

P, S waves and Tsunamis
Name: Pete
Status: Educator
Age: 68
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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Update: June 2012

