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

Abstract

Frequency-dependent reflection coefficients of P waves at the inner core boundary (ICB) are estimated from the spectral ratios of PKiKP and PcP waves observed by the high-sensitivity seismograph network (Hi-net) in Japan. The corresponding PKiKP reflection locations at the ICB are distributed beneath the western Pacific. At frequencies where noise levels are sufficiently low, spectra of reflection coefficients show four distinct sets of characteristics: a flat spectrum, a spectrum with a significant spectral hole at approximately 1 or 3 Hz, a spectrum with a strong peak at approximately 2 or 3 Hz, and a spectrum containing both a sharp peak and a significant hole. The variety in observed spectra suggests complex lateral variations in ICB properties. To explain the measured differences in frequency characteristics of ICB reflection coefficients, we conduct 2D finite difference simulations of seismic wavefields near the ICB. The models tested in our simulations include a liquid layer and a solid layer above the ICB, as well as sinusoidal and spike-shaped ICB topography with varying heights and scale lengths. We find that the existence of a layer above the ICB can be excluded as a possible explanation for the observed spectra. Furthermore, we find that an ICB topographic model with wavelengths and heights of several kilometers is too extreme to explain our measurements. However, restricting the ICB topography to wavelengths and heights of 1.0-1.5 km can explain the observed frequency-related phenomena. The existence of laterally varying topography may be a sign of lateral variations in inner core solidification.

Seismological evidence for a localized mushy zone at the Earth’s inner core boundary

“…2 – 4 ). If we attribute all the observed variations of PKiKP–PcP differential travel time residuals to ICB topographic variations, the estimated upper bound of ICB topographic changes in the study region is 1.5 km, similar to previous results obtained beneath the western Pacific 17 . Note that PKiKP–PcP travel time residuals exhibit slight negative correlations with PcP travel time residuals for events 2, 5, 6, 8, 10, and 11 (Supplementary Figs.…”

Dissimilarity of the Earth's Inner Core Surface Under South America and Northeastern Asia Revealed by Core Reflected Phases

“…It corresponds to the assumption that the moment tensor has no volumetric component, but in case of an event with focal depth of about 600 km, mechanics of the seismic source is strongly affected by chemical phase transitions taking place at the relevant depth. More detailed study of seismic sources as well as possible frequency dependence of ICB reflection coefficient (Tanaka & Tkalčić, ) is obviously beyond the scope of this paper, but we can summarize that in addition to favorable geometry with respect to the maximum in P ‐wave radiation pattern, successful observations of steeply reflected PKiKP and PcP waveforms can be bound to favorable E S / E P ratio of the analyzed seismic source.…”

“…This, however, may be difficult because the chief factor governing observability of steeply reflected PKiKP and PcP waveforms has not been established yet, despite efforts to examine various effects of seismic source, CMB (Tkalčić et al, ) and ICB (Krasnoshchekov et al, ). Although the total of claimed observations of PKiKP and PcP in some studies may reach few hundred (e.g., Koper & Pyle, ; Tanaka & Tkalčić, ; Tian & Wen, ), not all of the measurements are directly applicable to, for example, the problem of ICB density jump. Individual signal‐to‐noise ratios of the detected PKiKP and PcP waveforms are sometimes quite low or amplified by summation leading to distortions and biases; most of the observations still belong to the distance range of 15–60°, while the number of joint high‐quality PKiKP/PcP measurements below 10° is at least an order of magnitude as small.…”

Detection of small scale heterogeneities at the Inner Core Boundary

“…Finally, observations of amplitude anomalies in reflected PKiKP phases indicate the possible presence of short wavelength topography on the ICB (Cao et al, 2007;Zou et al, 2008;Dai et al, 2012;Tanaka and Tkalčić, 2015), with possible detection of temporal changes (Wen, 2006;Yao et al, 2015). Evidence for small scale scattering due to heterogeneities in the vicinity of the ICB have also been proposed from studies of the coda of core-sensitive phases (Vidale and Earle, 2000;Koper et al, 2004;Koper and Dombrovskaya, 2005;Cormier, 2007;Leyton and Koper, 2007;Adam and Romanowicz, 2015).…”