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A tsunami is a series of fast moving waves in the ocean caused by powerful earthquakes or volcanic eruptions. A tsunami has a very long wavelength. It can be hundreds of kilometers long. Usually, a tsunami starts suddenly. The waves travel at a great speed across an ocean with little energy loss. They can remove sand from beaches, destroy trees, toss and drag vehicles, damage houses and even destroy whole towns. Tsunamis can also be caused by meteorite impacts.
The water will draw back from the seacoast half of the wave period prior to the wave getting to the coast. If the slope of the coast is not deep, the water may pull back for hundreds of metres. People who do not know of the danger will often remain at the shore.
Tsunamis cannot be prevented. However, there are ways to help stop people dying from a tsunami. International and regional warning systems, especially for the Pacific Ocean, issue alerts before the big waves reach the shore. Because an earthquake that caused the tsunami can be felt before the wave gets to the shore, people can be warned to go somewhere safe.
The deadliest tsunami recorded was on December 26, 2004. It was caused by an earthquake. The earthquake was said to have a magnitude of 9.3 on the Moment magnitude scale. It was centered in the ocean near the coast of Sumatra, Indonesia. Over 215,000 people, mainly on the shores of the Indian Ocean, died from this disaster. The giant wave moved very quickly. Hundreds of thousands of people in Indonesia, Sri Lanka, Thailand, India, Somalia, and other nations, were killed or injured by it.
Tsunamis are often called tidal waves because they usually rise and fall more slowly than ordinary ocean surface waves. This name is misleading, because tsunamis are not related to tides; they merely rise slowly like tides do, though less slowly.