Tsunamis have occurred on many occasions all around the world. Volcanic eruptions, landslides, earthquakes, and other natural disasters have caused these massive waves. A tsunami is a towering ocean wave generated by tectonic displacements such as undersea earthquakes, landslides, cosmic collisions, or volcanic eruptions. There is a common misconception that a tsunami is a tidal wave. Although the impact of a tsunami on a coastline may change depending upon the tidal level at the time when one occurs, tsunami waves are unrelated to tides. In the open, the water level may rise and fall several feet as a tsunami passes by. But as a tsunami approaches the coastline, it can form a deadly wall of water that rises more than 100 feet high.
The most frequent cause of tsunamis is earthquakes. A tsunami that is caused by an undersea earthquake is also called seismic sea wave. When the sea floor abruptly deforms and vertically displaces the overlying water a tsunami can be generated. Consequently, as the displaced water mass tries to return to its original position, waves are created. On July 9, 1958, the largest tsunami waves produced by a landslide occurred at Lituya Bay in Alaska. The tsunamis of Krakatoa have been the cause of over 36,000 human fatalities. Tsunami waves that have been responsible for crossing entire oceans have almost always come from submarine faulting associated with extremely large earthquakes. Tsunami-producing earthquakes usually exceed 6.5 on the Richter scale. Since 1819, about 40 tsunamis have struck the Hawaiian Islands. Most tsunamis occur in the Pacific Ocean because of the seismic activity around its perimeter.
As the water depth decreases, the tsunami slows whereas the energy current stays almost constant despite its being reliant on both the wave speed and height. Tsunamis usually have wavelengths equal to the water depth (up to several thousand meters) at the point of origin. When tsunami waves finally encounter land, they begin to loose energy as they run up the shore. The shape of the near-shore seafloor, or local submarine topography, has an extreme effect on how tsunami waves behave. If a reflected wave interferes with another wave, large wave heights may occur at unexpected locations. The first sign of a tsunami would typically be a significant recede of increase of the water level depending upon which part of the tsunami wave train arrives at the shore first. A wave crest is known for bringing a rise in the water level. Tsunamis are generally very large in bays. This is very different from ordinary waves. The characteristics of tsunamis as they approach shore are greatly affected by wave refraction over the local bathymetry. The height and intensity of a tsunami wave+s impact on the shoreline may be significantly reduced if an extensively developed coral reef exists close to shoreline. Many people are confused about the difference between tsunamis and other waves. Although they are both waves, tsunamis are indeed very different from the wind-generated type. Wind -generated waves are predominately thought of as shallow-water waves with long periods and wavelengths; frequently viewed at a local beach or coastal beach. Tsunamis are on a much bigger scale. When the ratio between the water depth and its wavelength becomes very small a wave becomes a shallow-water wave. A strong earthquake would usually generate a tsunami capable of traveling vast distances.
The tremendous destruction from tsunamis can be produced in several ways. After a tsunami has hit, a city can be left devastated. Tsunamis have left beaches and coasts destroyed. Our best defense from tsunamis comes from detection systems. Scientists can calculate where and when a given undersea earthquake occurred and predict when the resulting seismic sea wave will reach shore. Seismic waves travel much faster than a tsunami travels through the water. The speed of a tsunami depends on the depth of the water. The tsunami warning system bases itself on the relationship between tsunamis and earthquakes. Most of the Pacific-wide tsunamis are created due to severe faulting on the ocean floor, and an earthquake magnitude of 6.5 or greater is almost always accompanied by the generation of tsunami. If the epicenter is located close enough to the ocean, a Tsunami Watch is declared. The arrival time of a tsunami has direct correlation to their wavelength. When the water depth is less than 1/20 if the wavelength the form is known as a shallow-water wave. Its speed is determined solely by the depth of the water. Because tsunamis occur in the Pacific, a basin less than 3 miles deep, they are considered shallow-water waves. One of the most memorable tsunamis in history to ever occur hit Hawaii on April 1, 1946. Today the Tsunami Warning System of the Pacific has grown.