Stratification of earth's outermost core inferred from SmKS array data

SmKS arrivals recorded by large-scale broadband seismometer arrays are analyzed to investigate the depth profile of P wave speed (V-p) in the outermost core. The V-p structure of the upper 700 km of the outer core has been determined using SmKS waves of Fiji-Tonga events recorded at stations in Europe. According to a recent outer core model (KHOMC), the V-p value is 0.45 % slower at the core mantle boundary (CMB) than produced by the Preliminary Reference Earth Model (PREM), and the slow anomaly gradually diminishes to insignificant values at similar to 300 km below the CMB. In this study, after verifying these KHOMC features, we show that the differential travel times measured for SmKS waves that are recorded by other large-scale arrays sampling laterally different regions are well matched by KHOMC. We also show that KHOMC precisely fits the observed relative slowness values between S2KS, S3KS, and S4KS (SmKS waves with m = 2, 3, and 4). Based on these observations, we conclude that SmKS predominantly reflect the outer core structure. Then we evaluate biases of secondary importance which may be caused by mantle heterogeneity. The KHOMC V-p profile can be characterized by a significant difference in the radial V-p gradient between the shallower 300 km and the deeper part of the upper 700 km of the core. The shallower part has a V-p gradient of -0.0018 s(-1), which is steeper by 0.0001 s(-1) when compared to the deeper core presented by PREM. The steeper V-p gradient anomaly of the uppermost core corresponds to a radial variation in the pressure derivative of the bulk modulus, K' = dK/dP. The K' value is 3.7, which is larger by about 0.2 than that of the deeper core. The radial variation in K' is too large to have a purely thermal origin, according to recent ab initio calculations on liquid iron alloys, and thus requires a thick and compositionally stratified layering at the outermost outer core.