According to a report by scientists from Columbia University's Lamont-Doherty Earth Observatory and the University of Illinois at Urbana-Champaign, the solid earth's core spins around one-quarter to one-half degree faster each year than the crust.
The researchers came up with their theory by measuring some 30 earthquakes and found differences in the travel times and shape of the waves as they traveled through the earth's core. The rate may vary over time, also.
"What we see right now is a snapshot of a long time process between the magnetic field and the inner core. I do expect to see this rate change with time. What is surprising for us is that we could actually see it in such a short time scale." source