The science of tsunamis

The tsunami launched by the magnitude 8.9 earthquake off the eastern shores of Japan was triggered at a site called a subduction zone, where one tectonic plate slowly pushes beneath another.

From there, the waves traveled across the sea, little higher than ripples, before piling up into powerful towers of water as they hit land.

Earthquake-caused tsunamis occur regularly in the Pacific Ocean, where subduction zones abound, said Robert Weiss, a tsunami scientist with Texas A&M University in College Station. When the pressure building between the two plates finally releases, one of the plates gets distorted and pushed very quickly into a tiny mountain, perhaps a yard in height.

Photos: Scenes from the earthquake

That upward movement disturbs the water, much as a pebble does when tossed into a pond. And just as a pebble causes many ripples, the disturbance sends a series of waves away from the fault line.

The rate at which the tsunami travels depends on the depth of the water, said Thomas O'Rourke, a geotechnical engineer at Cornell University in Ithaca, N.Y. The deeper the water, the less resistance there is to the waves' movement, and the faster the wave travels. A wave moving across the 14,000-feet deep Pacific Ocean will typically move at a rate of about 450 miles to 500 miles per hour — "airline speeds," said David Applegate, senior science advisor for earthquake and geologic hazards at the U.S. Geological Survey in Reston, Va.

In the open sea, the waves, though very long, may appear short in height, perhaps a few inches. But as the tsunami approaches a shore and the ocean becomes more shallow, there is less room for the rapidly moving water, causing a build-up — much like a car pile-up — of all the energy moving behind the wave front. With no way to go forward, all that water mass (and the energy it carries) gets pushed higher and higher, making the tsunami taller and taller.

Videos of the earthquake

The word tsunami, from Japanese, means "harbor wave" — because it isn't seen until it's too close to shore to do much about it.

The effects of Friday's tsunami so far have seemed less devastating than those from one generated by the 2004 Indian Ocean earthquake off Indonesia's Sumatra island, which killed more than 200,000 people.

Japan is better-prepared for seismic disasters, but circumstances were also in its favor, Weiss said. The Sumatra earthquake was of magnitude 9.1-9.3 — significantly more powerful than Japan's 8.9, Weiss said. In addition, the fault line in the Sumatra earthquake stretched for about 750 miles compared to 220 miles long for Friday's quake. It generated 100-foot-high waves in some places when they hit land. The waves reaching the shores of Japan following Friday's quake were 20 feet to 25 feet high.

The damage done by the tsunami depends on the topography of the seabed and land, said Jody Bourgeois, a geologist at the University of Washington in Seattle, who is currently in Sapporo, Japan.

On the United States' West Coast, Crescent City, at the northern tip of California, suffered the most damage. Crescent City is notorious among tsunami scientists because of ridges under the ocean that act as a lens, focusing the waves on the city.

Photos: Scenes from the earthquake