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.
Highlights
The inner core boundary (ICB) is one of the vital regions for understanding the Earth’s core dynamics (Loper and Roberts 1981; Loper 1983; Bergman and Fearn 1994; Shimizu et al 2005; Deguen et al 2007; Sumita and Bergman 2009; Deguen 2012)
The diameters of the 1 and 2 Hz Fresnel zones for PKiKP are approximately 80 and 60 km at the ICB, respectively. This is equivalent to the area of PKiKP reflection points at the ICB that is covered by a single event and all stations that detect PKiKP
We cannot rule out the possibility that the frequency characteristics of calculated reflection coefficients for results 3, 4, 5, and 7 are core–mantle boundary (CMB) effects, e.g., the high-frequency components of PKiKP may be lost by scattering at the CMB
Summary
The inner core boundary (ICB) is one of the vital regions for understanding the Earth’s core dynamics (Loper and Roberts 1981; Loper 1983; Bergman and Fearn 1994; Shimizu et al 2005; Deguen et al 2007; Sumita and Bergman 2009; Deguen 2012). To explain the hemispherical structure of the inner core, two models have been proposed: a large-scale asymmetric flow in the outer core (Sumita and Olson 1999; Aubert et al 2008; Gubbins et al 2011) and translational convection in the inner core (Alboussiere et al 2010; Monnereau et al 2010). The nature of these models is that they allow diametrically opposite scenarios of freezing and melting, i.e., one of the two hemispheres solidifies faster than the other. This has raised further questions about whether the surface of the inner core in Tanaka and Tkalčić Progress in Earth and Planetary Science (2015):
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