Tsunamis
A tsunami is a series of waves caused by the sudden displacement of a huge volume of water — most often when a megathrust earthquake lifts or drops the sea floor.
Speed = √(g × depth). Watch the wave slow down and grow taller as the sea shallows.
Fast and low in deep water
In the open ocean a tsunami travels at √(g × depth): about 700 km/h in 4,000 m of water, as fast as a jet. Its height may be under a metre, and ships may not notice it pass.
Slow and tall at the coast
As the water shallows the wave slows, and its energy is squeezed into less depth, so its height grows dramatically. The sea may first pull back and expose the sea floor — a natural warning.
What to do
If you are on the coast and feel strong shaking lasting 20 seconds or more, or see the sea withdraw, move to high ground or inland immediately. Don’t wait for an official warning, and stay away until authorities give the all-clear — later waves can be larger.