Tsunami and shallow water waves

Q: How fast is a tsunami?

In the deep ocean it can exceed 700 km/h, slowing near the coast.

Q: Why is it so destructive?

Its energy is spread through the whole water column, and it can carry a large amount of water inland.

Q: Why are tsunamis so fast in deep water?

Their speed depends on the depth, so a tsunami in deep ocean can travel at jet speeds, while it slows and grows near the shore.

Advanced

Speed depends on depth.

Key relationv = √(gh)

In the deep ocean a tsunami has a small amplitude (< 1 m) and huge wavelength (200 km); its speed √(gh) ≈ 200 m/s at 4 km depth. Near the shore the wave slows and the amplitude grows enormously.

Worked example

The 2004 Indian Ocean tsunami crossed the ocean in hours.

Common mistake

Energy conservation: slower speed gives a higher wave.

In more detail

Tsunamis are long waves with very long wavelengths that travel fast in deep ocean, slowing and growing taller in shallow water. Their speed depends on the square root of the water depth.

Real-world uses

Tsunami warning systems use deep-ocean sensors to detect waves early.
Coastal planners account for run-up when designing defences.
Tsunami warning systems estimate how fast a wave travels across the ocean from the depth of the sea floor.

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Frequently asked questions

How fast is a tsunami?