Intricate heterogeneous structures of the top 300 km of the Earth's inner core inferred from global array data: II. Frequency dependence of inner core attenuation and its implication

Abstract

We apply a nonlinear waveform inversion method that we developed previously (Iritani et al., 2010, 2014) to broadband (0.02–2 Hz) core phase data recorded by global seismic arrays in order to investigate the frequency dependence of inner core attenuation. The results show a prominent difference of the frequency dependence of attenuation between the eastern hemisphere and the western hemisphere: while attenuation is frequency dependent in the western hemisphere, not so in the eastern hemisphere. Moreover, in the western hemisphere, a stronger frequency dependence of the attenuation is observed beneath Africa compared to beneath North America. This intricate quasi-hemispherical property of the frequency dependence of the inner core attenuation, together with the quasi-hemispherical variability of seismic velocity and attenuation strength at around 1 Hz that we reported earlier, can be consistently understood in terms of the relative location of the seismic observation frequency band and the absorption band peaks that are different for regions. The variability of the absorption band peak may reflect the variability of the iron grain size of the inner core: the grain size is larger in the eastern hemisphere than the western hemisphere, and in the western hemisphere beneath North America, it gradually increases with depth. However, caution should be taken in this inference as the physical mechanism of attenuation in the inner core is still poorly constrained both theoretically and observationally.