Inner Core Rotation Captured by Earthquake Doublets and Twin Stations

The Earth's solid inner core (IC) is generally believed to rotate relative to the mantle, but the proposal remains controversial. Here we use seven waveform doublets in the South Sandwich Islands region with time lapses of 5.8–17.0 years that are recorded by two close stations in Kyrgyzstan with virtually the same epicentral distance. The fortuitous geometry allows precise measurements of the IC temporal changes and the underlying local structure at the same time. The remarkable observations in waveforms and spatial-temporal measurements show unequivocally that the IC must have shifted (rotated) eastward in 1991–2010 and help determine accurately the average rotation rate as 0.127 ± 0.006°/yr at 95% confidence level during the time span.