1/11/2006

Tsunami Myths

Myth 1: The word "tsunami" means "harbor wave" in Japanese.
Japanese Tsunami characters

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".
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.
Hawaii-5-O Wave

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.

Direction of wave movement 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: The government will warn you if there's a tsunami.

FALSE. As most people now know, the only ocean that currently has a tsunami warning system is the Pacific. The Atlantic Ocean has no warning system, and no one could issue a warning even if they suspected a tsunami could be coming. 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 will also probably get no warning.

Tsunami Myths

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