Richard Gross at 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 a change in day length of -0.000002676 seconds (-2.676 microseconds). In a 70 year lifetime this equates to a total slowdown of 0.0683718 seconds – not something that will be noticable by the average human.

More on the scientific analysis of the earthquake can be found on the USGS website.