Last May's magnitude 7.7 earthquake off Adak island in Alaska awakened a sickly fear in many coastal residents' minds, a sense of curiosity in others, and an indifferent shrug from some. For the first time in years, a public warning echoed around the Pacific: a wall of water was on the way, to arrive in several hours.
Westways magazine, October 1986 |
Almost all tsunamis occur in the Pacific Ocean, an area of high earthquake activity. When a tsunami is generated anywhere in the Pacific, every Pacific coastline is in danger, from Alaska down the west coast, South America, Hawaii and Japan.
When a tsunami is generated, it races across the ocean at up to 670 miles per hour, the speed being determined solely by the depth of the water. In open water, the wave cannot be seen or felt by boats, and doesn't even noticeably rise above sea level until it reaches land. Tsunami can not capsize ships in open water -- no matter what you saw in The Poseidon Adventure. In fact, one of the safer places to be during a tsunami is out in deep water. When a tsunami warning comes, most ships will head out to sea if there is time.
As a tsunami reaches shallower water and approaches land, its speed drops. In 60 feet of water, speed goes down to about 30 mph. But when the speed drops, the water behind the wave builds up and the wave rises in height. By the time it crests at a beach, the wave can be many feet high.
After the tsunami first hits land, the disaster isn't over. A tsunami is not just one wave but a series of waves, much like dropping a rock into a pool of water. From the point of the original earthquake, rings of waves travel in every direction. Waves then hit distant shores one after another up to two hours apart.
|
| A tsunami hits Hawaii on April 1, 1946, generated by an earthquake in Alaska five hours earlier. (Photo: NOAA) |
Between each of the waves, coastal waters often recede. They can rush away to a level lower than that of the lowest tides, and may do so even before the first wave hits. Sometimes when the water rushes away, people at the shore will walk onto the newly exposed area, fascinated by the flopping fish that suddenly find themselves out of water. When the people finally see the quickly approaching wave it's too late. If you see the water suddenly rise or drop, "you should take nature's warning on your own initiative" and run towards higher ground, says Commander Ken Lilly, the west coast marine officer of the National Oceanic and Atmospheric Administration (NOAA) corps, the federal agency responsible for the study of tsunami and the issuance of warnings when the waves are generated.
Copyright © 1986, 2004 by Randy Cassingham, All Rights Reserved worldwide. May not be copied, stored or redistributed without prior, written permission.