What are the geophysical observations that support (or contradict) the hypothesis that the Earth's inner core rotates at a faster rate than the Earth's mantle?
Summary of Answers:
1) Studies of seismic ‘body-waves’ travelling through the Earth’s inner core indicate the inner core is rotating 0.2 to 3 degrees faster than the mantle. These studies are sensitive to heterogeneities in the inner core’s structure.
2) Analysis of the free-oscillations of the Earth following great earthquakes are insensitive to local structure indicate that the differential rotation rate has been within 0.2 degrees over the past 20 years and that the inner core is probably gravitationally locked to the Earth’s mantle.
3) Comparisons of seismic ‘body-waves’ travelling through the Earth’s inner core from earthquake doublets (a pair of earthquakes that occur at the same place but different times,) should also be insensitive to local structure (because the ray-paths would be the same) and can be used to investigate changes over relatively short intervals of time. These studies indicate that the mean rate of rotation of the inner core has been 0.3 to 0.5 degrees per year faster than the mantle, but that the rate has also had decadal fluctuation about the mean of 1 degree per year.