Tsunami Myths --- Myths About Tsunamis
A great deal of misinformation is being spread about tsunamis, mostly by well-meaning but scientifically ill-informed members of the news media. As we saw last December, lack of knowledge about hazards of nature can have tragic consequences. This page attempts to correct some of these myths about "tidal waves".
Myth 1: The word "tsunami" means "harbor wave" in Japanese.
FALSE. The first character "Tsu" means "ferry crossing" or "harbor crossing," not harbor. The second character, "nami" means wave. "Harbor wave" may be close, and it's easier to say than "Ferry crossing wave," but it doesn't make it an accurate translation. The word for "harbor" in Japanese is minato, which bears no etymological relationship to either "tsu" or "nami". (Note: the characters at left are Japanese Kanji characters. The Chinese use different characters to represent a tsunami. In Chinese, the word for tsunami is haixiao or haizhen, which mean “sea roar” and “sea quake,” respectively. The word at left is not found in any of my Chinese dictionaries, but if it were used in Chinese, it would be pronounced "jinbo.")
Myth 2: A tsunami is a single wave. Once it is gone, you are safe.
FALSE. A Tsunami is a wave train. There may be 5 to 20 waves altogether. The first waves are typically small, and subsequent waves become larger. This means if you don't get washed away by the first wave, you still need to stay away from the shore to escape the others. Tsunami waves come at fixed periods, typically 10 minutes to 2 hours. This is much slower than normal waves, which come every few seconds.
Myth 3: A tsunami looks like a big Hawaii-5-O style curling wave or a 30-foot high wall of water.
FALSE. The news media and Hollywood show pictures of tall waves like this because they are visually impressive. However, while a tsunami could sometimes have this appearance, this type of wave is not a typical tsunami. Tsunamis were originally called "tidal waves" for a good reason. Their appearance resembles the tides much more than a wave. A good analogy to a tsunami would be if you had a large plastic tub full of water and picked up one side. The water would be displaced and slosh out the opposite side of the tub. The wave coming out doesn't have a particularly large slope, but there's a lot of it. Sometimes, a tsunami won't even 'break' when it reaches the shore. This makes it look smaller than it really is.
What makes a tsunami so destructive is not the height of the wave, but the large volume of water that pours out onto the beach. The distinguishing characteristic of a tsunami is that it is a long period wave--very broad but not necessarily particularly high, even at the seashore. A tsunami can have a wavelength as long as 125 miles (200 km).
Myth 4: There's not much that you can to to avoid a tsunami.
FALSE.
If you're near the seashore and the tide suddenly goes out, far enough
that you see fish flopping in the mud, this means a tsunami is
coming. Run to higher ground immediately. Fish are smart creatures
and never flop around at low tide. The reason the tide goes out so
far is that a tsunami is a very long wave. All waves try to move the
water vertically and horizontally in a circular pattern with a diameter
about equal to the wavelength. The amount of movement decreases
with depth, until some point is reached where a wave on the surface
has no effect. This depth is equal to 1/2 the wavelength. For example,
a wave with 100 meters between crests would be felt as deep as 50 meters.
As the water gets shallower, the circular movement is flattened out because
there is less room to move vertically.
Because of this lateral movement, for a long period wave like a tsunami,
the water will often recede as much as 1000 yards from the beach
before the first tsunami wave strikes. However, there's no guarantee
that this will always happen.
If you're near the seashore and feel an earthquake, move to higher ground immediately ... and just as important: warn the people around you.
Myth 5: Tsunamis only occur in the Pacific.
FALSE. As most people now know, a tsunami can occur in any ocean. Luckily, tsunamis find it more difficult to propagate in the Atlantic Ocean because of its shallower depth and the Mid-Atlantic Ridge. However, because of its smaller size, any tsunami in the Atlantic will also have less distance to travel. The strength of a wave is roughly inversely proportional to the distance from the source. So any tsunami in the Atlantic will be more localized, but you may get little warning.
Myth 6: Tsunamis travel fast because they are compression waves.
