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Tsunamis

Tsunamis


Tsunamis are large seismic sea waves that can cause major destruction in coastal regions. A tsunami (Japanese for "wave in bay") is caused by underwater seismic activity, such as an earthquake .

While tsunamis are commonly called "tidal waves," this is an erroneous term; these potentially catastrophic waves have nothing to do with the tides. Tides are the up and down movements of the sea surface at the shore, caused by the gravitational attraction of the moon and sun on our marine waters. Tsunamis are caused by the movements of Earth's crustal plates. Tides rarely cause major damage unless they are associated with a storm, while tsunamis can cause major loss of life and property.

It has been known for several hundred years that tsunamis are caused by seismic movements of the ocean floor. This occurs most commonly during submarine earthquakes, underwater landslides, and perhaps volcanoes, all of which release large amounts of energy. The sudden movement of the earth's crust caused by an underwater earthquake, for example, displaces or moves the water above it. This causes a high-energy wave to form, which is passed rapidly through the water.

Tsunamis are very long waves, with a period (the time for one complete wave to pass a fixed point) ranging from six to 60 minutes. These waves typically travel 450 mi/h (200 m/s or 720 km/h). Therefore, an earthquake in the Gulf of Alaska could result in a tsunami hitting Hawaii less than five hours later.

It is almost impossible to feel a tsunami out at sea in deep water, however, the form of the wave changes when it reaches shallow water. Since the water is shallower, the bottom of the wave begins to "feel" the ocean bottom. The friction that results causes the wave to slow down from about 450 mi/h (200 m/s) in very deep ocean water to 49 mi/h (22 m/s) in water 164 ft (50 m) deep. While the front part of the wave has been reduced in speed, the part at sea is still moving in quickly. As a result, the energy of the wave is compressed. As the wave enters shallow water, like that in a bay, the crest rises. It quickly builds up vertically as the wave moves onto the shore. This wall of water can be more than 100 ft (30.5 m) high, in extreme cases. Since gravity is acting on this huge wall of water, it cannot support itself and crashes or breaks onto the land, similar to a normal breaker in the surf zone of a beach. However, the huge amounts of energy released by a breaking tsunami are many times greater and more destructive than an ordinary breaker at the beach, and the tsunami can literally destroy anything in its path.

In Japan, where some of the most destructive tsunamis have occurred, there have been cases in which whole fishing villages were devastated. However, the fishermen, who were at sea plying their trade, did not feel the wave, which passed right under them. They did not discover the disaster until they returned home and found their homes and villages destroyed. Because these villages were often located within shallow bays, and the fishermen, being at sea, did not experience the wave, they assumed that the tsunami arose within the bay. Therefore, these waves were called tsunamis or "wave in bay."

While scientists are not yet able to predict submarine seismic activity with much accuracy , they can easily measure such events when they occur. Scientists use this information about the causes of tsunamis to predict when these destructive sea waves will occur. This is extremely important in reducing loss of life and property. After the destructive 1946 tsunami that hit Hawaii, a group of tsunami early warning stations was set up to monitor seismic activity throughout the Pacific Ocean. The geographical and administrative center of this monitoring system is in Honolulu, Hawaii. When an earthquake, underwater volcano , or landslide is sensed, its location is pinpointed. If a wave is generated and a change in the water height is measured at a nearby tide-measuring station, scientists can then accurately calculate the speed of the wave to determine when the wave will make landfall. The appropriate agencies can be alerted, and if necessary, evacuations and other preparations can be made. This early warning system has been very successful in reducing damage caused by tsunamis. For example, there were no deaths from a tsunami in Hawaii in 1957 because of an early warning, even though the tsunami was over 26 ft (8 m) tall.

Before the warning systems existed, the first indication of an approaching tsunami was the rapid movement of water in a bay out to sea. This exposed areas of the bay bottom that were rarely or never exposed. The water that rushed offshore rose to build the huge crest of the wave that would crash down a few minutes later.

Despite the success of the early warning systems, there are some problems. For example, not all seismic activity generates tsunamis; they more commonly result from shallow focus earthquakes, where the actual point of crustal movement is closer to the surface. It is during these earthquakes that major crustal movement is most likely. Deep focus earthquakes, which can be very strong but often result in less crustal movement, are less likely to trigger tsunamis. It has been estimated that only one out of ten large underwater earthquakes causes damage. In addition, the chances of a tsunami hitting any one spot directly and causing major damage are relatively small because the energy in the form of the tsunami is not passed along equally in all directions. Finally, there may be other factors that reduce or enhance chances of a tsunami striking. For example, major tsunamis are rare in regions with wide continental shelves, such as the Atlantic coast of the United States. A wide continental shelf is thought to both reflect the wave (with the energy being sent back out to sea) and absorb some of the energy of the wave through friction as it drags along the bottom. Thus, the early warning system, while essential, often gives false alarms.

Tsunamis are more common in the Pacific Ocean than other oceans of the world, primarily because there is so much seismic activity at the perimeter of the Pacific Ocean, where crustal plates meet. Thus, this region is where some of the world's most damaging tsunamis have occurred. For example, one of the most dramatic and destructive tsunamis occurred on August 27, 1883, when the volcanic island of Krakatoa , located in the Pacific Ocean, between Sumatra and Java, exploded and disappeared. The wall of water reached over 98 ft (30 m) in height and left catastrophic damage to coastal areas in the Sundra Strait in its wake. Over 36,000 people lost their lives. The energy from the tsunami was still measurable after it crossed the Indian Ocean, moved around the southern part of Africa, and headed north through the Atlantic Ocean into the English Channel. In addition to the tsunami, the sound of the explosion was heard 3,000 mi (4,827 km) away, and the dust that entered the atmosphere caused unusual sunsets for almost a year.

In 1896 there was a major tsunami in Japan that killed 27,000 people along the coast. In 1964, there was an earthquake in Alaska and the resulting tsunami caused major damage in some ports such as Kodiak and Seward. In addition, the tsunami traveled to the south, where four and a half hours later, despite warnings, it killed additional people and did major damage in Crescent City, California. A total of 119 people died and damage to property amounted to over $100 million.

Tsunamis also occur along the chain of Caribbean islands and in the Mediterranean. Both of these places, like the Pacific, are at the edges of Earth's crustal plates where earthquakes and other seismic activity are common.

[Max Strieb ]


RESOURCES

BOOKS


Knauss, J. A. Introduction to Physical Oceanography. Englewood Cliffs, New Jersey: Prentice-Hall, Inc., 1978.

Neshyba, S. Oceanography: Perspectives on a Fluid Earth. New York: John Wiley and Sons, 1987.

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