The term of “tsunami” comes from the Japanese which means harbour (“tsu”) and wave (“nami”). A tsunami is a series of waves generated when water in a lake or a sea is rapidly displaced on a massive scale.
A tsunami can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Such large vertical movements of the earth’s crust can occur at plate boundaries.
Subduction of earthquakes are particularly effective in generating tsunami, and occur where denser oceanic plates slip under continental plates.
As the displaced water mass moves under the influence of gravity to regain its equilibrium, it radiates across the ocean like ripples on a pond.
Tsunami always bring great damage. Most of the damage is caused by the huge mass of water behind the initial wave front, as the height of the sea keeps rising fast and floods powerfully into the coastal area.
(simplified from www.panda.org)