Answer: While this question is a little outside the earthquake role of
the USGS, scientists at NASA's Jet Propulsion Laboratory
(JPL) who work with the USGS, have told us that the
effects on the Earth's rotation from an earthquake even
of this magnitude is much too small to be observed. The
length of the day can be measured with an accuracy of
about 20 microseconds and calculations of the source
properties of the earthquake showed the change in the
length of the day to be -2.676 microseconds, or in other
words, less than can be effectively measured.
If you want a more complete and technical answer to this
question, Richard Gross at JPL offers the following:
JPL has modeled the coseismic effect on the Earth's rotation
of the December 26 earthquake in
Indonesia
by using the PREM model for the elastic properties of
the Earth and the Harvard centroid-moment tensor
solution for the source properties of the earthquake.
The result is:
change in length of day: -2.676 microseconds
polar motion excitation X : -0.670 milliarcseconds
polar motion excitation Y: 0.475 milliarcseconds
Since the length of the day can be measured with an
accuracy of about 20 microseconds, this model predicts
that the change in the length-of-day caused by the
earthquake is much too small to be observed. And, since
the location of the earthquake was near the equator,
this model predicts that the change in polar motion
excitation is also rather small, being about 0.82
milliarcsecond in amplitude. Such a small change in
polar motion excitation will also be difficult to
detect.
