Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one part of the process. The models are simplified to show the idea, not to forecast real tsunamis.
Tap the buttons and slide the sliders.
Switch between the two waves. Which bits of water move?
Wind-wave orbits shrink with depth; a tsunami moves the whole column back and forth.
Tap the buttons. Watch the little dots!
Press the plate edge down, then let it slip. What happens to the water?
Load the locked interface, release it, and watch the uplift displace the water column.
Press the ground down. Then let go!
Sizes are exaggerated so you can see them.
Change how deep the ocean is. How fast does the tsunami go?
In the open ocean a tsunami is a shallow-water wave, so depth alone sets its speed.
Make the sea deep or shallow. Watch the wave!
Wave height is drawn much bigger than real.
Drag the wave toward the beach. What happens to its speed and height?
As depth drops, speed and wavelength fall and height grows. Toggle trough-first arrival.
A simple model, not to scale.
Move the earthquake nearer or farther from the coast. How long until the waves arrive?
Travel time at average ocean depth (about 3,700 m). A local tsunami can arrive before any official warning.
Move the earthquake near or far. How soon do the waves come?
Uses the average ocean depth; real times depend on the sea floor.
A tsunami is a giant wave that moves the whole sea, from the top right down to the bottom. It is not made by the wind.
The ground under the sea is made of giant pieces. One piece slides under another and gets stuck. It bends, like a ruler you press down. Then, snap! It springs up and lifts all the water.
The water falls back down, and waves roll away in every direction. In the deep sea they are as fast as a jet plane, but not tall.
Near the beach the water is shallow. The wave slows down, the water piles up, and the wave gets taller.
More: what causes earthquakes? and why do plates move?
A tsunami is caused by a large and sudden push on the ocean, one that moves a huge amount of water at once.1 Most of the time, that push comes from a big earthquake under or near the seafloor. The seafloor jumps up or drops down, and the whole column of water above it moves with it. Gravity then pulls the water back toward its level, and the disturbance spreads outward in every direction as a series of very long, very fast waves.17
Earthquakes, or landslides set off by earthquakes, started 88% of the tsunamis in NOAA's Global Historical Tsunami Database.1 Landslides, volcanoes, certain kinds of weather and, very rarely, asteroids can do it too.12
A tsunami is a series of extremely long waves, with crests tens to hundreds of miles apart, caused by a sudden displacement of the ocean.1 The word is Japanese and means "harbor wave". It is pronounced soo-NAH-mee.12 Scientists call it a tsunami because of how it was made, not how big it is. A tsunami can be less than an inch tall or more than thirty feet tall.8 Sensitive instruments pick up 10 to 20 tsunamis every year, and most are too small for anyone to notice.8
Most ocean waves are made by wind, and they only stir the top of the sea. A tsunami moves the entire water column, from the surface to the seafloor.13
| Tsunami | Wind wave | |
|---|---|---|
| Cause | Earthquakes, landslides, volcanoes, some weather, asteroids | Wind blowing across the sea |
| Where the energy is | The whole water column, surface to seafloor | Near the surface |
| Distance between crests | 60–300 miles | 300–600 feet |
| Time between crests | 5 minutes to 2 hours | 5–20 seconds |
| Speed | 500–600 mph in deep water, 20–30 mph near shore | 5–60 mph |
Figures from the U.S. Tsunami Warning Centers.1
Not every earthquake does. Most tsunamis come from quakes stronger than magnitude 7.0, under or very near the ocean, and less than about 100 km (62 miles) deep.1 The quake has to move the seafloor up or down. Sideways "strike-slip" quakes rarely cause big tsunamis.16 USGS guidelines say quakes below magnitude 6.5 are very unlikely to cause one. At magnitude 7.9 and above, destructive local tsunamis are possible.6 For a dangerous tsunami to reach distant shores, a quake generally has to be stronger than magnitude 8.0.1
A tsunami's speed depends on how deep the water is, not on how big the wave is. The deeper the water, the faster it goes.18 Scientists work it out with a simple formula:1
speed = √(gravity × water depth)
In 15,000 feet of water, that works out to about 475 mph. At that speed, a tsunami gets from Alaska's Aleutian Islands to Hawaii in about five hours.1 In the deep ocean, tsunamis can go faster than 500 mph, as fast as a jet plane. Britannica gives a top speed of about 800 km (500 miles) per hour.111 Near land they slow to the speed of a car, about 20 to 30 mph, which is still faster than you can run.18
Out in the deep ocean, a tsunami is rarely more than about three feet tall. That small rise is spread over a crest many miles long, so ships usually don't notice it passing underneath them.13
