Seismological evidence for differential rotation of the Earth's inner core

The travel times of seismic waves that traverse the Earth's inner core show a small but systematic variation over the past three decades. This variation is best explained by a rotation of the inner core that moves the symmetry axis of its known seismic anisotropy. The inferred rotation rate is on the order of 1° per year faster than the daily rotation of the mantle and crust.