A tsunami is a series of long waves caused by the sudden displacement of a large volume of water. Most destructive tsunamis begin with large, shallow earthquakes beneath or near the ocean, especially where one tectonic plate moves beneath another. Landslides, volcanic eruptions and, very rarely, meteor impacts can also generate them.
Tsunamis are not ordinary surf and are not caused by tides. In deep water they may be low in height but travel across an ocean at speeds comparable with a jet aircraft. As water becomes shallow near a coast, the waves slow, shorten and grow.
How an earthquake creates a tsunami
An earthquake must move the seabed vertically enough to lift or drop the water above it. The disturbance spreads outward as a train of waves. A large earthquake that mainly slides horizontally may generate little tsunami, while seabed shape and the direction of rupture can focus energy toward some coasts.
Not every undersea earthquake creates a dangerous tsunami. Monitoring centres combine earthquake data, deep-ocean pressure sensors and coastal tide gauges to estimate whether waves formed and where they may arrive.
What happens when a tsunami reaches shore?
Wave speed depends mainly on water depth. The front slows as it enters shallow water while energy from behind continues to arrive. Water can surge inland as a rapidly rising flood, a wall of turbulent water or repeated powerful currents.
The first wave may not be the largest. The water can remain dangerous for hours, and debris makes the flow more destructive. Harbours and bays can amplify waves depending on their shape.
Natural tsunami warning signs
- A strong earthquake that makes standing difficult, or shaking that lasts a long time.
- A sudden, unusual rise or fall of coastal water.
- A loud roar from the ocean.
If you are in a tsunami hazard zone and observe a natural warning, move to high ground or inland immediately. Do not wait for an official alert, do not go to the shore to watch and follow signed evacuation routes. Return only when local authorities say it is safe.
Why “tidal wave” is the wrong term
Tides are driven by gravitational interactions among Earth, the Moon and the Sun. Tsunamis result from displaced water and can occur at any stage of the tide. Calling one a tidal wave hides its real cause and can create confusion.
Compare the mechanisms in our explainers on what causes tides and wind-driven ocean waves.
Can tsunamis be predicted?
Scientists cannot predict the exact time and location of an earthquake far in advance. Once an earthquake happens, warning systems can detect it, model possible waves and update forecasts with ocean observations. Near the source, waves may arrive within minutes, so natural warnings and prepared communities remain essential.
Frequently asked questions
Can a tsunami happen in the Mediterranean?
Yes. The Mediterranean has active faults, volcanoes and submarine slopes. Coastal communities should use national and local hazard guidance.
Is it safe to watch from a beach?
No. Even a wave that looks small can produce fast, debris-filled currents and later waves may be larger.