Investigation of time dependent inner core structure by the analysis of free oscillation spectra

Proposals that the seismic structure of the Earth's inner core is time-dependent are investigated using the splitting functions of free oscillations, determined from seismic data recorded over some 20 years. It is shown that the data support some time-dependence, although the paucity of the data far the 1970's and 1980's makes it difficult to be certain of the precise form of such time dependence. The data do not support a positive (i.e. West-to-East) rotation; rather, the inferred rotational component of the time-dependent inner core structure is negative (i.e. East-to-West), and varies according to the mode under consideration from less than 1 degrees per year to more than 2 degrees per year. The analysis suggests that inner core structure is time-dependent but that the dependence is not well explained by differential rotation of the inner core. This kind of analysis will become much more powerful with increasing quantities of data over a longer time span.