A tsunami is a series of waves generated by the sudden displacement of a massive volume of water. The word comes from the Japanese terms tsu (harbor) and nami (wave), translating to “harbor wave.” Tsunamis are often incorrectly called “tidal waves,” but they are unrelated to the gravitational forces that cause tides. A tsunami is a complex natural phenomenon that moves energy across an entire ocean basin, which is why scientists often refer to them as seismic sea waves.
Initial Displacement and Triggering Events
The formation of a tsunami requires an event capable of rapidly shifting the entire water column, from the ocean floor to the surface. The most frequent trigger is a large submarine earthquake, typically occurring in a subduction zone. These zones are convergent plate boundaries where one tectonic plate is forced beneath another, building immense stress over time.
When the stress exceeds the rock’s strength, the leading edge of the overriding plate suddenly snaps upward, a movement called vertical displacement. This abrupt shift of the seafloor pushes the water above it out of equilibrium, creating a bulge or depression on the ocean surface. Gravity acts on this displaced water, pulling it back down and generating the outward-radiating tsunami waves.
While earthquakes are the primary cause, accounting for about 72% of all tsunamis, other events can also displace the necessary water volume. Secondary triggers include massive underwater landslides, often triggered by earthquakes, violent volcanic eruptions, or the rare impact of a large meteorite. However, tsunamis generated by non-seismic mechanisms usually dissipate quickly and rarely affect distant coastlines.
Propagation Across the Deep Ocean
Once formed, the tsunami wave train moves outward from the source, distributing energy through the entire water column. Unlike wind-generated waves that only affect the surface, a tsunami is a shallow-water wave because its extremely long wavelength is much greater than the ocean’s depth. In the deep ocean, the wavelength, or distance between wave crests, can be enormous, often exceeding 100 to 500 kilometers.
The wave’s speed is directly related to the water depth; the deeper the ocean, the faster the tsunami travels. In the deepest parts of the ocean, where depths reach several kilometers, the wave can propagate up to 800 kilometers per hour, comparable to a jet airliner’s speed. Despite this speed, the wave height in the open ocean is negligible, often less than one meter.
Because of the small wave height and long wavelength, ships at sea rarely notice a tsunami passing beneath them. The wave loses very little energy as it travels across vast distances, allowing it to cross the entire Pacific basin in less than a day. The period (the time between successive wave crests) is also very long, ranging from a few minutes to over an hour.
Coastal Transformation and Run-up
The wave’s characteristics undergo transformation as it leaves the deep ocean and encounters shallower water near the coast, a process known as shoaling. As water depth decreases, friction with the seabed causes the front of the wave to slow down significantly. The wave’s speed can drop from hundreds of kilometers per hour to the speed of a car (approximately 30 to 50 kilometers per hour) as it approaches the shore.
Because the trailing part of the wave is still moving faster than the leading edge, the long wavelength compresses, forcing the energy into a smaller area. This compression converts the wave’s kinetic energy into potential energy, causing the wave’s amplitude (height) to increase rapidly. A wave barely a meter high in the deep ocean can quickly grow into a destructive wall of water up to 10 meters or more.
The first sign of an incoming tsunami is often “draw-down,” where the sea recedes significantly, sometimes revealing the seafloor. This occurs when the trough of the wave arrives first, pulling water away from the shore before the main crest. The final stage is the “run-up,” which describes the maximum vertical height the water reaches above sea level as it surges inland. This rush of water, which often resembles a fast-moving flood or a hydraulic bore, causes widespread destruction.