how tsunami happens
The
term tsunami has its origins in the Japanese language; an English
equivalent is "seismic sea waves." Tsunamis are often incorrectly
interchanged with the term" tidal waves," which refer to high
waves of water caused by changes in the flow and surge of the ocean.
Tsunamis, on the other hand, can be caused by a few different means:
1) the down drop or upthurst of the Earth’s crust which results in an
earthquake; 2) a large-scale undersea landslide; 3) a submarine volcanic
eruption of a certain degree; or potentially, 4) a large meteor impact
at sea. The vast majority of tsunamis result from earthquakes.
How Earthquakes Cause Tsunamis
The
illustration below shows a subduction earthquake (one where a denser
plates shifts below its neighboring plate, at left). Energy is transferred
and the displaced water forms a wave. As the wave travels and enters
shallower water in the coastal area, it begins to increase in amplitude.
Tsunamis
are not always colossal waves when they come into the shore. In fact,
"... most tsunamis do not result in giant breaking waves (like
normal surf waves at the beach that curl over as they approach shore).
Rather, they come in much like very strong and very fast tides (i.e.,
a rapid, local rise in sea level). Nevertheless, there is destruction of life and of
property by floating debris and impact of water. The tsunami produces
a series of rushing waves and also a series of withdrawals.
If
you think about throwing a rock in water, a ripple is formed. It is
the same principle involving a meteor or an earthquake, except they
form bigger ripples. If you think about that ripple, it seems to disappear
as time goes on--but in actuality, it does not stop. In the picture
below we can see that a tsunami is very fast (the speed often compared
with that of a jet) and it has a height of 20 inches.
However,
we need to take into consideration the effect of the tsunami reaching
the shore. The speed is diminished but the wave height is increased
drastically.
To
view a QuickTime visualization: Tsunami
Visualization
For
more information about earthquake formation see Tectonics.
How Volcanoes Cause Tsunamis
There
are two different ways that volcanoes can cause seismic waves. One
possibility is for a land-based volcano to break down and collapse,
forcing large amounts of ash and debris into the water. This sudden
change and displacement of the water column transfers to kinetic energy
and results in waves. More debris can create a bigger increase in
wave amplitude and number.
Tsunamis
can also be induced by submarine volcanoes. These underwater volcanoes
can collapse downwards or spew forth lava heating the surrounding
water quickly.
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