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 . 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]
Tsunami Propagation by National Oceanic and Atmospheric Administration. Propagation description: depth-dependent speed, long wavelength and coastal shoaling
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A tsunami is a series of ocean waves with very long wavelengths caused by large-scale disturbances of the ocean, such as underwater earthquakes, volcanic eruptions, or landslides. Unlike regular ocean waves that are generated by winds and are typically limited to the surface of the water, tsunamis can travel across entire oceans and reach immense heights and speeds. When a disturbance displaces a large volume of water, it creates waves that can propagate outward in all directions.