A tsunami is a series of powerful ocean waves caused by a large and sudden displacement of a significant volume of water. Unlike typical wind-generated waves that only affect the ocean’s surface, tsunamis involve the movement of the entire water column, from the seafloor to the surface.
Tsunamis from Underwater Earthquakes
Underwater earthquakes are the most frequent and powerful generators of tsunamis. These seismic events occur in subduction zones, where one tectonic plate slides beneath another. As these plates converge, stress builds up over long periods.
When the accumulated stress exceeds the strength of the rocks, the overriding plate can suddenly slip seaward. This abrupt vertical movement of the seafloor displaces the overlying water, which is the primary mechanism for generating the initial tsunami wave. Not all underwater earthquakes cause tsunamis; only those with a magnitude exceeding 7.0 that cause significant vertical seafloor displacement are likely to produce tsunamis.
Tsunamis from Other Natural Events
While less common than earthquake-generated tsunamis, other natural phenomena can also displace large volumes of water, leading to tsunami formation. Large underwater landslides, which can be triggered by earthquakes or volcanic activity, can cause tsunamis. These landslides rapidly move sediment and rock downslope, pushing the water in front of them.
Coastal landslides plunging into the ocean also generate tsunamis by forcefully displacing water upon impact. Major volcanic eruptions, both submarine and those occurring on land near the coast, can cause tsunamis. Underwater volcanic explosions or the collapse of volcanic edifices into the sea can displace water directly. Pyroclastic flows entering the water or large-scale flank collapses of volcanoes can similarly create waves.
How Tsunami Waves Travel and Transform
Once generated, tsunami waves radiate outwards from their source, capable of crossing entire ocean basins. In the deep ocean, tsunamis behave as shallow-water waves due to their long wavelengths, spanning hundreds of kilometers. Despite their energy, these waves have a low amplitude, less than a meter, making them virtually undetectable by ships at sea.
Tsunami waves travel at high speeds in the deep ocean, reaching velocities comparable to a jet plane, around 800 kilometers per hour (500 miles per hour). As a tsunami approaches shallower coastal waters, a process known as shoaling occurs. The wave’s speed decreases significantly, but its energy is conserved, causing its height to dramatically increase and its wavelength to shorten.
The transformation from a nearly imperceptible swell in the deep ocean to a large wave at the coast explains their impact. Sometimes, the trough of a tsunami wave arrives first, causing a noticeable recession of the shoreline, known as “drawdown,” before the crest of the wave crashes ashore. The maximum vertical height a tsunami reaches onshore above sea level is called “run-up,” and this can vary depending on coastal topography.