Degree one heterogeneity and hemispherical variation of anisotropy in the inner core from PKP(BC)–PKP(DF) times

Abstract

We have investigated the relationship between heterogeneity and anisotropy in the inner core. Heterogeneity was examined using PKP(DF) and PKP(BC) phases traveling in the east‐west direction (E‐W paths). The geographical pattern of the PKP differential travel time (BC‐DF) residuals was found to be a manifestation of degree one heterogeneity in the outermost 100 to 500 km of the inner core. Spherical harmonics analysis showed that the P velocities in the inner core are fast in the region from 43°E to 177°E (the quasi‐eastern hemisphere) with a peak value of +0.29±0.15% and slow in the region from 183°W to 43°E (the quasi‐western hemisphere) with a peak value of −0.66±0.20%. Regional variation of anisotropy in the inner core was examined using PKP phases traveling nearly parallel to the Earth's spin axis (polar paths). The averages of BC–DF time residuals were 2.4±0.8 s in the quasi‐western hemisphere and 0.5±0.4 s in the quasi‐eastern hemisphere. We found that the inner core anisotropy predominates only in the quasi‐western hemisphere in which the 0.66% slow P velocity is observed for the E‐W paths. Geophysical implications of the degree one pattern in the inner core were briefly discussed. We propose that the hemispherical structure of the inner core reflects ancient core dynamics immediately after core formation.