Regional Variations in Inner Core Heterogeneity and Anisotropy from Seismic Body Wave and Normal Mode Observations

Abstract:

Inner core anisotropy was discovered more than 25 years using both seismic body wave and normal mode observations (e.g. Morelli at al, 1986, Woodhouse et al, 1986). More recent studies have shown that the inner core may also display regional variations, which are often simplified as `hemispheres' (e.g. Tanaka & Hamaguchi, 1997), with stronger anisotropy and lower isotropic velocity seen in the west. The existence of hemispheres is key to our understanding of the dynamics of the inner core and its thermal and compositional structure.

Here, we combine seismic body wave and normal mode data and perform a combined inversion in order to reconcile the two data types. Our model shows evidence for hemispherical variation in isotropic velocity in the top 250 km. The anisotropy also varies regionally, but is confined to a narrower wedge region instead of a full hemisphere. The narrow anisotropic wedge region is clearly visible in both the normal mode and body wave data. Thus, the hemispherical variations in isotropic velocity may be unrelated to the much narrower wedge region in which the anisotropy is strongest and they may require different explanations.

Two mechanisms have been proposed to explain the hemispherical pattern and regional variations: either (a) inner core translation, wherein one hemisphere is melting and the other is solidifying, or (b) thermochemical convection in the outer core, leading to different solidification conditions at the inner core boundary. Neither is (yet) able to explain all seismically observed features, and a combination of different mechanisms is probably required.