Normal mode splitting function measurements of anelasticity and attenuation in the Earth's inner core

We have used the iterative spectral fitting method to measure both the elastic and anelastic splitting functions of 20 inner core sensitive normal modes. These modes show significant improvement in spectral fit when anelastic splitting function coefficients d(st) are introduced in addition to the elastic splitting function coefficients c(st). We employ two separate anelastic treatments: (i) fully anelastic measurement, in which a complete set of anelastic splitting function coefficients is measured in addition to the elastic coefficients, and (ii) zonal anelastic measurement, in which anelasticity is only allowed in zonal splitting function coefficients. Together, these two approaches confirm that normal modes sensitive to the Earth's inner core resolve zonally dominant elastic and anelastic structures. The zonal dominance of anelasticity suggests that the inner core exhibit cylindrical attenuation anisotropy in addition to cylindrical velocity anisotropy. In particular, the zonally dominant anelasticity correlates with zonal elastic structure, that is, directions of higher velocity in the inner core also appear more attenuating.