A tsunami (pronounced soo-nahm-ee) is a series of huge waves that happen after an undersea disturbance, such as an earthquake or volcanic eruption. (Tsunami is from the Japanese word for harbor wave.) The waves travel in all directions from the area of disturbance, much like the ripples that happen after throwing a rock. They are sometimes mistakenly called "tidal waves," but tsunami have nothing to do with the tides.
The waves may travel in the open sea as fast as 450 miles per hour. As a tsunami leaves the deep water of the open ocean and travels into the shallower water near the coast, it slows down and its height increases. When it reaches the coast, a tsunami may appear as a rapidly rising or falling tide, a series of breaking waves, or a tidal bore (see below). Video footage seen on television of the Indian Ocean tsunami on December 26th 2004 showed coastlines rapidly engulfed in rising water level, rather than the huge wall of water envisaged in the popular conception of tsunamis.
A tidal bore is a wall of water that moves up certain low-lying rivers due to an incoming tide. Tidal bores form when an incoming tide rushes up a river or inlet, developing a steep forward slope due to resistance to the tide's advance by the river, which is flowing in the opposite direction. The height of the tidal bore increases with the range of the tide and may vary in height from just a ripple to several metres.
The tsunami in the Indian Ocean was caused by an earthquake occurring under the ocean. Earthquakes commonly occur along the plate boundaries (see the Plate Tectonics page). The earthquake that triggered the tsunami occured at a convergent plate boundary. This is where plates meet and try to move together resulting in one plate attempting to ride up over the other and the other trying to descend beneath the other. The topographic expression of this is a deep trench beneath the ocean. Large vertical movements of the Earth's crust can occur at plate boundaries and earthquakes at subduction zones are particularly effective at generating tsunamis. The Magnitude 9.0 December 26th Sumatra earthquake caused 6m of vertical uplift of the Burma plate as it was pushed up over the descending Indian plate along 1000km of the Sunda Trench.
The Pacific Ocean, being encircled by subduction zones, has the most frequent occurrence of tsunamis. A Tsunami Warning System is operated by the U. S. National Oceanic and Atmospheric Administration (NOAA), with a centre in Hawaii serving as the international warning centre for tsunamis that pose a Pacific-wide threat. If the location and magnitude of an earthquake meet the known criteria for generation of a tsunami, a tsunami warning is issued predicting arrival times at coastal communities. If a significant tsunami is then detected by sea-level monitoring instrumentation, the tsunami warning is extended to the entire Pacific Basin.
Unfortunately there was no equivalent organisation in place for the Indian Ocean. While the Sumatra earthquake and its tsunami risk were detected by the NOAA and others, the simple lack of a proper communications network with the authorities in Sri Lanka and adjacent countries has been given as the reason that the two hour travel time of the tsunami across the ocean was inadequate to allow prevention of the tragic loss of life. Work is ongoing to install an Indian Ocean Tsunami warning system.
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