| ALL ABOUT TSUNAMIS >> |
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Usually caused by a massive earthquake under
the sea, tsunamis are in effect a series of travelling
ocean waves which can wreak severe destruction on low
lying coastal areas. |
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According to US geophysicists, the earthquake
that unleashed deadly tsunami waves on Asia, was so powerful,
it made the earth wobble on its axis and permanently altered
the regional map. |
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According to one expert, it may have moved
some of the smaller islands off Sumatra's coast, by as
much as 20 metres and USGS. Expert Ken Hudnut told the
AFP news agency, that a 20-metre shift is considered a
lot of a slip. He says the north-western tip of the Indonesian
territory of Sumatra may also have shifted to the south-west
by around 36 metres. |
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Another expert at the National Earthquake Information
Centre in Golden Colorado, said it was more likely the
islands off Sumatra had risen higher out of the sea.
|
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Tsunami is a Japanese word that translates as "harbour
wave". It is not strictly speaking a "tidal
wave" which describes the periodic movement of water
associated with the rise and fall of the tides. |
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Oceanographers call tsunamis seismic seawaves because
they are usually caused by a sudden rise or fall of part
of the earth's crust under or near the ocean. Less powerful
tsunami waves can also be triggered by volcanic activity.
They are most common in the Pacific Ocean. |
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A tsunami is not a single wave, but a series of waves
that can travel across the ocean at speeds of more than
800 km (500 miles) an hour. In the deep ocean, hundreds
of miles (km) can separate wave crests; many people have
lost their lives during tsunamis after returning home
thinking the waves had stopped. |
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As the tsunami enters the shallows of coastlines in
its path, its velocity slows but its height increases.
A tsunami that is just a few centimetres or metres high
from trough to crest can rear up to heights of 30 to 50
metres as it hits the shore, striking with devastating
force. |
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For those on shore there is little warning of a tsunami's
approach. The first indication is often a sharp swell,
not unlike an ordinary storm swell. |
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In 1883, a tsunami following the eruption of Krakatoa
volcano between the Indonesian islands of Java and Sumatra
killed 36,000. The tsunami's passage was traced as far
away as Panama. |
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In July 1998, two undersea quakes measuring 7.0 created
three tsunamis that killed at least 2,100 near the town
of Aitape on the north coast of Papua New Guinea. |
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