Science Puzzle
Why Don’t Ships Notice a Tsunami?
On 26 December 2004 an earthquake under the Indian Ocean set off a tsunami that struck coasts thousands of kilometres away, with waves more than 10 metres high in places. Yet ships out in the open ocean barely noticed it pass underneath them.
Why doesn't a ship at sea notice a tsunami?
The Answer
Out in the deep ocean a tsunami is very long and very low. It only grows tall when the shallow water near a coast slows it down.
A tsunami isn't like a wind-blown wave, which only stirs the top few metres of the sea. An earthquake heaves up the sea floor, so the whole depth of water, several kilometres of it, is set moving. In deep water the wave may be less than a metre high but hundreds of kilometres from one crest to the next. A ship rises by less than a metre over several minutes and gently sinks back. Nobody on board feels a thing.
A wave as long as that travels at a speed set by the depth of the water: the deeper the sea, the faster it goes. Over 4,000 metres of ocean a tsunami races along at 700 to 800 km/h, as fast as a jet airliner. As it reaches shallower water near the coast, the front of the wave slows down while the back is still moving fast and catches up. Energy that was spread through kilometres of deep water is squeezed into a shorter wave in shallow water, so it rears up, sometimes to tens of metres.
For a ship near a harbour when a tsunami warning comes, the safest move is often to head out to deep water, not in.
The principle: Shoaling. A long wave's speed depends on the depth of the water, so as it reaches shallow water it slows, bunches up and grows taller.