Vp structure of the outermost core derived from analysing large-scale array data of SmKS waves

The Earth'score is composed primarily of iron with several per cent by weight of lighter elements, which are thought to be progressively enriched in the outer liquid core as the core cools and the inner core crystallizes. Detailed profiles of P-wave speed (V_p) of the outermost core might be affected by this process. In this study, we constrain the radial V_p structure in the outermost 600 km of the core by analysing SmKS multiples with m up to 6, which are observed at large-scale broad-band seismometer arrays. Array processing of SmKS waves combined with investigation of their CMB piercing points clearly demonstrate that the array averaged SmKS differential traveltime anomalies primarily reflect the V_p structure in the outermost core. A p inversion is applied to the SmKS (m = 2, 3, 4 and 5) differential times measured by the array processing. The inversion reveals that the V_p is 0.45 per cent slower at the CMB than Preliminary Reference EarthModel (PREM) and the lower V_p anomaly gradually diminishes to zero at 300 km below the CMB. The deviation of the V_p gradient from PREM of the outermost 300 km of the core is nearly an order of magnitude larger than that of the deeper part of the core. The evaluation of Birch's parameter 1 - g_-1dφ/dr (φ = V² _p ) for the obtained V_p profile as well as the change in the V_p gradient with depth show that adiabatic self-compression of a homogeneous material cannot explain the observations. Waveforms corresponding to the arrivals of S6KS waves are consistent with the V_p profile of the topmost 50 km of the outer core, and agree with the results from the p inversion. This excludes the presence of a high V_p-low density layer thicker than 10 km immediately beneath the CMB.