Is there any structure inside the liquid outer core?

Abstract

The anomalous splitting of some core‐sensitive eigenmodes suggests the presence of a structure in the liquid core. Geodynamical considerations rule out density heterogeneities, however they do not rule out the possibility of anisotropy in the liquid core. The seismic polar paths through the anisotropic inner core, which are anomalously fast, also sample the liquid core inside the cylinder tangent to the inner core, a structure insulated from the rest of the core during rotation. We investigate the possibility that anisotropy be present in that part of the liquid core, using PKPbc waves which have a long path inside the tangent cylinder, but which do not sample the inner core. We also investigate the possibility of heterogeneities which could have deposited beneath the polar caps, using differential travel times of S3KS‐SKKS waves. The data do not reveal heterogeneity or anisotropy inside the tangent cylinder, nor the presence of anomalous polar caps.

Empirical transfer functions: Application to determination of outermost core velocity structure using SmKS phases

“…We sorted ETFs into bins according to SKKS ray parameter, discarded bins containing 4 or fewer traces, and stacked the results to create a global record section (Figure 3b). Our interpretation of the stacked ETF record section assumes that the outer core is much more homogeneous than the lowermost mantle [e.g., Souriau et al, 2003]. This implies that by stacking a sufficient number of observations, perturbations caused by lower mantle heterogeneity will tend to average out.…”

“…The phase shift arises from: (1) a Hilbert transform associated with the caustic at the turning point, (2) a small phase rotation, due to post-critical reflection from the base of the CMB, and (3) finite-frequency effects associated with the proximity of the turning point to the CMB [Choy, 1977]. While previous studies have usually incorporated the Hilbert transform [e.g., Souriau et al, 2003;Garnero et al, 1993;Tanaka, 2004], the remaining phase complications have generally been neglected. Here, we introduce ETF deconvolution as a way to simplify the analysis and inter-pretation of differential times between such closely related teleseismic phases.…”

“…Under the assumption that source and mantle path effects are the same for each of these SmKS phases, differential times may be used to infer velocity structure of the outer core above the respective turning depths. Although significant differences exist between various proposed models, all previous studies indicate that seismic velocities of the outermost core are generally overestimated by PREM [Souriau and Poupinet, 1991;Garnero et al, 1993;Garnero and Helmberger, 1995;Souriau et al, 2003;Tanaka, 2004;Tanaka, 2007]. Eaton and Kendall [2006] found that a better fit is achieved for all models by inserting a thin (12 km) high-velocity, low-density layer near the top of the core.…”

“…[4] Most previous SmKS studies have focused on differential times between SKS, SKKS and S3KS phases, either using single-station techniques [Souriau and Poupinet, 1991;Garnero et al, 1993;Garnero and Helmberger, 1995;Souriau et al, 2003] or array techniques [Tanaka, 2004;Eaton and Kendall, 2006]. Under the assumption that source and mantle path effects are the same for each of these SmKS phases, differential times may be used to infer velocity structure of the outer core above the respective turning depths.…”

“…[5] Global compilations of SmKS traveltime residuals along individual paths [Souriau et al, 2003] exhibit a high degree of scatter, likely caused by lateral velocity variations in the D 00 region [Garnero et al, 1993]. Tanaka [2007] stacked long-period (T > 10s) waveforms to mitigate this effect, but due to source-and path-dependent waveform variability did not stack shorter period signals, thus sacrificing some fine-scale resolution of outer core structure.…”