Limits on differential rotation of the inner core from an analysis of the Earth's free oscillations

Abstract

Differential rotation of the Earth's inner core has been inferred by several seismic 'body-wave' studies which indicate that the inner core is rotating at a rate between 0.2°and 3°per year faster than the Earth's crust and mantle. The wide range in inferred rotation rate is thought to be caused by the sensitivity of body-wave studies to local complexities in inner-core structure. Free-oscillation 'splitting functions', on the other hand, are insensitive to local structure and therefore have the potential to estimate differential rotation more accurately. A previous free-oscillation study, however, was equivocal in its conclusions because of the relatively poor quality and coverage of the long-period digital data available 20 years ago. Here we use a method for analysing free oscillations which is insensitive to earthquake source, location and mechanism to constrain this differential rotation. We find that inner-core differential rotation is essentially zero over the past 20 years (to within ±0.2°per year), implying that the inner core is probably gravitationally locked to the Earth's mantle.