A tsunami is a series of powerful waves in a body of water, most often the ocean, caused by the sudden displacement of a large volume of water. Unlike typical wind-driven waves, which only affect the ocean’s surface, a tsunami involves the entire water column from the seafloor to the surface. These events act more like a fast-moving, rapidly rising tide that pushes far inland rather than a normal breaking wave.
Giant Slips Under the Sea
The most common way a tsunami begins is with a sudden, vertical shift of the ocean floor, usually caused by a large underwater earthquake. Earth’s surface is covered by tectonic plates that are constantly moving. Where one plate slides beneath another, known as subduction, immense stress builds up over many years. When the locked plates suddenly slip, the seafloor is rapidly lifted or dropped.
This vertical movement acts like a giant, underwater paddle, pushing the entire column of water above it. This vertical displacement is necessary to generate a tsunami; earthquakes where plates slide horizontally past each other typically do not create these waves. The displaced water is pulled back down by gravity, creating a ripple that radiates outward across the ocean. For an earthquake to generate a significant tsunami, it generally needs to be powerful, at least magnitude 6.5, and occur at a relatively shallow depth beneath the ocean floor. These seismic events account for approximately 80% of all known tsunamis.
Other Big Splashes
Any event that abruptly displaces a massive amount of water can start a tsunami. Large landslides, both those occurring on land and falling into the water (subaerial) or those happening entirely underwater (submarine), are the second most common trigger. A massive volume of soil, rock, or debris crashing into the ocean pushes the water out of the way, much like dropping a huge pile of dirt into a swimming pool.
This can happen when an earthquake triggers a rock slide near a coast, or when large masses of unstable sediment on the continental slope suddenly collapse. Tsunamis caused by landslides are often larger near their source than those caused by seismic events, but they usually lose energy quickly and do not travel as far across the ocean. Volcanic eruptions can also cause tsunamis, either through the collapse of a volcano’s flank or by fast-moving pyroclastic flows and debris crashing into the sea.
The Wave’s Secret Journey
Once a tsunami is generated, it begins a journey across the ocean. In the deep ocean, where the water is thousands of meters deep, the wave travels incredibly fast, often reaching speeds similar to a jet airplane, around 550 miles per hour (890 km/hr). The wave has a very long wavelength but a very small height, making it virtually imperceptible to ships at sea.
This behavior changes dramatically as the wave approaches the coast and enters shallow water, a process known as shoaling. As the depth of the water decreases, the wave begins to slow down significantly. The decrease in speed forces the wave to compress, causing its height to increase dramatically. This transformation turns the low, fast wave into a much taller, slower, and more destructive force.
How to Stay Safe When the Water Goes Out
A strong earthquake that makes it hard to stand is a clear sign that a tsunami may be on its way. Another important sign is an unusual and sudden drop in sea level, where the water retreats far back from the shore, exposing the seafloor and reefs.
If the ground shakes intensely, or if you observe the water suddenly pulling back, you must immediately move to higher ground. Do not wait for an official warning, and never go to the shore to investigate the exposed seafloor, as the first wave may arrive within minutes. Once you reach a safe elevation, remain there until local authorities announce that the danger has passed, as a tsunami is a series of waves that can arrive hours apart.