A tsunami (pronounced in English as [su na me) is a natural
phenomenon consisting of a series of waves generated when an abrupt or
pulsating vertical
displacement occurs in a large body of water such as a lake, the sea or a coastal
inlet. Earthquakes, landslides, volcanic eruptions, explosions, and the impact
of extraterrestrial bodies such as meteorites, can generate tsunamis which can
rapidly and violently swamp coastlines, causing devastating property damage,
injuries, and loss of life due to injuries or drowning.
Tsunamis can travel great distances from the the original site of creation.
(see animation at right)
The term "tsunami" comes from the Japanese language meaning harbor
("tsu") and wave ("nami"). The term was created by fishermen
who returned to port to find the area surrounding the harbour devastated, although
they hadn't been aware of any wave in the open water. A tsunami is not a sub-surface
event in the deep ocean; it simply has much smaller amplitudes (wave heights)
offshore, and very long wave lengths (sometimes over 150 kilometers long), which
is why they generally pass unnoticed at sea, forming only a passing "hump" in
the ocean.
Tsunamis used to be called tidal waves because as they approach
land they look like a giant onrushing tide, rather
than the sort of cresting waves that are formed by wind action upon the ocean
(with
which people are more familiar).
(From Wikipedia: http://en.wikipedia.org/wiki/Tsunami)
The December 27 tsunami was caused by an earthquake magnitude 9.0, caused by movement along a transform boundary. . The Indo-Astralian plate has been sliding under the Eurasia plate at the rate of a few centimeters a year, but on December 27, the Eruasian plate suddenly lifted up, lifting perhaps 20 meters along a 1200 kilometer- ridge. Directly above, on the ocean surface maybe only ripples occured. In terms of the whole planet, the movement is "utterly insignificant," says geologist Simon Winchester, author of "Krakatoa," a recent best seller about a volcano that exploded off Sumatra in 1883, killing 40,000 people. "The earth shrugged for a moment. Everything moved a little bit."
This seismic bump was enough to move trillions of tons of water in a few seconds. Silently, invisibly, the water pushed outward at the speed of a jet plane 600 km/hr. (see animation at right) As it neared shore, the speed slowed, and large waves formed, in some places very large ones.
More Liunks on Tsunamis:
CNN, How a Tsunami forms.