Indian Ocean Tsunami-December 26 2004
|Indian Ocean Tsunami
The Tsunami in the Indian Ocean triggered by the most powerful earthquake on Earth in 40 years, generated the largest Tsunami in the Indian Ocean since 1883. Some of the tsunamis reached as far as 3,000 miles from the epicenter of the 9.0 magnitude quake, which was located about 100 miles (160 km) off the coast of Indonesia's Sumatra Island at a depth of about 6.2 miles (10 km).
The death toll will likely be in excess of 150,000 and the damage though restricted to the immediate coastal area is unparalleled. The earthquake, whose magnitude was a staggering 9.0, unleashed walls of water more than 10 meters high.
The regions has been struck by numerous aftershocks-note the table and map below. Much like a zipper the quakes initially progressed north along the fault to the Andaman Island Region.
A tsunami is not a single wave, but a series of traveling ocean waves generated by geological disturbances near or below the ocean floor. With nothing to stop them, these waves race across the ocean until they reach shore where they slow down and rise up in height
. Most tsunamis are triggered by large undersea earthquakes but they can be caused by landslides, volcanoes or even meteor impacts. The last large tsunami in the region was due to the eruption of Krakatoa in 1883, due to the collapse of that volcano during an explosive eruption.
In this case the ocean bottom shifted displacing sea water in the ocean basin. The bigger the earthquake, the more the Earth's crust shifts and the more seawater begins to move. A quake of this magnitude typically shifts the earth surface by up to 10-20 meters. In this case the rupture was up to 400 miles long, leading to a massive region of the ocean bottom shifting. The waves traveled outward just like those from throwing a rock into the water. Most tsunamis occur in the Pacific because the ocean basin is rimmed by the Ring of Fire, a long chain of the Earth's most seismically active spots. In a tsunami, waves typically radiate out in directions opposite from the seismic disturbance. In the case of the Sumatra quake, the seismic fault ran north to south beneath the ocean floor, while the tsunami waves traveled mainly west and east.
Tsunamis are distinguished from normal coastal surf by their great length, width and speed. A single wave in a tsunami series might be 100 miles long and race across the ocean at 600 mph. When it approaches a coastline, the wave slows dramatically, but it also rises to great heights because the enormous volume of water piles up in shallow coastal bays. Unlike ordinary waves tsunamis do not break on the coastline every few seconds. Because of their size, it might take an hour for another one to arrive.
There unusual speed and wavelength allow tsunamis to be identified by buoys moored in the ocean. Although seismic networks recorded Sunday's massive earthquake, there were no wave sensors in the Indian Ocean region, and no means to determine the existence or direction a tsunami would travel. Thus, no warnings were issued. A single wave station south of the earthquake's epicenter registered tsunami activity less than 2 feet high heading south toward Australia, researchers said.
The Pacific Ocean does have a Tsunami warning system. The international warning system was started in 1965, the year after tsunamis associated with a magnitude 9.2 temblor struck Alaska in 1964. It is administered by the U.S. National Oceanic and Atmospheric Administration. Member states include all the major Pacific rim nations in North America, Asia and South America, was well as the Pacific islands, Australia and New Zealand.
However, India and Sri Lanka are not members because tsunamis are not common in the Indian Ocean and the instruments are expensive. Both of these nations would have had several hours of time to respond to a warning, to evacuate coastal areas. It would be surprising if a network was not put in place now. Such a system would not have helped Indonesia since it was so close to the epicenter that the waves would strike very quickly.
|List of Aftershocks
Date Time Magnitude Location
The aftershocks occuring in the first twelve hours after the quake
The aftershocks occuring within the first 72 hours of the quake.