Tectonic history of the Earth's inner core preserved in its seismic structure

The Earth's inner core is structurally complex and its texture as well as the degree of its seismic anisotropy(1) varies with depth(2). In the modern inner core, an uppermost isotropic layer(3,4) surrounds a deeper and more anisotropic region that includes the seismically distinct innermost inner core(5-7). This structural complexity is probably related to strain resulting from the growth of the inner core(8). However, as most dynamic models of anisotropy generation(8-11) have considered only current deformation, how strain evolved through time and influenced texture is not fully understood. Here we use a numerical model to couple preferential crystallization in the equatorial region of the inner core(8) with density stratification arising from inner-core growth, allowing both to evolve over the entire history of the inner core. Our results suggest that the inner core evolves gradually from a regime in which deformation penetrates into the deepest parts to a regime in which it is confined to the uppermost region. The deep anisotropy is therefore best understood as a fossil anisotropy inherited from horizontal Maxwell stresses (arising because of the magnetic field)(10) or texture formation during solidification(12-14). The structure of the upper layers, on the other hand, probably results from active tectonics induced by heterogeneous growth of the inner core.