Abstract

Earth’s inner core (IC) is a central but poorly understood frontier for the Earth sciences. Within the inner core, fine-scale heterogeneity has previously been identified from the backscattering of high-frequency compressional waves. Here, we use the inner-core-scattered waves recorded by the Large Aperture Seismic Array, USA, between 1969 and 1975 to determine the geographical distribution of the scatterers using a novel back-projection method. Our model has two regions of strong scattering, one beneath eastern Asia and the other beneath South America, both located where past local surveys have identified scattering structures. We suggest that these loci of strong, fine-scale heterogeneities may be related to random alignments of small, inner-core crystals due to fast freezing. 

On the other hand, we re-examine the seismic records of two megaton nuclear tests in Novaya Zemlya, Russia, three years apart. We measure the slight time shifts between the inner-core-scattered waves from the two events. Based on the locations of scatterers in the inner core, we find the rotation of the inner core is 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 the best-fitting 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.