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Tsunamis or seismic tidal waves are produced because as a result of an earthquake a sudden change occurs in the bottom relief in the area of the earthquake. This may be a sudden subsidence of a fault block in the earth's crust of the seabed, a sudden lifting of such a fault block, or even a sudden subsidence of the bed along a continental shelf. This is accompanied by enormous water movements in the area of sea above it. As a result of this extremely long sea waves are produced that can cross a whole ocean virtually unsubdued. In the open sea these waves are not observed because of their length and relatively low height. The wave length in the open sea is around 500 - 1000 km, the period is in the order of ½ - 1 hour, and the speed at which the wave moves is for deep oceans 500 - 1000 km/hour. The amplitude of the wave in the open sea is usually not more than ½ m. When approaching coasts these waves may however reach a height of a few metres to 10, 20 m or more. This height depends on the configuration of the seabed off the coast and the shape of the coast itself. Narrow elongated bays are notorious in this respect. Observations of Tsunamis are then only limited to the immediate vicinity of the coast. Because of the dependence of the phenomenon on the local conditions, the observations may differ considerably for points located near one another.
tsunami Christmas 2004 (Sumatra-Andaman earthquake)
The 2004 Indian Ocean earthquake, known by the scientific community as the Sumatra- Andaman earthquake, was an undersea earthquake that occurred at 00:58:53 UTC (07:58:53 local time) on December 26, 2004. The earthquake triggered a series of lethal tsunamis that spread throughout the Indian Ocean, killing large numbers of people and devastating coastal communities in Indonesia, Sri Lanka, India, Thailand, and elsewhere. Initial estimates of the death toll were more than 283,100 people, however, more recent analysis indicates that the actual casualties was 186,983 dead, with 42,883 missing, for a total of 229,886. Nevertheless, this catastrophe is still one of the deadliest disasters in modern history. The disaster is known in Asia and in the international media as the Asian Tsunami, and also called the Boxing Day Tsunami in Australia, Canada, New Zealand, and the United Kingdom as it took place on Boxing Day.
The magnitude of the earthquake was originally recorded as 9.0 (Richter scale), but has been upgraded to between 9.1 and 9.3. At this magnitude, this is the second largest earthquake ever recorded on a seismograph, after the 9.5 magnitude Great Chilean Earthquake of May 22, 1960. The earthquake caused the ground to shake approximately 100 times harder than the Loma Prieta earthquake of 1989. This earthquake was also reported to be the longest duration of faulting ever observed, lasting between 500 and 600 s. It was large enough that it caused the entire planet to vibrate at least half an inch, or over a centimeter. It also triggered earthquakes in other locations, even as far away as Alaska.
The earthquake originated in the Indian Ocean just north of Simeulue island, off the western coast of northern Sumatra, Indonesia. The resulting tsunami devastated the shores of Indonesia, Sri Lanka, South India, Thailand and other countries with waves up to 30 m (100 ft). It caused serious damage and deaths as far as the east coast of Africa, with the furthest recorded death due to the tsunami occurring at Port Elizabeth in South Africa, 8,000 km (5,000 mi) away from the epicentre.
source: Wikipedia, the free encyclopedia
26-12-2004. Surface elevation registration of the fishfinder onboard the yacht Mercator anchored
one mile off the coast of Phuket (depth in metres)