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Is Earth's Rotation Slowing Down?

Earth's rotation

Q: Reports claim that certain events like the Chilean earthquake may have shortened the Earth's day by 1.26 microseconds. But in a previous Ask a Geologist response, 'Magnitude of Tides', Dr Russ Evans claims that the Earth's rate of rotation is slowing down making days longer by 2ms/century. Is the earthquake one such event that stops this process mentioned in the third paragraph? With events like the earthquake producing a counter quickening up, is the Earth slowing down slowly?

So which one is it? Is the Earth slowing down, or quickening up, with events like the Chilean eartquake and the quake causing the tidal wave in Sumatra?

From Vera Westbrook (July 2010)

Reply by Dr Russ Evans (BGS)

As you note, (large) earthquakes do indeed change the length of the day. This is because they make small changes to the distribution of mass (just the distribution, not the total mass) of the Earth, changing its moment of inertia. Parts of the Earth's surface and the material beneath can be lifted up, and other parts pushed down. Depending on the geometry of the earthquake fault and the slip along it, the change can either lengthen or shorten the day or leave it the same length. I haven't worked through the calculations myself, but the figure you mention for the recent Chilean earthquake seems quite reasonable. On average, the net long-term effect of earthquakes on the length of day is practically zero. "Practically zero" rather than "zero" only because the changing distribution of plates, of which earthquakes provide current evidence, plus the differences in density between continental and oceanic regions means that the Earth's moment of inertia does vary by some small amount over long time periods. The likely enlargement of the inner core and possible shrinking of the outer core diameter are other factors that need to be taken into account in very detailed calculations, but all these variations are simply small corrections to the rate of slowing caused by the principal long-term process of tidal locking that I discussed in my response to the earlier question.

In other words, the effect of large earthquakes, although observable on, say, very carefully operated transit telescopes, is to add a small short-term variations to the steady engthening of the day due to tidal locking.