Distinct seismic anisotropy at the centre of the Earth

Although arrival times of PKPDF (or PKIKP) phases from epicentral distances between 120° and 173° are consistent with an anisotropy model obtained from a joint inversion of normal-mode and body-wave data, PKPDF measurements from nearly antipodal distances (between 173° and 180°) exhibit deviations from this model. We demonstrate that this behaviour is coherent, and that the characteristics associated with these data are still those of transverse isotropy. We also perform a systematic search for the location of the symmetry axis. Results suggest that the axis may be significantly tilted away from the geographic pole; however, higher-quality data are required to draw a firm conclusion. Inversion of the data with the symmetry axis at the geographic pole yields a model of anisotropy that is considerably different from that of the inner-core between 300 and 1220 km radius. These observations imply that there is a seismically distinct region at the centre of the Earth with radius of about 300 km.