The stably‐stratified outermost core revisited

The P‐wave velocity structure of the outermost 200 km of the outer core (the E' region) is analyzed using a suite of differential travel‐times for SKS, SKKS and S‐waves bottoming below, within, and above this depth range. Core‐penetrating phases with paths beneath Alaska exhibit similar SKS‐SKKS differential travel‐time patterns to phases sampling the core beneath the central Pacific, but very different SKS‐S differential travel‐times. These patterns can be explained by a core velocity model in which the outermost 50 to 100 km has 1–2% slower P‐wave velocities than standard Earth models, perhaps due to a stably‐stratified chemical boundary layer, combined with 3% lateral variations in S‐wave velocity in the lowermost mantle.