Tsunamis: when an earthquake strikes the sea
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The worst earthquake disasters of recent times were caused not by the shaking itself but by the water that followed. A tsunami is the ocean's answer to an earthquake — and it can cross an entire ocean.
How a tsunami forms
When a large quake (typically magnitude 7+) happens beneath the seabed and the rupture lifts or drops the sea floor vertically, the entire column of water above it is pushed along. That displaced mass of water starts moving as waves — the tsunami. Most form in subduction zones, where one plate dives beneath another.
Invisible at sea, enormous at the coast
In deep water a tsunami travels at up to 700–800 km/h — airliner speed — yet is often less than a metre high, so ships barely notice it. When the wave reaches shallow water it slows down and the energy is forced upwards: the wave rises and can hit the coast as a wall of water metres high, or as a rapidly rising flood that keeps coming for minutes.
The warning signs
- A long, strong earthquake near the coast — if you can barely stand and the shaking lasts long, head for higher ground immediately
- The sea suddenly withdrawing, exposing the seabed — that is the wave trough arriving first
- An unusual roar or rumble from the sea
Never wait for an official warning if you sense the signs yourself — and remember a tsunami is several waves: the first is often not the biggest. Also read the general earthquake safety rules.
Monitoring today
Since the 2004 Indian Ocean disaster, the world's warning systems have been massively expanded with seismographs and ocean buoys. Large undersea quakes are detected within minutes — you can watch them appear on EarthquakeIndex's live map, where undersea quakes along the subduction zones are an everyday sight.