Seismic hemispheric asymmetry induced by Earths inner core decentering

In a first approximation the Earth’s interior has an isotropic structure with a spherical symmetry. Over the last decades the geophysical observations have revealed, at different spatial scales, the existence of several perturbations from this basic structure. Some of them are situated in the neighborhood of the inner core boundary (ICB). One of the best documented perturbations is the asymmetry at the top of the inner core (ATIC) characterized by faster seismic wave velocity in the eastern hemisphere than in the western hemisphere. All existing explanations are based on a hemispheric variation of the material properties near ICB inside the inner core. Using numerical simulations of the seismic ray propagation, we show that the ATIC can be explained as well by the displacement of the inner core towards east in the equatorial plane tens of kilometers from the Earth’s center, without modifying the spherical symmetry in the upper inner core. The hypothesis of a displaced inner core is also sustained by other observed hemispheric asymmetries at the top of the inner core and at the bottom of the outer core. A displaced inner core would have major implications for many mechanical, thermal, and magnetic phenomena in the Earth’s interior.

💡 Research Summary

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The paper challenges the conventional view that Earth’s inner core (IC) is perfectly centered and isotropic. It focuses on the well‑documented hemispheric asymmetry at the top of the inner core (ATIC), where seismic waves travel faster in the eastern hemisphere than in the western one. Existing explanations invoke lateral variations in material properties (velocity, attenuation) within the IC. The authors propose an alternative hypothesis: the entire inner core is displaced eastward by several tens of kilometres within the equatorial plane, while the velocity structure inside the IC remains spherical and unchanged.

Using ray‑tracing simulations, they examine the differential travel time Δt between the PKIKP and PKiKP phases. PKiKP reflects off the inner‑core boundary (ICB), whereas PKIKP refracts twice at the ICB and traverses the inner core. If the IC is offset, the geometry of the two rays becomes asymmetric: in the eastern hemisphere the PKIKP ray spends a longer path inside the faster‑velocity inner core (segment CD in their Fig. 1), while in the western hemisphere the path is shorter. Because the inner‑core P‑wave speed exceeds that of the outer core, the extra distance in the east reduces the travel time of PKIKP relative to PKiKP, producing a positive residual (Δt – Δt₀ > 0). The opposite occurs in the west, yielding a negative residual. This purely geometric effect reproduces the observed sign pattern of ATIC without invoking any lateral velocity or attenuation anomalies.

The numerical experiments adopt the standard 1‑D ak135 velocity model for both the inner and outer core, discretised into spherical shells of ≤1 km thickness. Rays are propagated as straight segments obeying Snell’s law at each shell interface. Simulations with inner‑core offsets ranging from 0 to 100 km show residuals up to ±0.5 s, matching the amplitude of observed ATIC residuals (≈ ±0.5 s). The authors also explore the longitudinal distribution of residuals by varying the epicentral angle of the simulated earthquakes in 10° steps and emitting rays in 10°‑spaced azimuthal planes. When the offset lies in the equatorial plane, positive residuals are confined to the eastern hemisphere and negative residuals to the western hemisphere, with a sharp boundary that is shifted eastward by about 20° relative to the geographic east‑west meridian. This pattern mirrors the observations reported by Waszek et al. (2011), suggesting an eastward displacement of roughly 100 km at a longitude near 10°–20° E.

Beyond ATIC, the displaced‑core model offers explanations for several other hemispheric asymmetries. First, seismic attenuation (Q⁻¹) is larger in the east because the PKIKP ray traverses a longer distance in the inner core, where Q is two orders of magnitude lower than in the outer core. Second, PKiKP arrivals sampled in the east are about 0.9 s earlier than those sampled in the west; the model reproduces this because the reflected PKiKP ray is shorter in the east when the core is shifted eastward.

The authors acknowledge several limitations. The ak135 model assumes a centered inner core; using it for a displaced core introduces systematic errors, especially near the ICB where the true radius would vary by ±100 km. The ray‑sampling scheme (10° steps) is coarse compared with the dense distribution of real earthquakes, and the study does not incorporate the full three‑dimensional heterogeneity of the mantle and outer core. Moreover, the physical mechanism capable of sustaining a tens‑of‑kilometre offset is only qualitatively discussed (e.g., electromagnetic coupling, outer‑core flow, angular‑momentum exchange) without quantitative modeling.

In conclusion, the paper presents a compelling geometric alternative to material‑property explanations of ATIC and related seismic hemispheric asymmetries. By showing that a modest eastward displacement of the inner core can reproduce observed travel‑time residuals, attenuation differences, and PKiKP timing offsets, it opens a new line of inquiry into the dynamics of Earth’s deepest interior. Future work should develop fully three‑dimensional Earth models that incorporate core displacement, test the dynamical feasibility of such offsets with magnetohydrodynamic simulations, and compare predictions against high‑resolution global seismic datasets.