Earth's inner core rotation, 1971 to 1974, illuminated by inner-core scattered waves

Abstract

The solid inner core is at the center of the Earth, gravitationally held within the liquid outer core. It is one of the most dynamic parts of Earth's interior. Since the initial claim of inner core differential rotation relative to the mantle, its existence and rate have been challenged for over two decades. Here, we re-examine the seismic records of two megaton nuclear tests in Novaya Zemlya, Russia, three years apart, from the Large Aperture Seismic Array in Montana, USA. Using an improved static time correction from an antipodal earthquake, we refine the resolution of the beamforming of PKiKP and its inner-core scattered coda. Then, we measure the slight time shifts (tenths of seconds) between the inner-core-scattered waves from the two events with moving-time-window cross-correlation. Applying a novel back-projection method, we locate the inner-core regions that scatter the energy within the PKiKP coda based on its slowness and the lapse time. We then measure the inner core rotation, first assuming alignment with Earth's rotation axis, then finding the best-fitting differential rotation axis with a grid search. The rotation rates here are robust and consistent across the many scattered arrivals throughout the inner core scattering wavetrain. Our results indicate 0.10°/year inner core super-rotation rate from 1971 to 1974 aligned with Earth's rotation axis, or 0.125°/year with the rotation axis tilting about 8° from the Earth's rotation axis, which yields a marginally better fit to the observed time shifts.