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What Causes a Tsunami?

Updated on June 13, 2016

Important Tsunami Facts

  • Tsunami is a Japanese word which means 'harbour wave'
  • Earthquakes under the sea are the main causes of Tsunamis
  • The biggest tsunami ever was at Lituya Bay, Alaska on July 9, 1958
  • Earliest tsunamis was in Sicily 8,000 years ago
  • Landslides and volcanoes can also cause tsunamis


A tsunami is a powerful series of waves caused by a disturbance under water. This is usually an earthquake under the sea.

The waves travel through the ocean and cause devastation when they reach land. Humans are often killed and buildings destroyed when the water hits the coast.

To fully understand how tsunamis are caused we must have an understanding of tectonic plates, earth quakes, and finally, water.

Panic: People flee as a Tsunami wave crashes into trees in Indonesia on Boxing Day, December 26, 2004
Panic: People flee as a Tsunami wave crashes into trees in Indonesia on Boxing Day, December 26, 2004
Wave: Japan has frequent Tsunamis
Wave: Japan has frequent Tsunamis

What Is It?

Tsunami is a Japanese word meaning 'harbour wave' that refers to a series of large ocean waves that hit a shoreline. The word originates from Japan because this is the country where tsunamis are most common.

These tsunami waves may be as long as 100km and travel across the ocean at speeds of up to 800kmh. There may be a constant stream of waves that that batter the shore for between 10 and 60 minutes.

Tsunamis are also known as tidal waves, due to their large and powerful nature. They have been depicted throughout history, art, television and film as something terrifying, cataclysmic and almost Armageddon-like.


Tsunamis are caused by sudden movements of the earth that happens under the sea. Often the most destructive Tsunamis are caused by earthquakes but causes can also include volcanic eruptions, landslides or even a comet hitting the sea.

Landslides cause tsunamis when the debris falls into the water. This has the same effect of dropping a large stone into a pool - big ripples are created. But when this happens in the sea and it is thousands of tonnes of rock and earth falling into the sea a very large ripple, more like a tidal wave is created. This travels across the sea until it comes into contact with land and a tsunami is formed.

Volcanoes cause tsunamis when there is an eruption. The volcano can either be on land or under the sea, in which case it is known as a submarine volcano. If the volcanic eruption happens on land, the tsunami is caused by debris and lava from the volcano flowing into the sea, which once again causes a bug ripple.

If the eruption happens under water, the enormous power of the eruption sends shudders through the earth and disrupts the water. The water in the sea then breaks into waves which travel across the ocean until they come into contact with a coast. Here, a tsunami is formed.

Friction: Tectonic plates rub against each other
Friction: Tectonic plates rub against each other

How Do Undersea Earthquakes Start?

The most common cause of a tsunami is fro earthquakes. This is what caused the Boxing Day tsunami in the Indian Ocean in 2004 and it is also the reason behind the 2011 Japan tsunami.

To understand how earthquakes cause tsunamis we must first fully understand what causes earthquakes. Remember, tsunamis are an after-effect of an earthquake.

The earth sits on about a dozen tectonic plates. These are large floating pieces of hard rock that are constantly moving and fit together around the world like a jigsaw.

Undersea earthquakes happen when one of these plates is rubbing against another at a plate boundary. The two plates may become stuck as the heavier plate tries to slide under the lighter other. This causes a build up of pressure in a process knows as subduction.

As the heavier plate continues to slide beneath the lighter plate, it causes the lighter plate to bend downwards with the pressure. A point comes when the lighter plate can no longer take the intense pressure and suddenly snaps back up to the surface where it had been before.

The incredible force of the earth's plate shooting upwards in the water causes a huge rise in sea level. A vast body of water moves upward - like a huge mountain of water in the sea.

How Does the Tsunami Develop?

Everybody knows that what goes up must come down. This is particularly true for water which always likes to form a nice flat surface. So once the mountain of water has risen up the next step is for the sea to level itself out.

The mountain of water comes back down. This pushes the water that was underneath it outwards. The force of the water moves through the ocean causing an underwater force that travels for hundreds of Kilometres. The force of the water can reach speeds of up to 800kmh as it surges through the ocean. The energy is underwater and is not noticeable on the surface.

As this force travels through the ocean it may eventually reach the shore. At this point, the sea becomes shallower. However, the energy in the water is still the same. The enegery is compressed and the water is pushed upwards. This is how the energy is transferred from being underater into waves on the surface.

Can Anything Be Done?

Unfortunately nothing can be done to prevent Tsunamis. However, there are several organisations that use complex technology to monitor movement of the earths plates and sudden changes in water movement. There are also warning and evacuation procedures in place around countries like Japan and Hawaii where Tsunamis are frequent.

Any sudden earthquake that happens underwater will be detected in the same manner of on on-shore earthquake. These are measured in the Richter scale. If this is recorded then warning systems can sometimes be activated to evacuate people.

Wreckage from Aleutian Islands
Wreckage from Aleutian Islands

Major Tsunamis

  • Japan - March 11, 2011
  • Indian Ocean - 26 December 2004.
  • Papua New Guinea - 17 July 1998
  • Sea of Japan - 26 May 1983
  • Alaska Britis Columbia - 27 March 1964
  • Chili - 22 May 1960
  • Aleutian Islands - 1 April 1946

Read more about the biggest tsunamis in history.

Tsunami videos

Tsunami videos


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