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Faster earth spin, forever

Los Angeles, Dec. 29 (Reuters): Sunday's devastating earthquake may have permanently accelerated the earth's rotation ' shortening days by a fraction of a second, US scientists said.

It was reported yesterday that as a consequence of the quake, caused when the India plate dived under the Burma plate off Sumatra, the earth may have spun on its axis 3 microseconds, or 3 millionths of a second, faster, shortening the day by that much.

Richard Gross, a geophysicist with Nasa's jet propulsion laboratory in California, said the change in the rate of rotation may be permanent.

When one plate went below another beneath the Indian Ocean 'it had the effect of making the earth more compact and spinning faster', Gross said.

Gross said changes predicted by his model probably are too minuscule to be detected by a global positioning satellite network that routinely measures changes in the earth's spin.

'The earth does slow down and change its rate of rotation,' he said.

When those tiny variations accumulate, planetary scientists must add a 'leap second' to the end of a year, something that has not been done in many years, Gross said.

Scientists have long theorised that changes on the earth's surface such as tide and groundwater shifts and weather could affect its spin but they have not had precise measurements to prove it, Caltech seismologist Hiroo Kanamori said.

'Even for a very large event, the effect is very small,' Kanamori said. 'It's very difficult to change the rotation rate substantially.'

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