# Wave with mass transport?

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.

• "On a sectional view we represented molecules of water going up and down as the wave travels but the molecules never moved horizontally." Hmmm...that's not entirely accurate. Near the surface they move backward and forward by a modest distance (less than one wavelength), but once the wave has passed they settle back near their original positions. See this nice answer for a animated picture, but take note that water is rather less compressible than the animation. – dmckee Apr 12 '12 at 14:59
• Silly me! They do indeed oscillate horizontally. But is that enough to explain the distance covered by a tsunami in the land? (some edge effect) – bear foot Apr 12 '12 at 15:44
• Hm, good question. And welcome to Physics Stack Exchange! – David Z Apr 12 '12 at 17:25

The fundamental difference with a tsunami is the size of the phenomena. For tsunamis, the wavelength can reach the order of 100km, which is $10^5$ larger than ordinary wind waves. This also comes with larger timescales, which is in the order of multiple minutes, as compared to regular water waves with periods of less than a second.