SEED Science

The Earth — A Living Planet



Images courtesy USGS

Click image for simulation of tsunami moving cross the Indian Ocean

Wave simulator

Click image here for simulation showing a tsunami similar to the one that occurred in the Indian Ocean on December 26, 2004.

On December 26, 2004 an earthquake jolted the ocean floor off the coast of Sumatra. The quake, registering 9.0 on the Richter scale, was the fourth strongest earthquake recorded in the world since 1900. The result was a devastating tsunami that raced across the Indian Ocean devastating coastal areas in Thailand, Malaysia, Indonesia, Sri Lanka and India. The effects were felt as far away as the east cost of Africa. What is a tsunami?

A tsunami is a large destructive ocean wave, or a series of waves, resulting from sudden vertical movement of the Earth’s crust. The most common cause is an earthquake occurring on the ocean floor. However, a tsunami may also be caused by volcanic action, a landslide, an explosion, or even a meteorite landing in the ocean.

The word tsunami (pronounced tsoo-NAH-mee) means "harbor wave" in Japanese. Some people call tsunamis "tidal waves", but this term is misleading. Tsunamis are not related to tides. The level of the tide at the time a tsunami reaches land has some effect on how much damage takes place, but that is the only connection.


Banda Aceh

Photo © DigitalGlobe, used with permission

Satellite images of a coastal village in Banda Aceh, Indonesia, before and after the December 26, 2004 tsunami. Click for larger version of image.


Another name for tsunamis is "seismic waves." This term is not accurate either, since earthquakes are not the only events that cause these waves.

When an earthquake or other major disturbance causes a section of the ocean floor to rise or sink abruptly, the mass of water above the affected area is also suddenly displaced. As the water tries to regain its equilibrium, waves are formed. Because of the depth of the water, the wavelengths of these waves are much greater than that of normal waves that occur where the ocean meets the land. A tsunami wave may be hundreds of kilometers (or miles) in length. It moves out in a circle from the spot where it started, similar to the way circles of water move out from the place where a rock has been dropped into a pond.

Because of the large size of its wavelength, a tsunami moves at high speed across great distances without losing much of its energy. In deep ocean, the surface swells marking the wave are hardly noticeable. In fact, they are often less than one meter (three feet) high. However, as the wave approaches land and moves into more shallow water, it slows down and increases in size. The features of the land beneath the water influence its actual height. The waves generated by the eruption of Krakatoa in 1883 reached a height of 37 meters (120 feet). In 1737, a tsunami estimated to be 64 meters (210 feet) high struck Cape Lopatka in northeast Russia.


Coast of India

Photo Courtesy NASA

Satellite images of the east coast of India.
Left Image: 10 December 2004.
Right Image: 29 December 2004.
(White areas in both images are clouds.)

In the open ocean, a tsunami can travel as fast as 700 km/hour (435 mph), approximately as fast as a jet aircraft. It loses speed as it approaches land, but it does not lose much energy. As its speed diminishes, the height of the wave increases. When it reaches land, it may behave like a series of breaking waves, a rapidly rising and falling tide, or a large, powerful wave called a bore. The tremendous energy of the wave can cause great quantities of water to surge inland, far beyond where even the highest of high tides would commonly reach.

Ordinary waves—the type that normally occur at an ocean beach or lakeshore—are generated by the wind. These waves, which typically roll in one right after another, might last about ten seconds and have a wavelength of 150 m (490 feet) or so. In contrast, when a tsunami wave reaches shore, its wavelength might be more than 100 km (62 miles), and it might last for more than an hour.

A tsunami moving onto the shore is usually traveling at about 70 km/hour (45 mph) with tremendous force that can strip sand off beaches, undermine trees, and crush buildings. People and boats are powerless against its turbulence. The quantity of water carried inland is capable of flooding vast areas of normally dry land.