Tidal Waves Often
Undetectable Until Last Minute
By Michael Perry

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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