Tsunamis: More than Big Waves

by Randy Cassingham

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The lighthouse on Unimak Island, Alaska, before the April 1, 1946 tsunami hit...
(click to enlarge) The remains of the Unimak lighthouse after the tsunami. It wasn't spared even though it was on a hillside. The dark area shows how far up the waves reached. (Photos: NOAA)
"Tsunamis are not that rare," says Lilly. Hawaii is hit by an average of 52 tsunamis per century; and the Pacific coast of the continental U.S. suffers about 34, or an average of one every three years. However, while tsunamis themselves are not that rare, Lilly continued, "the ones that are damaging are." There has not been a major tsunami to hit the west coast since 1964. "You might say we're overdue" for a destructive tsunami, he said.

Since 1906, several very destructive tsunamis have hit in the United States: three in Hawaii, three in Alaska, and five on the west coast of the lower 48 states. The last destructive tsunami to hit the continental U.S., in 1964, caused major damage in Crescent City, Calif., near the Oregon border. Much of the downtown area was wiped out and 11 people were killed. Those waves were generated by a magnitude 8.5 earthquake in the waters off Alaska. Several hours later, 18-foot waves hit Crescent City, causing $7.5 million in damages. Other areas on the west coast were also hit, but received less damage. Marin county, for instance, suffered $1 million in flood damages because of the waves.

Tsunami waves can be giants. The largest tsunami on record hit in 1737 on Cape Lopatka, on the north coast of Asia about 1,300 miles north of Japan. The waves there were about 210 feet high. Despite a common belief that tsunami waves are all giants, most are not. But tsunamis don't have to be big to cause widespread damage. "People that I've talked to often tell me, 'it wasn't very big, but it kept coming and coming'," Spaeth said. In such disasters, flooding can cause more damage and death than the sudden weight of the wave hitting.

o o o

In 1946, a serious tsunami hit Hawaii. Triggered by a magnitude 7.4 quake in Alaska's Aleutian Islands, the waves travelled at an average speed of 490 mph and hit Hawaii, 2,400 miles away, five hours later. On the big island, waves reached 55 feet above sea level. Many lives were lost.

Shortly after that tsunami hit, it was realized that if the waves took five hours to reach the island, a warning system could be put into effect to allow time for evacuation. The U.S. government built a seismographic system to determine the location and magnitude of earthquakes in the Pacific, and a network of tide gauges was added to watch for waves.

From their headquarters at the Honolulu Observatory, the Pacific Tsunami Warning System monitors the seismographs and tide gauges that ring the Pacific. The office is manned 24 hours-a-day. The system was first used on November 4, 1952 when a Pacific earthquake generated a tsunami. Six hours later, the waves hit Hawaii. But due to the advanced warning, no lives were lost.

When the system's monitors detect a major earthquake, its location and magnitude are quickly plotted by computer. If the magnitude is sufficient to generate a tsunami, a "tsunami watch" is issued to all weather stations (which relay the warnings to boaters) and local government offices on the coast. Since wave speed is based on the depth of the ocean and average depths are known, an approximate arrival time of the waves can be calculated by the computer. However, since not all earthquakes generate tsunamis, the actual existence of the waves can only be determined by the tide gauges or coastal observation.

Tsunami Safety Rules

Tsunamis follow no discernible pattern. When you receive a tsunami warning, you must assume a dangerous wave is on the way. You can help yourself by remembering these facts:

  1. All earthquakes do not cause tsunamis, but many do. When you hear that a strong earthquake has occured, stand by for a tsunami emergency.
  2. An earthquake in your area is a natural tsunami warning. Do not stay in low-lying coastal areas after a local earthquake.
  3. A tsunami is not a single wave, but a series of waves. Stay out of danger areas until an "all-clear" is issued by authorities. Waves can be an hour apart.
  4. Approaching tsunami are sometimes heralded by a noticeable rise or fall of coastal waters. This is nature's tsunami warning and should be heeded.
  5. A small tsunami at one beach can be a giant a few miles away. Don't let the modest size of one make you lose respect for all.
  6. The Tsunami Warning System does not issue false alarms. When an ocean-wide warning is issued, a tsunami exists. When a regional warning is issued, a tsunami probably exists. The tsunami of May, 1960 killed 61 people in Hilo, Hawaii who thought is was "just another false alarm."
  7. All tsunamis, like hurricanes, are potentially dangerous, even though they may not damage every coastline they strike.
  8. Never go down to the beach to watch a tsunami! When you can see the wave you are too close to escape it.
  9. Sooner or later, tsunamis visit every coastline in the Pacific. Warnings apply to you if you live in any Pacific coastal area.
  10. During a tsunami emergency, your local Civil Defense, police, and other emergency organizations will try to save your life. Give them your fullest cooperation.
  11. Potential danger areas are those less than 50 feet above sea level and within one mile of the coast for tsunami of distant origin, or less than 100 feet above sea level and within one mile of the coast for tsunami of local origin.
If the existence of tsunami waves are confirmed by the tide gauges or land near the earthquake is hit by a wave, a "tsunami warning" goes into effect. Coastal areas are told when they can expect waves, but the size of the approaching waves cannot be determined. Wave size depends on the intensity of the earthquake, the distance the waves travel, the direction the waves are coming from and, importantly, the underwater geography near the coast. A wave may be barely noticeable in one spot but may be very destructive only a few miles away.

In the United States today, the tsunami warning system is operated by the National Weather Service, a division of NOAA. A few years ago, NOAA sent Ken Lilly and Mark Spaeth to the west coast to confer with local government officials. NOAA wanted to remind coastal authorities of the tsunami threat and suggest that coastal communities have a tsunami disaster plan drawn up before it's too late.

"Admittedly, these are rare events," Spaeth told the disaster officials. "They are probably low on your list of priorities." But, he said, it is important for coastal areas to prepare a response for tsunami emergencies now, not when they are notified that waves are just a few hours away. It is only a matter of time until it happens.

Last May's Adak tsunami was a warning. The waves it generated turned out not to be large enough to see. If we are lucky, the scare made people more aware of the potential danger. If we weren't lucky, it made people think that the next tsunami warning can be ignored. And that could be a fatal mistake.

Page 3: Amazing satellite photos ->

To Send Aid

Two of the most effective international disaster relief organizations who do a great job and have low overhead (meaning they spend most of their donations on aid, not on advertising campaigns to get more donations) are:

More Info on SE Asia Tsunami

One of the cool things about the web is its ability to set up a communications and support system very quickly after a disaster. Perhaps one of the best single sources for information and "what to do to help" after the Asian tsunami is the SEA-EAT blog. It has up-to-date info and all sorts of links for more information.

Other links with background info (all open in new windows):

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