Water And Oceanography

How Tsunamis Form from Earthquakes



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Tsunami, from a Japanese word meaning "harbor wave," for those living in coastal areas, especially on the Pacific rim, conjures up nightmares and cold sweats. These towering tidal waves, coming with little warning, can change the shape of coastlines, and, like the tsunami that swept parts of Southeast and South Asia in December 2004, can kill hundreds of thousands of people and wipe human settlements out of existence.

Tsunamis are created by disturbances, such as undersea earthquakes or meteorite strikes on the ocean surface. The 2004 tsunami was caused by a tremor off the coast of Indonesia. When tectonic plates beneath the surface of the sea shift, they push the water above them up, causing a tremendous transfer of energy. Nature abhors a vacuum, so as water in one part of the ocean rises, it falls in others, causing tides to apparently rush out to sea, leaving vast expanses of exposed sea bottom along the shore. This, in fact, is one of the first signs of an impending tsunami - when the tide suddenly goes out, and goes out farther than normal.

As the water, under the influence of gravity, seeks to find a stable position, waves form. The size of these waves depends initially on the amount of water originally displaced. The waves radiate out from the point of origin at tremendous height and speed. Tsunamis start out small, but can reach heights of approximately 100 feet as they reach shallow water along coastlines; at least one 300 plus foot wave has been reported. In the open ocean, the waves travel at the speed of a jet plane. As they reach shallow water, they slow, with an average speed around 45 mph.

The energy in a tsunami is tremendous, and the destruction that can be wrought by one of these giant waves is beyond normal human imagination. Imagine if you will, a 100 foot high wall of water moving at 45 miles per hour. The weight of that volume of water is in the megaton range, and the destruction it can cause is roughly equivalent to a high-energy weapon. In fact, unlike bombs, a tsunami doesn't leave debris. As the water, continuing to seek stability, rushes back out to sea, it usually takes the debris with it. The actual death toll, for instance, of the 2004 Southeast Asia tsunami will probably never be known, as it washed whole villages out to sea as it withdrew. Officials can only estimate how many people actually perished in remote coastal areas where there were no survivors to report the event. Before and after satellite photos of the coast of Thailand illustrate the power of a tsunami. In some areas, the outline of the coast was radically altered, with islands and peninsulas disappearing, and new landforms appearing.

Because of the large number of active undersea volcanoes and frequently shifting tectonic plates in the Pacific Ocean, this area is subject to a large number of tsunamis annually, throughout the Pacific rim, including Alaska and Hawaii.

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