tsunami

Ocean wave generated by vertical movements of the sea floor resulting from earthquakes or volcanic activity or large submarine landslides. Unlike waves generated by surface winds, the entire depth of water is involved in the wave motion of a tsunami. In the open ocean the tsunami takes the form of several successive waves, rarely in excess of 1 m/3 ft in height but travelling at speeds of 650–800 kph/400–500 mph. In the coastal shallows tsunamis slow down and build up producing huge swells over 15 m/45 ft high in some cases and over 30 m/90 ft in rare instances. The waves sweep inland causing great loss of life and damage to property.

Before each wave there may be a sudden withdrawal of water from the beach. Used synonymously with tsunami, the popular term ‘tidal wave’ is misleading: tsunamis are not caused by the gravitational forces that affect tides.