Xiaoxia Xu and Xiaodong Song
Department of Geology, University of Illinois, Urbana, IL, USA
In an effort to confirm and constrain inner core rotation, we examine earthquakes from South Sandwich Islands (SSI) recorded at the Beijing Seismic Network (BSN). The SSI-BSN pathway has several characteristics that are amenable to study inner core rotation. (1) The epicentral distance is almost the same as the distance from SSI to College, Alaska, which was used previously to study inner-core rotation. This distance range is favorable for observing simultaneously DF and BC branches of PKP. Thus differential PKP BC-DF times can be used to reduce biases from mantle heterogeneities and source uncertainties. (2) BSN has a long history of operation (from 1966 to present). (3) The angle between this pathway and the spin axis is about $\rm 40^o$. This is a reasonably good geometry for detecting a shift of a possible laterally-varying local structure of the inner core, if the inner core rotates around the Earth's spin axis. We obtained a total of 109 high-quality measurements of BC-DF differential travel times by waveform cross-correlations from earthquakes spanning over 30 years. The results show robust temporal changes in BC-DF residuals along this pathway, providing new support for the differential inner core hypothesis. We also observed evidence for a lateral velocity gradient in this part of the inner core. Assuming that the temporal changes result from a shift of a lateral structure due to an inner core rotation, we estimate the inner core rotation rate to be about $0.41 \pm 0.12\rm ^o$/yr faster than the mantle.