Page last updated at 11:23 GMT, Thursday, 4 March 2010

How Chile's quake could have shortened a day

Nasa scientists believe Chile's devastating earthquake may have speeded up the Earth's rotation and shortened the length of a day.

Researcher Richard Gross and his colleagues at Nasa's Jet Propulsion Laboratory in California calculated that Saturday's 8.8-magnitude quake could have cut 1.26 microseconds off the length of a day.

Not that anyone would notice however - as that's one-millionth of a second.

But why does movement in the Earth's crust give the planet an apparent turbo boost?

How an earthquake could shorten a day

Earth spinning like a top
The Earth is not a perfect sphere. It is pinched in slightly at the poles and bulges at the equator. As such, it rotates with a wobble just like a spinning top. However, changes in the distribution of mass can affect this spin.
Ice skater spinning
In the same way a skater speeds up a spin by pulling in their limbs, a quake can make the Earth rotate faster by nudging some of its mass closer to the planet's axis. Movements in atmosphere and oceans can have a similar effect.
Chile earthquake location
Nasa calculates the quake in Chile shifted mass to such an extent that it changed the rotation rate to shorten a day by about 1.26 microseconds, or one millionth of a second. Source: Nasa, British Geological Survey
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Brian Baptie, of the British Geological Survey, explains how the Earth's lack of rigidity is what allows changes to the planet's rotation - and the length of a day.

"The earth is not rigid and movements of its constituent parts, including the atmosphere and oceans, occur. These effects introduce a wobble - a movement of the Earth's axis - which is small but detectable," he says.

"A small wobble can be caused as a result of great earthquakes like the recent event in Chile and Indonesia on 26 December 2004, due to the movement of mass close to the earth's surface, but it will only be marginally detectable by the most sensitive instruments and is not a cause for concern."

Using the same mathematical model as it used for Chile, the Nasa team had estimated the 9.1 Sumatran earthquake in 2004 would have shortened a day by 6.8 microseconds.

But, as Nasa and other scientists also point out, the Earth's rotation rate changes all the time as a result of variations in winds and ocean currents, and doesn't have much practical consequence for people because of the tiny amount of time involved.

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