Tsunami

 

Cross section of a tsunami as its long waves move through the ocean and compress as they approach the coast.

Tsunami Propagation

Once generated, tsunamis radiate outward in all directions from their source. Unlike wind waves that affect just the surface of the ocean, tsunamis propagate (move) through the entire depth of the ocean, from the surface to the floor. They move at great speeds and have tremendous energy. Large tsunamis can move across entire oceans.

The speed of a tsunami depends on the depth of the water it is traveling through. The deeper the water, the faster the tsunami.

In the deep ocean, tsunamis can move as fast as a jet plane, over 500 mph (800 km/h), and can cross entire oceans in less than a day.

Enlarge tsunami energy/propagation map from the February 27, 2010, Pacific-wide tsunami.

The distance between waves is the wavelength. Because of a tsunami's long wavelengths, which can be hundreds of miles, a tsunami is barely noticeable in the deep ocean and rarely more than three feet (one meter) high.

Mariners at sea will not normally notice a tsunami as it passes beneath their hulls.

As a tsunami enters shallow water near land, it slows down, wavelengths decrease, waves grow in height, and currents intensify. At the shore, most tsunamis slow to the speed of a car, approximately 20 to 30 mph (30 to 50 km/h).

Enlarge tsunami energy/propagation map from the May 22, 1960, Pacific-wide tsunami.

Fast Facts

Tsunami speed can be computed by taking the square root of the product of the acceleration of gravity, which is 32.2 feet (9.8 meters) per second squared, and water depth. In 15,000 feet (4,600 meters) of water, this works out to almost 475 mph (765 km/h).

At rates like this, a tsunami will travel from Alaska's Aleutian Islands to Hawaii in about five hours; or from the Portugal coast to North Carolina in eight and a half hours.

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