2004 Tsunami Relief

How a Tsunami Occurs

  1. An earthquake, volcanic eruption, or landslide occurs beneath the ocean (submarine). A tsunami could also be caused if an asteroid struck the earth.
  2. Great ocean waves are started and roll along the ocean floor as fast as a jet plane. In the open ocean, tsunamis may have wavelengths of up to several hundred miles and travel at speeds up to 450 mph (720 kph), yet have wave heights of less than three feet (one meter), which pass unnoticed beneath ships at sea.
  3. The period between crests of tsunami waves varies from five minutes to about one hour.
  4. When these great ocean waves reach shallow coastal waters, they are slowed, causing their length to shorten and their height to form waves about 100 feet (30 meters) high.
  5. The first indication is often a sharp swell, followed by a sudden outrush of water that often exposes offshore areas as the first wave trough reaches the coast.
  6. After several minutes, the first huge wave crest strikes and rushing inland to flood the coast. Generally, the third to eighth wave crests are the largest.
  7. When they break, they often destroy piers, buildings, and beaches and take human life. The wave height as they crash upon a shore depends almost upon the submarine topography offshore.

Tsunami came into English at the end of the 19th century. It is a Japanese word from two elements, tsu "harbor" and nami "waves." A tsunami is series of great ocean waves created by submarine earth movements caused by earthquakes, volcanic eruptions, or landslides beneath the ocean. A tsunami may also be caused by an asteroid striking the earth. The plural is tsunamis or tsunami and the adjective form is tsunamic. (Dictionary.com entries for tsunami.)

The most destructive tsunami before the one on December 26, 2004 occurred following the eruption of the volcano Krakatoa in the East Indies on August 27, 1883, when over 36,000 people were killed. The U.S. Geological Survey site offers complete information for those interested in learning more about this and previous tsunamis.