Inner-core shear-wave anisotropy and texture from an observation of PKJKP waves

Abstract

Since the discovery of the Earth’s core a century ago¹, and the subsequent discovery² of a solid inner core (postulated to have formed by the freezing of iron³) seismologists have striven to understand this most remote part of the deep Earth. The most direct evidence for a solid inner core would be the observation of shear-mode body waves that traverse it, but these phases are extremely difficult to observe. Two reported observations in short-period data^4,5 have proved controversial⁶. Arguably more successful have been studies of longer-period data^6,7, but such averaging limits the usefulness of the observations to reported sightings. We present two observations of an inner-core shear-wave phase at higher frequencies in stacked data from the Japanese High-Sensitivity Array, Hi-Net⁸. From an analysis of timing, amplitude and waveform of the ‘PKJKP’ phase we derive constraints on inner-core compressional-wave velocity and shear attenuation at about 0.3 Hz which differ from standard isotropic core models⁹. We can explain waveform features and can partially reconcile the otherwise large differences between core wavespeed and attenuation models that our observations apparently suggest if we invoke shear-wave anisotropy in the inner core. A simple model of an inner core composed of hexagonal close-packed iron with its c axis aligned perpendicular to the rotation axis¹⁰ yields anisotropy that is compatible with both the shear-wave anisotropy that we observe and the well-established 3 per cent compressional-wave anisotropy.