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The Shuffling Rotation of the Earth's Inner Core 

Hrvoje Tkalčić, Mallory Young, Thomas Bodin, Silvie Ngo and Malcolm Sambridge

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

We present strong observational evidence from a newly observed collection of earthquake doublets that the Earth’s inner core “shuffles” exhibiting both prograde and retrograde rotation in the reference frame of the mantle. 

This discovery is significant on several levels. First, the observed pattern consists of intermittent intervals of quasi-locked and differentially rotating inner core with respect to the Earth’s mantle. This means that the angular alignment of the inner core and mantle oscillates in time over the past five decades. Jolting temporal changes are revealed, indicating that during the excursions from the quasi-locked state, the Earth’s inner core can rotate both faster and slower than the rest of the planet, thus exhibiting both eastward and westward rotation. According to our results, a short time interval (on the order of one to two years) is needed for the inner core to accelerate to a rotation rate of several degrees per year, and typically a slightly longer time is needed to decelerate down to a negligibly small differential rotation rate. These time scales are in agreement with experimental spin-up times obtained when the magnetic torque alone is used to accelerate the inner core.

Second, the correlation between the observed acceleration in the inner core rotation rate and the observed geomagnetic jerks is statistically highly significant. If we assume a very conservative time width of 2 years for an overlap between an observed geomagnetic jerk and the observed change in the rotation rate in the period from 1969 and 2008, the probability for an individual overlap is 1/20. We then obtain a chance of about 1/500 that the observed geomagnetic jerks and the observed changes in the rotation rate are coincidental. Namely, all three of the most dramatic changes in the rotation rate that we observe correlate with known geomagnetic jerks. Because there is also a documented correlation between the geomagnetic jerks and the Length of Day time series, this all points to the same source and works in favor of a differential rotation rather than processes at the inner core boundary. 

Last but not least, when we integrate the rotation rate over different time intervals, it is possible to explain discrepancies between the body wave and normal modes results for the rate of the inner core rotation found by previous authors. We show that the integrated shift in angular alignment and average rotation rates (previously determined to be constant) in normal mode studies are much smaller that those for the body waves. This reconciliation between the differences in the estimated rotation rates from body wave and normal modes studies is one of the most significant results of this study.

The repeating earthquakes from the South Atlantic generate elastic waves that traverse the Earth’s mantle and core, and are recorded by the seismographs located in the northern hemisphere. The waveform doublets produced by repeating earthquakes present a reliable probe, which can reveal temporal changes exhibited by the inner core due to the fact that the mantle effects are minimized. We observe new waveform-doublets at the College station, Alaska, and analyse all existing doublets recorded at that station using state of the art mathematical methods. The complex temporal pattern of differences in travel times between the first and the second event of a doublet is impossible to explain with a simple linear-fit approach. An ensemble approach utilizing transdimensional and hierarchical Bayesian analysis proves to be a powerful approach in this case, relaxing the choices on model parameterization and revealing hitherto unseen complex dynamics of the Earth’s inner core.