Sabine,
Earthquakes redistribute the Earth’s mass on a global scale.
Eventually, after an earthquake the Earth mass can be slightly closer to the Earth axis of rotation than before the earthquake.
Despite these changes, the rotation momentum of the Earth will not change.
This is a consequence of the Newton's law of motion.
Without external forces on a mechanical system, the rotation momentum never changes.
During an earthquake, internal forces are on display, but no external forces.
The rotation momentum that doesn't change is given by R=I\omega.
I is the moment of inertia which represents the distribution of mass around the axis of rotations of the earth.
I decreases when the mass gets closer to the axis of rotation.
\omega={2\pi}/{T} is the angular rotation speed of the earth. (T is the period of rotation: roughly 86400 seconds)
\omega increases when I decreases, to keep R constant.
The relation between mass distribution and angular speed is well known, see http://www.exploratorium.edu/snacks/momentum_machine.html for a famous school experiment.
On wikipedia you can find the maths behind the conservation of rotation momentum:
look here .
If you want some specialised information, http://www.ecgs.lu/pdf/jlg92/JLG92_Gross.pdf
You can also find details on the Earth rotation http://www.agu.org/reference/gephys/24_dickey.pdf .
If you can have fun with calculations, you could try to re-evaluate the 3µs you mentioned in your question.
To do this, you will need the moment of inertia of the earth, an evaluation of the mass displaced (assume a Earth crust thickness of 5km and a density of 5 maybe, and some geographical area), an evaluation of the displacement in direction of the axis of rotation.
The last reference explain other sources of changes in the Earth rotation speed. The 3 µs are small in comparison.
Have fun !
