MORE information about the killer of many!

1 JUNE 2008

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. 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.

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.

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.

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.