Tilted Transverse Isotropy in the Earth's inner core
Abstract
Inner core anisotropy was discovered in 1986, elegantly proven by the publication of two companion papers, one on short period PKIKP body waves observations (Morelli, Dziewonski & Woodhouse, 1986) and the other one using long period normal mode splitting functions (Woodhouse, Giardini & Li, 1986). The anisotropy appears to be aligned with the Earth's rotation axis with a larger wave velocity in the polar (North-South) direction than in the equatorial (East-West) direction. Ever since this discovery, our ideas on the structure of the inner core have become more and more detailed. Recent models include an isotropic layer at the top, anisotropy separated into a strongly anisotropic western hemisphere and more weakly anisotropic eastern hemisphere and an innermost inner core. Most previous studies assumed that the symmetry axis of the anisotropy is aligned with the rotation axis axis and then attributed regional variations to variations in the magnitude of the anisotropy. Here, we make a tomographic model of inner core anisotropy using seismic body waves observations using a different approach. We assume that the inner core is made of cylindrically symmetric anisotropy crystals that all have the same magnitude of anisotropy, and instead we allow the symmetry axis to vary. Thus, we make a model of Tilted Transverse Isotropy in the inner core. We find that our model fits the body wave data equally well as models in which the magnitude varies, with the advantage that our model requires fewer parameters. In our model, the anisotropy in the central part of the inner core is still mainly aligned with the rotation axis. In the upper part of the inner core we find two caps around the Indian Ocean/Indonesia and the mid-Atlantic with the fast symmetry axis aligned parallel to the equatorial plane. Inner core anisotropy is most likely due to alignment of hcp iron crystal formed either (i) during solidification at the inner core boundary or (ii) afterwards by deformation deeper in the inner core. Thus, our new model may be related to flow in the inner core or solidification processes at the inner core boundary and constrain geodynamic processes in the inner core.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2023
- Bibcode:
- 2023AGUFMDI21A..05D