Chile earthquake might have shortened Earth’s day, but how?

The massive 8.8-magnitude earthquake that struck Chile in February 2010 might have shortened Earth’s day by 1.26 microseconds, according to calculations by a NASA scientist. An EarthSky Facebook friend asked us how. And the answer is for the day to get shorter, Earth’s rotation has to speed up. EarthSky spoke to Richard Gross, of NASA’s Jet Propulsion Laboratory. He is the scientist who performed the calculations indicating the minuscule shift in Earth’s day length.

The Earth’s rotation changes all the time. It speeds up and it slows down and it wobbles as it rotates. These changes are caused by anything that moves mass around on the Earth.

He said the Chilean earthquake re-arranged what he called the mass balance of Earth. He explained:

This is just like an ice skater, a spinning ice skater who will spin faster as she brings her arms closer to her body. The net result of the mass motion that was caused by the earthquake everywhere within the earth was to rearrange the Earth’s mass in such a manner that it brought it a bit closer to the Earth’s rotation axis causing the Earth to rotate a bit faster and the length of the day to be a bit shorter.

Dr. Gross said there won’t be any practical consequences from this shortening of our day. In fact, winds and oceans currents have a bigger effect on the length of the day than this February 2010 earthquake in Chile. In the end, the change is too small to detect even with the most sophisticated instruments. Dr. Gross added that the way an earthquake effects the Earth’s rotation differs based on the latitude where it occurs.

In order to change the Earth’s rotation you have to move mass vertically, up and down, and it turns out that if the earthquake is located on the equator, that vertical mass motion is most effective in changing the length of the day, and if the earthquake is located at mid-latitudes it’s most effective at changing the position of the figure axis. The Chilean earthquake was located at mid-latitudes and so it was quite effective in changing the figure axis of the Earth.

The figure axis, Gross went on to explain, is the axis about which the Earth’s mass is balanced. This is different from Earth’s rotational axis. This difference of axes causes the Earth to literally wobble as it rotates.

So the Earth is wobbling slightly differently than it was before the earthquake.

March 15, 2010

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