Inner core differential rotation inferred from antipodal seismic observations

Abstract

The Earth's Inner Core may rotate at a different speed than Earth's crust–mantle. We use observations for antipodal earthquake and station pairs to examine the arrival time shift of the whispering-gallery wave which propagates tangentially along the top of the Inner Core. Observations of these waves for earthquakes which occurred at the same location reveal that travel times vary by ≥0.2 s over the past 20 years. We propose that these time shifts are caused by the interaction of a zero rigidity patch (lateral heterogeneity) at the Inner Core surface with the Inner Core differential rotation. The simplest interpretation infers a rotation rate of 0.05°/year, which is one order of magnitude smaller than those reported by previous studies. Our result may also give constraint to the provenance of a zero rigidity patch at the Inner Core surface. The Fresnel zone coverage of whispering-gallery PKIIKP+… antipodal measurements beneath the Inner Core boundary extend over ~60% of the boundary surface, in contrast to PKiKP–PKP paths which individually measure specular reflections (~1 Hz) from the top of the ICB over Fresnel zones each encompassing 0.002% of the Inner–Outer Core boundary.