Precise seismic-wave velocity atop Earth's core: No evidence for outer-core stratification

Alexandrakis, C.; Eaton, David W.

doi:10.1016/j.pepi.2010.02.011

Cited by 41 publications

(23 citation statements)

References 27 publications

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“…His model (called Tanaka-1 hereafter) is similar to that presented in an earlier study by Garnero et al (1993). Alexandrakis and Eaton (2010) investigated composite globally observed record sections of S2KS to S4KS for the distance range shorter than 140°and showed a permissible range of V p profiles for the top 200 km of the outer core. The range of permissible models centers around slightly slower V p values (from ∼0.1 to 0.4 %) than PREM and falls between PREM and the IASP91 velocity model (Kennett and Engdahl 1991).…”

Section: Fig 1 Smks Ray Paths Ray Paths Of Smks Waves (S2ks S3ksmentioning

confidence: 72%

“…Although PREM is known to be a good global model, there may be other reference models as good as PREM. We perform the τ − p inversion using IASP91 as the starting model, as did Alexandrakis and Eaton (2010). By using only the Fiji-Tonga to European stations data, as in the case of KHOMC, we obtained a profile of V p anomaly relative to IASP91 (called KOCTI).…”

Section: Array Measurementsmentioning

confidence: 99%

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Stratification of earth’s outermost core inferred from SmKS array data

Kaneshima

Matsuzawa

2015

Prog. in Earth and Planet. Sci.

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SmKS arrivals recorded by large-scale broadband seismometer arrays are analyzed to investigate the depth profile of P wave speed (V p ) in the outermost core. The V p structure of the upper 700 km of the outer core has been determined using SmKS waves of Fiji-Tonga events recorded at stations in Europe. According to a recent outer core model (KHOMC), the V p value is 0.45 % slower at the core mantle boundary (CMB) than produced by the Preliminary Reference Earth Model (PREM), and the slow anomaly gradually diminishes to insignificant values at ∼300 km below the CMB. In this study, after verifying these KHOMC features, we show that the differential travel times measured for SmKS waves that are recorded by other large-scale arrays sampling laterally different regions are well matched by KHOMC. We also show that KHOMC precisely fits the observed relative slowness values between S2KS, S3KS, and S4KS (SmKS waves with m = 2, 3, and 4). Based on these observations, we conclude that SmKS predominantly reflect the outer core structure. Then we evaluate biases of secondary importance which may be caused by mantle heterogeneity. The KHOMC V p profile can be characterized by a significant difference in the radial V p gradient between the shallower 300 km and the deeper part of the upper 700 km of the core. The shallower part has a V p gradient of −0.0018 s −1 , which is steeper by 0.0001 s −1 when compared to the deeper core presented by PREM. The steeper V p gradient anomaly of the uppermost core corresponds to a radial variation in the pressure derivative of the bulk modulus, K = dK/dP. The K value is 3.7, which is larger by about 0.2 than that of the deeper core. The radial variation in K is too large to have a purely thermal origin, according to recent ab initio calculations on liquid iron alloys, and thus requires a thick and compositionally stratified layering at the outermost outer core.

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Section: Fig 1 Smks Ray Paths Ray Paths Of Smks Waves (S2ks S3ksmentioning

confidence: 72%

Section: Array Measurementsmentioning

confidence: 99%

Stratification of earth’s outermost core inferred from SmKS array data

Kaneshima

Matsuzawa

2015

Prog. in Earth and Planet. Sci.

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“…We are, therefore, now extending calculations to alloy systems (with oxygen, silicon, sulfur, and nickel), which will be reported elsewhere. These might produce fundamental physical data to give tighter constraints for the thermochemical properties of the Earth's outer core including its upper [ Lay and Young , ; Garnero et al ., ; Tanaka , ; Alexandrakis and Eaton , ; Helffrich and Kaneshima , ] and lower [ Cormier et al ., ; Gubbins et al ., ] boundary layers.…”

Section: Geophysical Implicationsmentioning

confidence: 99%

The P‐V‐T equation of state and thermodynamic properties of liquid iron

2014

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The equation of state (EoS) and thermodynamic properties of non-magnetic liquid iron were investigated from energy (E)-pressure (P)-volume (V)-temperature (T) relationships calculated by means of ab initio molecular dynamics simulations at 60-420 GPa and 4000-7000 K. Its internally consistent thermodynamic and elastic properties, in particular, density, adiabatic bulk modulus, and P wave velocity, were then analyzed. Compared to the seismological data of the Earth's outer core, pure liquid iron is found to have an 8-10% larger density and 3-10% larger bulk modulus than the Earth's values. Results also show that the P wave velocity of liquid iron has marginal temperature dependence as the bulk sound velocity of solid iron. The new EoS model and thermodynamic properties of liquid iron may serve as fundamental data for the thermochemical modeling of the Earth's core.

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“…A stratified layer with a lower

V p

gradient than the Preliminary Reference Earth Model (PREM; Dziewonski & Anderson, ), labeled the

F

layer, has been documented using body seismic wave observations (Ohtaki & Kaneshima, ; Souriau & Poupinet, ; Song & Helmberger, ; Zou et al, ). Another stratified layer, the

E^{'}

layer, is hypothesized to exist at the top of outer core, and its properties may be constrained by geomagnetic secular variations (Buffett, ; Gubbins, ), but the seismic evidence, especially

S m K S

differential arrival times, for this layer is contradictory and controversial (e.g., Alexandrakis & Eaton, ; Eaton & Kendall, ; Helffrich & Kaneshima, ; Kaneshima & Helffrich, ).…”

Section: Introductionmentioning

confidence: 99%

Array‐Based Iterative Measurements of Travel Times and Their Constraints on Outermost Core Structure

Irving

2020

JGR Solid Earth

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Vigorous convection in Earth's outer core led to the suggestion that it is chemically homogeneous. However, there is increasing seismic evidence for structural complexities close to the outer core's upper and lower boundaries. Both body waves and normal mode data have been used to estimate a P wave velocity, Vp, at the top of the outer core (the E′ layer), which is lower than that in the Preliminary Reference Earth Model. However, these low Vp models do not agree on the form of this velocity anomaly. One reason for this is the difficulty in retrieving and measuring SmKS arrival times. To address this issue, we propose a novel approach using data from seismic arrays to iteratively measure SmKS‐ SKKS differential travel times. This approach extracts individual SmKS signal from mixed waveforms of the SmKS series, allowing us to reliably measure differential travel times. We successfully use this method to measure SmKS time delays from earthquakes in the Fiji‐Tonga and Vanuatu subduction zones. SmKS time delays are measured by waveform cross correlation between SmKS and SKKS, and the cross‐correlation coefficient allows us to access measurement quality. We also apply this iterative scheme to synthetic SmKS seismograms to investigate the 3‐D mantle structure's effects. The mantle structure corrections are not negligible for our data, and neglecting them could bias the Vp estimation of uppermost outer core. After mantle structure corrections, we can still see substantial time delays of S3KS, S4KS, and S5KS, supporting a low Vp at the top of Earth's outer core.

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Precise seismic-wave velocity atop Earth's core: No evidence for outer-core stratification

Cited by 41 publications

References 27 publications

Stratification of earth’s outermost core inferred from SmKS array data

Stratification of earth’s outermost core inferred from SmKS array data

The P‐V‐T equation of state and thermodynamic properties of liquid iron

Array‐Based Iterative Measurements of Travel Times and Their Constraints on Outermost Core Structure

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