What Causes a Tsunami — In Plain Words

General information only.

Short answer

Most tsunamis are caused by earthquakes on converging tectonic plate boundaries.

A tsunami is a series of extremely long waves caused by a large and sudden displacement of the ocean, usually the result of an earthquake below or near the ocean floor.

What this page covers

This page explains what a tsunami is and what causes it, in plain words. It is based on one page from NOAA: the education page “Tsunamis.”

Cross-reference, in plain text only: See also the companion page on storm surge. Storm surge is wind-driven water rise from a storm; a tsunami is displacement-driven. They are different things.

What is a tsunami?

A tsunami is a series of extremely long waves caused by a large and sudden displacement of the ocean, usually the result of an earthquake below or near the ocean floor.

This force creates waves that radiate outward in all directions away from their source, sometimes crossing entire ocean basins.

Unlike wind-driven waves, which only travel through the topmost layer of the ocean, tsunamis move through the entire water column, from the ocean floor to the ocean surface.

What causes tsunamis?

Most tsunamis are caused by earthquakes on converging tectonic plate boundaries.

According to the Global Historical Tsunami Database, since 1900, over 80% of likely tsunamis were generated by earthquakes.

However, tsunamis can also be caused by landslides, volcanic activity, certain types of weather, and — possibly — near-earth objects (e.g., asteroids, comets) colliding with or exploding above the ocean.

How a tsunami moves

Once a tsunami forms, its speed depends on the depth of the ocean.

In the deep ocean, a tsunami can move as fast as a jet plane, over 500 mph, and its wavelength, the distance from crest to crest, may be hundreds of miles.

Mariners at sea will not normally notice a tsunami as it passes beneath them; in deep water, the top of the wave rarely reaches more than three feet higher than the ocean swell.

NOAA Deep-ocean Assessment and Reporting of Tsunami (DART) systems, located in the deep ocean, are able to detect small changes in sea-level height and transmit this information to tsunami warning centers.

An example from weather: Lake Michigan, 2018

On the afternoon of April 13, 2018, a large wave of water surged across Lake Michigan and flooded the shores of the picturesque beach town of Ludington, Michigan, damaging homes and boat docks, and flooding intake pipes.

Thanks to a local citizen’s photos and other data, NOAA scientists reconstructed the event in models and determined this was the first ever documented meteotsunami in the Great Lakes caused by an atmospheric inertia-gravity wave.

Tsunami safety

A tsunami only becomes hazardous when it approaches land. As a tsunami enters shallow water near coastal shorelines, it slows to 20 to 30 mph. The wavelength decreases, the height increases, and currents intensify.

Tsunami warnings come in different forms. There are official warnings issued by tsunami warning centers that are broadcast through local radio and television, wireless emergency alerts, NOAA Weather Radios, NOAA websites, and social media. They may also come through outdoor sirens, local officials, text message alerts, and telephone notifications.

There may not be time to wait for an official warning, so it is important to be able to recognize natural tsunami warnings. These include strong or long earthquakes, a loud roar (like that of a train or an airplane) coming from the ocean, and a sudden rise or fall of the sea level that is not related to the tide.

Official and natural warnings are equally important. Be prepared to respond immediately to any tsunami warnings. Move quickly to a safe place by following posted evacuation signs. If you do not see an evacuation route, go to high ground or as far inland as possible.

When a tsunami strikes land

When they strike land, most tsunamis are less than 10 feet high, but in extreme cases, they can exceed 100 feet near their source.

A tsunami may come onshore like a fast-rising flood or a wall of turbulent water, and a large tsunami can flood low-lying coastal areas more than a mile inland.

Rushing water from waves, floods, and rivers is incredibly powerful. Just six inches of fast-moving water can knock adults off their feet, and twelve inches can carry away a small car.

Tsunamis can be particularly destructive because of their speed and volume. They are also dangerous as they return to the sea, carrying debris and people with them.

The first wave in a tsunami may not be the last, the largest, or the most damaging. Stay out of the tsunami hazard zone until local officials tell you it is safe, as the danger may last for hours or days.

Effects on humans

Large tsunamis are significant threats to human health, property, infrastructure, resources, and economies. Effects can be long-lasting, and felt far beyond the coastline.

Tsunamis typically cause the most severe damage and casualties near their source, where there is little time for warning. But large tsunamis can also reach distant shorelines, causing widespread damage.

The 2004 Indian Ocean tsunami, for example, impacted 17 countries in Southeastern and Southern Asia and Eastern and Southern Africa.

Can scientists forecast tsunamis?

Scientists cannot predict when and where the next tsunami will strike.

But the tsunami warning centers know which earthquakes are likely to generate tsunamis and can issue messages when one is possible. They monitor networks of deep-ocean and coastal sea-level observation systems designed to detect tsunamis and use information from these networks to forecast coastal impacts and guide local decisions about evacuation.

Tsunami warning capabilities have become dramatically better since the 2004 Indian Ocean tsunami. NOAA scientists are working to further improve warning center operations and to help communities be prepared to respond.

General information only.