How tall does a wave have to be to be a tsunami?

Answer 1

Actually a tsunami is not really a wave in the normal sense. A wave is a localized region of distrurbance the propagates through the water. Typical storm waves, which can of course get quite big, are really just very localized regions where the water is humped up a bit above the surrounding level. In the direction the wave is travelling, that raised region of water is only a few feet long even in a big storm, and if the wind is strong it might get quite large. Typically waves get bigger as they approach shore since the leading edge of wave starts to slow down and the tail end catches up and the height of the wave appears to increase. These can get quite large, but the total volume of water is not huge and so the wave will crash onto the shore and perhaps do some damage, but generally will just roll some limited distance up the beach.A tsunami is completely different. This is due usually to an earthquake which lifts a huge area of groung under the sea and causes the level of the sea often over many miles to suddenly rise. Now imagine that you were standing on the beach and the entire level of the sea went up by 3 feet. The sea would advance onto the land till it reached the point where the land was about 3 feet higher. In coastal regions the land can be very flat and this might be many miles inland. Some tsunamis are only a few inches tall and don't do much damage. If the earthquake is very violent or the water tries to flow into a restricted area like a bay, then the leading edge of the disturbance can get quite violent and large and like any disturbance on water it will start to build up as it flows into shallower water. If you look on the internet at the movies of the tsunami in Thailand in 2004 you see some cases where the water has built up into a wave and other examples where it just appears to be flowing rather gently but what is unique about the tsunami is that once it starts, it just will not stop flowing in until everything is flooded.So you can see that it is not the height of the wave that defines a tsunami. If you want to get really technical, it is depth of the disturbance and its wavelength that really determines a tsunami. Storm waves are very short wavelengths whose effects are really just biggle ripples on the surface whereas tsunamis are very, very long wavelengths and often correspond to whole sections of the ocean being displaced all the way from the surface to the ocean floor. It is like a whole section of ocean being lifted up.

Answer 2

It is not a matter of height. A tsunami is very different from an ordinary wave, and not just in size. A tsunami wave is not wind-driven but is usually caused by an earthquake or landslide. They have a much longer wavelength, and so carry far more water than an ordinary wave of similar height. In open ocean a tsunami may be a couple feet high an hundreds of miles long, becoming higher in the shallow water near shore. While a storm wave washes in and out in a matter of seconds, a tsunami comes in as a continuous surge for several minutes.