A tsunami, sometimes called a tsunami wave train, is a series of waves that are triggered by the sudden displacement of large volumes of sea water. Tsunamis usually occur in the ocean, but have been reported in lakes and rivers. Tsunamis occur frequently in Japan. Over the course of Japanese history, Japan has been struck by a major tsunami 195 times. As a result of the ability to displace very large volumes of water, and their ability to strike with great energy, tsunamis frequently devastate low-lying coastal regions.
What Does the Word "Tsunami" Mean?
The term tsunami comes from the Japanese words "tsu," which means harbour, and "nami" which means wave. Tsunamis are sometimes referred to as a tidal wave. However, this term has fallen out of use, particularly in the scientific community. The reason for this is because the generation of tsunamis is not related to the tides. Rather, the term "tidal wave" comes from the tsunami’s similar appearance to a very high tidal bore. Both produce very high waves that reach very far inland, however the damage done by a tsunami is much more extensive.
How Are Tsunamis Generated?
The primary method of tsunami generation is the displacement of a large volume of seawater as a result of the sudden disturbance of the sea floor. Such a disturbance of the sea is usually caused by underwater earthquakes, landslides or volcanic eruptions. On rare occasions, tsunamis have been triggered by asteroid impacts and underwater nuclear tests.
Seismically generated tsunamis occur when the ocean floor suddenly deforms, displacing the overlying column of water. When an earthquake occurs underwater, the equilibrium of the water column is disturbed. Tsunamis are often generated along submerged thrust faults as a consequence of sudden plate boundary movement. This results in water displacement due to the vertical component of seismic events. Strikeslip faults are also capable of causing the seabed to move, but not enough to generate a large tsunami.
Tsunami Wavelenghts and Amplitudes
Tsunamis have a small wave height, or amplitude, and very long wavelength. In the open ocean, tsunamis can be hundreds of miles long, but only a foot high. As a result, they often pass unnoticed by ships. Tsunamis grow in height as they reach shallow water. Tsunamis can occur at any tidal state. Even at low tide, tsunamis can still flood low-lying coastal areas.
Everyday wind waves have an average wavelength of 300 feet and an amplitude of six feet. In contrast, a tsunami in the open ocean can have a wavelength of 200 kilometres. Tsunamis in the open ocean travel at speeds upward of 500 miles per hour. Due to their high speed and the wave’s very low amplitude, it often only takes half an hour or less to complete a cycle. As a result, tsunamis are nearly impossible to detect in the open ocean without a tsunami warning system.
As a tsunami approaches the coast, the sea floor begins to rise and wave shoaling forces the wave to compress. It also slows the velocity of the wave to 50 miles an hour. The tsunami’s wave length compresses to 12 miles while its amplitude begins to increase, as a result, the tsunami becomes visible for the first time. Since the tsunami still has a very long wavelength, it may take several minutes for the tsunami to reach its full height. With the exception of the very largest tsunamis, most tsunamis do not break; rather they take the appearance of a fast moving tidal bore. It is for this reason that tsunamis are sometimes erroneously called tidal waves.
When the peak of the tsunami comes ashore, it triggers a temporary rise in sea level known as a run up. The run up is measured in meters above sea level. A very large tsunami may come in multiple waves over a period of several hours. Additionally, the first wave that reaches shore may not be the largest and may not have the highest run up.
Approximately 80% of all tsunamis occur in the Pacific Ocean, however, tsunamis can occur in any large body of water.
Sources
Fradin, Judith Bloom and Dennis Brindell (2008). Witness to Disaster: Tsunamis. Witness to Disaster. Washington, D.C.: National Geographic Society. pp. 42, 43.
Smid, T. C. (Apr., 1970). 'Tsunamis' in Greek Literature. 17 (2nd ed.). pp. 100–104
[a. Jap. tsunami, tunami, f. tsu harbour + nami waves.— Oxford English Dictionary]
Jakarka Tsunami Information Centre Mar.11/11
Thucydidies. The Peloponnesian War. Mar. 11/11
Haugen K, Løvholt F, Harbitz C, K; Lovholt, F; Harbitz, C (2005). "Fundamental mechanisms for tsunami generation by submarine mass flows in idealised geometries". Marine and Petroleum Geology 22: 209–217
Voit, S.S (1987). "Tsunamis". Annual Review of Fluid Mechanics 19: 217–236.
Terry Long - I hold degrees in history and journalism. In addition to my contributions to Suite 101, I have also written for the Brampton Bulletin, the ...
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