teep reflections from the earth ’ s core reveal small-scale heterogeneity in the pper mantle

We investigate arrivals of seismic phases reflected from the core–mantle boundary (PcP waves) and those reflected from the inner-core boundary (PKiKP waves) at subcritical angles, with the goal of measuring their amplitude ratios. This adds invaluable data points required to study the density jump across the inner-core boundary (ICB). One nuclear explosion and one earthquake, both with favourable focal mechanisms are identified as sources that produce an excellent and abundant record of arrivals of both PcP and PKiKP waves at multiple stations. There is only a limited number of detections of both types of waves on the same seismogram, while more frequently, either one or another of the two phases is detected. Thus, for those cases where at least one phase is above a detectable level, we observe a highly significant negative correlation (anti-correlation) of phase appearances on seismograms, where PcP and PKiKP phase-detections are treated as dichotomous, categorical random variables that can take values detected or undetected. The fact that similar anti-correlation in detection is observed for both explosive and tectonic sources makes less likely the possibility that source effects or a specific near source structure is responsible for this phenomenon. Although laterally varying structure near the core–mantle boundary (CMB) can account for the magnitude of observed fluctuations in the amplitude ratio of PKiKP to PcP, the Fresnel volumes surrounding their ray paths are well separated at the CMB at the frequencies of interest. This separation excludes the possibility that complex structure at or near the CMB is the dominant effect responsible for the observed anti-correlation. We demonstrate that the interaction of the wavefield with near-receiver heterogeneities is an important additional source of amplitude fluctuations across arrays of stations, and the likely cause of the anti-correlation between PKiKP and PcP detections and their spatial coherences at regional arrays. The combined effects of heterogeneities near the surface and the CMB have estim B. a profound impact on the the density jump at the IC . Introduction and motivation PKiKP waves have been typically difficult to observe at subcritcal distances. It has been argued that arrivals occur only in rare ases, with just enough energy to get barely recorded above noise, nd hence only providing estimates of the upper bound of the ICB ensity jump (e.g., Souriau and Souriau, 1989; Shearer and Masters, 990). Souriau and Souriau (1989) stated that PKiKP emerges only lightly out of the noise, which may arise from focusing by some eterogeneity. Shearer and Masters (1990) made similar remarks, dentifying only one convincing simultaneous arrival of PKiKP and cP waves on the global scale. Attempts to collect PcP waves globlly sometimes reveal very clean reflections of the CMB, but at other imes PcP is completely buried in noise or hardly visible (Tkalčić et ∗ Corresponding author. Tel.: +61 2 6125 3213; fax: +61 2 6257 2737. E-mail address: Hrvoje.Tkalcic@anu.edu.au (H. Tkalčić). 031-9201/$ – see front matter © 2009 Elsevier B.V. All rights reserved. oi:10.1016/j.pepi. 2009.08. 004 ates of the PKiKP/PcP amplitude ratios and the subsequent estimates of © 2009 Elsevier B.V. All rights reserved. al., 2002; Tkalčić and Romanowicz, 2002). The existence of high quality PcP arrivals at one station does not guarantee the existence of PcP arrivals on another nearby station of a similar quality. Although this could be attributed to the topography at the CMB and the variability in source radiation pattern, this “patchy” pattern of PcP observations is still not well understood. Here we present unusually high quality-observations of a large number of PcP and PKiKP phases—from an earthquake and a nuclear explosion. Such high quality records are rare and previously relatively unexploited for the purposes of studying the deep earth. Our primary motivation was to compile a dataset of PcP and PKiKP waves observed on the same seismogram for the purpose of studying the density jump at the ICB. In the course of that study we found that such observations of PcP and PKiKP at a single station were rare, the more common situation being that when PKiKP is observed, PcP is not, and vice versa. Observations of PcP and PKiKP waves on the same seismogram, however, could still be used to provide some bounds on the density jump at the ICB (Tkalčić et al., H. Tkalčić et al. / Physics of the Earth and Planetary Interiors 178 (2010) 80–91 81