tsunamis (sometimes called seismic sea waves) are large sea waves that are created by underwater earthquakes, volcanic eruptions, or even nonseis-mic events such as landslides and meteorite impacts. Tsunamis are also known as tidal waves, even though this is a misnomer because the waves have nothing to do with tides. The word tsunami is a Japanese word meaning "harbor wave." Tsunamis are not easily seen on the open water, as they have extremely long wavelengths on the order of tens of kilometers. The speed of the wave is directly related to the depth of the water; therefore, as the water depth decreases, the tsunami moves slower. As the waves propagate toward the coast, the speed will decrease, but the amplitude or the height of the waves can achieve extraordinary levels. Tsunamis lose energy as they approach the coast, but they still have incredible amounts of energy, as they often cause beach erosion and undermine trees and other types of coastal vegetation. The fast-moving water is capable of flooding several hundreds of meters inland, well above normal flood levels, and destroying buildings and other structures. Tsunamis can extend to heights well above sea level, in extreme cases sometimes as high as 30 m. or 100 ft.

Volcanic activity and earthquakes are the prime causes of tsunamis. When the seafloor starts to buckle, the overlying water will begin to displace. As the seafloor rises and sinks, the displaced water will form waves because of the effects of gravity. Most of the major earthquakes occur at plate boundaries.

There are three different types of plate boundaries. A divergent boundary takes place where two plates move away from each other. At this type of boundary, volcanoes will form, out of which molten material will flow. Also, weaker, shallow-focus earthquakes can occur along these boundaries. Divergent boundaries are very common in the midocean such as at the Mid-Atlantic Ridge, the East Pacific Ridge, the Mid-Indian Ridge, and the Southeast-Indian Ridge. Convergent boundaries occur where two plates moving in opposite directions collide. One plate will be denser and will subduct underneath the other. These subduction zones are a very common location for earthquake activity. There are three different types of convergent boundaries: oceanic-continental convergence, oceanic-oceanic convergence,

and continental-continental convergence. At an oceanic-continental convergent boundary, the oceanic crust is denser and will subduct underneath the continental crust. Volcanoes will form along the continental boundary, whereas deep trenches will form off the coast. Shallow-focus earthquakes often form along these subduction zones, such as along the west coast of South America. At an oceanic-oceanic convergent boundary, two ocean plates collide, forming a volcanic island arc on the ocean floor. Examples of this type of boundary include the Aleutian Islands, the Mariana Islands, and Japan. At a continental-continental convergent boundary, two continental plates collide, typically forming huge mountain ranges such as the Himalayas or the Alps. Under this type of convergence, volcanic activity is rare, but earthquake activity is very common. The final type of boundary is called a transform boundary, where two plates slide past each other. Transform boundaries occur along vertical fractures called faults, which are noted for great magnitudes of earthquake activity. Most faults are found near midoceanic ridges, but they can also extend through continents, as evidenced by the San Andreas Fault in California.

Tsunamis can be formed by anything that displaces a large volume of water from its equilibrium state. When earthquakes or volcanoes generate tsunamis, water is displaced as a result of the uplift or subsidence of the seafloor and water column. Sometimes submarine landslides, which are common with large earthquakes as well as volcanic collapses, can displace great volumes of water. However, these types of disturbances disturb the water from above, rather than from below. Tsunamis derived from these types of mechanisms usually do not last long and have minimal impacts on the coastlines.

The most recent deadly tsunami was the Asian tsunami that occurred after the 2004 Indian Ocean earthquake on December 26 of that year. The epicenter, which is the location at the Earth's surface directly above the focus of the earthquake, took place off the coast of Sumatra, Indonesia. The magnitude of this earthquake has been estimated to be between 9.1 and 9.3 on the Richter scale (the scale devised to estimate the amount of energy released in an earthquake). This made it the fourth most powerful recorded earthquake since 1900. The earthquake lasted almost 10 minutes and was the second most powerful ever recorded on a seismograph (an instrument that measures the seismic waves from an earthquake). Waves were reported as high as 30 m. or 100 ft., and it was the deadliest in recorded history, with an approximate 230,000 lives lost, mostly in the countries of Indonesia, India, Sri Lanka, and Thailand. Before this, the 1782 Pacific Ocean tsunami was the deadliest in recorded history, with an estimated 40,000 casualties in the South China Sea. Other powerful tsunamis include the 1883 tsunami following the eruption of Krakatoa, a volcanic island in Indonesia, and the 1908 tsunami that occurred in the Mediterranean Sea near Messina, Italy.

There has been speculation about possible effects of global warming after the 2004 tsunami, with proponents suggesting that the increase in average temperature allows the atmosphere and the oceans to gather energy, which may cause more earthquake activity. Critics, however, claim that if this were the case, there would be more of a correlation between El Niño and tsunamis, as El Niño warms the ocean over an active region with many plate boundaries. Therefore, links to global warming and tsunamis are unsubstantiated because it is difficult to associate what is happening at the surface of the ocean with the depth at which the focus of the earthquakes takes place.

sEE ALso: Climate Change, Effects; Oceanic Changes.

BIBLioGRAPHY. Patrick L. Abbott, Natural Disasters, 6th ed. (McGraw Hill, 2007); Thomas A. Easton, Taking Sides (McGraw Hill/Dushkin, 2005); Tom McKnight and Darrel Hess, Physical Geography (Prentice Hall, 2005).

Kevin Law Marshall University

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