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Name: Justin
Status: Student
Grade: 6-8
Location: CA
Country: United States
Date: Spring 2010


Question:
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?



Replies:
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 crying wolf!

Nigel Skelton


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
Instructor
B.S. Earth Sciences; M.S. Geophysics
Oklahoma State Univ. Inst. of Technology



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