What Causes a Tsunami, How Are They Formed?

tsunami wave

Written by Marisa Wilson

Updated: September 13, 2022

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Tsunamis can be awful and often lead to loss of life and damage to property. They are giant waves caused by sudden disturbances in the ocean. Earthquakes, volcanoes, and landslides can all cause tsunamis. When an earthquake occurs underwater, it creates a fast-moving disturbance that propagates outwards as a wave. 

A tsunami can also be caused when a landslide displaces a large volume of water. This disturbs the water column and creates a wave that travels outwards from the point of disturbance. Volcanic eruptions can also cause tsunamis. If a volcanic eruption is powerful enough, it can move a large amount of water and create a tsunami. 

All these events can cause terrible damage to coastal areas, so it is vital to be aware of their dangers. In this post, you’ll learn more about how these disasters can cause tsunamis and the deadliest tsunamis in history. Understanding the other hazards involved during and after one is crucial, especially if you’re near an area prone to them is essential. 


When the sea floor suddenly changes shape, and the water above it moves up or down, this can cause a tsunami. Tectonic earthquakes are caused by the deformation of the earth’s crust. When these happen under the sea, the water above the deformed area moves out of its usual place. Waves are made when a mass of water moves and tries to return to its normal state under the influence of gravity. 

When large parts of the sea floor rise or fall, it can cause a tsunami. At plate boundaries, the earth’s crust can move up and down by a great deal. Plates interact along what are called faults. Around the edge of the Pacific Ocean, for example, a process called subduction causes denser oceanic plates to slide under continental plates.

 Subduction earthquakes are especially good at making tsunamis happen. The Hokkaido-Nansei-Oki earthquake caused one of the enormous ones ever recorded in Japan on July 12. This massive tsunami was caused by an earthquake that shook the west coast of Hokkaido and the island of Okushiri. The central west coast of Hokkaido and the Okushiri coastline were engulfed in huge waves within two to five minutes. There was significant damage on the southernmost point of Okushiri Island, in the town of Aonae.

earthquake aftermath

Tectonic deformations by earthquakes can cause tsunamis.

Volcanic Eruptions

Despite being relatively infrequent, violent volcanic eruptions represent impulsive disturbances that have the potential to move a significant amount of water and produce highly destructive tsunami waves close to the source. According to this mechanism, waves may be made by the abrupt displacement of water brought on by an explosion, the collapse or engulfment of the volcanic magmatic chambers, or, more likely, by the explosion and slope failure. 

Pyroclastic flows can cause tsunamis when they interact with bodies of water. However, the conditions needed to produce a tsunami and how pyroclastic flows and water interact are still not fully understood. Volcanic explosions that happen underwater can cause tsunamis, but the size and depth of the blast affect how giant the tsunami is. An underwater explosion that makes a crater can cause tidal bores and smaller waves by making the water expand, rise, and fall back down due to gravity. 

After the Indonesian volcano, Krakatoa (also known as Krakatau) erupted and collapsed on August 26, 1883, one of the most significant and most destructive tsunamis ever recorded. On the islands of Java and Sumatra, this explosion caused waves to reach a height of 135 feet, destroyed coastal towns and killed 36,417 people. It is also thought that the eruption and subsequent collapse of Santorini’s volcano in the Aegean Sea in 1490 B.C. led to the end of the Minoan civilization in Greece.

Despite being relatively infrequent, violent


eruptions represent impulsive disturbances that have the potential to produce highly destructive tsunami waves.


“Landslide” is a broad term for several different types of ground movement that can cause tsunamis. These include rock falls, slope failures, debris flows, and slumps. Even though icefalls/avalanches and glacial calving (when large chunks of ice break off from a glacier) are not technically landslides, they act the same and are considered in this discussion. A tsunami may result when a landslide shifts the water from below (subaerial) or above (surface). 

The amount of debris that moves the water, its speed, and the depth it travels to all affect the generation of tsunamis. Landslide-generated ones can strike nearby coasts within minutes with little to no notice, and they may be larger nearby than tsunamis caused by earthquakes. These rarely affect far-off coasts because they typically lose their energy quickly. Earthquakes bring on most of the landslides that result in tsunamis, but unstable slopes can also suddenly give way due to other forces. 

Even earthquakes that are not powerful enough to produce a tsunami on their own may be powerful enough to trigger a landslide which may create a tsunami. On July 10, 1958, in Alaska’s Southeast, five people were killed by tsunamis produced by several submarine landslides. Rock falls, and ice falls were sparked by a magnitude 7.8 earthquake. The water rushed over the other shore when a rock dropped into Lituya Bay. It reached up to a height of 1,720 feet, removing trees from the area. The tsunami is regarded as the tallest one ever seen.

Even earthquakes that are not powerful enough to produce a tsunami on their own may be powerful enough to trigger a


which may create a tsunami.

The Dangers of Tsunamis 

The most severe damage and deaths from tsunamis occur close to the source. Tsunamis with runups greater than 3.28 feet are dangerous for people and property. Smaller ones can be harmful as well. Strong currents can destroy boats and harbor infrastructure, harming and drowning swimmers. Flooding, wave impacts, erosion, powerful currents, and floating debris account for most tsunami damage and destruction. 

The danger posed by the water is equal to, if not greater, as it carries people and debris back to the sea with it. Other possible effects include destroying homes, businesses, and other infrastructures. Often the loss of life and widespread injuries. Access to services like water, sewer, and power may be lost. This increases the risk of waterborne illnesses like cholera. Public safety, health, and transportation services may all be affected. 

A giant local tsunami will probably have effects in addition to the previous earthquake’s effects. Both could result in secondary hazards even more harmful than direct damage. Earthquakes and tsunamis can result in fires, vehicle mishaps, and environmental releases of hazardous materials. This can contaminate water sources and endanger public health. These effects may make leaving, responding, and recovering even more difficult.


Tsunamis are massive waves caused by sudden shifts in the ocean floor. These shifts can be caused by earthquakes, volcanic eruptions, or landslides. Tsunamis can travel faster than a cheetah and cause devastating damage when they make landfall. 

As a result, it is essential to be aware of the potential for tsunamis in prone areas. If you are near an area at risk for tsunamis, heed weather alerts. Familiarize yourself with your community’s emergency plan. By being prepared, you can help to protect yourself and your loved ones in the event of a tsunami. 

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About the Author

Creepy-crawly creatures enthrall Marisa. Aside from raising caterpillars, she has a collection of spiders as pets. The brown recluse is her favorite spider of all time. They're just misunderstood. You don't have to worry about squishing the creatures as her catching, and relocating abilities can safely move stray centipedes or snakes to a new location that's not your living room.

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