Super-rotation, Translation and Growth of the Inner Core: A Step Towards a Coherent Model of Seismic Structures

Abstract:

The inner core hemispherical difference in seismic velocity, attenuation and anisotropy is well-established from seismic studies. Recent observations reveal complex features, yet current geodynamical models generally only attempt to explain the simple hemispheres. Two major theories invoke lateral translation and differential growth to generate a hemispherical difference in age and growth rate of inner core material. Here, we simulate growth and translation, and growth plus super-rotation of the inner core to investigate whether either model can produce the sharp and shifting hemisphere boundaries. We generate age models, then forward model seismic travel times and calculate growth rate models. We then compare these results with inner core seismic data observations. Although both geodynamical models generate an east-west difference, neither produce distinct hemispheres with sharp boundaries. Instead, they generate a gradual transition across the boundaries over more than 30 degrees of longitude. Within the hemispheres, the age and growth rate calculated show no correlation to seismic properties. Super-rotation successfully creates an eastward shift in properties with depth, as observed seismically. Our results are compatible with inner core super-rotation, and indicate that further mechanisms and compositional differences are required to generate the discontinuous boundaries.