A tsunami (say it like “soo-NAH-mee”) is a series of giant ocean waves caused by a sudden, powerful disturbance under or near the sea. Unlike the waves surfers ride, which are pushed by wind, tsunamis are triggered by events like underwater earthquakes, volcanic eruptions, or massive landslides. They can cross entire oceans and cause serious damage when they reach land.
How a Tsunami Starts
Most tsunamis begin with an earthquake on the ocean floor. When large sections of the Earth’s crust suddenly shift underwater, they push an enormous amount of water upward. That displaced water starts moving outward in every direction, forming waves that spread across the ocean like ripples in a bathtub, only much, much bigger.
Earthquakes aren’t the only cause, though. Volcanic eruptions can blast water aside or cause parts of an island to collapse into the sea. Giant landslides, where huge chunks of rock or earth tumble into the ocean, can also set off a tsunami. Sometimes an earthquake triggers a landslide, and the landslide triggers the tsunami, so one event can set off a chain reaction.
Faster Than a Jet Plane
Here’s the wildest part: in the deep ocean, a tsunami can travel over 500 miles per hour. That’s as fast as a commercial jet airplane flying overhead. At that speed, a tsunami can cross the entire Pacific Ocean in less than a day.
But you wouldn’t even notice it passing beneath a boat. Out in deep water, a tsunami wave might only be a foot or two tall. It’s spread out over a very long distance, sometimes hundreds of miles from one wave crest to the next, so it feels like a gentle rise in the water. Ships in the open ocean often don’t feel a thing.
What Happens Near the Shore
As a tsunami enters shallower water near the coast, everything changes. The wave slows down, but all that energy has to go somewhere. The water piles up higher and higher. A wave that was barely visible in the deep ocean can grow into a wall of water 30, 50, or even 100 feet tall by the time it reaches shore.
Most tsunamis don’t curl and crash the way surfing waves do. Instead, they often look like a fast-rising flood or a powerful surge of water that just keeps coming and coming. The water pushes far inland, carrying debris, cars, trees, and anything else in its path. And because a tsunami is a series of waves, not just one, the water may rush in and pull back several times over the course of hours. The first wave isn’t always the biggest.
Tsunamis Are Not Tidal Waves
You might hear people call tsunamis “tidal waves,” but scientists don’t use that term. Tidal waves are the regular rise and fall of the ocean caused by the gravity of the Moon and Sun. Tsunamis have nothing to do with tides. They’re caused by earthquakes, landslides, and volcanic eruptions. The word “tsunami” comes from Japanese: “tsu” means harbor, and “nami” means wave.
Normal ocean waves are created by wind blowing across the water’s surface. They only move the top layer of the ocean. A tsunami, on the other hand, moves the entire column of water from the surface all the way down to the ocean floor. That’s why it carries so much more energy and power than even the biggest storm waves.
The Tallest Tsunami Ever Recorded
In 1958, an earthquake shook a remote bay called Lituya Bay in Alaska. The shaking knocked a massive chunk of mountainside loose. About 90 million tons of rock, enough to fill 8 million dump trucks, plunged into the narrow bay. The splash sent water surging up the opposite mountainside to a height of 1,720 feet. That’s taller than the Empire State Building. It remains one of the tallest tsunami waves ever recorded by scientists.
That was an unusual case because the wave happened in a small, enclosed bay. Tsunamis in the open ocean don’t typically reach heights like that. But even a tsunami a few feet tall carries tremendous force because of the sheer volume of water behind it.
How Scientists Detect Tsunamis
Scientists use a system of special buoys placed across the ocean floor called DART buoys. Each one has a pressure sensor sitting on the seafloor connected to a floating buoy on the surface. The sensor can detect changes in water pressure caused by a passing tsunami wave, picking up waves as tiny as 1 centimeter (less than half an inch). When the sensor detects something unusual, it sends a signal up to the buoy, which transmits the data to satellites and then to warning centers on land.
This system gives coastal communities precious time to evacuate before the waves arrive. Since tsunamis can take minutes to hours to cross an ocean, early detection can save thousands of lives.
Warning Signs You Can Spot
Nature gives its own warnings before a tsunami hits. If you’re near the coast, watch for these signs:
- A strong earthquake that makes it hard to stand. Any powerful earthquake near the ocean could mean a tsunami is on the way.
- The ocean suddenly pulls back from the shore, exposing the seafloor far beyond where the water normally sits. This can look fascinating, but it means a huge wave is coming.
- A loud roaring sound from the ocean, like a freight train or a jet engine.
If you notice any of these signs, don’t wait for an official warning. Move to higher ground or as far inland as you can, right away. A tsunami can arrive within minutes of an earthquake. Stay away from the coast until officials confirm it’s safe, because more waves can keep coming for hours after the first one.
How to Stay Safe
The most important rule during a tsunami is simple: get to high ground fast. Don’t stop to collect belongings or take pictures of the water. Move uphill or inland as quickly as possible. If there’s no high ground nearby, the upper floors of a sturdy concrete building can work as a last resort.
Never go to the beach to watch a tsunami come in. The waves move far faster than any person can run. And if you see the water suddenly drain away from the shore, resist the urge to walk out and explore. That exposed seafloor is the most dangerous place to be. The water will come back, and it will come back with incredible force.