Seismic structure near the inner core boundary in the south polar region

It is crucial to obtain good spatial coverage of seismic data points for better understanding the Earth's core, but the core beneath the polar regions remains largely unexplored. We analyzed differential traveltimes and amplitude ratios of core phases whose raypaths run beneath Antarctica for determining the V-p and Q(p) structure near the inner core boundary in the south polar region. The model we obtained (south polar region model, SPR) is described relative to the preliminary reference Earth model (PREM) as follows: a 0.05 km/s lower V-p value at the top of the inner core, a 1.5 times steeper V-p gradient in the upper 300 km of the inner core, a smaller Q(p) (300) in the upper 300 km of the inner core, and a 0.04 km/s lower V-p at the bottom of the outer core. The V-p values of SPR in the lowermost outer core lie between those of PREM and AK135, being closer to those of AK135. The lowermost outer core V-p inside the tangent cylinder is thus close to the global average. In the upper inner core, SPR has lower V-p than AK135 and PREM. The SPR V-p profile is close to that of previous models for the Western Hemisphere, although most of our data sample the Eastern Hemisphere of the inner core. Our results indicate that the inner core does not have a simple hemispherical variation as usually supposed. Our data support an eyeball-shaped high-V-p anomaly with compressional velocity higher than in 1-D reference Earth models, concentrated to a smaller region beneath eastern Asia.