As the wave reaches shallow water, the seafloor slows its front. The faster water behind it keeps coming and piles up, like traffic bunching behind a slow truck.9 The wave gets shorter and much taller, and its currents get stronger.1 This change as waves move into shallower water is called shoaling.14 Most tsunamis are less than 10 feet high when they reach land. Close to where they start, they can be taller than 100 feet and flood more than a mile inland.13 The shape of the coast matters too: a small tsunami at one beach can be much bigger a few miles away.17
A tsunami has high parts (crests) and low parts (troughs). In about half of tsunamis, a trough reaches the coast first. The water drains away and exposes the seafloor, reefs and fish, like a very low tide.18 Then the crest rushes in. In 1755 in Lisbon, Portugal, curious people walked out onto the newly exposed bay floor. Many of them drowned when the crest arrived minutes later.11 In the other half of tsunamis the crest arrives first, with no drawdown at all. So never wait for the sea to pull back.8
Usually not like a surfer's wave. A tsunami rarely becomes a tall, curling breaker. It tends to come ashore as a fast-rising flood or a wall of churning water called a bore.15 It is also a series of waves. They can arrive minutes to hours apart and keep coming for hours or even days, and the first wave may not be the biggest.18 Even a 6-foot tsunami has currents strong enough to knock a person off their feet.5
Tsunamis can happen in any ocean, and in large lakes too, at any time of year: there is no tsunami season.1 About 78% of the 754 confirmed tsunamis between 1900 and 2015 happened in the Pacific Ocean, around the earthquake-prone "Ring of Fire". Japan produced 21% of them, then Russia and Indonesia with 8% each.1 Tsunamis that cause damage near their source happen about twice a year. Ones that do damage on distant shores happen about twice a decade.1
| Year | Where | Cause | What happened |
|---|---|---|---|
| 2004 | Indian Ocean | M9.1 earthquake off Sumatra | Deadliest tsunami in history: about 230,000 deaths in 15 countries1 |
| 2011 | Japan | M9.1 earthquake | Waves up to 128 ft, nearly five miles inland1 |
| 1960 | Chile | M9.5, the largest earthquake ever recorded | A devastating tsunami across the Pacific1 |
| 1958 | Lituya Bay, Alaska | Rock fall into a bay | Water surged up to 1,720 ft1 |
| 1883 | Krakatau, Indonesia | Volcanic explosion and collapse | Waves up to 135 ft1 |
Scientists can't predict when the next tsunami will happen. But warning centers know which earthquakes are likely to cause one.1 Seismic waves from an earthquake travel about 100 times faster than a tsunami, so news of the quake arrives first.1 Then DART stations confirm whether a tsunami is on its way. A pressure recorder on the deep seafloor feels the change in the weight of water passing above it. It sends that signal by sound to a buoy at the surface, which relays it by satellite to the warning centers.1
Nature's warnings may be your first, best or only warning:1
If you notice even one, don't wait for an official warning. Go to high ground or inland straight away, on foot if you can (at least 100 feet above sea level or a mile inland). Stay there until officials say it's safe, because more waves are coming.15
Most tsunamis begin with an undersea earthquake at a place where two of Earth's plates meet. See what causes earthquakes and what makes tectonic plates move for the story underneath this one.
Most tsunamis are caused by large earthquakes under or near the ocean floor that suddenly lift or drop the seafloor. Underwater and coastal landslides, volcanic eruptions, sudden air-pressure jumps in fast storms (meteotsunamis) and, very rarely, asteroid impacts can also cause them.
When an undersea earthquake moves the seafloor up or down, the entire column of water above it moves too. Gravity pulls that hump or dip of water back toward its level, and the disturbance spreads outward in all directions as a series of very long, fast waves.
Nature's warnings are strong or long earthquake shaking, a loud roar from the ocean (like a train or an airplane), and unusual sea behaviour such as the water suddenly pulling back and exposing the seafloor, or rising very fast. If you notice even one, head to high ground or inland straight away.
In the Pacific Ocean. About 78% of confirmed tsunamis between 1900 and 2015 happened there, around the earthquake-prone Ring of Fire. Japan generated the most (21%), followed by Russia and Indonesia.
The deadliest was the Indian Ocean tsunami of December 26, 2004, caused by a magnitude 9.1 earthquake off Sumatra. It killed about 230,000 people in 15 countries. The highest wave run-up ever recorded was in Lituya Bay, Alaska, in 1958, when a rock fall sent water surging 1,720 feet up the hillside. That was a local splash in a narrow bay, not an ocean-crossing tsunami.
Tides are caused by the gravity of the Moon and Sun. Tsunamis have nothing to do with tides: they are caused by a sudden push on the water, usually from the seafloor. That's why scientists say calling a tsunami a tidal wave is incorrect.
In the deep ocean a tsunami can travel over 500 miles per hour, as fast as a jet plane. Its speed depends on the water depth, so it slows to about 20 to 30 miles per hour near the coast, which is still faster than people can run.