Constraining shear wave velocity and density contrast at the inner core boundary with PKiKP/P amplitude ratio

Abstract

Shear velocity and density contrast across the inner core boundary are essential for studying deep earth dynamics, geodynamo and geomagnetic evolution. In previous studies, amplitude ratio of PKiKP/PcP at short distances and PKiKP/P at larger distances are used to constrain the shear velocity and density contrast, and shear velocity in the top inner core is found to be substantially smaller than the PREM prediction. Here we present a large dataset of PKiKP/P amplitude ratio measured on 420 seismic records at ILAR array in Alaska for the distance range of 80°–90°, where the amplitude ratio is sensitive to shear velocity and density contrast. At high frequency (up to 6 Hz), mantle attenuation is found to have substantial effects on PKiKP/P. After the attenuation effects are taken into account, we find that the density contrast is about 0.2–1.0 g/cm3, and shear velocity of inner core is 3.2–4.0 km/s, close to the PREM (Preliminary Reference Earth Model) prediction (0.6 g/cm3 and 3.5 km/s, respectively). The relatively high shear velocity in inner core does not require large quantities of defects or melts as proposed in previous studies.