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

Abstract

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.

Seismological Evidence for Laterally Heterogeneous Lowermost Outer Core of the Earth

“…On the other hand, like the case for the South America events, the frequency dependence of DF arrival times is nearly constant, and close to zero (Figure c). These results clearly indicate that the origin of the dispersion does not lie in the inner core but in the F layer, as discussed in Ohtaki and Kaneshima (), and suggest somewhat complicated structure in the F layer beneath Australia compared with the region beneath the northeast Pacific. This feature is summarized in Figure , which shows the means of the observed BC – DF dispersion times for the events in Table .…”

“…We correct the effect of the reference model on differential times using ray‐theoretical traveltimes between the two models to compare observed values between the two regions. However, as discussed in Ohtaki and Kaneshima (), the choice of the reference model does not affect the results.…”

“…First, we briefly summarize our method to determine the V P structure of the F layer, which analyzes BC dispersion and CD – BC traveltimes as described in detail in Ohtaki and Kaneshima (). Next, we discuss minor differences from the previous study.…”

“…Their results have slower velocities in the western F layer than in the eastern layer, which may suggest the existence of chemical heterogeneity. However, their observations should have poor resolution in the F layer and are insufficient to resolve detailed structures (Ohtaki & Kaneshima, ).…”

Array‐Based Iterative Measurements of Travel Times and Their Constraints on Outermost Core Structure

“…However, there is increasing seismic evidence for structural complexities close to its top and bottom boundaries. A stratified layer with a lower gradient than the Preliminary Reference Earth Model (PREM; Dziewonski & Anderson, ), labeled the layer, has been documented using body seismic wave observations (Ohtaki & Kaneshima, ; Souriau & Poupinet, ; Song & Helmberger, ; Zou et al, ). Another stratified layer, the layer, is hypothesized to exist at the top of outer core, and its properties may be constrained by geomagnetic secular variations (Buffett, ; Gubbins, ), but the seismic evidence, especially differential arrival times, for this layer is contradictory and controversial (e.g., Alexandrakis & Eaton, ; Eaton & Kendall, ; Helffrich & Kaneshima, ; Kaneshima & Helffrich, ).…”