Major tsunami thrashes SE Asia
A massive tsunami swept across southeast Asia Sunday, killing more than 8,000 people.
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All warnings are based on such information as the scale of the earthquake as measured by special cables on the seabed. A warning takes about 10 minutes to issue, and is flashed on TV screens nationwide in Japan, and sent to disaster-prevention agencies at local government offices that sound evacuation alarms. Many of Japan's coastal towns also have water gates to shut out waves that might head inland via low-lying river networks.
But even this technology has been criticized as slow and able to provide only a rough projection of the height of a wave. After a government study in 2003 showed that a tsunami resulting from an 8.6 magnitude quake in the Pacific south of Japan could kill up to 8,600 people if evacuations were slow, both government and private industry groups renewed efforts to improve warning systems.
A Japanese research agency has since developed a method to accurately predict the height of a tsunami just three seconds after an earthquake hits. More seabed cables are currently being laid to broaden the area this system covers.
The company Hitachi has developed another system using Gobal Positioning System (GPS) satellite technology that can detect the presence of a tsunami several kilometers offshore by measuring how much a giant 13-meter buoy rises or falls on the ocean surface.
• Simon Montlake contributed reporting from Bangkok, Thailand, Tom McCawley from Jakarta, Indonesia, Janaki Kremmer from Delhi, Bennett Richardson from Tokyo, and staff writers Robert Marquand from Beijing.
What is a tsunami?
A tsunami is a series of large waves usually generated by a violent undersea disturbance. The word tsunami (pronounced tsoo-nah'-mee) is composed of the Japanese words "tsu" (which means harbor) and "nami" (which means "wave").
How big - and fast - are they?
In deep ocean, the height of the tsunami may be only a few centimeters to a meter or more. Tsunami waves in deep water can travel at high speeds thousands of miles and lose very little energy. The deeper the water, the greater the speed.
For example, at the deepest ocean depths, the tsunami wave speed will be as much as 497 m.p.h, about the same as that of a jet aircraft. In 1960, great tsunami waves generated in Chile reached Japan, more than 10,439 miles away in fewer than 24 hours, killing hundreds of people.
What role do earthquakes play?
By far, the most destructive tsunamis are generated from large, shallow earthquakes with epicenters or fault lines near or on the ocean floor.
The quakes can disturb the ocean's surface, displace water, and generate destructive waves that can travel great distances. Not all earthquakes generate tsunamis. Usually, it takes an earthquake with a Richter magnitude exceeding 7.5 to produce a destructive tsunami.
What else causes tsunamis?
Less frequently, tsunamis can be generated from displacements of water resulting from rock falls, icefalls, and sudden submarine landslides or slumps. Major earthquakes are suspected of causing underwater landslides, which may contribute significantly to tsunami generation. For example, many scientists believe that a major 1998 tsunami along the northern coast of Papua-New Guinea was generated by a large underwater slump of sediments, triggered by an earthquake.
The largest tsunami wave ever observed was caused by a rock fall in Lituya Bay, Alaska, on July 9, 1958. Triggered by an earthquake along the Fairweather fault, a massive rock fall at the head of the bay generated a wave that reached the incredible height of 1,720 feet on the opposite side of the inlet.
- The International Tsunami Information Center, Honolulu, Hawaii. http://www.prh.noaa.gov/pr/itic/library/about_tsu/faqs.html#7
