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

Abstract

Differential rotation of the inner core has been inferred by several body-wave studies with most agreeing that a superrotation may exist with a rate between 0.2◦ and 3◦ per year. The wide range of inferred rotation rate is caused by the sensitivity of such studies to local complexities in structure which have been demonstrated to exist. Freeoscillation “splitting functions” are insensitive to local structure and are therefore better candidates for estimating differential IC rotation more accurately. We use a recently developed method for analyzing free oscillations which is insensitive to earthquake source, location and mechanism to constrain this differential rotation. In a prior study, we found that inner core differential rotation has been essentially zero over the last 20 years. We revisit this issue, including additional earthquakes and modes in our analysis. Our best estimate is a barely significant superrotation of 0.13±0.11◦/yr, which is still consistent with the idea that the inner core is gravitationally locked to the mantle.