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. 2017 Aug 1;8(1):165.
doi: 10.1038/s41467-017-00229-9.

Seismological Evidence for a Localized Mushy Zone at the Earth's Inner Core Boundary

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Free PMC article

Seismological Evidence for a Localized Mushy Zone at the Earth's Inner Core Boundary

Dongdong Tian et al. Nat Commun. .
Free PMC article

Abstract

Although existence of a mushy zone in the Earth's inner core has been hypothesized several decades ago, no seismic evidence has ever been reported. Based on waveform modeling of seismic compressional waves that are reflected off the Earth's inner core boundary, here we present seismic evidence for a localized 4-8 km thick zone across the inner core boundary beneath southwest Okhotsk Sea with seismic properties intermediate between those of the inner and outer core and of a mushy zone. Such a localized mushy zone is found to be surrounded by a sharp inner core boundary nearby. These seismic results suggest that, in the current thermo-compositional state of the Earth's core, the outer core composition is close to eutectic in most regions resulting in a sharp inner core boundary, but deviation from the eutectic composition exists in some localized regions resulting in a mushy zone with a thickness of 4-8 km.The existence of a mushy zone in the Earth's inner core has been suggested, but has remained unproven. Here, the authors have discovered a 4-8 km thick mushy zone at the inner core boundary beneath the Okhotsk Sea, indicating that there may be more localized mushy zones at the inner core boundary.

Conflict of interest statement

The authors declare no competing financial interests.

Figures

Fig. 1
Fig. 1
Raypaths of PKiKP and PcP phases. PKiKP (red) and PcP (blue) are compressional waves reflected off the inner core boundary (ICB) and the core–mantle boundary (CMB) respectively. Raypaths are calculated based on PREM, for a seismic source at a depth of 300 km (black star) and receivers (black triangles) at two example epicentral distances of 30° and 35°
Fig. 2
Fig. 2
ICB study region and PKiKP–PcP differential travel time residuals. a Map view of the ICB study region (solid black box, with detailed results shown in Fig. 2b), along with seismic events (red stars, labeled by event ID listed in Supplementary Table 1) and stations (black triangles) used in the analyses. b Geographic distribution of PKiKP–PcP differential travel time residuals in the study region (corresponding to the solid black box in Fig. 2a), averaged over caps with a radius of 1° covering the PKiKP reflected points at the ICB (Supplementary Fig. 1 for a complete version). Cap-averaged residuals are plotted at the center of each cap and color-coded with each event, with positive and negative values denoted by circles and triangles respectively and the size of the symbol proportional to the absolute value of travel time residual. The ICB is grouped into regions according to their structural characteristics inferred based on seismic data, with black boxes (labeled as A: A1, A2, A3, and A4) indicating flat and sharp regions and blue box (labeled as B) a region with a laterally varying double-layered structure (mushy zone) across the ICB
Fig. 3
Fig. 3
Comparisons of stacked PKiKP and PcP waveforms of two example events sampling two regions of ICB with different seismic characteristics. a Comparisons for event 5 which samples a sharp ICB beneath central China (black box labeled as A1 in Fig. 2b). b Comparisons for event 11 which samples a double-layered mushy ICB beneath southwest Okhotsk Sea (black box A4 and blue box B in Fig. 2b). Stacked PcP (black traces) and PKiKP (blue traces) waveforms are bandpass filtered in a frequency range of 2–3 Hz. The number of waveforms used in each stacking is indicated at the right of each trace
Fig. 4
Fig. 4
Seismic model and waveform modeling for the PKiKP data sampling the ICB region beneath southwest Okhotsk Sea. a Compressional velocity profile of the best-fitting double-layered model across the ICB (red line) represented by two parameters: d the thickness of the layer and δV P percentage compressional velocity jump of the top layer with respect to PREM velocity jump at ICB, along with PREM (black line). b Synthetic seismograms in a frequency range of 2–3 Hz for a series of double-layered ICB models (red traces, labeled accordingly with two model parameters: d and δV P) and for PREM (bottom black trace). c Comparisons of stacked observed PKiKP waveforms (blue traces) of event 11 in a frequency range of 2–3 Hz and synthetic seismograms (red traces) of the best-fitting models. The number of waveforms used in each stacking is labeled at the right of each trace, while the thickness d and percentage compressional velocity jump of the top layer δV P of the best-fitting models are labeled above the red traces. The sampled ICB region also corresponds to Boxes B and A4 in Fig. 2b

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