Tsunami

Tsunami can be generated when earth quakes happen under the sea. The sea floor deforms at a tectonic plate boundary causing stresses to pile up at the boundary. These give rise to strains that lift up the plate. At the time of a large earth quake, the plates which are stuck against each other slip suddenly, and the huge stresses are released in a few minutes. This causes part of the ocean floor to abruptly rise. This vertically displaces the overlying water. The energy released during the slip is given to the water. The energy released causes the tsunami waves.
Tsunamis have a small amplitude (wave height) offshore, less than half a metre or so. They have a very long wavelength (distance from crest to next crest), often hundreds of kilometers long, whereas normal ocean waves have a wavelength of only 30 or 40 metres, which is why they generally pass unnoticed at sea.
When they reach shallower water, the wave slows down and this causes the water to pile up to great heights in a process called shoaling. This is what causes the great devastation associated with a tsunami. The larger the earthquake, the more is the water displaced and hence greater the resulting tsunami. Moreover, there can be many tsunami waves from the same earthquake, as was seen in the Dec 26, 2004 tsunami caused by the Sumatra-Andaman earthquake. The first three waves of this tsunami were very powerful, and spaced about half-hour apart.