How Do Tsunamis Form and Travel Across Oceans?
A tsunami forms when the seafloor suddenly moves and pushes a whole column of water, sending waves across oceans at jet speed that grow tall only near the coast.
A tsunami is a series of long ocean waves, most often produced when a large undersea earthquake suddenly lifts or drops the seafloor. The moving seabed displaces the whole column of water above it, and waves spread outward in every direction. In the deep ocean a tsunami can travel at several hundred kilometres per hour yet be only a metre or so high. It becomes dangerous only when it reaches shallow water near the coast.
What makes a tsunami?
Most tsunamis come from earthquakes at subduction zones, where one tectonic plate dives beneath another. When the fault slips, a large block of seafloor can rise or sink by several metres, and the water above it moves with it. Earthquakes of magnitude 7 or more can generate damaging tsunamis, though the amount of seafloor movement matters more than magnitude alone. The 2004 Indian Ocean earthquake off Sumatra and the 2011 Tōhoku earthquake in Japan both produced devastating tsunamis this way. The 2004 tsunami killed more than 200,000 people across more than a dozen countries.
Landslides, volcanic collapses and eruptions can also displace water. The 1883 eruption of Krakatau in Indonesia generated tsunamis that killed tens of thousands of people. Impacts from space can also generate tsunamis, as the Chicxulub impact probably did. Earthquakes remain the most common cause, and their seismic waves are often the first warning that a tsunami may follow.
Why do tsunamis move so fast in deep water?
A tsunami behaves as a shallow-water wave, even in the deep ocean, because its wavelength is far longer than the water is deep. Often hundreds of kilometres long, the wave moves the whole column like a long, low swell. For such waves the speed depends only on gravity and depth: speed equals the square root of gravity multiplied by the depth. Gravity is about 9.8 metres per second squared, so at 4,000 metres of depth the speed is about 200 metres per second, or roughly 700 kilometres per hour.
Because the energy is spread through the whole water column, the surface rises only slightly. A ship in deep water may pass over a tsunami without noticing it. The energy also spreads over the whole ocean, so a tsunami loses little strength across long distances. Waves from a single Pacific earthquake have crossed the ocean and damaged distant coasts.
What happens when a tsunami reaches the coast?
As the wave enters shallow water, its front slows down while its back is still moving quickly, so the wavelength shrinks. The energy that was spread through kilometres of depth is now packed into a thinner layer, and the wave grows taller. This process is called shoaling. The sea may first draw back sharply, exposing the seabed, or the crest may arrive as a rapidly rising flood rather than a breaking wave. Some tsunamis arrive as a series of waves over hours, and the first is not always the largest.
The table compares the same wave in the two settings. The figures are approximate.
| Property | Deep ocean (about 4,000 m) | Near shore (about 10 m) |
|---|---|---|
| Speed | About 700 km/h | About 35 km/h |
| Wavelength | Hundreds of kilometres | About 10 km |
| Height | Often under a metre | Can reach many metres where bays and river mouths funnel the water |
How do warning systems detect them?
Pressure sensors on the seafloor, linked to surface buoys, can detect the small rise in water pressure as a tsunami passes. The United States operates such a network, called DART. Warning time for distant coasts is often hours. Coasts near the source may get only minutes, which is why the sea drawing back, or a strong earthquake felt on the coast, is itself a reason to move to high ground. Coastal tide gauges then confirm the wave and help forecasters update their warnings.
For the deep-sea setting, see the deep ocean, and for the volcanic cause, see volcanoes.
Tags
earthquakes oceanography plate tectonics tsunamis waves
Related articles
- Why Is the Sky Blue? Rayleigh Scattering Explained
- Earthquakes and Seismic Waves
- Doppler Effect
- Electromagnetic Spectrum
- Ocean Currents
- The Rock Cycle