LOS ANGELES – The tsunami caused by the 8.9 earthquake off the eastern shores of Japan Friday was triggered at a site called a subduction zone, where one tectonic plate is slowly pushing beneath another. These powerful waves, which are caused by underwater earthquakes or volcanoes, can travel across the sea at 500 mph at ripple-height, then pile up into powerful towers of water when 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 little ridge, perhaps a meter in height.

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

The rate at which the wave 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 wave’s movement, and the faster the wave travels. A wave moving across the 4,000-meter-deep Pacific Ocean will typically move at a rate of about 450 mph — “airline speeds,” said David Applegate, senior science adviser for earthquake and geologic hazards at the U.S. Geological Survey in Reston, Va.

In the open sea, the wave may appear shallow: perhaps a few inches in height. But when it approaches a shore and the ocean grows more shallow, the increased water resistance slows it down, causing a pile-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 up and up, making the wave taller and taller.

Tsunami, translated from Japanese, means “harbor wave” — because it isn’t seen until it’s already too close to shore to do much about it.

The effects of this tsunami are less devastating than those from one generated by the 2004 Sumatra earthquake, which killed more than 200,000 people, for several reasons. Japan is well-prepared for seismic disasters, but circumstances were also in their 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 20-25 feet high.

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

In California, Crescent City at the northern tip of the state suffered more damage than other parts of the West Coast; the city is notorious among tsunami scientists because of ridges under the ocean that act as a lens, focusing the waves on the city.In the open sea, the wave may appear shallow — perhaps a few inches in height. But when it approaches a shore and the ocean grows more shallow, the increased water resistance slows it down, causing a pile-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 up and up, making the wave taller and taller.