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Japan earthquake Q & A: The tsunami wave

More from Charles Lewis | @holycharlie

Sendai Airport is flooded after a tsunami following an earthquake in Sendai, northeastern Japan,  March 11, 2011

Ioan Nistor is a professor of civil engineering at the University of Ottawa. His expertise is in tsunami waves and the kind of damage they can cause when they hit land. He has traveled around the world gathering information on the impact of tsunamis. He spoke to the National Post to give us an idea of what happened in Japan on Friday.

Q. How does a tsunami form?

A giant mound of water forms in the middle of the ocean just above the area where an earthquake occurs and that translates into an uplift of the sea surfaces, several thousand of metres above. But a ship directly above would barely feel anything because the height of the wave in the open ocean would be just a few centimetres. The mound of water begins to move across the ocean floor at speeds similar to a jet plane. As it approaches the coast the wave is squeezed up by the sloping bottom of the ocean that pushes the wave higher but slows the wave’s speed. So the wave height is increasing as the water depth decreases. When the height of the wave equals the depth of the water below it breaks, usually a few metres offshore. Because of the huge amounts of water behind the wave it advances inland and floods the coastal area.

Q. So a ship at sea is actually in the safest place?

That’s true. In fact if there is time ships will be ordered out of port and out to sea.

Q. What do we know about this wave that hit Japan?

Reports show that the wave was moving at tens of kilometres and hour and the wall of water was about 10 metres high. Because the area is flat the water can move kilometres inland. No one could outrun it, but someone in a car might be able to get away, assuming there were not traffic jams. If you were on the ground you would see a foamy wall of water advancing towards you at high speed. You’d hear just a rumbling noise and you would feel the ground shaking.

Q. Can buildings and other structures withstands the kind of tsunami that hit Friday?

Even large structures, two or three stories high, made of concrete can be washed out by the impact of a tsunami wave. In the case of Japan, the houses near the coast are made of wood, making the situation worse. None of these buildings could withstand the impact of a tsunami. From an earthquake engineering point of view, the design codes are quite advanced. So there is a lot of information to construct buildings to withstand earthquake motion. The problem, though, there is no guidelines for the design of buildings to withstand tsunamis. The forces are too extreme. In Japan, they concentrate on evacuation plans rather than try to design the right kind of building.

Q. Does Japan have a good warning system?

Japan has a very good alert system to warn coastal areas of tsunamis. The problem is, in Friday’s event, the tsunami took very little time to hit the coastline, not more than 30 minutes after the earthquake, rather than the several hours it usually takes. That’s because the earthquake was relatively close to shore.

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