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On the inner—outer core density contrast from PKiKP/PcP amplitude ratios and uncertainties caused by seismic noise
The inner core boundary (ICB) of the earth is characterized 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 ICB is given by the amplitude ratio of P waves reflected from the core—...
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Published in: | Geophysical journal international 2009-10, Vol.179 (1), p.425-443 |
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Main Authors: | , , |
Format: | Article |
Language: | English |
Subjects: | |
Citations: | Items that this one cites Items that cite this one |
Online Access: | Get full text |
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Summary: | The inner core boundary (ICB) of the earth is characterized 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 ICB is given by the amplitude ratio of P waves reflected from the core—mantle boundary (CMB; PcP waves) and the ICB (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°–20° from recording stations. The resulting uncertainties are high precluding precise estimates of the ICB density contrast, but provide a robust estimate of an upper bound from body waves of about 1100 kg m−3. Median values of two amplitude ratios observed around 17° epicentral distance indicate a small density contrast of 200–300 kg m−3 and suggest the existence of zones of suppressed density contrast between the inner and the outer core, a density contrast stronger than 5000 kg m−3 at the CMB, or a combination of both. |
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ISSN: | 0956-540X 1365-246X |
DOI: | 10.1111/j.1365-246X.2009.04294.x |