Geography
What is a tsunami?
Quick answer
A tsunami is a series of very long, fast ocean waves, usually caused by an underwater earthquake. They are small in deep water but can grow tall and powerful when they reach the coast.
In one sentence
A tsunami is a set of enormous waves made when a huge amount of sea water is suddenly pushed, most often by an earthquake under the sea.
Explained
The word tsunami comes from Japanese and means "harbour wave". Tsunamis are different from the normal waves you see at the beach, which are pushed by the wind. A tsunami moves the whole depth of the ocean, from the surface to the sea floor.
Most are caused by underwater earthquakes, when part of the sea floor jerks up or down at a plate boundary (see plate tectonics and earthquakes). That pushes the water above it and sends waves out in all directions, like ripples from a stone in a pond. Tsunamis can also be caused by underwater landslides, volcanic eruptions or, very rarely, an asteroid hitting the sea.
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In the deep ocean, a tsunami may be less than a metre high, and ships may not even notice it. But it travels incredibly fast, up to about 800 kilometres per hour, as fast as a jet plane, with crests that can be hundreds of kilometres apart. As it nears the coast, the sea gets shallower, so the wave slows down and its water piles up, growing taller. It may arrive not as one breaking wave but as a rapidly rising tide that floods the land. There are usually several waves, sometimes tens of minutes or hours apart, and the first is not always the largest.
A tsunami in the Indian Ocean in 2004 and another in Japan in 2011 caused terrible damage, and they taught the world a lot about safety. Now many countries have warning systems: sensors on the sea floor and buoys detect waves, and alerts and sirens warn people. On coasts at risk, there are signs showing evacuation routes.
If you ever feel strong shaking near the sea, see the water pull back unusually, or hear a roar from the ocean, move straight away to high ground or as far inland as you can. Do not go to look at the waves, and stay away until officials say it is safe. Tsunamis are related to other dangerous sea events like hurricanes, but they are caused by very different things; see oceans.
Examples
| Cause | How it makes a tsunami |
|---|---|
| Undersea earthquake | the sea floor moves and pushes water |
| Landslide | falling material displaces water |
| Volcanic eruption | explosion or collapse moves water |
Common mistakes
- "A tsunami is one giant breaking wave." It often looks like a fast-rising flood.
- "The first wave is the biggest." Later waves can be larger.
- "You can outrun it on a beach." Move away early, to high ground.
Quick quiz
Tsunamis
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What usually causes a tsunami?
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