Water And Oceanography

How Tsunamis Form as a Result of Earthquakes



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A tsunami, Japanese for "great harbor wave," is a seismic sea wave, formed as the result of sudden changes in the sea floor brought on by an earthquake.

There are four basic stages to a tsunami. Although the cycle starts with an earthquake, and sometimes a landslide, this is only the beginning of what can result in a deadly disaster for those on land, perhaps many miles from the site of the original disturbance.

Initiation is the first stage. An earthquake is the shaking of the ground from waves of movement through solid earth. A submarine earthquake has the capability of lifting and then dropping the floor of the ocean causing the displacement of a huge quantity of water. This column of water is lifted up and down and in the process, the potential energy from raising water above sea level becomes a sideways or horizontal movement setting a tsunami in motion.

Almost immediately the wave "splits" with what is termed the "distant" tsunami traveling out to deep ocean and the "local" tsunami traveling to the nearest coast. Each of these is half the height of the original wave. While the local tsunami will arrive on land first the distant tsunami will also reach land eventually with the same characteristics as the local tsunami as it hits land.

Next follows "amplification" whereby the amplitude of the wave changes as it travels over the continental slope on its approach to land. Amplification is a change in the oscillation or back and forth motion of the wave. In this case, the amplification increases at the same time the wavelength decreases causing the leading wave to become very steep. Since the first part of the wave to reach shore is the trough, the characteristic and dramatic action of the sea drawing well back from the shore is the most common warning sign of a tsunami.

The last stage is commonly referred to as "run-up" and much as it sounds describes the events after a tsunami makes landfall. The tsunami travels on shore and achieves a height comparable to its own height above sea level. It is not a curling wave as such, breaking on the shore, but rather more like a strong, fast moving tide, although it is not a tidal wave, having nothing to do with tidal motion. Strong currents and accumulated debris cause most of the damage. After initial landfall, some of the energy is transferred back into the ocean where it scatters depending on the form of the coastline. Some waves may be trapped between formations causing multiple tsunamis often larger than the first wave. These may continue for hours.

Not all earthquakes cause tsunamis. They must be large earthquakes, under or near the ocean and cause a vertical movement of the ocean bed. These earthquakes may go on for minutes rather than seconds giving an opportunity for tremendous amounts of energy to transfer to the water.

A tsunami may not seem very impressive if spotted in the open ocean as the wave is low, often under three feet. At this point, the energy of the wave is evident in the large wavelength, up to several hundred miles. In shallow water the waves become gigantic, 100 feet or more. The highest ever recorded was 1640 feet in July of 1958 in Lituya Bay, Alaska.

In addition, the wave can achieve a speed of 500 mph in the open ocean which is slowed to as low as 45 mph upon reaching the continental shelf transferring this potential energy to the height of the wave.

While all parts of the world can have tsunamis the Pacific is particularly at risk with its many earthquakes associated with the "Ring of Fire." The more earthquakes, the more potential for tsunamis to form in this volatile area.

Thirty-five years ago, seismic monitoring stations and tide gauges were established in 24 countries. These stations can alert those who may be in danger from a resulting tsunami but it is difficult to predict how large the final wave will be. Early warnings can serve to alert those who live in the area of predicted landfall to be wary and perhaps secure or remove vulnerable sea craft. Tide gauges will give the alert as the tsunami is just coming on shore as the water begins to recede.

The final warning that disaster is about to strike is the retreat of the water from the shore in preparation for the peak of the wave to achieve landfall. Those unfortunate enough to view this phenomenon must try to head directly away from shore inland to try and escape the force of the water that is arriving in order to survive.

http://walrus.wr.usgs.gov/tsunami/basics.html

http://www.ussartf.org/tsunamis.htm

http://www.geophys.washington.edu/tsunami/general/physics/earthquake.html (contains simulation footage)

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