A huge sea wave, or series of waves. Most are formed from earthquakes of 5.5 or more on the Richter scale. Other causes include the eruption of submarine volcanoes, very large landslides off coastal cliffs, or the calving of very large icebergs from glaciers in fiords. Fault-generated tsunamis may spread thousands of kilometres across ocean basins with velocities up to 800 km h−1. In mid-ocean the height of a tsunami may be no more than 30–50 cm, but this can increase up to 500 m when the tsunami hits the shore. The most active source region of tsunamis between 1900 and 1983 was along the Japan–Taiwan island arc, where over a quarter of all tsunamis were generated. Wikipedia is very good on the Japanese 2011 Tohoku earthquake, which triggered tsunami waves up to 40.5 m high, and travelled up to 10 km inland. The earthquake which caused the tsunami on 26 December 2004 was caused when a portion of the Indian tectonic plate slid under the Burma plate. The US Geological Survey estimates that the ensuing rupture was more than 1 000 km long, displacing the sea-floor above by perhaps 10 m horizontally. The great volume of the ocean displaced thereby created the tsunami, which hit the coastline of eleven Indian Ocean countries, with wave heights up to 15 m. Estimates of the death toll range between 188 000 and 320 000. See Krom in J. Holden (2012), pp. 42-3 for an excellent summary of this tsunami.
The largest recorded earthquake in the world occurred in May 1960, with a magnitude of 9.5. The damage caused by the resulting tsunami caused:
These statistics reflect the population densities in the affected zones, the economic value of the properties affected, and the importance of warning systems and evacuation planning (Keys et al. (2004) Antipode 38, 2).