What is a Tsunami?

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A tsunami is a series of catastrophic ocean waves which may be caused by earthquakes, volcanic eruptions, landslides beneath the ocean, or an asteroid striking the earth. The movement under the sea causes big waves to form, that travel very fast.

In the open ocean, tsunamis may have waves hundreds of kilometres long, and travel at speeds up to 800 km per hr. But ships at sea mostly don't notice them, because the wave may only be a metre high. The time gap between the crests of a tsunami's waves varies from 5 minutes to about 1 hour.

When tsunamis approach shallow water along a coast, they are slowed down by the rising land under the sea, and this is what causes the waves to get very high. If the bay is shallow, the waves rise higher than if the water is very deep close in to shore. When they break, they often destroy piers, buildings, and beaches and take human life.

There is little warning of a tsunami coming; when a train of tsunami waves approaches a coastline, the first indication is often a sharp swell, followed by a sudden outrush of water that often exposes offshore areas as the first wave trough reaches the coast. After several minutes, the first huge wave crest strikes, covering the newly exposed beach and rushing inland to flood the coast. The first wave is almost never the largest and damaging waves may keep coming for hours.