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Tsunami Formation

Tsunamis are characterized as shallow-water waves due to their long periods and wavelengths. A wind-generated wave might have a period of ten seconds and a wavelength of 150 metres, but it is possible for a tsunami to have a wavelength more than 100 kilometres and a period on the order of one hour, with a wavelength of only a few centimeters.

Due to their long wavelengths, tsunamis behave like shallow-water waves. A wave becomes a shallow-water wave when the ratio between the water depth and its wavelength gets very small. Shallow-water waves move at speeds equivalent to the square root of the product of the acceleration of gravity and the water depth, i.e. In the Pacific Ocean, where the depth of water is about 4000 metres, tsunamis travel at about 200 m/s, or more than 700 km/hr. Because the rate of energy loss by the wave is inversely proportional to its wavelength, tsunamis not only spread at high speeds, they can also travel large distances i.e. across the ocean, with little energy loss.


Receding Shoreline
Source: Photoduck

Tsunamis can be generated when the sea floor suddenly deforms and vertically displaces the overlying water. Tectonic earthquakes are a particular kind of earthquake that is associated with the earth's crustal deformation. When they occur beneath the sea, the water above the deformed area is displaced from its equilibrium position. Waves are formed as the displaced water mass, acting under the influence of gravity, tries to regain equilibrium. When large areas of the sea floor elevate or subside, a tsunami can be created.

Large vertical movements of the earth's crust can occur at plate boundaries. Plates move along these boundaries called faults. For example, at the boundaries of the Pacific Ocean, denser oceanic plates slip under the less dense continental plates in a process known as subduction. Subduction earthquakes are particularly effective in generating tsunamis because they provide much force and disturbances deep underwater.

As a tsunami leaves the deep ocean and travels toward the shallow coast, it transforms. A tsunami moves at a speed relative to the water depth, therefore the tsunami slows as the water depth decreases. The tsunami's energy flux, being dependent on both its wave speed and wave height, remains nearly constant. As a result, the tsunami's speed decreases as it travels into shallower water, and its height increases. Because of this shoaling effect, a tsunami, unobvious at sea, may heighten to several meters or more near the coast. When it reaches the coast, it may appear as a rapidly rising or falling tide or a series of breaking waves.


Tsunami Formation
Source: Wikipedia.org

As a tsunami reaches the shore, it begins to lose energy whereby part of its wave energy is reflected offshore, while the shoreward-propagating wave energy is dissipated through friction and turbulence. In spite of this, tsunamis still reach the coast with tremendous amounts of energy.

References

  • USGS Earthquake Hazards Program-Latest Earthquakes. US Geological Survey. Retrieved March 5, 2005, from http://earthquake.usgs.gov/eqinthenews/2004/usslav/
  • 26 December 2004 Tsunam. National Institute of Oceanography, India. Retrieved March 5, 2005, from http://www.nio.org/jsp/tsunami.jsp
  • Tsunami!: The WWW Tsunami Information Resource. Earth and Space Sciences (Geology and Geophysics) at UW. Retrieved March 5, 2005, from http://www.ess.washington.edu/tsunami/toc.html
  • Sumatra Earthquake 26 December 2004. British Geological Survey. Retrieved from http://www.earthquakes.bgs.ac.uk/latest_info.htm

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