NASA geophysicist Richard Gross calculated that Earth’s rotation sped up by 1.6 microseconds. That’s because of the shift in Earth’s mass caused by the 8.9-magnitude earthquake in Japan on 11th March 2011. A microsecond is one-millionth of a second.
Why It Happens ?
You might have read in your school physics that moment of inertia (I) of a rotating object is sum of mass chunks x distance of chunks from center of rotation. Whether the mass or radius is changed, change in moment of inertia happens. In a situation where angular momentum (which is product of moment of inertia (I) and rotational speed (ω, pronounced as omega) is conserved i.e it doesn’t undergo changes, other quantity changes to compensate for the loss/gain in neighbouring quantity.
Now of course, if earth’s mass distribution changes, so does it’s moment of inertia (I). In case of I (which is I=(2/5)m x r x r for a spherical body), moving mass towards the center decreases I. To conserve Angular momentum, the quantity ω increases. We know the earth’s day is defined as the duration in which earth completes one rotation. Increase in speed takes lesser time to complete rotation and hence the day becomes shorter.
Earlier the Haiti earthquake had caused the similar reduction in length of a day by an amount of 1.26 microseconds, a bit lesser than one caused by Japanese earthquake. The largest amount by which a day was shortened was 6.8 microseconds in 2004 Sumatra earthquake. Misconception
Earthquakes do not always shorten the length of days, they can increase it too. Ponder on above explanation and you will figure out how and why it can happen !