Localized boundary layer below the mid‐Pacific velocity anomaly identified from a PcP precursor

Dense record sections from deep earthquakes in Fiji and Argentina recorded on hundreds of short‐period stations in California at distances of 81° to 85° are used to investigate the detailed P wave velocity structure above the core‐mantle boundary (CMB). In the Fiji data a secondary phase arriving 2 to 4 s after the direct P is identified as a precursor to PcP . This phase provides good evidence for a reflection off the top of a thin low‐velocity layer above the CMB. Comparisons to synthetic seismograms indicate a layer thickness of 10 km and a velocity reduction of 5%–10% compared to the overlying mantle. A record section from an Argentina event does not show the PcP precursor, indicating that the low‐velocity layer is not a global feature. This thin low‐velocity layer is in the same place as a much larger S wave velocity anomaly in the lower mantle and is probably indicative of a boundary layer just above the CMB under the mid‐Pacific.