What Is a Tsunami?

A tsunami (Japanese for "harbour wave") is triggered when a large volume of water is suddenly displaced, most often by a powerful undersea earthquake at a subduction zone where one tectonic plate thrusts beneath another. This displacement sends waves radiating outward in all directions. Unlike normal wind-driven waves, a tsunami involves the entire depth of the ocean, carrying immense energy. In the deep open ocean, a tsunami may be only 30-60 cm high but travel at speeds up to 800 km/h — as fast as a jet plane — and pass ships almost unnoticed. As it approaches shallow coastal water, friction with the seafloor slows the front of the wave while the water behind piles up, dramatically increasing its height — sometimes to 10, 30, or even more metres. Tsunamis often arrive not as a single breaking wave but as a fast, relentless surge of water that floods far inland. A warning sign is the sudden withdrawal of the sea from the shore, as the trough arrives before the crest. The 2004 Indian Ocean tsunami killed around 230,000 people across 14 countries.