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Using a complex mathematical model, Richard Gross of NASA's Jet Propulsion Laboratory (Pasadena, California), has calculated how the magnitude 9.0 earthquake that struck Japan on 11th March has effected the rotation of the Earth.
As is usual after events such as these, the media began to talk of a shift in the Earth's axis, implying that the planet will lean one way or the other, something which can actually only happen as the result of an external gravitational influence. What actually changes position slightly is something called the "figure axis". This axis traces the centre of the mass distribution in the planet, and is a line that runs close to, but not parallel to, the rotation axis.
If we imagine slicing the Earth into many horizontal disks, along lines of latitude, we would find that the centre of mass of each disk was not actually at the geometrical centre, but is instead slightly offset due to the distribution of mass in the tectonic plates, oceans and mantle. The geometrical centre corresponds to a point on the spin axis, the centre of mass to a point on the figure axis.
These two axes within the planet are displaced, on average by 10 metres, and the offset causes the Earth to wobble about its spin axis.
According to Gross' calculations, the earthquake in Japan has caused the figure axis to shift by 17 cm towards 133° east, and has also shortened the day by 1.8 microseconds (millionths of a second). This shortening of the day is caused by a lowering of a section of the Pacific plate (the same movement that caused the tsunami).
When a mass moves closer to its rotation axis the rotational velocity increases in order to conserve angular momentum; just as in the classical example of a spinning skater who brings their arms closer to their body.
Previous earthquakes in Sumatra and Chile moved the figure axis by 7 and 8 cm and shortened the day by 6.8 and 1.26 microseconds respectively. These changes are actually quite insignificant, the figure axis changes by about 1 metre and the length of the day by about a millisecond as a result of seasonal variations in oceanic currents and atmospheric wind. So, for all the devastating power of an earthquake, it has very little influence on the rotation of the planet.
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