Robust Normal Mode Constraints on Inner-Core Anisotropy from Model Space Search

Beghein, Caroline; Trampert, Jeannot

doi:10.1126/science.1078159

Cited by 132 publications

(137 citation statements)

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“…The data from the three earliest of these events were used by Durek & Romanowicz (1999) and the Bolivia and Kuril events were part of the splitting function dataset used by Beghein & Trampert (2003) The presence of an isotropic layer, of thickness ranging from 0km to 1150km was imposed upon the anisotropy models in each calculation. The different thickness of the isotropic layer used were separated by 25km intervals between 0km and 600km, and by 50km intervals between 600km and 1150km.…”

Section: Methodsmentioning

confidence: 99%

“…We use these four normal mode models of inner core anisotropy -the B&T model (Beghein & Trampert, 2003), the D&R model (Durek & Romanowicz, 1999), the Tr model (Tromp, 1993) and the W,G&L model ) -to examine the apparent discrepancy between the anisotropic structure at the top of the inner core observed using normal modes and the isotropic M a n u s c r i p t structure found by previous body wave studies. All of these models have been made using the self-coupling (SC) approximation, which we have shown (Irving et al, 2008) to be inaccurate when inner core anisotropy is considered.…”

mentioning

confidence: 99%

“…They found that the fit of the model to the data was degraded if an isotropic uppermost layer of thickness greater than 200km was introduced into the model. Beghein & Trampert (2003) developed an inner core anisotropy model by inverting splitting functions. Like the models of Durek & Romanowicz (1999) and Woodhouse et al (1986) this model contains anisotropic structure at both the ICB and centre of the inner core; none of these models contains isotropic structure at the top of the inner core.…”

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confidence: 99%

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Stratified anisotropic structure at the top of Earth's inner core: A normal mode study

Irving

Deuss

2011

Physics of the Earth and Planetary Interiors

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Please cite this article as: Irving, J.C.E., Deuss, A., Stratified anisotropic structure at the top of Earth's inner core:a normal mode study, Physics of the Earth and Planetary Interiors (2010), doi:10.1016/j.pepi.2011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Page 1 of 40A c c e p t e d M a n u s c r i p tWe probe the isotropic layer at the top of Earth's inner core using normal modes. Normal mode data are compatible with an isotropic layer of up to 250km thick. Normal modes can therefore be reconciled with previous body wave studies. We further show seismic data support the presence an S-wave isotropic layer. *HighlightsPage 2 of 40 A c c e p t e d M a n u s c r i p t AbstractBody wave studies support the presence of an isotropic layer at the top of the inner core. Recent normal mode models of inner core anisotropy do not contain such a layer, but instead have anisotropic structure extending to the inner core boundary. Here, we use full-coupling of normal mode oscillations sensitive to inner core structure in order to investigate the discrepancy between models of the inner core developed using these two different types of data. We impose an isotropic layer onto existing normal mode models of inner core anisotropy, and calculate frequencies, quality factors and synthetic seismograms for radial modes, PKIKP modes and PKJKP modes. Using full-coupling allows us to make the first simulations of the effect of an isotropic layer on radial modes.The presence of an uppermost isotropic layer has an effect on the frequencies and attenuation of normal modes. By calculating the misfit between data from four large earthquakes and synthetic seismograms created for models of the inner core with isotropic layers of varying thickness, we find that normal mode data are compatible with the presence of an isotropic layer of up to 250km thickness at the top of the inner core. Thus, normal modes can be reconciled with previous body wave studies. The influence of such a layer on PKJKP modes is the only way that S-wave isotropy at the top of the inner core can currently be studied;we show, for the first time, that seismological data support the presence an S-wave isotropic layer.

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Section: Methodsmentioning

confidence: 99%

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Stratified anisotropic structure at the top of Earth's inner core: A normal mode study

Irving

Deuss

2011

Physics of the Earth and Planetary Interiors

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“…It was shown that the calculated free energy of bcc-Fe at V=7.2 Å/atom and 5500 K is only marginally higher than that for hcp (by ~35 meV). The assumption that iron must have the hcp structure at core conditions, especially in the presence of lighter elements, had already been challenged (Beghein and Trampert, 2003;Ishii and Dziewonski, 2002), and so Vočadlo et al (2003) presented calculations that suggested that a small amount of light element impurity could indeed stabilise the bcc phase at the expense of the hcp phase in the Earth's inner core (see also Côté et al, 2007). We note that nickel, by contrast, crystallises in the fcc structure and stabilises fcc Fe-Ni alloys relative to pure Fe (e.g., Mao et al, 2006).…”

Section: Page 4 Of 21mentioning

confidence: 99%

The stability of bcc-Fe at high pressures and temperatures with respect to tetragonal strain

Vočadlo

Wood

Gillan

et al. 2008

Physics of the Earth and Planetary Interiors

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This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. AbstractThe phase that iron adopts at the conditions of the Earth's inner core is still unknown. The two primary candidates are the hexagonal-close-packed (hcp) structure and the bodycentred-cubic (bcc) structure polymorphs. Until recently, the former was favoured, but it now seems possible that bcc iron could be present. A remaining uncertainty regarding the latter phase is whether or not bcc iron is stable with respect to tetragonal strain under core conditions. In this paper, therefore, we present the results of high precision ab initio free energy calculations at core pressures and temperatures performed on bcc iron as a function of tetragonal strain. Within the uncertainties of the calculations, direct comparison of free energy values suggests that bcc may be unstable with respect to tetragonal strain at 5500 K; this is confirmed when the associated stresses are taken into account. However, at 6000 K, the results indicate that the bcc phase becomes more stable, although it is unclear as to whether complete stability has been achieved. Nevertheless, it remains distinctly possible that the addition of light elements could stabilise this structure convincingly. Therefore, bcc-Fe cannot be ruled out as a candidate inner core phase.

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“…Observations of such type of modes, for example, gave evidence of the rigidity of the Earth's inner core (Dziewonski & Gilbert 1971) or, more recently, provided constraints on its anisotropic structure (Laske & Masters 1999;Beghein & Trampert 2003;Deuss et al 2010). This would not have been possible without stable, long time-series of vertical component displacements corresponding to high-quality spectra at very long periods.…”

Section: Introductionmentioning

confidence: 99%

Normal mode coupling observations with a rotation sensor

Nader

Igel

Ferreira

et al. 2015

Geophysical Journal International

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S U M M A R YWe present observations of cross coupled spheroidal modes in the Earth's free oscillation spectrum recorded by the vertical component G-ring laser (Geodetical Station Wettzell) of the 2011 M w 9.0 Tohoku-Oki earthquake. In an attempt to determine which are the mechanisms responsible for spheroidal energy in a vertical axes rotational spectra, we first rule out instrumental effects as well as the effect of local heterogeneity. Secondly, we carry out simulations of an ideal rotational sensor taking into account the effects of the Earth's rotation, its hydrostatic ellipticity and structural heterogeneity, which results in a good fit to the data. Simulations considering each effect separately are performed in order to evaluate the sensitivity of rotational motions to various phenomena compared to traditional translation measurements.

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Robust Normal Mode Constraints on Inner-Core Anisotropy from Model Space Search

Cited by 132 publications

References 35 publications

Stratified anisotropic structure at the top of Earth's inner core: A normal mode study

Stratified anisotropic structure at the top of Earth's inner core: A normal mode study

The stability of bcc-Fe at high pressures and temperatures with respect to tetragonal strain

Normal mode coupling observations with a rotation sensor

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