Earthʼs inner core: Innermost inner core or hemispherical variations?

Lythgoe, Karen; Deuss, Arwen; Rudge, John F.; Neufeld, Jerome A.

doi:10.1016/j.epsl.2013.10.049

Cited by 53 publications

(93 citation statements)

References 45 publications

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“…Hemispherical dichotomy is recognized as a global phenomenon near the ICB (Niu and Wen 2001;Waszek et al 2011) and possibly near the center of the inner core (Lythgoe et al 2014). 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).…”

Section: Introductionmentioning

confidence: 99%

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

Tanaka

Tkalčić

2015

Prog. in Earth and Planet. Sci.

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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.

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Section: Introductionmentioning

confidence: 99%

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

Tanaka

Tkalčić

2015

Prog. in Earth and Planet. Sci.

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“…cate up to 4.4% and 1% anisotropy in the western and eastern hemispheres, respectively (Tanaka and Hamaguchi, 1997;Garcia and Souriau, 2000;Niu and Chen, 2008;Deuss et al, 2010;Irving and Deuss, 2011), with an almost isotropic layer -60 km deep in the most recent studies -at the top of the inner core surrounding a more anisotropic region (Shearer, 1994;Song and Helmberger, 10 1995; Creager, 1999;Garcia and Souriau, 2000;Waszek and Deuss, 2011), and, possibly, an innermost inner core with yet different properties (Ishii and Dziewoński, 2002;Beghein and Trampert, 2003;Calvet et al, 2006;Niu and Chen, 2008;Sun and Song, 2008b,a), although the presence of an innermost inner core is not required in the analysis of more recent datasets (Lythgoe et al, 2013). In this paper, we present models aiming at reproducing global NS anisotropy and its depth 15 variation.…”

Section: Introductionmentioning

confidence: 99%

“…As such, the interpretation of seismic data 30 is often simplified by introducing "structural" inner core models, with different layers of distinct anisotropy (e.g. Bergman, 1997;Calvet et al, 2006;Sun and Song, 2008a;Lythgoe et al, 2013).…”

Section: Introductionmentioning

confidence: 99%

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Seismic response and anisotropy of a model hcp iron inner core

Lincot

Deguen

Merkel

et al. 2014

Comptes Rendus. Géoscience

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A Lincot, R Deguen, Sébastien Merkel, Philippe Cardin To cite this version:A Lincot, R Deguen, Sébastien Merkel, Philippe Cardin. AbstractWe present a framework for simulating the measurement of seismic anisotropy in a model inner core by computing travel time residuals of synthetic seismic rays propagated through the model.The method is first tested on simple inner core structural models consisting of layers with distinct anisotropy, as often proposed in the literature. Those models are not consistent with geodynamics.Hence, we extend the method to a numerically grown inner core composed of ε-Fe with flow generated from an excess of crystallization in the equatorial belt, inducing polycrystalline textures.The global inner core anisotropy is 7 times smaller than that of the single crystal. Compositional stratification amplifies the global anisotropy by 15% while the addition of solidification textures reduces it by a factor of 2. As such, and within the tested geodynamical models, no published elastic model of ε-Fe at inner core conditions allows for reproducing the 3% cylindrical anisotropy reported in seismology publications. In addition, our models demonstrate that additional information such as the depth dependence and the spread of the observed anisotropy is a key for revealing the dynamics and history of the inner core.

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“…In particular, the upper inner core is seismically isotropic and has a Western Hemisphere with an approximately 1 per cent slower isotropic P-wave velocity and greater attenuation than in the east (Niu & Wen 2001;Cao & Romanowicz 2004;Waszek et al 2011). Cylindrical anisotropywith compressional waves travelling fastest along Earth's rotation axis and slowest along the equatorial plane-appears from a depth of around 100 km below the inner core boundary (ICB) and is concentrated in a region in the western hemisphere (Tanaka & Hamaguchi 1997;Creager 2000;Garcia & Souriau 2000;Deuss et al 2010;Lythgoe et al 2014). The eastern region remains isotropic throughout the inner core (Lythgoe et al 2014).…”

Section: Introductionmentioning

confidence: 99%

The feasibility of thermal and compositional convection in Earth's inner core

Lythgoe¹,

Rudge²,

Neufeld

et al. 2015

Geophysical Journal International

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S U M M A R YInner core convection, and the corresponding variations in grain size and alignment, has been proposed to explain the complex seismic structure of the inner core, including its anisotropy, lateral variations and the F-layer at the base of the outer core. We develop a parametrized convection model to investigate the possibility of convection in the inner core, focusing on the dominance of the plume mode of convection versus the translation mode. We investigate thermal and compositional convection separately so as to study the end-members of the system. In the thermal case the dominant mode of convection is strongly dependent on the viscosity of the inner core, the magnitude of which is poorly constrained. Furthermore recent estimates of a large core thermal conductivity result in stable thermal stratification, hindering convection. However, an unstable density stratification may arise due to the pressure dependant partition coefficient of certain light elements. We show that this unstable stratification leads to compositionally driven convection, and that inner core translation is likely to be the dominant convective mode due to the low compositional diffusivity. The style of convection resulting from a combination of both thermal and compositional effects is not easy to understand. For reasonable parameter estimates, the stabilizing thermal buoyancy is greater than the destabilizing compositional buoyancy. However we anticipate complex double diffusive processes to occur given the very different thermal and compositional diffusivities.

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Earthʼs inner core: Innermost inner core or hemispherical variations?

Cited by 53 publications

References 45 publications

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

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

Seismic response and anisotropy of a model hcp iron inner core

The feasibility of thermal and compositional convection in Earth's inner core

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