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.

[1] It is crucial to obtain good spatial coverage of seismic data points for better understanding the Earth's core, but the core beneath the polar regions remains largely unexplored. We analyzed differential traveltimes and amplitude ratios of core phases whose raypaths run beneath Antarctica for determining the V p and Q p structure near the inner core boundary in the south polar region. The model we obtained (south polar region model, SPR) is described relative to the preliminary reference Earth model (PREM) as follows: a 0.05 km/s lower V p value at the top of the inner core, a 1.5 times steeper V p gradient in the upper 300 km of the inner core, a smaller Q p (300) in the upper 300 km of the inner core, and a 0.04 km/s lower V p at the bottom of the outer core. The V p values of SPR in the lowermost outer core lie between those of PREM and AK135, being closer to those of AK135. The lowermost outer core V p inside the tangent cylinder is thus close to the global average. In the upper inner core, SPR has lower V p than AK135 and PREM. The SPR V p profile is close to that of previous models for the Western Hemisphere, although most of our data sample the Eastern Hemisphere of the inner core. Our results indicate that the inner core does not have a simple hemispherical variation as usually supposed. Our data support an eyeball-shaped high-V p anomaly with compressional velocity higher than in 1-D reference Earth models, concentrated to a smaller region beneath eastern Asia.

A model of P‐wave velocity of the Earth's core 500 km above and below the inner core boundary (ICB) beneath North America is constructed from travel time analysis and broad‐band waveform modeling of core phases at distances from 130° to 160°. Differential travel times between PKPBC (Cdiff) and PKPDF (TBC–TDF) are about 0.5 sec shorter than those computed from PREM at distances from 146° to 152° and increase with distance to nearly the same as those from PREM at 155°. Differential travel times between PKPCD and PKPDF (TCD–TDF) at distances from 130° to 143° are approximately 0.2 sec shorter than those of PREM. Amplitudes of PKPBC (Cdiff) relative to PKPDF (ABC/ADF) at distances 150° to 156° are about 40% larger than those from PREM. These observations require a smaller P‐wave velocity gradient (0.0005 sec−1) in the lowermost 300 km of the outer core than that from PREM (∼0.00065 sec−1), a slower P‐wave velocity (10.96 km/sec) at the top of the inner core, and a larger velocity gradient (0.0005 sec−1) in the uppermost 300 km of the inner core.

We investigated the spatial distribution of lateral heterogeneity in the upper mantle in the western Pacific region analyzing transverse components of teleseismic P‐coda recorded by broad‐band seismic networks. Large amplitude transverse components are observed at stations close to plate boundaries such as island arcs for the frequency range of 0.04–2.56 Hz, while small amplitudes are observed at stations on stable continents. Our envelope inversion applied to the observed P‐coda reveals large scattering coefficients at depths from 100 to more than 300 km beneath the island arcs, which are interpreted as strong heterogeneities originating from subducting slabs and mantle diapirs. Analyses of a dense seismic network further show a significant difference of heterogeneity between East and West Japan: the former is more heterogeneous than the latter. In contrasts, scattering coefficients less than 20% of the maximum beneath the island arcs are obtained beneath the stable continents, which represents homogeneous and transparent upper mantle.