Probing the Inner Core Anisotropic Structure Using Global Coda-correlation Wavefield.

Abstract:

A major challenge in exploring the anisotropic structure of the Earth’s inner core (IC) is the limitation of its volumetric sampling, restricted by the uneven distribution of earthquakes and receivers. The poor coverage of ray paths sensitive to the deep IC, especially in the polar direction, leads to uncertainties in the strength and orientation of its anisotropy. Yet, improved constraints on the anisotropy are required to understand the crystallographic structure of iron in the IC, and the evolution of its solidification and deformation processes. In this study, we investigate the IC anisotropy properties using the recently-established technique based on Earth’s coda-correlation wavefield constructed from the late coda of large earthquakes. We perform travel-time analysis of I2*, a correlation feature sensitive to the Earth’s IC that resembles PKIKPPKIKP compressional wave in the direct seismic wavefield. I2* is a mathematical manifestation of cross-terms of multiple seismic phases with similarities in slowness, and therefore, substantially different from PKIKPPKIKP. We show that the travel-time of these core-correlated phases is highly dependent on the location of the earthquakes and receiver pairs and this guides our selection of data. We then present a travel time analysis of correlation stacks obtained from podal-antipodal source-station geometry. The travel-time variability of I2* with the direction within the IC supports anisotropy with a slow axis around ksi = 50 to 60 degrees, with azimuthal variations, where ksi is the angle between the ray path in the IC and the Earth’s rotation axis. This observation indicates a more complex anisotropic pattern in the deepest portion of the IC than previously proposed.