Uppermost inner core seismic structure – new insights from body waveform inversion

Differential travel times and waveforms of PKIKP and PKiKP phases in the 129°–141° distance range, deconvolved for the effects of source time functions and average mantle attenuation operators, are inverted for velocity and attenuation in the uppermost 80 km of the inner core, and the velocity gradient and attenuation in the lowermost 200 km of the outer core (F region). Results confirm degree-one velocity structure in the inner core whose fast and slow regions appear to be separated by transitions with a velocity contrast of about 1%. Although attenuation in the inner core also exhibits degree-one pattern, a high attenuation region ( Q p − 1 = [ 0.002 , 0.0067 ] ) extends to the central Pacific, which has been previously considered a part of the western hemisphere characterized by slower velocity and lower attenuation. The rest of the inner core exhibits a positive correlation between velocity and attenuation as observed previously. An observed mantle-like relationship between velocity and attenuation in the central Pacific can be interpreted as originating from higher than average homologous temperature, consistent with a model of inner core solidification driven by outer core convection coupled to core–mantle boundary thermal heterogeneity. Alternative mechanisms, including differences in grain size and impurity concentration, and lateral differences in solidification rate might be required to explain the structure of the inner core. The F region velocity and attenuation are best explained by the AK135-F velocity model and Q p − 1 = [ 0 , 0.001 ] respectively.