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Western States Prepare for Tsunamis
Associated Press, June 14, 1998; 12:02 p.m. EDT

The Word:
Tsunami is a Japanese word that translates as "harbor wave." The term "tidal wave" is inaccurate, since tides have nothing to do with their formation. The term "seismic sea wave" is also misleading, since tsunamis can also be generated by such non-seismic events as landslides and meteorites. Such tsunamis generally dissipate more quickly than those that follow earthquakes.
What It Is:
Unlike wind-caused waves, which are fairly short and only a few seconds apart, a tsunami may stretch dozens of miles in length, and the time between one tsunami and the next can be an hour or so. In the deep ocean, where they move fastest, they can travel at speeds of up to 500 miles per hour.
Earthquakes and Tsunamis:
An earthquake can generate a tsunami if the quake involves a sudden rising or lowering of part of the sea floor. That abrupt change displaces the water above, and a tsunami can form as the water moves to regain its normal surface.
How Tsunamis Look:
In the deep ocean, a tsunami is imperceptible to humans. As a tsunami reaches shallower depths near shore, its height can rise several meters or more. When it reaches shore, it may appear as a rapidly changing tide, a series of breaking waves, or a bore -- a high, abrupt wave in a narrow channel.
On Shore:
Tsunamis can inundate areas hundreds of yards past the normal high-water mark, stripping out soil and crushing structures. They can reach heights of 100 feet.

Sources: University of Washington Geophysics Center; National Oceanic and Atmospheric Administration