TSUNAMI

Disaster warning beamed out in minutes

IT WAS 6.47 yesterday morning when David Jepsen, a seismologist at Geoscience Australia, was woken by his mobile phone.

A computer at his Canberra office was phoning to report that automated seismographs had recorded an earthquake measuring 8.1 on the Richter scale, seven minutes earlier under the sea floor 40 kilometres south-east of Gizo, in the Solomon Islands.

Dr Jepsen immediately called the Bureau of Meteorology, which runs Australia's tsunami warning system, before putting in a second call to the national disaster authority, Emergency Management Australia.

Minutes later the warning system, established last year in the wake of Sumatra's Boxing Day 2004 disaster, was beaming out its first alert to Australians.

For eons the tectonic plate on which the Australian continent rides has been pushing north-east. Below the sea, off the Solomon Islands, it is being forced down and under the Pacific plate. As the Australian plate dives into the earth, it drags down the edge of the Pacific plate.

"So much pressure builds up," said Dr Jepsen, "it can't stand the stress any more."

At 6.40am Sydney time, 10 kilometres below the surface of the planet, up to 150 kilometres of the Pacific plate rebounded. As it forced its way up, the sea above lifted too, sending a wave towards Australia.

Gary Gibson, senior seismologist at the private Melbourne company Environmental Systems and Services, estimated more than 10,000 square kilometres of sea floor might have been lifted in the rupture.

Tony Leggett, of the Bureau of Meteorology's National Meteorology and Oceanographic Centre in Melbourne, said an 8.1 quake could trigger "an ocean-wide destructive tsunami".

But computer modelling suggested such a wave would brake as it rolled around reefs and islands dotting the Pacific. Wave heights along the Queensland coast would be no more than 20 to 25 centimetres, and probably much less by the time they reached Sydney.

However, detailed modelling has been completed only for a tsunami travelling over deep water. Work to determine exactly how it would behave in coastal waters has not been completed, so, deciding it would be better to be cautious than sorry, the bureau issued a tsunami alert for Queensland and NSW.

A wave-monitoring station at Yorkeys Knob, near Cairns, recorded the sea surging 35 centimetres. While a buoy off Sydney failed to spot any tsunami, a 10-centimetre wave was observed off Port Kembla at 12.40pm. It had travelled some 2800 kilometres in six hours.

Yesterday's quake, at 8.1 on the Richter scale, was about 40 times weaker than the one that triggered the Boxing Day 2004 disaster.

The quake rupture, 10 kilometres down, may have been 150 kilometres long, lifting 10,000 square kilometres of sea floor two to three metres.

A 22-centimetre wave was recorded at Cape Ferguson, near Townsville.