Abstract
We investigate the differential rotation of Earth's inner core relative to the mantle using pairs of precisely located nuclear explosions. We find that the inner core subrotated at least 0.1° from 1969 to 1971, in contrast to superrotation of ~0.29° from 1971 to 1974. These observations contradict models of steady inner core rotation and models that posit much faster rotation rates. The reversal of polarity, timing, and rotation rates is consistent with a model of oscillations about an equilibrium with gravitational locking of the mantle and inner core due to lateral density variations. The model, which has a 6-year period, can explain the variation in the length of day, which has oscillated fairly steadily for the past decades. Inner core oscillation would also allow interpretations of causal connections between inner core and mantle lateral variations, which are problematic if the inner core consistently superrotates.