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What is a tsunami?

"Tsunami" is a Japanese word meaning "harbor wave." Some people call them tidal waves. But these deadly waves are not related to tides and they are not restricted to harbors. Few words can express the horror these waves can bring.

Tsunami as Waves

Tsunami are deadly ocean waves from the sharp jolt of an undersea earthquake. Less frequently, these waves can be generated by other shocks to the sea, like a meteorite impact. Fortunately, few undersea earthquakes, and even fewer meteorite impacts, generate tsunami.

Wave Height

Tsunami waves have small wave heights relative to their long wavelengths, so they are usually unnoticed at sea. When traveling up a slope onto a shoreline, the wave is pushed upward. As with wind waves, the speed of the bottom of the wave is slowed by friction. This causes the wavelength to decrease and the wave to become unstable. These factors can create an enormous and deadly wave.

How a tsunami forms is shown in this animation: http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::640::480::/sites/dl/free/0072402466/30425/16_19.swf::Fig.%2016.19%20-%20Formation%20of%20a%20Tsunami .

Landslides, meteorite impacts, or any other jolt to ocean water may form a tsunami. Tsunami can travel at speeds of 800 kilometers per hour (500 miles per hour).

A video explanation of tsunami is here: http://www.youtube.com/watch?v=StdqGoezNrY&feature=channel .

Wavelength

Since tsunami are long-wavelength waves, a long time can pass between crests or troughs. Any part of the wave can make landfall first.

In 1755 in Lisbon, Portugal, a tsunami trough hit land first. A large offshore earthquake did a great deal of damage on land. People rushed out to the open space of the shore. Once there, they discovered that the water was flowing seaward fast and some of them went out to observe. What do you think happened next? The people on the open beach drowned when the crest of the wave came up the beach.

Large tsunami in the Indian Ocean and more recently Japan have killed hundreds of thousands of people in recent years. The west coast is vulnerable to tsunami since it sits on the Pacific Ring of Fire. Scientists are trying to learn everything they can about predicting tsunamis before a massive one strikes a little closer to home.

See more at http://science.kqed.org/quest/video/scary-tsunamis/.

Summary

  • Tsunami have relatively low wave heights, so they are not noticeable until they move up a shore.
  • Tsunami have long wavelengths. The time between two crests or two troughs can be many minutes.
  • Tsunami warning systems have been placed in most locations where tsunami are possible.

Practice

Use this resource to answer the questions that follow.

  1. What does the word tsunami mean?
  2. Why has Japan had so many tsunamis?
  3. What causes a tsunami?
  4. How fast do the waves travel?
  5. What happens to the tsunami as it reaches the continental shelf?
  6. How do tsunamis differ from regular waves?
  7. What was the deadliest tsunami ever recorded?
  8. What does the Pacific Tsunami Warning Center do?

Review

1. Why is a wave that is so powerful and tall on land unnoticeable at sea?

2. What should you do if you are at the beach and the water suddenly is sucked offshore?

3. Describe tsunami as waves in the way they travel up a shoreline and may strike as crests or troughs.

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At Grade

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Date Created:

Feb 24, 2012

Last Modified:

Sep 11, 2014
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