Seismological Evidence for Laterally Heterogeneous Lowermost Outer Core of the Earth
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Abstract The Earth's outer core is believed to be laterally homogeneous because of its low viscosity. However, a hemispherical difference in the inner core likely exists due to its uneven growth, which may be accompanied by localized light‐element releases to the outer core. A few seismological studies proposed heterogeneous lowermost outer core (called F layer) but using methods that are not very sensitive to F layer structures. In a previous study we developed a new method sensitive to the F layer structure and insensitive to others. The method analyzes differences in P wave traveltimes reflected on the inner core boundary and those that turn above the boundary as well as dispersion in waves bottoming or diffracting in the F layer, and was applied to obtain an F layer model of P wave velocity for the northeastern Pacific Ocean. In this paper, we examine the F layer structure beneath Australia using the same method. The observed dispersion requires a lower‐velocity gradient beneath Australia than beneath the northeastern Pacific, whereas the observed traveltime differences require higher average velocities beneath Australia. The results obtained for the two regions indicate that the F layer is laterally heterogeneous and that the layer beneath Australia has a higher velocity and velocity gradient in its upper part and a much smaller gradient in its lower part than beneath northeastern Pacific. The maximum velocity difference between the two regions is 0.04 km/s, which corresponds to 0.8 wt% excess oxygen according to ab initio calculations of elastic properties. These results suggest regional light‐element concentration beneath Australia.Keywords:
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This article investigates the effects of a mushy inner core boundary on the eigenperiods of the Slichter modes for a simple, but realistic, earth model (rotating, spherical configuration, elastic inner core and mantle, neutrally stratified, inviscid, compressible liquid core). It is found that the influence of the mushy boundary layer is substantial compared with some other effects, such as those from elasticity of the mantle, non-neutral stratification of the liquid outer core and ellipticity of the Earth and centrifugal potential. The results obtained here may set a lower bound on the eigenperiods of the Slichter modes for a realistic earth model. For example, for a PREM model, the lower bound of the central period of the Slichter modes should be about 5.3 hr.
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This thesis addresses the fine structure, both radial and lateral, of compressional wave velocity and attenuation of the Earth's core and the lowermost mantle using waveforms, differential travel times and amplitudes of PKP waves, which penetrate the Earth's core. The structure near the inner core boundary (ICB) is studied by analyzing waveforms of a regional sample. The waveform modeling approach is demonstrated to be an effective tool for constrainning the ICB structure. The best model features a sharp velocity jump of 0.78km/s at the ICB and a low velocity gradient at the lowermost outer core (indicating possible inhomogeneity) and high attenuation at the top of the inner core. A spherically symmetric P-wave model of the core, is proposed from PKP differential times, waveforms and amplitudes. The ICB remains sharp with a velocity
jump of 0. 78km/ s. A very low velocity gradient at the base of the fluid core is demonstrated to be a robust feature, indicating inhomogeneity is practically inevitable. The model also indicates that the attenuation in the inner core decreases with depth. The velocity at D is smaller than PREM. The inner core is confirmed to be very anisotropic, possessing a cylindrical symmetry around the Earth spin axis with the N-S direction 3% faster than the E-W
direction. All of the N-S rays through the inner core were found to be faster than the E-W rays by 1.5 to 3.5s. Exhaustive data selection and efforts in insolating
contributions from the region above ensure that this is an inner core feature. The anisotropy at the very top of the inner core is found to be distinctly different from the deeper part. The top 60km of the inner core is not anisotropic. From 60km
to 150km, there appears to be a transition from isotropy to anisotropy. PKP differential travel times are used to study the P velocity structure in D. Systematic regional variations of up to 2s in AB-DF times were observed, attributed primarily to heterogeneities in the lower 500km of the mantle. However, direct comparisons with tomographic models are not successful.
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Abstract Free oscillation data are used to determine the average compressional and shear wave velocity structure of the Earth's inner core and place tight constraints on their values. The new data reinforce the argument for a solid inner core, and the best data fit is obtained for average inner core velocities close to the PREM reference model: υ s = 3.55 ± 0.05 km/s, υ p = 11.15 ± 0.05 km/s. These values limit the time windows in which inner core shear waves, like PKJKP, may be observed. Inner core velocities for hcp and bcc iron at inner core conditions from ab initio molecular dynamics simulations and diamond anvil cell experiments are found to be incompatible with the seismological data, in particular for the shear wave velocity. When light elements are taken into account, the misfit is worse than for a fluid inner core model. The discrepancy between seismic and mineral physical values for the inner core shear wave velocity may be explained by an inner core temperature in excess of 6500 K, the existence of fluid inclusions in the inner core, the effect of viscoelastic weakening or crystal defects.
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We review the current state of the seismological constraints on core structure. The evidence supports a simple two‐layer model of the core. The fluid outer core seems to have properties which are consistent with those of a vigorously convecting region. The inner core has a sharp boundary characterised by a 0.5–0.6 g/cc density jump, a 0.6–0.7 km/s jump in P velocity, and a jump in shear velocity to 2.5 km/s or greater. There is weak evidence for an anomalously steep shear velocity gradient at the top of the inner core. Finally, the attenuation of both body waves and free oscillations indicate that the inner core is strongly attenuating near its surface though the data suggest that the attenuation is frequency dependent
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Seismic tomographic models based solely on wave velocities have limited ability to distinguish between a temperature or compositional origin for variations in Earth’s structure. Attenuation or damping, which is the loss of energy as a wave travels through the Earth, is able to make that distinction because it is directly ... read more sensitive to temperature, partial melt and water content. In this thesis, we use whole Earth oscillations, or normal modes, to study 1D variations in inner core attenuation and 3D variations in mantle attenuation. Focussing and scattering, which are problematic when measuring seismic attenuation, are automatically included in our normal mode calculations when jointly modelling elastic and anelastic structure using first order perturbation theory. For the inner core, we measure the controversial mode pair 10S2-11S2, and find that our measurements indeed agree with a strongly attenuating inner core, just as seen in previous measurements of other inner core sensitive modes, and that our new observations can be used in future inner core tomographic studies. We attribute the previous disagreement to the strong energy exchange between 10S2 and 11S2, that changes the characteristics of the modes as a result of a 0.5% perturbation to either inner core shear velocity or radius. We also measure radial modes, both in isolation and resonating with other modes, to study bulk attenuation and inner core structure with an extended data set. We find indications of lower global bulk attenuation and lower inner core radial anisotropy than previously suggested. For the mantle, the main focus of this thesis, we measure lateral variations in attenuation for both the upper and lower mantle. In the upper mantle, we find anti-correlation between velocity and attenuation, with strong attenuation found in regions with low seismic velocity. These results suggest a thermal origin for the low-velocity oceanic spreading ridges, agreeing with previous studies. In the lower mantle, we find the strongest attenuation in the ‘ring around the Pacific’ high velocity region, which is thought to be the ‘graveyard’ of subducted slabs, and not in the Large Low Shear Velocity Provinces (LLSVPs) under Africa and the Pacific. These findings might potentially be explained by either a small grain-size in combination with cold temperatures in the slabs resulting in strong attenuation, or the presence of the strongly attenuating post-perovskite mineral in the slab regions. The weakest lower mantle attenuation is found near Hawaii and southern Africa, at the edges of the LLSVPs. This suggests that these two low attenuation regions might be dominated by compositional variations instead of temperature, potentially due to iron enrichment in agreement with larger density found in the same places. show less
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