The word "Tsunami" is derived from the Japanese meaning "Harbour Wave". They are often described as tidal waves but this is inaccurate as they have nothing to do with tides. In fact they are generated by offshore earthquakes, submarine landslides and also occasionally by undersea volcanic activity.

		Debris scattered near harbour at Aonae, Okushiri Island, Japan following 1993 Hokkaido Nansei-Oki Tsunami

In each case, a large water disturbance is created by large-scale underwater displacement of sediment or rock on the seabed, usually as a result of a fault or a landslide. The initial water movement is often characterized by a rapid draw-down or lowering of the sea surface at the coast as the water moves into the area of seabed displacement. Thereafter large kinematic waves are propagated outwards from the zone of seabed disturbance. The waves travel across the ocean at very high velocities, often in excess of 450km/hr, and possess very long wavelengths and periods. At the coast, the tsunami flood level (run-up) associated with a tsunami is partly a function of the dimensions of the propagated waves but it is also greatly influenced by the topography and bathymetry of the coastal zone and, as such, the waves can reach considerable distances inland.