Tsunamis

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, [Page 985]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.

...