About Tsunamis

Frequently Asked Questions:

What does the name “tsunami” mean and where did the term come from?

Tsunami is a Japanese word 津波 that comes from two Chinese characters (kanji), 津 “tsu” which means harbor and 波 “name”, meaning wave. Like many compound words, it no longer strictly means a harbor wave, but now internationally is used to describe the series of surges in oceans or lakes following the displacement of the sea or lake floor, which moves the entire column of water above it. Impacts are often greatest in harbors where people and structures are concentrated at sea level.

How does a tsunami happen?

Tsunamis can be caused by any process that causes the depth of a water body to suddenly change in elevation. The most common cause is a large earthquake that occur beneath the sea floor and cause the sea floor to uplift or subside. Aa much smaller percentage of tsunamis is caused by landslides that either originate in the ocean or fall from the land into the water. Volcanoes can also produce tsunamis by explosive eruptions, caldera collapse and/or flank collapse. Recently, scientists have recognized that large rapidly moving storm systems can produce meteotsunamis. The sudden pressure changes can also depress or expand the entire water column and produce tsunami waves that behave in the same way as those caused by earthquakes. Much rarer are tsunamis caused by gas pockets from beneath the sea or lake floor or meteor impacts.

Tsunami caused by earthquakes

How fast do tsunamis travel?

Tsunamis are caused by the potential energy of water when it is suddenly uplifted or dropped down relative to the water around it. Under the force of gravity, the water wants to return to a stable position. All tsunamis involve the entire column of water from the sea floor to the ocean surface. The deeper the water, the more potential energy and driving force for the tsunami. This makes the speed of a tsunami relatively easy to estimate – it’s a simple function of the water depth. In the deep ocean, a tsunami is about as fast as a jet airplane, traveling around 500 miles per hour. In the very deepest parts of the oceans, the speed can be over 700 miles per hour. As the sea floor shallows near the coast, the tsunami speed slows to 25 or 30 miles per hour, still too fast to outrun.

NOAA Tsunami Animation

Where is a tsunami biggest?

Tsunamis are always largest at the coast. In the deep ocean, the height of a tsunami is usually too small to be noticed, even by small boats in the water. As the tsunami approaches shallower water, the speed slows and the water begins to build up. A tsunami that is only a few feet high in deep water can build to tens of feet along the closest coasts. A tsunami spreads out as it travels, losing energy, but if the tsunami is very large, it can still be big enough to cause damage thousands of miles away. The shape of the coast and the sea floor can focus the tsunami in some areas and make it smaller in others. Scientists studying tsunami hazards consider all these factors – the size and nature of the source, and the coastal and sea floor shape in determining the size of a tsunami.

How big do the waves have to be before it is called a tsunami?

The term tsunami refers to the mechanism of generation not how big or small the waves are. If surges are caused by deformation of the sea floor, it’s a tsunami whether it is less than an inch or more than 30 feet high. Every year between 10 and 20 tsunamis are detected. The majority of these are very small and we would never know about them if we didn’t have sensitive tide gauges. And all tsunamis, whether large or small, travel at the same speed. The speed of a tsunami depends only on how deep the water is and is independent of how large or the type of source that caused it.

Are tsunamis and tidal waves the same thing?

Some people think “tidal wave” is an English substitute for tsunami. It isn’t. There are true tidal waves. They are caused by the gravitational attraction of the sun and moon and can be predicted many years in advance, just by knowing orbital positions and local site conditions. There are some places where these conditions can produce a truly tsunami-like waves. The Qiantang River in East China is the site of a famous tidal bore, a surge of water that rushes from the river mouth inland. When the tidal conditions are just right, the bore can reach heights of 30 feet and rush inland at speeds of 25 miles per hour. It has become a tourist attraction and some even try to surf it. Tsunamis are not caused by tidal attraction and it is a misnomer to call one a tidal wave.

World's Largest Tidal Bore Forms in China's Qiantang River

How are earthquakes and tsunamis related?

The majority of tsunamis are caused by earthquakes. Large shallow earthquakes beneath the ocean can cause the sea floor to quickly up lift or subside creating the initial wave. The larger the earthquake and the more vertical fault motion or slip it produces, the larger the tsunami. It typically takes a magnitude in the mid 7 range to produce a tsunami capable of doing damage and the great ocean-wide tsunamis like 2011 Japan, 2004 Indian Ocean and 1960 Chile have magnitudes in the upper 8 to 9 range.

Tsunamis Generated by Megathrust Earthquakes

Do all earthquakes cause tsunamis?

The overwhelming majority of earthquakes don’t produce tsunamis. Most are too small, or too deep and centered under land. In a typical year, about 15 to 20 earthquakes in the magnitude 7 or larger range occur and only one or two of these may produce a tsunami large enough to cause any impacts. But whenever you feel an earthquake at the coast, it is a natural warning that a tsunami could follow and you should immediately head inland or to higher ground. Much better to be safe than sorry, and practicing your evacuation skills means you will know what to do when a bigger earthquake causes a major tsunami.

How long does a tsunami last?

Tsunamis last for hours or sometimes days. They always consist of many surges or waves and the first surge is almost never the largest. Unlike typical wind-caused waves and swells which arrive on the order of every 10 to 20 seconds, tsunami surges are usually many minutes apart and sometimes more than an hour can pass between successive crests. Tsunamis can trick you. They are irregularly spaced in time and just when you might think a tsunami is over, larger surges may arrive. The arrival time of the first surge in a tsunami can be accurately predicted within minutes as it only depends on where the source is located and the distance and sea floor depths to coastal areas. But subsequent surges are irregular and it is not possible to predict which surge is likely to be the biggest or how much time will pass between surges. In some coastal locations like Crescent City, tsunami energy is trapped in the harbor and can continue for many days. Our recommendation – never return to coastal areas after a tsunami until authorities give you the all clear.

Does the water always retreat before a tsunami arrives?

In about half of tsunami cases, the initial wave arrival at the coast is a trough, meaning that the water will draw down and appear to retreat, exposing the sea floor over a much larger area that an extremely low tide. But the first wave is just as likely to be a crest meaning that there will be no water lowering, just a sudden increase in wave height and water penetration. Whether the first arrival is negative (drawdown) or positive (wave increase) depends on the source characteristics and where your are relative to the source. If you suddenly notice that the water has gone out much further than normal, treat this as a warning that the water could suddenly rush back in and evacuate. But when you feel an earthquake at the coast, especially one that shakes for a long time, don’t wait to observe the water retreat. It might not do so and you can’t outrun a tsunami surging in.

Where and how frequently are tsunamis generated?

The majority of historic tsunamis have been located in the Pacific Ocean basin. It is earth’s largest ocean and is almost entirely ringed by subduction zones, the type of geologic feature that is most likely to produce very large earthquakes with vertical sea floor deformation.