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The rate that Earth's inner core rotates relative to the mantle and crust has been debated for decades. Nonrotational processes, including internal deformation and flow in the outer core, have also been proposed to explain observed seismic changes. The observed changes thus far have been so inconsistent and weak as to hamper convincing interpretation. Here, we examine waves backscattered from within the inner core, which can more robustly evaluate rotation, from two nuclear tests 3 years apart in Novaya Zemlya, Russia. We have extended our previous analysis of these explosions using precise station corrections and the full Large Aperture Seismic Array, thus revealing how the time shifts depend on slowness and lag time and halving our rotation rate estimate. Our derived 0.07 degrees/year inner core superrotation rate from 1971 to 1974 is more robust and slower than most previous estimates and may require interesting reinterpretations of localized signals previously interpreted as inner core rotation.

Plain Language Summary The Earth's solid inner core, which is surrounded by the liquid outer core, is not attached to the overlying mantle. It is only inhibited from differentially spinning by gravitational, magnetic, and viscous coupling through the outer core. Rotation has been inferred, but studies do not agree on the rate nor the time variation in the rate nor even on whether differential spinning takes place. By comparing backscattering from the inner core due to two large nuclear tests in the same place 3 years apart, we measure the motion of the inner core with unprecedented resolution and find very slow motion, 0.07 degrees superrotation per year from 1971 to 1974. This slow rate, if persistent, has difficulty explaining observations in other studies of localized core changes, and so may require additional dynamic physics be active near the inner core boundary.