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A tsunami is a series of waves caused by the displacement of a large volume of water, usually by an earthquake, volcanic eruption or other disturbance. Learn about the terminology, history, characteristics, warnings, mitigation and impacts of tsunamis from this comprehensive encyclopedia article.
- Overview
- Origin and development
- Notable tsunamis
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A tsunami is a catastrophic ocean wave, usually caused by a submarine earthquake, an underwater or coastal landslide, or a volcanic eruption. Waves radiate outward from the generating impulse at speeds of up to 500 miles (800 km) per hour, reaching maximum heights of 100 feet (30 metres) near coastal areas. Although often called tidal waves, the occurrence of tsunamis have no connection with tides. The word tsunami is Japanese for “harbour wave.”
What have been some of the worst tsunamis in history?
Perhaps the most destructive tsunami in recorded history was the Indian Ocean Tsunami of 2004. A 9.1-magnitude earthquake occurred off the coast of Sumatra in Indonesia. Waves as high as 30 feet (9 metres) struck the eastern coasts of India and Sri Lanka—some 750 miles (1,200 km) away—and traveled more than 1,800 miles (3,000 km) to East Africa. The final death toll was at least 225,000, mostly in Indonesia, Thailand, India, and Sri Lanka. The affected countries also reported extensive economic and infrastructural damage.
What are the signs of a tsunami?
Because of frequent tsunamis in the Pacific Basin, many adjacent countries have established tsunami warning systems that look for large earthquakes (magnitude 7.0 or higher) and unusual changes in sea level. Depending on the distance from the seismic disturbance, this warning system may give people several hours to evacuate coastal areas.
Where is the safest place to go during a tsunami?
After an earthquake or other generating impulse occurs, a train of simple, progressive oscillatory waves is propagated great distances over the ocean surface in ever-widening circles, much like the waves produced by a pebble falling into a shallow pool. In deep water a tsunami can travel as fast as 800 km (500 miles) per hour. The wavelengths are enormous, sometimes exceeding 500 km (about 310 miles), but the wave amplitudes (heights) are very small, only about 30 to 60 cm (1 to 2 feet). The waves’ periods (the lengths of time for successive crests or troughs to pass a single point) are very long, varying from five minutes to more than an hour. These long periods, coupled with the extremely low steepness and height of the waves, enables them to be completely obscured in deep water by normal wind waves and swell. A ship on the high seas experiences the passage of a tsunami as an insignificant rise and fall of only half a metre (1.6 feet), lasting from five minutes to an hour or more.
As the waves approach the coast of a continent, however, friction with the rising sea bottom reduces the velocity of the waves. As the velocity lessens, the wavelengths become shortened and the wave amplitudes (heights) increase. Coastal waters may rise as high as 30 metres (about 100 feet) above normal sea level in 10 to 15 minutes. The continental shelf waters begin to oscillate after the rise in sea level. Between three and five major oscillations generate most of the damage, frequently appearing as powerful “run-ups” of rushing water that uproot trees, pull buildings off their foundations, carry boats far inshore, and wash away entire beaches, peninsulas, and other low-lying coastal formations. Frequently the succeeding outflow of water is just as destructive as the run-up or even more so. In any case, oscillations may continue for several days until the ocean surface reaches equilibrium.
Much like any other water waves, tsunamis are reflected and refracted by the topography of the seafloor near shore and by the configuration of a coastline. As a result, their effects vary widely from place to place. Occasionally, the first arrival of a tsunami at a coast may be the trough of the wave, in which case the water recedes and exposes the shallow seafloor. Such an occurrence took place in the bay of Lisbon, Portugal, on November 1, 1755, after a large earthquake; many curious people were attracted to the bay floor, and a large number of them were drowned by the wave crest that followed the trough only minutes later.
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One of the most destructive tsunamis in antiquity took place in the eastern Mediterranean Sea on July 21, 365 ce. A fault slip in the subduction zone beneath the island of Crete produced an earthquake with an estimated magnitude of 8.0–8.5, which was powerful enough to raise parts of the western third of the island up to 10 metres (33 feet). The earthquake spawned a tsunami that claimed tens of thousands of lives and caused widespread damage throughout the Mediterranean, from islands in the Aegean Sea westward to the coast of present-day Spain. Tsunami waves pushed ships over harbour walls and onto the roofs of houses in Alexandria, Egypt, while also ruining nearby croplands by inundating them with salt water.
Perhaps the most destructive tsunami in recorded history took place on December 26, 2004, after an earthquake of magnitude 9.1 displaced the ocean floor off the Indonesian island of Sumatra. Two hours later, waves as high as 9 metres (30 feet) struck the eastern coasts of India and Sri Lanka, some 1,200 km (750 miles) away. Within seven hours of the quake, waves washed ashore on the Horn of Africa, more than 3,000 km (1,800 miles) away on the other side of the Indian Ocean. More than 200,000 people were killed, most of them on Sumatra but thousands of others in Thailand, India, and Sri Lanka and smaller numbers in Malaysia, Myanmar, Bangladesh, Maldives, Somalia, and other locations.
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On March 11, 2011, seafloor displacement resulting from a magnitude-9.0 earthquake in the Japan Trench of the Pacific Ocean created a large tsunami that devastated much of the eastern coast of Japan’s main island of Honshu. Waves measuring as much as 10 metres (33 feet) high struck the city of Sendai and other low-lying coastal regions of Miyagi prefecture as well as coastal areas in the prefectures of Iwate, Fukushima, Ibaraki, and Chiba. The tsunami also instigated a major nuclear accident at the Fukushima Daiichi power station along the coast.
One of the most notable prehistoric tsunamis took place during the K-T extinction, a global extinction event that eliminated approximately 80 percent of all animal species about 66 million years ago. Many scientists argue that the event was mostly caused by the impact of a large meteor or comet on the Yucatán Peninsula near Chicxulub, Mexico. The impact caused an enormous 1.6-km- (1-mile) tall tsunami that washed up on the shores of the Gulf of Mexico and the islands of the Caribbean before propagating across the Atlantic Ocean and other ocean basins.
Learn what a tsunami is, how it is caused by earthquakes, landslides, or volcanoes, and how it affects coastlines and people. Explore the history, science, and examples of tsunamis around the world.
- The Editors of Encyclopaedia Britannica
Dec 14, 2015 · Learn what causes tsunamis and how they affect the world in this video from National Geographic. See examples of tsunamis and their devastating effects, and how to prepare for them.
- 3 min
- 1.6M
- National Geographic
Learn what tsunamis are, how they are formed by underwater earthquakes or other events, and how they can cause devastation on land. Find out how to recognize the signs of a tsunami and how to...
- 3 min
A tsunami is a series of waves caused by earthquakes or undersea volcanic eruptions. Learn how tsunamis travel, how they affect different ocean depths, and why they are not tidal waves.
Sep 17, 2024 · A tsunami is a large wave caused by movements in Earth's outer layer, or crust, such as earthquakes or volcanoes in the ocean. Learn how tsunamis form, how they can be dangerous, and how NASA satellites watch for them from space.
Aug 23, 2023 · A tsunami is a giant wave or series of waves produced by an earthquake, volcano, or other event that displaces a large amount of water. Learn how tsunamis differ from regular waves, what causes them, how they work, and how to recognize and mitigate them.
