What are they and why do they occur?

Tsunami, a Japanese word meaning “harbor wave,” is a wave in the ocean or lake created by a geologic event. Often a tsunami is incorrectly referred to as a tidal wave, which, strictly speaking, describes the periodic movement of water associated with the rise and fall of the tides. The term tsunami was adopted for general use in 1963 by an international scientific conference.

Oceanographers call tsunamis seismic seawaves because they are usually caused by earthquakes, landslides or marineslides under or near the ocean. These push the water upward, sideways or downward to create the tsunami waves. Volcanic eruptions can also cause tsunamis. They are more common in the Pacific Ocean.

A tsunami is not a single wave, but a series of waves that can travel across the ocean at speeds of more than 500 miles an hour. In the deep ocean, hundreds of miles can separate wave crests; many people have lost their lives during tsunamis after returning home thinking the waves had stopped.

As the tsunami enters the shallows of coastlines in its path, its velocity slows but its height increases. A tsunami that is just a few centimeters or meters high from trough to crest can rear up to heights of 30 to 50 meters as it hits the shore, striking with devastating force.

For those on shore there is little warning of a tsunami’s approach. The first indication is often a sharp swell, not unlike an ordinary storm swell.

In 1883, a tsunami following the eruption of Krakatoa volcano between the Indonesian islands of Java and Sumatra killed 36,000. The tsunami’s passage was traced as far away as Panama.

In July 1998, two undersea quakes measuring 7.0 created three tsunamis that killed at least 2,100 near the town of Aitape on the north coast of Papua New Guinea.