Detection of inner core solidification from observations of antipodal PKIIKP

Abstract:

PKIIKP waves, reflected from the underside of the inner core boundary, are very sensitive to the S velocity in the uppermost 80 km of the inner core at antipodal distances, undergoing a phase change and a factor of 4 amplification as the shear modulus in the uppermost inner core approaches zero. PKIIKP waveforms, modeled by a superposition of Gaussian beams in laterally varying models of the inner core, demonstrate that a near-zero shear modulus exists in the uppermost inner core in a 20-40 km thick patch benearth the eastern equatorial hemisphere. This bright spot of PKIIKP reflection correlates with a thin zone of low P velocity inferred from the complexity of PKIKP+PKiKP waveforms sampling this region. Estimates of grain sizes from seismic attenuation, the absence of backscattered PKiKP coda from this region, and a prediction for enhanced heat flow through this patch suggest that it is a region of solidification rather than melting.