Observational evidence for differential rotation of the inner core has recently been reported from time-dependent travel times of the seismic inner core phase, PKP(DF). These analyses are hindered by potential biases from the mantle heterogeneity and systematic earthquake location errors, which have raised doubts about the reality of the inferred differential rotation. Here we report time-dependent PKP(DF)-PKP(BC) differential travel times of a total of 611 measurements obtained from 92 earthquakes in the South Sandwich Islands (SSI) over the past nearly half a century recorded at the College, Alaska station (COL) and Alaska Seismic Network (ASN). The events are relocated using the joint hypercenter determination (JHD) technique [Dewey, 1971]. Travel time residuals at COL from SSI earthquakes over a period of 45 years and at three additional stations from SSI earthquakes over periods of about 22 years show steady increases with time; the temporal trends are demonstrated to be statistically very significant. With dense samples of the pathway at different time periods, we use a joint inversion technique that allows us to separate the time-dependent inner core structure from the time-independent mantle biases. Surprisingly, the inner core rotation seems resolvable even from digital data observed at ASN in the 1990s (spanning over 8 years) alone in such a joint inversion. The results provide strong support for a differential inner core rotation. The inferred rotation rate ranges from 0.3 degrees to 1.1 degrees per year faster than the mantle when we take into account the uncertainty in the tilt of the anisotropy axis of the patch, of the inner core that was sampled. A westward inner core rotation can be ruled out.
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