FALSE. Tsunamis propagate by up and down movement like a
regular wave, not by horizontal pressure. Of course, the situation
is not 100% clear cut because up and down movement causes changes
in pressure as water builds up under the wave. Tsunamis travel
fast because of their long wavelength. In shallow water, a wave's
speed is given by
where c = the speed of the wave (or phase velocity) in meters/sec, g = the
gravitational acceleration (9.8066 meters/s2 )
and d = wave depth in meters. This means that as the depth doubles, the
speed of the wave is quadrupled.
In deep water, a wave's speed is described by linear wave theory,
which assumes that transport of water is small enough to be
ignored. The speed is given by
or
and
where c = the speed of the wave (or phase velocity) in meters/second, g = the gravitational acceleration (9.8066 meters/second2), l = wavelength in meters, and t = time between crests (the wave period) in seconds. This equation means that at a constant depth, the speed is proportional to the wave period. A tsunami traveling at 450 miles per hour would have a wave period of 129 seconds and a wavelength of 16 miles. The waves will sharpen up somewhat and lose their sinusoidal shape as they approach the shore. The wavelength will also decrease because the waves are moving more slowly. However, the time between crests remains the same. The distance between wave peaks is always much longer than a regular wave.
Myth 7: Tsunamis travel at or near the speed of sound.
MISLEADING. This myth is probably the reason some people think a tsunami is a compression wave. In fact, the speed of sound in salt water is about 1500 meters/second, or 3355 miles per hour. This is about 4.4 times faster than the speed of sound in air at sea level, which is about 742 miles per hour at 0 degrees C. (For the sake of comparison, the speed of sound in steel is 17 times faster than in air, or 13,332 miles per hour. At 30,000 feet, the speed of sound in air drops to 676 miles per hour.)
A tsunami traveling at 600 miles per hour is only going about 1/5 the speed of sound in water.
Myth 8: If you can grab onto a solid object, you are likely to survive a tsunami.
FALSE. You may not drown or get washed away, but you may well be struck by piece of wood, branches, rocks, vehicles, and other objects traveling in the water at 30 miles per hour. A 30 mph log hidden under the surface of the water would be almost invisible and, obviously, hazardous to your health. One cubic yard of sea water weighs 1,727 pounds. A tsunami wave moving at 30 miles per hour has kinetic energy equivalent to a solid mass of Jeep Libertys traveling at 20 miles per hour. Even a strong person would have trouble hanging on to a support with this much energy pushing against them. Holding on to an object would be even more difficult if the water was cold.
Myth 9: Volcanoes and underwater landslides can produce tsunamis.
TRUE. Tsunamis are caused by sudden, large-scale vertical displacement of water. Underwater landslides can produce a tsunami, but tsunamis produced by landslides falling into the water tend to be larger. (Update: Cindi Preller of NOAA recently pointed out to me that this is not always true, and that there is NO warning for landslide tsunamis, which may arrive very very quickly.) However, it is not clear how far a landslide tsunami can propagate, because they are also sharper than a tsunami produced by an earthquake.
For example, the large collapses of Krakatau and Santorin created catastrophic, intense waves in the immediate area, but computer simulations showed that these waves were too sharp to propagate long distances. The principle is similar to propagation of sound waves. You can hear low-pitched sound waves, like the thumping bass of your neighbor's boom box, quite a distance away, while the high-pitched crashing of your neighbors smashing empty beer bottles over each other's heads is more rapidly dispersed and absorbed by obstacles like empty Dorito bags and moth-eaten sofas in its path.
Myth 10: You can surf a tsunami.
FALSE. You can't surf the tide. People can and do surf tidal bores, but surfing a tsunami in the open ocean would be very difficult, if not impossible. The "wave" is just too broad. The myth is like the fictional story about a guy who surfed a tsunami moving at 300 miles per hour all the way from Hawaii to California. The story goes, “if it hadn't been for that three-story condo in Sausalito ... ” Unfortunately, since a typical tsunami wave is 16 miles long and only 1 meter high in the open ocean, the surfer would have needed a surfboard at least half a mile long to capture enough energy from the wave to move forward.
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