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Name: Pete
Status: Educator
Age: 6-8
Location: CA
Country: United States
Date: April 11, 2005


Question:
I have noticed that tsunami waves all seem to travel at the same speed. I do not understand why this happens since the earthquake or plate shift event that sets them off generate "S" and "P" waves that are different for each event.
Is there some "normaling" factor that I am not considering?



Replies:
The speed of a tsunami wave varies with the depth of the water it is traveling through. The deeper the water, the faster it travels. As the wave approaches shallow water, it slows down. But as it slows down it simultaneously gets taller, which makes the wave more destructive when it strikes land.

According to the University of Washington's tsunami web site

(http://www.ess.washington.edu/tsunami/index.html)

the speed of a tsunami wave is equal to the square root of the acceleration of gravity (32 feet per second per second) times the depth of the water. In the open ocean, if the water depth is 13,000 feet, the speed of the wave would be 645 feet per second or about 440 miles per hour. If the depth of the water is decreased to 100 feet, the speed is decreased to 57 feet per second or 39 miles per hour.

The laws of thermodynamics require that energy be conserved. So, if the wave is moving at hundreds of miles per hour, it has enormous kinetic energy. When it suddenly slows down the wave piles up and that kinetic energy is transferred into gravitational potential energy that shows up as a much greater wave height.

The speed of the wave is constant at a uniform water depth, but the height and length of the wave is dependent on the severity of the earthquake that generates it. So, different tsunami waves may show up at different heights and smaller ones may hardly be noticeable, but they arrive at the same speed.

As an analogy, think of sound. The speed of a sound wave does not vary with the volume or frequency.

Andy Johnson



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