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On The Inner-Outer Core Density Contrast From New PKiKP/PcP Amplitude Ratios And Uncertainties Caused By Seismic Noise

Hrvoje Tkalčić 1, Brian L. N. Kennett1 and V. F. Cormier2

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 Department of Physics, University of Connecticut, Storrs, 06269 - 3046United States

 

Figure 1. PKiKP/PcP amplitude measurements and their uncertainties (the median values are shown by diamonds, and the uncertainties are shown by error bars) plotted as a function of epicentral distance for: a varying density contrast at the inner core boundary (top) and the core mantle boundary (bottom). Theoretical values are indicated with lines of various thicknesses. Red line indicates the theoretical ratio from the ak135 model.

 

The inner core boundary of the earth is characterised by a discontinuous change in elastic properties between the liquid outer and solid inner core. In the ray theory approximation, a measure of the density contrast at the inner core boundary is given by the amplitude ratio of P waves reflected from the core-mantle boundary (PcP waves) and the inner core boundary (PKiKP waves), since that ratio conveniently appears in an explicit form in the transmission/reflection coefficient equations. The results for inner-outer core density contrast derived from direct amplitude picks of these waves in the time domain have varied significantly among different authors.

The transmission/reflection coefficients on the liquid-solid and solid-liquid boundaries derived from ground displacements enable a direct comparison between the amplitude measurements on displacement seismograms in the time domain and theoretical values. A new approach is proposed and applied to integrate effects of microseismic and signal-generated noise with the amplitude measurements, thus providing a direct maximal uncertainty measure. To suppress the effects of varying radiation pattern and distinctively different ray-paths at longer epicentral distances, this new method was applied to high-quality arrivals of PcP and PKiKP waves from a nuclear explosion observed at epicentral distances 10° to 20° from recording stations. The resulting uncertainties are high precluding precise estimates of the inner core boundary density contrast, but provide a robust estimate of an upper bound from body waves of about 1100 kg/m3.

Median values of two amplitude ratios observed around 17° epicentral distance indicate a small density contrast of 200-300 kg/m3 and suggest the existence of zones of suppressed density contrast between the inner and the outer core, a density contrast stronger than 5000 kg/m3 at the core-mantle boundary, or a combination of both (Figure 1). Such a small estimate of the density contrast across the inner-core boundary from body waves could still produce the desired effect on the compressional velocity profile in the thermo-chemical boundary layer at the bottom of the outer core and return a modest heat flux from the inner core with a substantial inner core age, but only if accompanied by a small estimate of the density contrast of about 400 kg/m3 from normal modes. If the inner-core boundary is characterized with such a mosaic of variable density contrast to which seismic body waves are sensitive, it is more likely that the density fluctuations are restrained to the top of the inner core. It has been argued that at least the top of the inner core is a dendritic mushy zone, in which interdendritic fluid pockets likely coexist to explain the observed nature of attenuation and attenuation in anisotropy of body waves. If PKiKP waves reflect from the inner core at the places where less dense features at the top of the inner core reduce the density contrast, than this could explain the observed lower values for stations at around 17° (Figure 1).