How tsunamis form
Wind waves move only the surface of the sea, but a tsunami begins when the sea floor suddenly rises or drops and the whole water column, from bottom to surface, is set in motion. Most are triggered by shallow undersea earthquakes of magnitude 7.5 or more in subduction zones where tectonic plates meet; volcanic eruptions (Krakatoa, 1883) and submarine landslides can also cause them. The word is Japanese for "harbour wave": fishermen at sea felt nothing, yet returned to find their harbour destroyed. Scientists adopted it internationally after the 1946 Hawaii disaster, replacing the misleading "tidal wave".
Speed and height
A tsunami's speed is roughly √(g × depth). Over 4,000 m of ocean it travels at about 700 km/h, as fast as a jet, yet its height may be under a metre, so ships barely notice it. As the water shallows near the coast the wave slows, the water behind piles up, and the height can grow to several metres or even tens of metres. Later waves are often larger than the first, and the sea may draw back dramatically before the wave arrives. This behaviour was put on a quantitative footing only in the late 20th century, alongside plate tectonics and modern ocean monitoring.
| Year | Region | Quake magnitude | Dead/missing |
|---|---|---|---|
| 2004 | Indian Ocean (Sumatra) | 9.1 | ~230,000 |
| 2011 | Tōhoku, Japan | 9.0–9.1 | ~18,000 |
| 1960 | Valdivia, Chile | 9.5 | thousands |
| 1755 | Lisbon, Portugal | 약 8.5 | tens of thousands |
Warnings and evacuation
The 2004 Indian Ocean tsunami killed nearly 230,000 people in regions without a warning system, prompting UNESCO's Intergovernmental Oceanographic Commission to build new networks for the Indian Ocean and the Mediterranean. The Pacific Tsunami Warning Center has operated in Hawaii since 1949, and deep-ocean pressure sensors (DART buoys) now detect sea-level changes in real time. If you are on the coast and feel strong shaking, or see the sea retreat suddenly, do not wait for an official alert: move at once to high ground or the upper floors of a sturdy building. Japan's Sanriku coast teaches "tsunami tendenko" — everyone flee on their own, immediately.
FAQ
- Is a tsunami the same as a tidal wave or storm surge?
- No. Tsunamis have nothing to do with tides; they are caused by sea-floor displacement. A storm surge is pushed ashore by the low pressure and winds of a typhoon or hurricane.
- How much warning is there before a tsunami?
- It depends on distance. A tsunami from a nearby earthquake can arrive within minutes, while one crossing the Pacific may take many hours, which is why natural warning signs matter so much near the source.
Sources reviewed: 2026-10-26
References
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- Lay, T., et al. (2005). The great Sumatra-Andaman earthquake of 26 December 2004. Science, 308(5725), 1127–1133.
- Bernard, E., & Titov, V. (2015). Evolution of tsunami warning systems and products. Phil. Trans. R. Soc. A, 373, 20140371.
- UNESCO-IOC. (2019). Tsunami Glossary (4th ed.). IOC Technical Series 85.
- Okal, E. A. (1988). Seismic parameters controlling far-field tsunami amplitudes: A review. Natural Hazards, 1, 67–96.
- Ward, S. N. (2001). Landslide tsunami. Journal of Geophysical Research: Solid Earth, 106(B6), 11201–11215.
- Synolakis, C. E., & Bernard, E. N. (2006). Tsunami science before and beyond Boxing Day 2004. Phil. Trans. R. Soc. A, 364, 2231–2265.
- Simons, M., et al. (2011). The 2011 magnitude 9.0 Tohoku-Oki earthquake: Mosaicking the megathrust from seconds to centuries. Science, 332(6036), 1421–1425.