How and why tsunami kill (some) people

Slightly edited by RC from an 1994 Internet exchange

An Internet contributor had written:

My new wife and I were in Hawaii last week for our honeymoon. While we were there in Kauai, our plans to visit Secret Beach were trashed when the waves from the 7.4 Honshu quake hit the islands. While there was no single tsunami, along the northwestern side of the island the waves reached as high as 30 feet at Ke'e beach.

Gerard Fryer of the University of Hawaii wrote in reply:

The recent big surf in Hawaii was storm generated and had nothing to do with tsunamis. The tsunami from the recent Japanese EQ was less than 4 inches high in Hawaii (indeed, the tsunami from the massive Shikotan Island earthquake on October 4 was only one foot, thank heavens). But the confusion between storm waves and tsunamis gives me an excuse to vent my BIG PEEVE:

People often think a tsunami is identical to a normal storm-generated wave, only bigger. That's just not true. People have died because of that error and people will continue to die because of it. There are really two errors:

1. people think a tsunami is a single wave. It isn't. It is a series of waves, typically lasting for hours (61 people died in the Hilo tsunami in 1960 because of that mistake).

2. People think that a 30-foot tsunami is just like 30-foot storm-generated surf. That is absolutely wrong.

Tsunami waves are not like normal ocean waves. The wavelength of the tsunami (distance from one peak to the next) on the open ocean can be hundreds of miles long. But as the wave slows down (from about 500 mph to about 20 mph) in shallow water, the wavelength shortens to something like five miles. Waves in a tsunami sequence, then, are somewhere from nine minutes to half-an-hour apart. Remember the old name "tidal wave?" While the waves have nothing to do with tides, the old name at least gave the idea of variation in sea level. During Hawaii's tsunami false alarms of 1986 and 1994, visitors and local residents alike made jokes about the tsunami that didn't happen, but for the many thousands who had positioned themselves with a view of the beach, the tsunamis were obvious if you just knew what to look for: slow variations in sealevel.

So if a tsunami is just a variation in sealevel, where does the classic image of a massive crashing wave come from? The speed of a tsunami depends on water depth. If the tsunami is so large that its height is a significant fraction of the water depth, then the crest will move faster than the trough and you'll end up with a vertical wave of water approaching land. The effect is accentuated by the effects of the previous wave in the sequence. As water from a previous wave floods off the land it interacts with the next wave coming in and the top of the incoming wave can shear off and accelerate towards land. (While a normal wave's speed is controlled by water depth, a tsunami's is not, and acceleration is possible.)

For a tsunami from a distant source, the first wave is invariably small enough that a vertical wave doesn't form (and there isn't a previous wave to interact with). So the first wave often goes unnoticed. The tsunami that spawned the Tsunami Warning System, the huge waves of April Fool's Day, 1946, didn't start killing people until wave six, more than an hour after the first wave had arrived.

That brings me to another pet peeve (propagated by the awful article in Smithsonian magazine some months back): it's a myth that you can't run away from a tsunami. It all depends, of course, on how close the wave is to you. As it hits the beach, its speed will never be greater than 35 mph. And it's going to decelerate rapidly as it advances over land above sealevel. Can you run away from a car traveling at 35 mph? Sure, if the driver is braking hard. More important, if you see signs that a tsunami has arrived - sealevel higher or lower than usual, for example - then you definitely do have a chance to save yourself - just walk away from the ocean. The massive death toll in 1946 was just because people didn't understand what they were looking at. Judging from what I hear on radio and the questions the TV people throw at me, common knowledge is little better nearly fifty years later.

One more complaint, then I'll shut up. People who should know better (surfers, news people) repeatedly ask questions like "Five foot surf is common here, why should I be worried about a five foot tsunami?" The wavelength. A five-foot tsunami is like a five-foot rise in sealevel. Since the wave has such great wavelength, it's going to reach much further ashore too. The biggest storm waves are spent after a hundred feet or so. In low-lying areas a tsunami can reach half-a-mile from the ocean. Five-foot surf is fun to play in, but a five-foot tsunami can knock down the building you are in, drag you out to sea, and batter you to death.

Gerard Fryer, Hawai`i Institute of Geophysics & Planetology

gerard@soest.hawaii.edu

January 1995

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