Country: United States
Date: Spring 2010
With the recent Chile earthquake (February 2010) there were
many tsunami warnings, but little came of it. How are tsunami's
predicted, and why are we so bad at predicting them?
A Tsunami is wave which appears in shallow water. The name comes from
the Japanese for Harbour Wave.
Traditionally a tsunami would show first as the water recedes - often a
long way from the shore. The water then rushes back in as a huge wave
perhaps several metres high.
Tsunami are caused by seismic movements - earthquakes- underwater.
Some aspects of the tsunami are easy to predict. As soon as the
earhquake has been located, seismologists can predict the possibility of
a tsunami - If the earthquake is underwater a tsunami is POSSIBLE. If
the earthquake is in deep water a tsunami is more likely. And predicting
the travel time and arrival time of the tsunami is fairly simple too -
simple math. Arrival time = Distance to Epicentre x Speed of the wave -
approximately 700 kilometers per hour
The thing that is most difficult to predict are how big the wave is
going to be when it arrives on the other side of the ocean.
The size and direction of the movement under the sea have a huge
influence. Even a large quake a great depth BELOW the sea floor may
cause little movement ON the sea floor. Little movement means small
wave. Then the direction of the movement - even a large movement
parallel to the sea floor - a sideways movement - will cause a small
wave. A movement which either lifts or drops the sea floor - even a
relatively small amount - will cause a big wave. Lastly the dimensions
of the quake itself - An earthquake is often a linear occurrence rather
than a single point. If the line of movement is long the wave generated
will be more significant than one generated by a smaller event. The
Christmas 2004 quake was approximately 1600 kilometers long and sent
out a wave
which STARTED 1600 kilometers wide.
As the wave travels some energy can be lost - more if the water is
shallow. More if there are numerous underwater features to scatter the
energy. More if there is violent storm waves working against or across
the path of the tsunami. As the path is different for every earthquake
it is impossible to calculate how much energy will be lost on the way
quickly enough to make the information useful.
And lastly the receiving shore. The topography of the receiving shore is
critical. The wave will have far less impact on a gently sloping open
beach than it will in an enclosed harbour or bay (hence the name harbour
wave) If the wave impacts the shore at a right angle it will have a much
greater effect than if it impacts at a low angle.
So the result of all that is that unless we know how the wave starts,
how the wave travels and how it arrives it is not possible to make any
meaningful predictions in anything like the time it takes the wave to
travel (which we know!) Rather than risk huge losses, such as in
Thailand in 2004, authorities now will always err on the side of caution
and advise people to move away from the shore.
The next problem is that people start to accuse the authorities of
Actually Justin, we are pretty good in telling when there is a
tsunami. We need more work on deciding just how high the wave will
be when it hits land.
But we do have a system that saves as many lives as possible.
What we do is send out an alert whenever a big enough quake happens
in areas that have the right geology to start tsunamis. This tells
harbors near the quake to be on the lookout for unusual rises and
falls in sea level. Scientists also check buoys in the ocean and
sea floor detectors to see if a tsunami wave is traveling through the water.
If there is no evidence of a tsunami, the alert is cancelled. So we
are actually pretty good at deciding whether a tsunami has been created.
The big problem is deciding how large the wave will be when it hits
land, such as California, Japan or islands in the Pacific. There
are many things that control the size of the tsunami wave both near
land and as it crosses the ocean. We are still figuring these
out. The best we can do is to make estimates and be sure everyone
is out of danger of the highest waves. If our prediction is wrong
at least no one is hurt.
Oceanographers, geophysicists and physicists are among those who
study tsunamis in case you want to look deeper into these very
dangerous and uncommon waves.
R. W. "Bob" Avakian
B.S. Earth Sciences; M.S. Geophysics
Oklahoma State Univ. Inst. of Technology
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Update: June 2012