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

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.

Measuring the seismic velocity in the top 15 km of Earth’s inner core

“…This is in agreement with previous studies which found complexity beneath the Pacific Ocean (Stroujkova and Cormier, 2004;Miller et al, 2013;Cormier, 2015;Irving and Deuss, 2015;Yu et al, 2017). The inner core boundary beneath the northern Pacific Ocean is also found to display small scale features including topography and melting (Waszek and Deuss, 2015;Tian and Wen, 2017;Shen et al, 2016).…”

“…Alternatively, the observations may result from the inner core hemispheres, whereby the deeper ray paths spend more time in the low velocity west hemisphere, and latitudinal variation in the hemisphere boundary is reported (Cormier, 2015;Irving and Deuss, 2015;Yu et al, 2017). The lower velocity we detect in the northern part of our dataset may correspond to small scale mushy zones on the inner core boundary observed here (Tian and Wen, 2017;Shen et al, 2016).…”

“…Further irregularities and variation in the location of the Pacific boundary have been observed (Miller et al, 2013;Yee et al, 2014;Irving and Deuss, 2015;Yu et al, 2017). Small scale features on the inner core boundary have been detected in this area (Waszek and Deuss, 2015;Tian and Wen, 2017;Shen et al, 2016), which may be linked to regional solidification of the inner core (Cormier, 2015).…”

Irregular Transition Layer Beneath the Earth's Inner Core Boundary From Observations of Antipodal PKIKP and PKIIKP Waves

“…They proposed a model of mosaic ICB structure, in which regions having a liquid-liquid transition are interspersed with those having a liquid-solid transition. Lateral variations observed in PKiKP/PcP amplitude ratios and PKiKP-PcP travel times, as well as complexity observed in precritical PKiKP waveforms, might also support the presence of such a complex ICB structure (Tian & Wen, 2017;Waszek & Deuss, 2015). Seismic and thermodynamic modeling constrains the thickness of transition patches as described by Krasnoshchekov et al (2005) to less than 10 km (Cummins & Johnson, 1988b;Deguen et al, 2007;Tian & Wen, 2017).…”

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

“…At the Earth's radii of about 1,220 km, the temperature falls below the solidus of the Fe‐Ni alloy of the outer core (OC) and IC freezes (Jacobs, ). The up‐to‐date description of this outward crystallization is very far from complete and admits a number of scenarios including formation of slurry, dendritic, and non‐dendritic morphologies (e.g., Fearn et al, ; Loper & Roberts, ; Tian & Wen, ). It was shown that interfacial heat flux at either boundary of the OC has a significant effect on solidification regime and IC texture formation, so the cooling rate during phase transition is not uniform throughout the inner core boundary (ICB) and may give rise to multi‐scale heterogeneities (e.g., Aubert et al, ; Gubbins et al, ).…”