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In a first approximation the Earth's interior has an isotropic structure witha spherical symmetry. Over the last decades the geophysical observations haverevealed, at different spatial scales, the existence of several perturbationsfrom this basic structure. Some of them are situated in the neighborhood of theinner core boundary (ICB). One of the best documented perturbations is theasymmetry at the top of the inner core (ATIC) characterized by faster seismicwave velocity in the eastern hemisphere than in the western hemisphere. Allexisting explanations are based on a hemispheric variation of the materialproperties near ICB inside the inner core. Using numerical simulations of theseismic ray propagation, we show that the ATIC can be explained as well by thedisplacement of the inner core towards east in the equatorial plane tens ofkilometers from the Earth's center, without modifying the spherical symmetry inthe upper inner core. The hypothesis of a displaced inner core is alsosustained by other observed hemispheric asymmetries at the top of the innercore and at the bottom of the outer core. A displaced inner core would havemajor implications for many mechanical, thermal, and magnetic phenomena in theEarth's interior.

Seismic hemispheric asymmetry induced by Earth's inner core decentering