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How a Tsunami Grows
a scientific process or phenomenon
In deep water, these waves travel as fast as a jet aircraft, yet a ship at sea may hardly notice them, because their height is often less than a metre.
Because the first sign of a tsunami on shore is sometimes the sea drawing back far beyond the usual low-tide line, people in coastal areas are taught to treat a sudden retreat of the sea as a signal to move to higher ground at once.
Most ocean waves are driven by the wind and disturb only the top few metres of water. A tsunami is different: it is set in motion when a huge volume of water is suddenly pushed out of place, most often by an earthquake beneath the sea floor.
That changes as the waves approach the coast. In shallower water they slow down, the waves behind catch up with those in front, and the water piles up, sometimes to more than ten metres.
When such an earthquake lifts or drops a section of the seabed, the whole column of water above it moves too. The disturbance spreads outward in all directions as a series of waves whose crests may be hundreds of kilometres apart.