Insights on Inner-Core Anisotropy Obtained from the Earth's Correlation Wavefield

We demonstrate the feasibility of constraining seismic anisotropy of the Earth's Inner Core (IC) based on global correlation stacks of the late-coda seismic wavefield (the correlation wavefield) that reverberates within the Earth up to 10 hours after large earthquakes. We develop a novel method that is sensitive to the seismic properties of the deepest portion of the IC by recovering correlated waveforms retrieved via an optimal configuration of podal and antipodal sources and receivers (i.e. Δ < 5° or Δ > 175°). The new spatial sampling of the IC provided by our method overcomes the shortage of seismic ray paths in conventional seismology based on direct seismic wavefield, and offers an originally independent and complementary dataset to explore the planet's very centre. Anisotropy in seismic velocities that is likely to be present in iron-nickel alloy at high pressures is one of the highly unresolved physical parameters for the IC. Different models have been proposed to fit the travel time observations of IC-sensitive seismic phases, but the non-uniqueness of the solutions demands further investigation. In particular, an uneven distribution of seismic ray paths sensitive to the innermost IC leads to controversies in the estimation of its elastic parameters. We show how to obtain reliable constraints on timing properties of IC-sensitive correlated signals. The availability of 20 years of data coverage and seismic networks antipodal to large events incorporated in this study enhances the volumetric sampling of the IC, especially in the near-polar direction. Our method is able to detect weak IC seismic phases that reverberate at high latitudes and are not clearly observed in the conventional seismology.