A series of very long, fast ocean waves generated by the sudden displacement of a large body of water, most often by an undersea earthquake, but also by submarine landslides or volcanic eruptions; the word is Japanese for "harbour wave."
- The most common trigger is a strong, shallow undersea earthquake at a subduction zone that suddenly raises or drops the sea floor.
- In the deep open ocean a tsunami has a low height but a very long wavelength and travels extremely fast (comparable to a jet aircraft); it is barely noticeable to ships.
- On reaching shallow coastal water it slows and piles up, so the wave height grows enormously and it surges far inland; the sea often withdraws first (a warning sign).
- The 26 December 2004 Indian Ocean tsunami, from a great earthquake off Sumatra, killed huge numbers across coasts including India (Tamil Nadu, Andhra Pradesh, the Andaman and Nicobar Islands); India then set up a tsunami early-warning system at INCOIS, Hyderabad.
- A tsunami is not a tidal wave (it has nothing to do with tides) and is unrelated to ordinary wind-driven waves.
The earthquake (subduction) trigger, the deep-ocean speed and coastal build-up, the 2004 Indian Ocean event, and the INCOIS warning system are standard disaster-management and geography facts.
A tsunami is caused by sea-floor displacement (earthquake), not by tides or wind, so calling it a "tidal wave" is wrong; it is low and fast in the deep ocean but tall and slow near the shore.
Long, fast waves from undersea earthquakes that build to huge height at the coast; the 2004 Indian Ocean tsunami led to the INCOIS warning system.