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

The finding that the Earth's inner core might be rotating faster than the m antle(1) has important implications for our understanding of core processes , including the generation of the Earth's magnetic field(2,3). But the repo rted signal is subtle-a change of about 0.01 s per year in the separation o f two seismic waves with differing paths through the core. Subsequent studi es of such data have generally supported the conclusion that differential r otation exists(4-6), but the difficulty of accurately locating historic ear thquakes(7) and possible biases induced by strong lateral variations in str ucture near the core-mantle boundary(8) have raised doubt regarding the pro posed inner-core motion(9). Also, a study of free oscillations(10) constrai ned the motion to be relatively small compared to previous estimates and it has been proposed that the interaction of inner-core boundary topography a nd mantle heterogeneity might lock the inner core to the mantle(11). The re cent detection of seismic waves scattered in the inner core(12) 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, U SSR, in 1971 and 1974. The data show small but coherent changes in scatteri ng which point toward an inner-core differential rotation rate of 0.15 degr ees per year-consistent with constraints imposed by the free-oscillation da ta(10).