Using PKiKP coda to determine inner core structure: 2. Determination of QC

Abstract

Despite the similar and rather uniform composition of two components of the core, the solid inner part presents a much larger attenuation compared to the liquid outer part. More over, several authors have reported evidence of scattered seismic energy coming from the inner core (IC). This implies that the IC has lateral variations in structure or composition with a scale length from one to tens of kilometers. These seismic scatterers could be caused by changes in crystal grain size, variations in the orientation of anisotropy, or the presence of partial melt and/or impurities. This work pursues the characterization of inner core scattering (ICS) using a global data set of high‐frequency PKiKP waveforms recorded at short‐period, small‐aperture seismic arrays. We apply a methodology used in the study of heterogeneities in the crust and upper mantle that consists of curve fitting of the observed scattered energy with a standard model, along with a stripping technique to isolate the PKiKP coda from the rest (e.g., P, PcP, and PP codas). From the observed PKiKP codas we found an average QC ∼ 500 for the inner core, reflecting a scattering attenuation at least comparable to the intrinsic attenuation. We found a geographical dependence in the observations of PKiKP coda, with most of them coming from the Pacific Ocean and Asia, and relatively few observations coming from the Atlantic Ocean. The simplest explanation is that the ICS has a quasi‐hemispherical variation, similar to other properties such as P wave velocity jump at the inner core boundary, attenuation in the upper IC, strength of anisotropy and/or thickness of an overlaying isotropic layer. This phenomenon is probably related to the solidification texturing of iron crystals, showing hemispherical differences in the growth of the inner core.