Attenuation in the inner core from the broadband PKIKP data

Date of Completion

January 2001






Broadband velocity waveforms of PKIKP in the distance range 150° to 180° are inverted for inner core attenuation using Reference Source Time Functions (RSTF). A total of 46 earthquakes with Mw of 5.5∼7.0 worldwide during 1994–1999 were processed. The Reference Source Time Functions (RSTF), which incorporate an average mantle attenuation operator, are determined by inverting P-waves in the distance range 30° to 90° for a far-field point source representation. The model parameters for both intrinsic relaxation and scattering attenuation mechanisms are obtained. ^ For an intrinsic attenuation model, a mean Q at 1 Hz of 307 ± 90 is determined from 345 available PKIKP ray paths. Both global and regional results suggest a depth dependent attenuation in the deep inner core, and anisotropic attenuation is resolved on the regional results. The attenuation is much stronger in the upper part of the inner core than that in the deeper part of the inner core. The results show that the attenuation in the inner core is frequency dependent with the very weak velocity dispersion. No apparent hemispherical difference of attenuation in the deep inner core can be seen in our data. ^ For a scattering model, a mean velocity perturbation of 8.4% ± 1.8% and scale length of 9.8 ± 2.4 km are determined from 262 available PKIKP ray paths. The results for velocity perturbation show strong depth dependence and anisotropy in both global and regional data, while those for scale length have these features just on regional data. The apparent attenuation shows similar features as the results from an intrinsic relaxation model, except a stronger difference between upper and deeper parts of the inner core. ^ The depth dependence, anisotropy, and frequency dependence of attenuation in the inner core can be interpreted by the scattering effects of heterogeneities having different scale length and velocity perturbation with increasing depth and variations in directions between parallel to the rotation axis and parallel to the equatorial plane. An inner core structure is constructed with incorporation of the solidification model, the isostatic adjustment model, and the mushy zone model to account for the depth dependence of the attenuation in the inner core. ^