On the inner-outer core density contrast from PKiKP/PcP amplitude ratios and uncertainties caused by seismic noise

Abstract

S U M M A R YThe 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.

Complex inner core boundary from frequency characteristics of the reflection coefficients of PKiKP waves observed by Hi-net

“…Recently, the same class of PKiKP waves was observed in the eastern hemisphere on Chinese and Japanese short-period and broadband stations (J-array) (Tkalčić et al, 2009(Tkalčić et al, , 2010 and by the high-sensitivity seismograph network (Hi-net) in Japan (Dai et al 2012, Jiang andZhao 2012). Table 1 To contribute to a better understanding of inner core dynamics and to constrain ICB structure in the eastern hemisphere, here, we collect an extensive dataset of PKiKP waves recorded by Hi-net in Japan.…”

“…Encircled numbers are explained in Fig. 6 Simulations for topography at the ICB Earlier PKiKP and PcP amplitude ratio analyses made sporadic seismological observations of a lower density contrast at the ICB than predicted values for spherically symmetric Earth models, as low as 200-300 g/m 3 (Koper and Pyle 2004;Tkalčić et al 2009). In addition, Gubbins et al (2008) inferred a low density contrast from geodynamical considerations.…”

“…Studies in the twentieth century usually analyzed PKiKP recorded on short-period seismographs with a predominant frequency of 1 Hz (Engdahl et al 1970;Buchbinder 1972;Engdahl et al 1974;Souriau and Souriau 1989;Shearer and Masters 1990). These pioneering studies may have been hampered by the small number of observations and the large scatter in PKiKP/ PcP amplitude ratios due to the nature of noisy amplitude measurements (Tkalčić et al 2009). Another factor influencing the amplitude ratios, and possibly the anticorrelation between the two phases, is the occurrence of near-receiver crustal and mantle heterogeneity (Tkalčić et al 2010).…”

Irregular topography at the Earth’s inner core boundary

“…PKiKP phase has been enigmatic in its observability in the precritical distances (16)(17)(18)(19)(20)(21)(22)(26)(27)(28)(29). Although its observability was predicted based on the elastic parameters of Earth's models, it was only occasionally observed when the seismic station coverage was sparse, which can be explained by the characteristics of the ICB topography inferred here.…”

Constraints from material properties on the dynamics and evolution of Earth’s core

“…Body waves have much better resolution of a few kilometers, but the estimates vary widely because PKiKP is a noisy phase [34][35][36] ; they give an upper bound 36 of 1.1 gm cc −1 . There is also evidence for an anomalously dense layer in the lowermost 150 km of the outer core 37 , which probably has a chemical origin 38 .…”