Slow differential rotation of the Earth's inner core indicated by temporal changes in scattering

The finding that the Earth's inner core might be rotating faster than the mantle1 has important implications for our understanding of core processes, including the generation of the Earth's magnetic field2,3. But the reported signal is subtle—a change of about 0.01 s per year in the separation of two seismic waves with differing paths through the core. Subsequent studies of such data have generally supported the conclusion that differential rotation exists4,5,6, but the difficulty of accurately locating historic earthquakes7 and possible biases induced by strong lateral variations in structure near the core–mantle boundary8 have raised doubt regarding the proposed inner-core motion9. Also, a study of free oscillations10 constrained the motion to be relatively small compared to previous estimates and it has been proposed that the interaction of inner-core boundary topography and mantle heterogeneity might lock the inner core to the mantle11. The recent detection of seismic waves scattered in the inner core12 suggests a simple test of inner-core motion. Here we compare scattered waves recorded in Montana, USA, from two closely located nuclear tests at Novaya Zemlya, USSR, in 1971 and 1974. The data show small but coherent changes in scattering which point toward an inner-core differential rotation rate of 0.15° per year—consistent with constraints imposed by the free-oscillation data10.