might have been trapped.

The result was the destruction of a series of villages along a stretch of coastline that was, until recently, home to roughly 10,000 people.

How does the Papua New Guinea disaster compare to past killer tsunamis?

more


Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

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