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

Abstract

The finding that the Earth's inner core might be rotating faster than the mantle has important implications for our understanding of core processes, including the generation of the Earth's magnetic field. But the reported signal is subtle--a change of about 0.01 s per year in the separation of two seismic waves with differing paths through the core. Subsequent studies of such data have generally supported the conclusion that differential rotation exists, but the difficulty of accurately locating historic earthquakes and possible biases induced by strong lateral variations in structure near the core-mantle boundary have raised doubt regarding the proposed inner-core motion. Also, a study of free oscillations constrained the motion to be relatively small compared to previous estimates and it has been proposed that the interaction of inner-core boundary topography and mantle heterogeneity might lock the inner core to the mantle. The recent detection of seismic waves scattered in the inner core 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, USSR, in 1971 and 1974. The data show small but coherent changes in scattering which point toward an inner-core differential rotation rate of 0.15 degrees per year--consistent with constraints imposed by the free-oscillation data.

The Earth's free oscillations and the differential rotation of the inner core

“…Our result is also consistent with recent body wave studies that give eastward rotation rates between 0.15 and 0.25 • /yr (e.g. Creager, 2000;Vidale et al, 2000).…”

“…Creager (2000) re-analyzed a large dataset of differential travel times and now sets the lower limit of the IC rotation rate to 0.15 • /yr which is actually consistent with his observations of 1997. This result would also be consistent with the small rate of 0.15 • /yr found in a study that investigated the time-dependence of inner core scattered waves generated by sources at the Novaya Zemlya test site and observed at the LASA (Large Aperture Seismic Array) in Montana (Vidale et al, 2000). Song (2000) re-analyzed his dataset in a joint inversion for inner core structure and superrotation and now finds that the superrotation is 0.6 • /yr which is less than 1 • /yr but still the highest recently published value.…”

A theory of the Earth's magnetic field and of sunspots, based on a self-excited dynamo incorporating the Hall effect

“…The theory to be explored here is that the metallic solid inner core of the Earth is rotating with respect to the mantle, as has been confirmed by Song and Richards (1996) and Vidale et al (2000). This rotation maintains an angular velocity gradient across the outer core.…”

“…If the initial slope were greater than that of F i vs. d, it would certainly denote a stable offset from the Earth's spin axis. However, the estimate ω = 8.3 × 10 −11 rad s −1 (Vidale et al, 2000) gives…”

Localized temporal variation of Earth’s inner-core boundary from high-quality waveform doublets

“…However, seismological studies have suggested structural and topographic complexities of the ICB. The existence of strong scattering from the inner core (Vidale et al 2000;Koper et al 2004) suggests heterogeneity at a scale length of tens of kilometers, which may be partly responsible for the attenuation of the waveforms passing through the inner core (Cormier et al 1998). The variability of pre-critical PKPcd (or PKiKP) amplitudes has been used to infer the mosaic structure of the ICB (Krasnoshchekov et al 2005), which includes patches of a thin partially liquid layer and patches of a sharp transition.…”