Implication of the lopsided growth for the viscosity of Earth's inner core

Abstract

Two main seismic features characterize the Earth's inner core: a North-South polar anisotropy and an East-West asymmetry of P-wave velocity and attenuation. Anisotropy is expected if shear deformation is induced by convective motions. Translation has recently been put forward as an important mode of convection of the inner core. Combined with a simple diffusive grain growth model, this mechanism is able to explain the observed seismic asymmetry, but not the bulk anisotropy. The source of anisotropy has therefore to be sought in the shear motions caused by higher modes of convection. Using a hybrid finite-difference spherical harmonics Navier-Stokes solver, we investigate the interplay between translation and convection in a 3D spherical model with permeable boundary conditions at the inner core boundary. Three parameters act independently: viscosity, internal heating and convection velocity in the outer core. Our numerical simulations show the dominance of pure translation for viscosities of the inner core higher than $10^{20}$ Pas. Translation is almost completely hampered by convective motions for viscosities lower than $10^{18}$ Pas. Between these values, translation and convection develop, but convective downwellings are restricted to the coldest hemisphere where crystallization occurs. On the opposite side, shear is almost absent, thereby allowing grain growth. We propose that the coexistence of translation and convection observed in our numerical models leads to a seismic asymmetry but localizes deformation only in one hemisphere.