Seismic velocity and anisotropy of the inner core

Since the discovery of the inner core via observational seismology in 1936, the nature of its internal structure and dynamics has been at the forefront of deep Earth research. Increasingly complex models are being developed to accommodate both small and large scale features observed in multiple seismic datasets. One such model, first proposed in the mid-80s, is that of a cylindrically anisotropic inner core. That is, seismic rays traveling through the inner core parallel to the Earth’s rotation axis travel faster than their equatorial counterparts. However, in the years following this observation, there have been continuing discrepancies in estimates of both strength and orientation of anisotropy, with some evidence suggesting such a model may not be supported by the data at all. This is in part due to the fact that a simple model of anisotropy fails to fit all seismic data optimally.

 

This presentation will address one particular model of inner core anisotropy - the inner-most inner core, where I will be presenting results from rigorous testing of the hypothesis: “can we observe a depth-dependent change in the strength of anisotropy within the inner core?” As a means of testing, I have assembled a large global data set of the seismic phase PKIKP (the phase that traverses the inner core), and employed a robust methodology, the Neighbourhood Algorithm, to search for all possible solutions of two layered inner core anisotropy. The findings demonstrate that data needs to be averaged spatially in order for structure to be visible in the deepest part of the inner core, and show that our current understanding of inner core structure is somewhat limited by the present day seismic data coverage and quality.