A wave that is all the way down
Why Are Tsunamis So Dangerous?
A tsunami moves the entire depth of the ocean, not just the surface.
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Step 1 of 5
Ordinary waves are made by wind.
Wind pushes the surface. Just a few metres down the water is barely disturbed — which is why divers do not feel storms above them.
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Step 2 of 5
A tsunami is made by the seabed moving.
An earthquake lifts or drops the ocean floor, displacing the entire column of water above it — from seabed to surface, all at once.
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Step 3 of 5
In deep water it is nearly invisible.
Out at sea it may be under a metre high but hundreds of kilometres long. Ships pass over it without noticing anything.
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Step 4 of 5
It crosses oceans at jet speed.
Because it involves the full depth, it travels around 800 km/h in deep water — roughly the speed of an airliner.
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Step 5 of 5
Shallow water makes it rear up.
As the seabed rises, the front slows while the back keeps coming. All that energy compresses into much less depth, so the height climbs sharply.
If the sea suddenly draws far out, that is a trough arriving first. It is a warning to run inland, not to look.
The short version
A tsunami moves the entire ocean depth, so it carries vastly more water than its height at sea suggests.
Try it yourself
Push a tray of water from underneath rather than blowing on the surface. The whole body moves — that is the difference.
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