A tsunami forms when a massive volume of ocean water is suddenly displaced, usually by the seafloor lurching upward or downward during an underwater earthquake. The displaced water creates waves that radiate outward in all directions, traveling across the open ocean at speeds over 800 km/h before slowing down and growing dramatically taller as they approach shore. The whole process, from trigger to impact, can unfold in minutes for nearby coasts or take hours to reach distant shorelines.
Step 1: The Seafloor Shifts
Most tsunamis begin with an earthquake beneath the ocean. When tectonic plates grind past or under one another, the seafloor can snap upward or drop downward in seconds. This vertical movement is the critical ingredient. An earthquake that only shifts the ground sideways won’t push enough water to generate a tsunami. Earthquakes greater than about magnitude 6.5 to 7 that produce predominantly vertical displacement are the typical threshold, though the largest and most destructive tsunamis tend to come from quakes above magnitude 8.
The displacement doesn’t need to be enormous. Even a few meters of uplift across a large stretch of ocean floor transfers a staggering amount of energy into the water column above it. The entire depth of the ocean, from the seafloor to the surface, gets shoved upward almost instantly, creating a broad dome or depression of water that gravity immediately tries to flatten out.
Step 2: Waves Radiate Outward
Once the water surface has been disturbed, gravity pulls the raised water back down, and the resulting waves spread in all directions like ripples in a pond, but on a vastly larger scale. In the deep ocean, these waves travel at roughly 800 km/h, comparable to the cruising speed of a commercial jet. A tsunami can cross an entire ocean basin in less than a day.
What makes a tsunami different from a wind-driven wave is its wavelength. A normal ocean wave might have a few hundred meters between crests. A tsunami’s wavelength can stretch hundreds of kilometers. That means the energy isn’t concentrated at the surface. It extends all the way to the ocean floor, which is why tsunamis carry so much more destructive power than even the largest storm waves. Paradoxically, this also makes them nearly invisible in open water. Out in the deep ocean, a tsunami is rarely more than about one meter tall. A ship at sea might pass over one without anyone on board noticing.
Step 3: The Wave Slows and Grows
As a tsunami enters shallower water near a coastline, friction with the rising seafloor slows the front of the wave. But the water behind it is still moving fast, so it stacks up. The wave compresses horizontally and grows vertically. A wave that was barely noticeable in deep water can build to 10 meters or more by the time it reaches shore.
This process is called shoaling, and it explains why coastal areas face such extreme danger from an event that’s essentially invisible in the open ocean. The wave also refracts, bending around headlands and into bays, which can focus energy in unexpected places. Some harbors and inlets funnel the incoming water into a narrower space, amplifying the wave height even further.
Step 4: The Water Arrives at Shore
A tsunami rarely arrives as a single towering wall of water, though that can happen. More often, it looks like a rapidly rising tide that just keeps coming. The sea may first recede dramatically, exposing seafloor that’s normally underwater. This drawback is one of the most recognizable warning signs, and it happens because the trough of the wave sometimes reaches shore before the crest does.
When the crest arrives, water surges inland with tremendous force, carrying debris, vehicles, and anything else in its path. The flooding can penetrate hundreds of meters or even kilometers inland on flat terrain. And because a tsunami is a series of waves rather than a single pulse, the first surge is not always the largest. Subsequent waves can arrive minutes to over an hour apart, each one potentially more powerful than the last. The back-and-forth flow of water rushing inland and then draining seaward causes destruction in both directions.
Triggers Beyond Earthquakes
Earthquakes cause the vast majority of tsunamis, but they aren’t the only trigger. Submarine landslides can displace enough water to generate dangerous waves, especially in confined areas like fjords or volcanic island coastlines. A large chunk of underwater slope material sliding downhill pushes water ahead of it much the way a hand sweeping through a bathtub does. These landslide-generated tsunamis behave differently from earthquake-generated ones. They tend to have shorter wavelengths, meaning their energy is more concentrated and they can produce extremely tall waves near the source, but they lose energy more quickly and don’t travel as far across open ocean.
Volcanic eruptions can also trigger tsunamis, either by collapsing into the sea, by generating underwater explosions, or by setting off landslides on the volcano’s flanks. The 2022 eruption of Hunga Tonga-Hunga Ha’apai generated tsunamis that were recorded across the entire Pacific. In rare cases, asteroid impacts in the ocean could theoretically displace enough water to create massive tsunamis, though no such event has occurred in recorded history.
How Tsunamis Are Detected
The open-ocean invisibility of tsunamis made early detection nearly impossible for most of human history. That changed with the development of the DART system (Deep-ocean Assessment and Reporting of Tsunamis), a network of seafloor pressure sensors paired with surface buoys. These instruments can detect a tsunami as small as one centimeter in wave height and relay that data in real time to warning centers. There are currently 74 DART buoy systems deployed around the world, most of them concentrated near coastlines in the Pacific Ocean, where tsunami risk is highest.
When an earthquake strikes underwater, seismic monitoring stations pick it up within minutes. If the quake has the right characteristics (large magnitude, shallow depth, vertical displacement beneath the ocean), warning centers issue alerts to coastal populations. The DART data then confirms whether a tsunami was actually generated and helps forecasters predict its size and arrival time at specific coastlines. For communities close to the earthquake’s epicenter, there may be only minutes of warning. For distant coasts, the warning window can be several hours, which is enough time for evacuation if systems work as designed.