Complex inner core of the Earth constrained by differential travel times and differential ray parameters

Abstract

Studies of velocity structure in the Earths inner core and its volumetric variations illuminate our understanding of inner core dynamics and composition. Here I use an extensive number of seismograms recorded by the Hi-net array to construct complete empirical curves of PKPbc-PKPdf differential traveltimes. The nature of these curves implies that significant variations in traveltimes are accumulated during the passage of PKPdf waves through the uppermost inner core, and rules out outer core structure effects. Uniform cylindrical anisotropy of a plausible strength in the uppermost inner core can also be ruled out as a cause of the observed travel-time variations because the range of sampled ray angles is too narrow. The configuration and strength of inhomogeneities from a recent tomographic model of the lowermost mantle cannot account for the observed travel-time variations. Therefore, I infer that either variations of P-wave isotropic velocity on the scale of about hundred km and less are present in the uppermost inner core or the material may be organized in distinctive anisotropic domains, and both of these features may be superimposed on long-wavelength hemispherical structure. If the former holds true, the absolute magnitude of required P-wave velocity perturbations from referent values is 0.60±0.10% in the quasi-eastern and 1.55±0.15% in the quasi-western hemisphere (0.85±0.05% and 1.10±0.10%, respectively, with the lowermost mantle correction). The existence of these variations is a plausible physical outcome given that vigorous compositional convection in the outer core and variations in heat-exchange across the inner core boundary may control the process of solidification. Although I show the existence of heterogeneity on regional scale, the precise location and size of heterogeneities could not be constrained using exclusively the perturbed differential travel times. To overcome this limitation, here I utilize the PKPbc-PKPdf differential ray parameters in addition to the corresponding differential travel times to study the Vp structure of the uppermost inner core. I find that the differential ray parameters measured by phase-weighted stacking of waveforms recorded by the Hi-net array also display regional variations in both quasi-hemispheres. This observation supports previous results and provides additional constraint on complex Vp structure. I examine a number of 2-D models with different nature of heterogeneities including the inner core boundary topography and a mushy zone to predict the observed regional variation of the differential travel times and differential ray parameters. Our estimates of the heterogeneity spectrum represent invaluable new constraints on inner core structure.