Physics 12

On the plane I got quized by a neighboring passenger about tsunami dynamics (“oh, you’re a physicist?”) Here is what I recall from Physics 12:
The waves created by tsunamis are very long wavelength. While typical ocean waves are around a hundreds of meters long, tsunamis produce wavelengths of up to a hunderds of kilometers. Since the wavelength of the tsunami is on the order of the depth of the ocean, tsunami waves are shallow water waves (most ocean waves have wavelength of hundreds of meters and are so are different beasts called deep water waves.) The speed of this type of wave (if you want to be fancy you say “celerity” here) is around the square root of the accleration due to gravity times the water depth (typically a few kilometers). This is why tsunami waves move at speeds of a few hundred meters per second and is also why tsunamis which hit the land aren’t moving at this speed (because the ocean depth gets shallower as you approach land and so the tsunami slows down.)