- SYDNEY (Reuters) - Tidal waves start as an insignificant ripple on the
ocean's surface capable of passing under a ship unnoticed, but they become
giants as they approach land, destroying everything in their path. A tidal
wave, or tsunami, three of which smashed into the Papua New Guinea coast
on Friday killing at least 700 people, can be caused by an undersea earthquake,
a submarine landslide or a volcanic eruption. They are most common in the
Pacific Ocean. Geological centres in Hawaii and Australia on Friday night
monitored a quake measuring 7.0 on the Richter scale in the Bismarck Sea
just off Papua New Guinea's remote northwest coast. ``That's a major earthquake,''
Mal Summerville at the Australian Geological Survey unit told Reuters.
The multinational Pacific Tsunami Warning Centre in Hawaii also monitored
the earthquake but in a recorded message said:
``No Pacific-wide tsunami was generated.'' But what the earthquake did
generate were three localised tidal waves, the biggest 10 metres (30 feet)
high, which hit the Papua New Guinea coast near the town of Aitape in darkness,
killing perhaps 1,000 people or more and wiping entire villages off the
map. Villagers said the waves sounded like a jet fighter landing. They
struck about 10 to 15 minutes after their flimsy houses had shuddered from
the earthquake's shockwaves. The massive walls of water crushed the timber
huts that lined the remote coast, trapping some villagers in their beds,
and burying others under sand and mangroves. But most of the dead drowned
as tonnes of water, which washed two kilometres (1.2 miles) inland, swept
them away. Tsunamis are normally generated when an earthquake shakes the
ocean floor.
They can travel across the ocean at speeds of up to 1,000 km (620 miles)
an hour, but often go unnoticed. ``Its length from crest to crest may be
a hundred kilometres or more, its height from trough to crest only a few
centimetres or metres. It cannot be felt aboard ships in deep water,''
said Hawaii's Tsunami Warning System on its Internet site. It is when the
tidal wave nears shore in shallower waters that it gains its destructive
force. ``A tsunami is not a single wave, but a series of waves. As the
tsunami enters the shoaling waters of coastlines in its path, the velocity
of its waves diminishes and wave height increases,'' said the warning centre.
``It is in these shallow waters that tsunamis become a threat to life and
property, for they can crest to heights of more than 30 to 50 metres and
strike with devastating force,'' it said. 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, followed by a sudden outrush
of water that often exposes the shore area as the first wave trough approaches.
The third to eighth wave crests of a series of tsunamis are often the most
destructive. Probably the most destructive tsunami on record occurred on
August 27, 1883, following an eruption of the Krakatoa volcano between
the Indonesian islands of Java and Sumatra. Over 36,000 people were killed
by the resulting tidal waves, which reached a height of up to 30 metres
and travelled at 560 to 720 km per hour. The tsunami's passage was traced
as far away as Panama.
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