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A tsunami is a train of long surface gravity waves generated by a rapid displacement of water, for example by seabed motion during an earthquake or by a landslide. Its wavelength can be much greater than the ocean depth, so the shallow-water approximation can apply even in a deep ocean.

In that limit the wave speed is $c=\sqrt{gH}$. The long wave can cross an ocean while its surface height remains small offshore. Entering shallower water lowers its speed and shortens its wavelength; coastal geometry, focusing and nonlinear flow then control amplification and inundation. Source displacement and bathymetry are therefore needed alongside an earthquake magnitude to predict local effects.^[ref]