*EPF209 12/28/2004
Only Largest Earthquakes Generate Tsunami Waves
(In the last decade, destructive tsunamis have hit nine countries) (1120)

By Cheryl Pellerin
Washington File Staff Writer

Washington -- The monster waves, or tsunamis, that destroyed life and property December 26 in nine countries bordering the Indian Ocean resulted from a magnitude 9.0 earthquake centered off the west coast of northern Sumatra, according to the U.S. Geologic Survey.

A tsunami is a series of large waves usually generated by a sudden, violent undersea disturbance near a seacoast or in the ocean. Earthquakes, landslides, volcanic eruptions, nuclear explosions, and even the impacts of meteorites, asteroids, and comets can generate a tsunami.

Tsunami is a Japanese word that means "harbor wave."

According to geologists, earthquakes and other violent upheavals of the earth are possible because the earth is made up of different layers. From the inside to the outside, the layers are -- a solid inner core, a liquid outer core, and a solid mantle. The solid mantle contains a soft, semi-molten layer called the asthenosphere, the solid lithosphere and the crust.

The lithosphere is made up of massive plates, like pieces of a jigsaw puzzle, that float on the soft asthenosphere and support the Earth's continents and oceans. This theory, called plate tectonics, says that hot currents of molten rock under the plates move them constantly in different directions.

Sometimes the plates are close together and pull apart (spreading boundary) or grind past each other (transform boundary), or one plate slides under another (subduction zone).

The most destructive tsunamis are generated by large, shallow earthquakes with an epicenter or fault line near or on the ocean floor. These usually occur in regions of the world characterized by tectonic subduction along tectonic plate boundaries.

When the plates move past each other under the ocean, they generate large earthquakes that tilt, offset or displace large areas of the ocean floor, from a few kilometers to 1,000 kilometers or more. This disturbs the ocean's surface, displaces water, and generates a series of tsunami waves.

Once a tsunami is generated, its energy is distributed throughout the water column above the earthquake, regardless of the ocean's depth.

Tsunami waves may go unnoticed by ships on the open ocean, but as they approach land and the water shallows, they can grow to great heights and rush far inland. Tsunami waves can travel across the ocean at speeds up to 800 kilometers an hour -- about as fast as a jet airliner travels.

A tsunami is made up of a series of very long waves. The waves travel outward on the surface of the ocean in all directions away from the source area, like ripples caused when a rock is thrown into a pond.

The period of the tsunami waves (the time it takes for two successive wave crests to pass a given point) can range from 5 minutes to 90 minutes. A tsunami's wave crests can be a thousand kilometers long, and from a few to a hundred kilometers or more apart as they travel across the ocean.

On the open ocean, the wavelength of a tsunami may be as much as 200 kilometers, many times greater than the ocean depth, which is on the order of a few kilometers.

Tsunami waves in the deep ocean can travel at high speeds for long periods of time for distances of thousands of kilometers and lose very little energy in the process. The deeper the water, the greater the speed of tsunami waves will be.

Tsunamis arrive at a coastline as a series of successive crests (high water levels) and troughs (low water levels) -- usually occurring 10 to 45 minutes apart. As they enter the shallow waters of coastlines, bays or harbors, their speed decreases to 50-60 kilometers per hour.

For example, in 15 meters of water, a tsunami's speed will be only 45 kilometers per hour. But 100 or more kilometers away, another tsunami wave travels in deep water toward the same shore at a much greater speed, and still behind it there is another wave, traveling at even greater speed.

As tsunami waves become compressed near the coast, the wavelength is shortened and the wave energy is directed upward, increasing their heights considerably. Just as with ordinary surf, the energy of the tsunami waves must be contained in a smaller volume of water, so the waves grow in height.

Even if a tsunami wave was 1 meter or less in the deep ocean, it may grow into a huge 30-35 meter wave when it sweeps over the shore. If the tsunami waves arrive at high tide, or if there are concurrent storm waves in the area, the effects will be cumulative and the inundation and destruction will be even greater.

Not all earthquakes generate tsunamis. It usually takes an earthquake with a magnitude of more than 7.5 to produce a destructive tsunami.

The December 26 devastating earthquake was centered in the town of Padangsidempuan in North Sumatra Province, about 1,300 kilometers northwest of Indonesia's capital Jakarta and had an estimated magnitude of 9.0.

Tsunamis can be generated in any world ocean, inland sea or large body of water, but most occur in and near the Pacific Ocean. This happens because the Pacific covers more than a third of Earth's surface and is surrounded by a series of mountain chains, deep-ocean trenches and island arcs called the "ring of fire," where most earthquakes occur (off the coasts of Kamchatka, Japan, the Kuril Islands, Alaska and South America). Many tsunamis are also generated in seas that border the Pacific Ocean.

One of the largest and most destructive tsunamis ever recorded was generated in 1883 after the explosion and collapse of the volcano of Krakatau in Indonesia. The explosion generated waves that reached 41 meters and destroyed coastal towns and villages along the Sunda Strait in the islands of Java and Sumatra, killing 36,417 people.

Some geologists believe the explosion or collapse of the volcano of Santorin in the Aegean Sea destroyed the Minoan civilization in Greece in 1490 B.C.

In the last decade, destructive tsunamis have occurred in Nicaragua (1992), Indonesia (1992, 1994, 1996), Japan (1993), Philippines (1994), Mexico (1995), Peru (1996, 2001), Papua-New Guinea (1998), Turkey (1999), and Vanuatu (1999).

General and educational material on tsunamis is available at http://www.tsunami.org/

National Weather Service International Tsunami Information Center
http://www.prh.noaa.gov/itic/library/about_tsu/faqs.html

Information about tsunamis and earthquakes from the U.S. Geological Survey (USGS) is available at http://walrus.wr.usgs.gov/tsunami/

USGS tsunami and earthquake links are available at http://walrus.wr.usgs.gov/tsunami/links.html

The Great Waves, a brochure about tsunamis from the National Oceanic and Atmospheric Administration, is available at http://www.prh.noaa.gov/pr/hq/itic/library/pubs/great_waves/tsunami_great_waves_1.html

(The Washington File is a product of the Bureau of International Information Programs, U.S. Department of State. Web site: http://usinfo.state.gov)

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