Hemispherical anisotropic patterns of the Earth's inner core

Abstract

It has been shown that the Earth's inner core has an axisymmetric anisotropic structure with seismic waves traveling approximately 3% faster along polar paths than along equatorial directions. Hemispherical anisotropic patterns of the solid Earth's core are rather complex, and the commonly used hexagonal-close-packed iron phase might be insufficient to account for seismological observations. We show that the data we collected are in good agreement with the presence of two anisotropically specular east and west core hemispheres. The detected travel-time anomalies can only be disclosed by a lattice-preferred orientation of a body-centered-cubic iron aggregate, having a fraction of their [111] crystal axes parallel to the Earth's rotation axis. This is compelling evidence for the presence of a body-centered-cubic Fe phase at the top of the Earth's inner core.

Fig. 1.

PKiKP-PKIKP travel-time residuals versus epicentral distances for ray paths spanning a turning depth point ranging from 1.72 to 107.14 km (Δ = 121.29°–143.82°). The uncertainty in picking is ± 0.1 s. Differential travel-time residuals have been calculated with respect to the isotropic PREM (3). Blue and red symbols indicate the eastern and the western hemisphere, respectively. Green open symbols refer to the polar data beneath Africa (46). Positive (negative) travel-time residuals show higher (lower) seismic velocity in the top part of the Earth’s IC compared to PREM. The thick solid (dashed) line indicates the predicted PKiKP-PKIKP travel-time residuals by using a PREM-like model 0.5% faster (0.3% slower).

Fig. 2.

V_p velocity models for both bcc (solid blue) and hcp (dashed red) iron relative to their equatorial velocities [V_p(ξ = 90°)]. The ray angle ξ defines the angle between the inner-core leg of the PKIKP ray path and the Earth’s rotation axis.

Fig. 3.

(A) Absolute PKiKP-PKIKP differential travel-times (|δt|) as a function of ray angles for a cylindrically averaged bcc aggregate (). The solid gray lines show the Transversely Isotropic Model curves, where the t parameter ranges between 7.799 and 123.234 s, so as to cover the entire interval of observed epicentral distances (Δ = 125°–150°). The shallower (deeper) curve refers to a PKIKP ray path that has the smallest (largest) travel time inside the inner core. The blue (red) open diamonds show the same eastern (western) differential travel-time data of Fig. 1. The polar African anomalies (46) and the datasets of Creager (8) and Song and Helmberger (59) are also shown as green and black open diamonds, respectively. Recall that the ray paths analyzed in refs.  and turn at ∼100–300 km beneath the Earth ICB and are centered near Δ = 150°. The solid (dashed) black lines are the predicted travel-time anomalies for Δ = 150° (Δ = 130°) when considering 50% (25%) of the Fe-bcc crystals aligned along the Earth’s spin axis. (B) Absolute PKiKP-PKIKP travel-time residuals versus ray angles for a cylindrically averaged hcp aggregate. The solid gray lines are referring to the transversely isotropic hcp model, and the seismic datasets are the same as in A. The solid (dashed) black lines are the predicted absolute δt anomalies for Δ = 150° (Δ = 130°) when considering 85% (50%) of LPO for the Fe-hcp crystals.

Fig. 4.

Plot of PKiKP-PKIKP travel-time anomalies as a function of ray angles for a bcc polycrystalline aggregate. Note the specularity between the eastern and western hemispherical patterns. Solid gray lines show the anisotropy pattern predicted by the transversely isotropic model for both eastern (blue open diamonds) and western (red open diamonds) hemispheres. We used the same t parameter range as in Fig. 3, as well as the same seismic observables. The solid (dashed) blue lines are the predicted eastern travel-time anomalies for Δ = 150° (Δ = 130°), when assuming 25% of LPO for iron bcc and a hemispherical velocity correction of +0.056 km/s. The solid (dashed) red lines indicate the computed western anomalies for Δ = 150° (Δ = 130°) at 25% of LPO and with μ^hem = -0.034 km/s.

Fig. 5.

PKiKP-PKIKP travel-time anomalies as a function of ray-angles for an hcp iron aggregate. Both eastern (blue open diamonds) and western (red open diamonds) hemisphere anomalies were tentatively adjusted by using 85% (50%) of LPO iron hcp for Δ = 150° (Δ = 130°). The solid (dashed) lines are the calculated Transversely Isotropic Model travel times for Δ = 150° (Δ = 130°). The polar African anomalies (46) and the datasets of Creager (8) and Song and Helmberger (59) are also shown as green and black open diamonds, respectively.