Independent estimate of velocity structure of Earth's lowermost outer core beneath the northeast Pacific from PKiKP - PKPbc differential traveltime and dispersion in PKPbc

The presence of a low-velocity layer at the base of Earth's outer core has been proposed. However, the seismic profile of the basal layer indeed has been poorly constrained. In previous seismic studies the model parameters of the layer are substantially nonunique and there are tradeoffs between the seismic velocity of the layer and the properties of the mantle and inner core. A more tightly constrained profile of the layer helps further examine the composition and dynamics of the layer. In this study we obtained the P wave velocity profile of the basal layer beneath the northeast Pacific based on two new seismic observations by analyzing seismograms of the Hi-net in Japan. The new observations are particularly sensitive to the layer structure and are relatively insensitive to the structure of the other parts of the Earth: (1) the frequency dispersion in P waves that graze or are diffracted at the inner core boundary (PKPbc and PKPc-diff) and (2) differential traveltimes between the P waves reflected from the inner core boundary (PKiKP) and those that turn above the boundary (PKPbc). The resulting velocity model of the lowermost outer core (called "F layer velocity model for the Western Hemisphere" (FVW)) has P wave velocities that lie between those of AK135 and the preliminary reference Earth model (PREM), and a velocity gradient that is slightly gentler than that of PREM. Models with a uniform P wave velocity value within the layer are not supported by the observations for the region investigated, which appears not to support the presence of a thick basal layer that is Fe rich and dense there.