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Last updated on April 23, 2014 at 11:40 EDT

What is a Tsunami?

February 14, 2013

Hi, I’m Emerald Robinson, and in this “What is” video, we’re going to discuss Earth’s most powerful waves: tsunamis.

A tsunami is a series of fast moving waves triggered by an underwater shock. Tsunamis are usually caused by earthquakes or volcanic activity, but landslides or underwater explosions can also be the cause.

Tsunamis were once called tidal waves, but this name isn’t very accurate because tsunamis aren’t related to earth’s normal tides. Instead, tsunamis form when water is somehow displaced by force. Let’s look at an example.

Shifting lava beneath the earth’s crust can cause the ocean floor to bulge, and then suddenly flatten. As this occurs, energy is released into the water, creating a series of waves. In deep water, the waves have a small height, or amplitude, a very long length, and move at an extremely high rate of speed – up to 500 miles per hour. Because these waves are usually less than a foot high, tsunamis are barely noticeable at this stage.

Tsunamis gain their power when they get close to shore. As the water gets shallower, the rapidly moving waves are forced to suddenly slow down, and the energy that has been carrying the tsunami causes the wave to increase in amplitude. Most tsunamis reach heights of about 10 feet, but some can be up to 100 feet tall. When a tsunami makes landfall, it usually takes the form of a sudden and rapidly rising tide that moves at about 50 miles an hour.

Tsunamis are very unpredictable. Not all undersea earthquakes cause tsunamis, and about 75% of tsunami warnings turn out to be false alarms. Still, scientists continue to monitor areas of high earthquake or volcanic activity such as the Pacific Ocean’s Ring of Fire, where about 85% of the world’s tsunamis occur. Their studies will improve our ability to predict these natural disasters and efficiently warn areas at risk.