I don\'t think this question has been asked on this forum before (at least I did

Question

I don't think this question has been asked on this forum before (at least I didn't find it).

In the case of a tsunami, an earthquake generates a wave which will travel with the sea/ocean as the medium. However, what I remember from high school is that mechanical waves transfer energy but energy only. On a sectional view we represented molecules of water going up and down as the wave travels but the molecules never moved horizontally.

Then why does a tsunami can flood half of a country? in that case the water does move horizontally. what am I missing?

I have read that for regular waves, the problem is different and their momentum comes from wind streams, but I don't think this is the case for tsunami.

I would appreciate some pieces of answer.

Explanation / Answer

Far away from shore the water is moving in a roughly circular motion. If you put a ping pong ball or some other warker on the water you'll find it flows towards the approaching wave and away from the retreating wave. There are loads of animations of this out there; a quick Google found http://www.youtube.com/watch?v=7yPTa8qi5X8 and this is a pretty good description. As you move down in the water the size of the circles decreases until at about one wavelength down the water is no longer moving. Again courtesy of Google, a more technical discussion is .ihad.tmd.go.th/lect18.html.

The problem comes when the depth gets shallower than one wavelength because the sideways oscillation becomes larger and larger as the water gets shallower. When any wave (a tsunami is just an extreme example) hits the shore the oscillation means the water first moves away from the shore then back onto the shore as evey peak hits the shore. In the case of the tsunami the wave is so big that you get first the characteristic sea withdrawal, then the flooding over the shore and onto the land.

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