KILLER WAVES - TSUNAMI

A tsunami at one time referred to as a tidal wave, is a series of water waves caused by the displacement of a large volume of a body of water, usually an ocean, though it can occur in large lakes. Tsunamis are a frequent occurrence in Japan, approximately 195 events have been recorded. Owing to the immense volumes of water and the high energy involved, tsunamis can devastate coastal regions. Earthquakes, volcanic eruptions and other underwater explosions glacier calvings and other mass movements, meteorite ocean impacts or similar impact events, and other disturbances above or below water all have the potential to generate a tsunami.

Some meteorological conditions, such as deep depressions that cause tropical cyclones can generate a storm surge called a meteotsunami, which can raise tides several metres above normal levels. The displacement comes from low atmospheric pressure within the centre of the depression. As these storm surges reach shore, they may resemble tsunamis, inundating vast areas of land. Tsunami can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Tectonic earthquakes are a particular kind of earthquake that are associated with the Earth's crustal deformation. When these earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position. More specifically, a tsunami can be generated when thrust faults associated with convergent or destructive plate boundaries move abruptly, resulting in water displacement, owing to the vertical component of movement involved. Movement on normal faults will also cause displacement of the seabed, but the size of the largest of such events is normally too small to give rise to a significant tsunami.Tsunamis have a small amplitude offshore and a very long wavelength ,which is why they generally pass unnoticed at sea, forming only a slight swell usually about 12 inches above the normal sea surface. They grow in height when they reach shallower water, in a wave shoaling process described below. A tsunami can occur in any tidal state and even at low tide can still inundate coastal areas. On April 1, 1946, a magnitude 7.8 earthquake occurred near the Aleutian Islands, Alaska. It generated a tsunami which inundated Hilo on the island of Hawai with a 14 metres high surge. The area where the earthquake occurred is where the Pacific Ocean floor is subducting under Alaska. Examples of tsunami originating at locations away from convergent boundaries include Storegga about 8,000 years ago, Grand Banks 1929, Papua New Guinea 1998. The Grand Banks and Papua New Guinea tsunamis came from earthquakes which destabilized sediments, causing them to flow into the ocean and generate a tsunami. They dissipated before traveling transoceanic distances.

The cause of the Storegga sediment failure is unknown. Possibilities include an overloading of the sediments, an earthquake or a release of gas hydrates like methane. The 1960 Valdivia earthquake 9.5, 1964 Alaska earthquake 9.2, 2004 Indian Ocean earthquake 9.2 and 2011 Tohoku earthquake 9.0 are recent examples of powerful megathrust earthquakes that generated tsunamis that can cross entire oceans. Smaller 4.2 earthquakes in Japan can trigger tsunamis that can only devastate nearby coasts but can do so in only a few minutes.