Inner Core Rotation Captured by Earthquake Doublets and Twin Stations

Temporal changes of PKIKP waves through the inner core from earthquake waveform doublets provide compelling evidence for the temporal variability of the inner core. However, the general interpretation of a differential rotation of the inner core relative to the mantle has still been debated. Here we use seven pairs of waveform doublets in the South Sandwich Islands (SSI) region in 1991-2010 with time separation of 5.8 to 17.0 years. The doublets were recorded by two close stations (AAK and KZA, separated by 0.79°) in Kyrgyzstan that have virtually the same distance from any SSI event. The ray paths in the inner core are separate by about 0.80-0.82° in longitude. The fortuitous geometry captures the inner core motion and the underlying structure at the same time. While clear temporal changes are observed in AAK between the doublet events and clear local lateral inner core variations are observed between AAK and KZA for a same event, the AAK waveform of old event agrees better with the KZA waveform of the new event for the doublets. For three doublets with time separations of 7-10 years, the waveforms agree quite well, suggesting the KZA station (east of AAK) was capturing the inner core structure sampled by the AAK station earlier. Measurements of the temporal changes at AAK show they are proportional to the lateral spatial changes between AAK and KZA. These spatial and temporal correlations demonstrate unequivocally that the inner core rotates eastwards and allow us to determine the rate accurately at 0.12°/year with two standard deviations of less than 0.03°/year in 1991-2010.