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. 2012 May 15;109(20):7654-8.
doi: 10.1073/pnas.1116342109. Epub 2012 Apr 30.

Irregular Topography at the Earth's Inner Core Boundary

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Free PMC article

Irregular Topography at the Earth's Inner Core Boundary

Zhiyang Dai et al. Proc Natl Acad Sci U S A. .
Free PMC article

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.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
(A) Ray paths of PKiKP (red) and PcP (blue) waves at epicentral distances of 40° and 48°. CMB and ICB represent the core-mantle boundary and the inner core boundary, respectively. (B) Map view of great-circle ray paths (gray lines) for the PKiKP waves from earthquakes occurring in the Banda Sea (red stars) to the Hi-net stations (blue triangles) in Japan. Red circles indicate the PKiKP bouncing points at the ICB, and light blue crosses and green squares represent PKiKP entrance and exit points across the CMB. Only those ray paths with both high-quality PcP and PKiKP waveforms are plotted. Boxes A, B, C correspond to the CMB regions studied in Fig. 4 AC, respectively. (C and D) Waveforms of PKiKP (C) and PcP (D) waves observed for event January 27, 2006. The dashed lines indicate the theoretical PKiKP and PcP arrival times with respect to PREM (23). Waveforms are band-pass filtered from 1–3 Hz and with the worldwide standard seismic network short-period instrument response.
Fig. 4.
Fig. 4.
Analyses of small-scale seismic heterogeneities in the CMB regions near the PcP reflected points (A), PKiKP exit points (B), and PKiKP entrance points (C) for the seismic phases from the Banda Sea events to the Hi-net stations. A, B, and C correspond to the boxed regions labeled as A, B, C in Fig. 1B. (A) Difference between PcP travel time residuals (with respect to PREM) between neighboring events, plotted at the PcP reflected points of the CMB. Red symbols are for event pair of January 15, 2006 and August 28, 2009, green symbols for event pair January 27, 2006 and February 15, 2010, blue symbols for event pair July 1, 2007 and August 4, 2008, and black symbols for event pair December 15, 2007 and June 6, 2008. The top left inset shows PcP ray paths for an event pair whose distance separation is 2°. (B) Variations of PcP travel time residual with respect to PREM, plotted at the PcP reflected points at the CMB. Symbols are color-coded with different events (Table 1). Green squares represent PKiKP exit points at the CMB from the Banda Sea events. (C) PKP precursory amplitudes with respect (w.r.s.) to the main PKP phases (the ratio between the two amplitudes) obtained from migration of the observations recorded in 12 high-quality stations in the United States (Table S1) from an event in the region (star labeled June 27, 2011 and Table 1). The light-blue crosses represent the PKiKP entrance points at the CMB of the three events in the Banda Sea to the Hi-Net stations.
Fig. 2.
Fig. 2.
(A) PKiKP-PcP differential travel time residuals with respect to PREM, plotted at the PKiKP bouncing points of the ICB. (B) PKiKP/PcP amplitude ratios, plotted at PKiKP bouncing points of the ICB; circles denote the amplitude ratios of the reliably observed PKiKP and PcP phases; plus symbols show the amplitude ratios of invisible PKiKP and clear PcP, which should be considered as a lower bound of the amplitude ratio. The data are color-coded as earthquakes: January 27, 2006 (red), August 4, 2008 (green), and August 28, 2009 (blue). The amplitude ratios are also plotted in the inset as a function of epicentral distance, along with the predictions based on PREM (thick line).
Fig. 3.
Fig. 3.
Relationship between PKiKP-PcP differential travel time residuals and PKiKP travel time residuals (both with respect to PREM) for events January 27, 2006 (A), August 4, 2008 (C), and August 28, 2009 (E). (B, D, and F) Same as A, C, and E, except for the relationship between differential PKiKP-PcP travel time residuals and PcP travel time residuals.
Fig. 5.
Fig. 5.
(A) Synthetic vertical component of PKiKP seismograms at the distance range from 35° to 55° for PREM (red traces) and a model with ICB topography shown in C (black traces). The black traces are normalized with respect to (wrt) the amplitudes of PKiKP synthetics based on PREM. Traces are band-passed filtered with the same filtering procedures applied to the data and aligned along the predicted PKiKP arrival times based on PREM (t = 0). (B) PKiKP travel time residuals (red dots, left scale) and amplitudes (blue stars, right scale) with respect to the PREM predictions at various epicentral distances. (C) ICB topographic model plotted as a function of distances from the seismic source, which also correspond to the PKiKP reflected points for the distances of the synthetics at A and B. (DF) Same as AC, except that the ICB topographic model has a superimposed small-scale topography with a height variation of 4 km repeated with a horizontal length scale of 3 km. Note the amplitude differences in the distance range of 43° to 52° between the synthetics in A and D.

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