Seismological Constraints on Inner Core Properties

We constrain seismic properties of the inner core using various body wave observations. In the epicentral distance range of 10−70 o , our high quality measurements of the amplitude ratio of PKiKP and PcP provide a new estimate of the density contrast (0.6-0.9 g/cm 3) at the Inner Core Boundary (ICB). This estimate is compatible with a recent reevaluation of normal mode data, and thus reconciles the long-term discrepancy between body wave and normal mode measurements. Our Q α study based on the amplitude ratio of PKIKP and PKiKP reveals an attenuation transition at the top of the inner core. In the western hemisphere Q α decreases first with depth from the ICB and then turns to increase at a depth of ∼ 85km, and in the eastern hemisphere we infer that the attenuation transition takes place at the top ∼ 32km beneath the ICB. We find high Q α and low velocity in the western hemisphere, and low Q α and high velocity in the eastern hemisphere. This remarkable hemispherical pattern starts to disappear at a depth of ∼ 85km. This might constrain the depth extent of the mushy zone at the top of the inner core. Taking advantage of the long-term high quality broadband Gräf enberg seismic array, we observed a reliable inner core shear wave phase PKJKP. Four kinds of evidence (travel time, slowness, back-azimuth, and comparison with a pseudo-liquid inner core model) are jointly presented. The high signal-to-noise waveform of PKJKP, for the first time, gives us an opportunity to constrain the inner core shear wave velocity and attenuation directly using body waves. The envelope modeling results suggest that, in the inner core, the shear wave velocity 2 may be slightly faster (∼ 1.5%) and Q β may be significantly larger (∼ 315 ± 150) than obtained from normal mode observations. Taking into account the fact that normal mode data mainly sample the shallow portion of the inner core and PKJKP samples the central part, this implies that both shear wave velocity and Q β increase with depth in the inner core.