Day the Earth was shaken to its core
Science writer Leigh Dayton
December 29, 2004

WHEN it hit, the earthquake that unleashed Sunday's killer waves caused the Earth to wobble on its axis and changed the regional map forever.

According to experts, the 9.0-magnitude earthquake that hit 250km southeast of Sumatra was so powerful that some of the smaller islands off the southwest coast of the island may have shifted southwest by 20m.

"That is a lot of slip," said geophysicist Ken Hudnut of the US Geological Survey.

Further, Dr Hudnut claimed that the northwestern tip of Sumatra might also have moved southwest - possibly as much as 36m - when the Indian Plate dived beneath the Burma plate, causing earth-shattering uplift and triggering the tsunami. Geoscience Australia seismologist Cvetan Sinadinovski said the exact displacement remained to be confirmed by measurements made by global positioning satellites.

Dr Sinadinovski was sceptical of Dr Hudnut's claim that the event rocked the Earth off its axis, claiming instead that it was more a "glitch" in planetary rotation.

Still, Dr Sinadinovski agreed that the quake had made the Earth vibrate.

"It made the Earth ring like a bell," he said.

Dr Sinadinovski added that the Sumatra quake was worsened by an 8.1-magnitude earthquake that had struck on Christmas eve, about 800km southeast of Hobart, near Macquarie Island and the underwater "Macquarie Rise".

"That earthquake was not a direct trigger to the Sumatran event, but it had the potential to speed up the release of energy; it was released more quickly," he explained.

The fact that a major earthquake occurred just three days before the one on Sunday has raised scientific speculation that major shakes may come in clusters, either on a global basis or along fault lines.

In Pasadena, California, geologist Kerry Sieh noted that clumps of quakes hit Turkey in the 1930s and 40s and whoppers slammed Alaska in the 50s and 60s.

"There were other periods of time when there were global flurries of earthquakes," he said, noting that five of the 10 largest earthquakes of the 20th century had hit between 1957 and 1965.

Two more struck in 1950 and 1952. Earth was quake-free from 1989 to 1994, then it was rocked by four in the next two years, said Professor Sieh.

The biggest known earthquake, was a 9.5-magnitude event that hit Chile in May 1960.

The next largest was the 9.2-magnitude event that hit in Alaska's Prince William Sound in March 1964.

Despite historical clusters - and 31 aftershocks since Sunday of between 5.5 and 7.3 magnitude - it's impossible to know if Sunday's plate-buster will spawn more major quakes, Dr Sinadinovski said.

"We can expect large earthquakes to continue occurring in these zones, but the exact timing is nearly impossible to predict," he concluded.