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Tsunami
Fact File
3561/ A tsunami is a series of
great sea waves caused by an underwater earthquake, landslide,
or volcanic eruption. More rarely, a tsunami can be generated
by a giant meteor impact with the ocean.
3562/ Tsunami (pronounced soo-NAH-mee)
is a Japanese word which translates as "harbour wave".
Tsunamis are fairly common in Japan and many thousands of
Japanese have been killed by them in recent centuries.
3563/ The Tsunami Warning System
(TWS) in the Pacific, comprised of 26 member countries, monitors
seismological and tidal stations throughout the Pacific region.
The system evaluates potentially tsunamigenic earthquakes
and issues tsunami warnings. There is currently no international
warning system for tsunamis in the Indian Ocean.
3564/ Tsunamis are formed by
a displacement of water -- a landslide, volcanic eruption,
or, as in this case, slippage of the boundary between two
of the earth's tectonic plates -- slabs of rock 50 to 650
feet (15 to 200 km) thick that carry the Earth's continents
and seas on an underground ocean of much hotter, semi-solid
material. The December 26 tsunami
was caused by slippage of about 600 miles (1,000 km) of the
boundary between the India and Burma plates off the west coast
of northern Sumatra.
3565/ The deadliest earthquake
recorded since 1900 occurred on July 27, 1976, in Tangshan,
China, when the official death count reach 255,000 for a 7.5
magnitude quake. Estimated death counts, however, reached
as high as 655,000.
3566/ Tsunami waves are distinguished
from ordinary ocean waves by their great length between wave
crests, often exceeding a 100 km (60 miles [mi]) or more in
the deep ocean, and by the time between these crests, ranging
from 10 minutes to an hour.
3567/ Tsunamis, also called seismic
sea wave or incorrectly tidal waves, are caused generally
by earthquakes, less commonly by submarine landslides, infrequently
by submarine volcanic eruptions and very rarely by large meteorite
impacts in the ocean. Submarine volcanic eruptions have the
potential to produce truly awesome tsunami waves. The Great
Krakatau Volcanic Eruption of 1883 generated giant waves reaching
heights of 40 meters above sea-level, killing more than 30,000
people and wiping out numerous coastal villages.
3568/ Where the ocean is over
6,000 m deep, unnoticed tsunami waves can travel at the speed
of a commercial jet plane, over 800 km per hour (~500 mi per
hour). They can move from one side of the Pacific Ocean to
the other in less than a day.
3569/ When the tsunami reaches
the coast and moves inland, the water level can rise many
meters. In extreme cases, water level has risen to more than
15 m (50 ft) for tsunamis of distant origin and over 30 m
(100 ft) for tsunami waves generated near the earthquake’s
epicenter.
3570/ The force of some tsunamis
is enormous. Large rocks weighing several tons, along with
boats and other debris, can be moved inland hundreds of meters
by tsunami wave activity, and homes and buildings destroyed.
All this material and water move with great force, and can
kill or injure people.
3571/ Impacts by comets or asteroids
can also generate giant tsunamis. No one has actually witnessed
such an event, except perhaps in films like DEEP IMPACT. But
computer simulations show that the giant tsunamis unleashed
by Hollywood special effects wizards -- large enough to swamp
the Manhattan skyline -- are possible and have almost certainly
happened in the distant past.
3572/ Scientists at the Los Alamos
National Laboratory in New Mexico calculated that if an asteroid
three miles across hit the middle of the Atlantic Ocean, the
tsunami would swamp the upper East Coast as far inland as
the Appalachian Mountains and drown the coasts of France and
Portugal.
3573/ Twenty-four tsunamis have
caused damage in the United States and its territories during
the last 204 years. Just since 1946, six tsunamis have killed
more than 350 people and caused a half billion dollars of
property damage in Hawaii, Alaska, and the West Coast.
3574/ On April 1, 1946, a tsunami
was generated by a magnitude 7.8 earthquake in Alaska's Aleutian
Island chain. As a result, two years later the United States
established a Pacific Tsunami Warning Center in Hawaii.
3575/ The Pacific Tsunami Warning
Center has issued a total of 20 warnings since it was first
established in 1948. Of these 20, 5 resulted in significant
Pacific-wide tsunamis. Even though all significant Pacific-wide
tsunami events have been detected since 1948, 61 people perished
when they failed to heed the warning for the 1960 tsunami
that struck Hilo. Since 1964, there have been no significant
Pacific-wide tsunami events.
3576/ When a tsunami is generated
offshore the wave will behave as a shallow water wave. A shallow
water wave is one that travels through water having a depth
less than 1/20 of its wavelength. Knowing that the average
ocean depth is roughly three miles, oceanographers can determine
the speed of the tsunami, and calculate the time it will take
to travel between any two points. This information has led
to the development of travel-time charts that make it possible
to predict the arrival time of a tsunami wherever it is generated.
Due to the high speeds of these waves, a tsunami can travel
across the Pacific Ocean is less than one day!
3577/ Boats are safer from tsunami
damage while in the deep ocean rather than moored in a harbor.
U.S. Coast Guard guidelines suggest deployment to water depths
of at least 1,200 feet (200 fathoms).
3578/ One cubic yard of water
weighs nearly a ton, and a tsunamis come ashore at speeds
of about 30 miles an hour. An oncoming tsunami can hit a building
with millions of pounds of force, said Dr. Peter E. Raad,
a professor of mechanical engineering at Southern Methodist
University in Dallas.
3579/ In 1998, seismologists
were surprised when a modest magnitude 7.0 earthquake off
Papua New Guinea was followed by a 30-foot-high tsunami that
killed more than
2,100 people. The earthquake, it turned out, had caused nearly
a cubic mile of sediment to give way.
3580/ In 1987, Dr. Brian F. Atwater,
a geologist with the United States Geological Survey, discovered
near the mouth of the Columbia River and in several other
estuaries in Washington the scars of a large tsunami, including
spruce tree forests that had suddenly turned into salt water
tidal flats when the land elevation dropped several feet.
"There must be tens of thousands of stumps in the estuaries,"
he said.
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