Possible heterogeneity of the Earth's core deduced from PKIKP travel times

The core of the Earth is usually described by spherically-symmetrical velocity models. The core is made of two main spherical layers: the fluid outer core with a radius close to 3,480 km and a P-velocity increasing with depth from 8 km s^-1 to 10.3 km s^-1, and the solid inner core with a radius of 1,220 km and a P-velocity close to 11 km s^-1 (refs 1, 2). Station residuals of the seismic core phase PKIKP have been computed for 400 seismological observatories worldwide using 5 yr of the International Seismological Centre (ISC) Bulletins. PKIKP travel times can be corrected for upper mantle propagation by subtracting P delays; thus PKIKP-P residuals are a measurement of the average vertical travel times in the lower mantle and in the core of the Earth beneath seismic stations. A spherical harmonic development of PKIKP-P delays up to degree 4 explains 58% of the variance in the data. PKIKP-P exhibit a latitudinal dependence: polar stations tend to be faster than equatorial stations. We show here that this pattern may reflect a departure from spherical symmetry in the P-velocity distribution in the vicinity of the inner core boundary of the Earth.