Equatorial anisotropy in the inner part of Earth’s inner core from autocorrelation of earthquake coda

Wang, Tao; Song, Xiaodong; Xia, Hao

doi:10.1038/ngeo2354

Cited by 111 publications

(116 citation statements)

References 30 publications

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“…We exploit vertical component seismograms of earthquakes with Mw ≥ 6.8 according to the global centroid moment tensor catalog (Ekström et al, 2012) and extract the coda waveforms for the time interval 10,000-35,000 s (~3-10 hr) after the origin time. The time window and the magnitude threshold we have employed are those recommended in previous studies that analyzed the emergence of few individual correlation phases that are sensitive to the core (e.g., Wang et al, 2015;Xia et al, 2016). We find that this window does indeed produce results of high quality.…”

Section: Global Waveform Coda Cross Correlogrammentioning

confidence: 92%

Earth's Correlation Wavefield: Late Coda Correlation

Phạm

Tkalčić

Sambridge

et al. 2018

Geophysical Research Letters

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Cross correlation of seismograms provides new information on the Earth both through the exploitation of ambient noise and specific components of earthquake records. Here we cross‐correlate recordings of large earthquakes on a planetary scale and identify a range of hitherto unobserved seismic phases in Earth's correlation wavefield. We show that both arrivals with the timing expected for the regular seismic wavefield and previously unexplained phases are produced by interference between seismic paths having the same ray parameter but with only a subset of propagation legs in common. This insight explains the origin and generation mechanism of the features of Earth's correlation wavefield and opens up new ways of addressing issues in global seismology. Strong similarity between observed and synthesized correlation wavefields indicates that the Earth's radial structure is remarkably well constrained in the intermediate period range.

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Section: Global Waveform Coda Cross Correlogrammentioning

confidence: 92%

Earth's Correlation Wavefield: Late Coda Correlation

Phạm

Tkalčić

Sambridge

et al. 2018

Geophysical Research Letters

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“…In particular, the compressional ( P wave, V P ) and shear ( S wave, V S ) wave velocities from seismic observations are low when compared to observations from mineral physics [e.g., Vočadlo , ; Vočadlo et al ., ; Belonoshko et al ., ; Martorell et al ., ]. This makes even a basic understanding of the inner core difficult, let alone the interpretation of the far more complex characteristics of the inner core revealed by recent studies [ Alboussière et al ., ; Olson and Deguen , ; Tkalčić , ; Wang et al ., ].…”

Section: Introductionmentioning

confidence: 99%

Thermoelasticity of Fe₇C₃ under inner core conditions

Vočadlo

Brodholt

et al. 2016

JGR Solid Earth

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It has recently been reported, on the basis of extrapolated experimental data, that the iron carbide, Fe7C3, has shear wave velocities and a Poisson's ratio consistent with the seismological values for the Earth's inner core and thus that Fe7C3 is a strong candidate for the inner core composition. In this study, using ab initio molecular dynamics simulations, we report the thermoelastic properties of Fe7C3 at 350 GPa up to its melting temperature. Due to significant elastic softening prior to melting, the calculated elastic properties, including wave velocities, do indeed agree well with those from seismology. However, the density was found to be much too low (by ~8%) when compared to geophysical data, and therefore, Fe7C3 must be ruled out as a major component of the Earth's inner core.

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“…Stiffness tensors calculated ab initio for body‐centered cubic iron (bcc‐Fe) crystals have been used to fit body‐wave data for the IMIC in previous studies (Romanowicz et al, ; Wang et al, ). In Appendix B, we apply our effective‐media models to a hexagonal stiffness tensor derived by averaging a bcc‐Fe simulation at 6000 K and 360 GPa (Belonoshko et al, ) about its fast [111] crystallographic axis.…”

Section: Resultsmentioning

confidence: 99%

Effective‐Medium Models of Inner‐Core Anisotropy

Song

Jordan

2018

JGR Solid Earth

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Body‐wave and normal‐mode observations of Earth's inner core show cylindrical anisotropy consistent with an alignment of hexagonal close‐packed iron (hcp‐Fe) crystals along the rotation axis. We quantify the degree of alignment by comparing seismic observations longitudinally averaged over the outer 600 km of the inner core with stochastic rotation models based on ab initio calculations of hcp‐Fe stiffness tensors. Likelihood functions are constructed for the Woodhouse anisotropy ratios from an ensemble of normal‐mode models and for the Backus ray‐angle parameters from the Irving‐Deuss traveltime data set. The tensor at each inner‐core location is modeled as a random rotation of the hcp‐Fe stiffness tensor, and the rotations are assumed to have transversely isotropic statistics with correlation lengths that are small compared to the seismic wavelengths. Two stochastic rotation models are used, one based on Fisher statistics (F model) with concentration parameter κ and one by Jordan (J model) with orientation ratio ξ. The first‐order estimates, obtained by Voigt averaging, imply that hcp‐Fe tensors by Martorell and others considering premelting softening can account for the seismic data if the hcp‐Fe axes are moderately aligned with Earth's rotation axis ( κ=2.1−0.6+1.9, ξ=0.60−0.15+0.13). The J model is extended to include second‐order Born scattering, which in the low‐frequency limit depends on the aspect ratio η of the transversely isotropic correlation tensor. Although η is not usefully constrained by the existing data, we show that second‐order scattering acts to increase the required bipolar concentration of hcp‐Fe axes. Integrating over η yields the marginal estimate ξ=0.52−0.12+0.19, corresponding to a 50% dispersion angle of 42°−7°+9°.

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Equatorial anisotropy in the inner part of Earth’s inner core from autocorrelation of earthquake coda

Cited by 111 publications

References 30 publications

Earth's Correlation Wavefield: Late Coda Correlation

Earth's Correlation Wavefield: Late Coda Correlation

Thermoelasticity of Fe₇C₃ under inner core conditions

Effective‐Medium Models of Inner‐Core Anisotropy

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Equatorial anisotropy in the inner part of Earth’s inner core from autocorrelation of earthquake coda

Cited by 111 publications

References 30 publications

Earth's Correlation Wavefield: Late Coda Correlation

Earth's Correlation Wavefield: Late Coda Correlation

Thermoelasticity of Fe7C3 under inner core conditions

Effective‐Medium Models of Inner‐Core Anisotropy

Contact Info

Product

Resources

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Thermoelasticity of Fe₇C₃ under inner core conditions