Irregular topography at the Earth’s inner core boundary

Abstract

Compressional seismic wave reflected off the Earth's inner core boundary (ICB) from earthquakes occurring in the Banda Sea and recorded at the Hi-net stations in Japan exhibits significant variations in travel time (from −2 to 2.5 s) and amplitude (with a factor of more than 4) across the seismic array. Such variations indicate that Earth's ICB is irregular, with a combination of at least two scales of topography: a height variation of 14 km changing within a lateral distance of no more than 6 km, and a height variation of 4-8 km with a lateral length scale of 2-4 km. The characteristics of the ICB topography indicate that small-scale variations of temperature and/or core composition exist near the ICB, and/or the ICB topographic surface is being deformed by small-scale forces out of its thermocompositional equilibrium position and is metastable.inner core growth | geodynamo | outer core convection T he Earth's inner core grows from the solidification of the liquid outer core (1). The solidification process releases latent heat and expels light elements, providing driving forces for the thermocompositional convection in the outer core (2, 3) and the geodynamo that is responsible for the Earth's magnetic field (4-6). Information about the inner core boundary (ICB) is thus the key to the understanding of driving forces in outer core convection. The ICB has always been thought to be flat, simple, and smooth, due to the presumed extremely small variation in temperature in the outer core (7). Although recent seismic observations provide indirect evidence for the existence of topography at the ICB based on the localized temporal changes of the inner core surface (8-10), direct seismic observations about the existence of inner core topography are still nonexistent. Here, we used PKiKP (a seismic compressional wave reflected from the ICB, Fig. 1A) recorded in precritical distances to study inner core topography. ResultsIn the precritical distances, PKiKP travel times and amplitudes are sensitive to topography, geometry, and property contrast across the ICB (11-16). We adopted PcP waves [a compressional wave that is reflected off the core-mantle boundary (CMB)] as a reference phase and used PKiKP-PcP differential travel time and relative amplitude to study the ICB property. The use of differential PKiKP-PcP travel time and relative PKiKP/PcP amplitude ratio minimizes the effects of shallow Earth's structure and uncertainties in source origin time, location, magnitude, and radiation pattern Fig. 1A).We collected all available PKiKP-PcP data from earthquakes occurring in the Banda Sea and recorded at the Hi-net stations in Japan during 2004-2010. The selected earthquakes have a depth range from 100 to 650 km, with magnitudes ranging between 5.8 and 7.6 (Table 1). We measured PKiKP-PcP differential times on the vertical components of seismic data. The seismic data were filtered with a two-pole causal Butterworth band-pass filter of 1-3 Hz and the worldwide standard seismic network short-period instrument response. These fi...

Deep Earth Structure: The Earth’s Cores

“…Seismic waves reflected at different latitudes at the ICB show that the ellipticity is consistent with the equilibrium figure in a rotating Earth ). However, local topographies of about 100 m to more than 10 km, with horizontal extension of a few kilometers, are reported from observations of waves reflected at the ICB by several authors Dai et al, 2012;Song and Dai, 2008). Mantle loading and inner core loading give each a stable topography of the order of 100 m peak to peak (Defraigne et al, 1996); thus, it could not be detected by seismological methods.…”

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

“…Of particular interest is the possibility of sharp edges between the solidification and melting areas. Recent waveform modeling suggests significant topography (Dai et al 2012); however, determination of the amplitude is a more difficult problem than determination of the wavelength.…”

“…Using amplitude of precursors to PKIKP, Dai et al (2012) and Yao and Wen (2014) showed that several regions exhibit weak scattering in the lowermost mantle beneath the southwestern Pacific. PKiKP phases from events that occurred in the Banda Sea (events 4 and 9), Sumatra (events 3 and 10), and the Philippines (event 11) enter a "normal" CMB.…”

Detectability of temporal changes in fine structures near the inner core boundary beneath the eastern hemisphere

“…The shape of the IC can be adjusted by viscous relaxation or either melting or solidification at the inner core boundary (ICB) [Buffett, 1997;Monnereau et al, 2010]. Aspherical core structures and density and/or shear velocity increases across the ICB can be inferred from observations of the differential times of PKiKP-PcP (P wave reflected off the CMB) and amplitude ratios of PKiKP/PcP [Souriau and Souriau, 1989;Shearer and Masters, 1990;Koper et al, 2003;Cao and Romanowicz, 2004;Koper and Pyle, 2004;Dai et al, 2012]. At precritical distances (Δ < 110°), PKiKP amplitudes are sensitive to the sharpness of the ICB [Engdahl et al, 1970;Buchbinder et al, 1973;Cummins and Johnson, 1988].…”