A tsunami is one of nature’s most powerful forces. The word itself comes from the Japanese language, meaning “harbor wave.” A tsunami is not just a single huge wave, but a continuous train of waves caused by a massive, sudden disturbance in the ocean. This phenomenon involves the entire vertical column of water, from the seafloor all the way to the surface, creating a surge that can cross entire ocean basins.
How a Tsunami Gets Started
The primary way a tsunami begins is with an underwater earthquake that causes the seafloor to move suddenly. The Earth’s crust is made up of large pieces called tectonic plates that are constantly shifting. When two plates meet and one slides forcefully beneath the other, tension builds until the rock snaps upward.
This massive vertical movement of the seafloor acts like a giant paddle, displacing the enormous volume of water resting above it. This sudden displacement of ocean water is the beginning of the tsunami.
About 80% of all tsunamis are triggered by these powerful underwater earthquakes, especially those occurring in areas like the Pacific Ocean’s “Ring of Fire.” While earthquakes are the main cause, other extreme events can also generate tsunamis. A large underwater landslide or a violent volcanic eruption near or beneath the sea surface can also displace enough water to create this giant water push.
The Difference Between a Tsunami and a Regular Wave
Most waves seen at the beach are created by wind blowing across the water’s surface. These regular waves only move the upper layers of the ocean and have a relatively short distance between their crests, usually about 30 to 40 meters.
A tsunami is fundamentally different because it is a movement of the entire ocean column, not just a surface ripple. Tsunami waves have extremely long wavelengths, meaning the distance from one wave crest to the next can be hundreds of kilometers long. This long wavelength means the tsunami involves a tremendous volume of water.
In the deep ocean, this long wave travels incredibly fast, often moving at speeds comparable to a jet plane, around 800 kilometers per hour (500 miles per hour). Despite this speed, the wave height is often less than a meter, making it virtually unnoticeable to ships. The tsunami only begins to get tall when it reaches shallow coastal waters.
As the wave moves into shallower areas, the seafloor creates friction, forcing the wave to slow down dramatically, sometimes to less than 80 kilometers per hour. The energy of the fast-moving water behind the wave piles up as the front slows down, causing the height of the wave to increase rapidly. Instead of a typical breaking wave, a tsunami often looks like a rapidly rising tide or a fast-moving river, with an immense amount of water surging inland and continuing to rise for several minutes.
Tsunami Warnings and Safety Rules
Knowing the warning signs is the most important step in staying safe, as you cannot outrun a tsunami. There are two types of warnings to pay attention to: official alerts and natural signals. Official warnings come from government agencies and may be delivered through sirens, phone alerts, or emergency radio broadcasts.
If you are near the coast and feel a very strong earthquake, that is your first natural warning, and you should not wait for an official alert. A second natural warning is the ocean pulling back from the shore much farther and faster than a normal low tide, exposing large areas of the seafloor. You may also hear a loud, roaring sound approaching, similar to a train or jet.
If you experience any of these natural signs or hear an official warning, you must evacuate immediately. The safest action is to move quickly and calmly to higher ground, meaning a location at least 30 meters (100 feet) above sea level or 3 kilometers (2 miles) inland. If the water has pulled back, the first wave is on its way. Since tsunamis are a series of waves, stay on high ground until authorities announce that the danger has completely passed, which may take several hours.