word "tsunami" is actually a Japanese word.
The first part, tsu, means harbor and nami means wave.
So, in its original usage, tsunami meant a harbor wave.
However, the term is now used to refer to waves caused
by seismic (earthquake) or volcanic activity in or near
the ocean floor. When there is an undersea disturbance,
waves form and travel away from the center of the disturbance
kind of like when you throw a rock in a pond. These waves
can travel as fast as 450 miles per hour. Deep in the
ocean, tsunami can pass undetected under ships. However,
as they approach land, the water becomes more shallow
and they rise up and crash on the shore. Tsunami can be
very dangerous. They can damage or destroy coastal towns
and villages. (Note: In the image above, notice the man
on the left. The photo was taken in April 1946 in Hilo,
Hawaii. Click the image for a full size photo.)
Most tsunami are caused by earthquakes
with epicenters near or on the ocean floor. Not all earthquakes
generate tsunami and some tsunami can be small and cause
little or no damage. Because of this, people and surfers
sometimes go to the beach when there is a tsunami warning.
This is not a good idea because it is difficult to know
when tsunami will be small and when they will be deadly.
It usually takes an earthquake greater than 7.5 on the
Richter scale to
produce dangerous tsunami. Click here
for a tsunami animation (you will need MPEG viewing software).
Sometimes people use the words tidal
wave and tsunami to mean the
same thing. However, the two are not related. While tsunami
refers to dangerous waves caused by underwater disturbances,
tidal waves are simply the crest of tides as they travel
around the Earth. Tsunami have nothing to do with tides.