Tomographic errors from wave front healing: more than just a fast bias

Malcolm, Alison; Trampert, Jeannot

doi:10.1111/j.1365-246x.2011.04945.x

Cited by 42 publications

(37 citation statements)

References 39 publications

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“…Some studies question whether the seismic data require an anti-correlation (e.g. Malcolm and Trampert 2011;Schuberth et al 2012). We note that the partial correlation between bulk-sound and shear-wave velocities inferred from imaged seismic structure (Fig.…”

Section: High ∂Ln V S /∂Ln V P and Anti-correlated ∂Ln V S And ∂Ln V φmentioning

confidence: 80%

Thermally Dominated Deep Mantle LLSVPs: A Review

Davies

Goes

Lau

2015

The Earth's Heterogeneous Mantle

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The two large low shear-wave velocity provinces (LLSVPs) that dominate lower-mantle structure may hold key information on Earth's thermal and chemical evolution. It is generally accepted that these provinces are hotter than background mantle and are likely the main source of mantle plumes. Increasingly, it is also proposed that they hold a dense (primitive and/or recycled) compositional component. The principle evidence that LLSVPs may represent thermo-chemical 'piles' comes from seismic constraints, including the following: (i) their longwavelength nature; (ii) sharp gradients in shear-wave velocity at their margins; (iii) non-Gaussian distributions of deep mantle shear-wave velocity anomalies; (iv) anti-correlated shear-wave and bulk-sound velocity anomalies (and elevated ratios between shear-and compressional-wave velocity anomalies); (v) anti-correlated shear-wave and density anomalies; and (vi) 1-D/radial profiles of seismic velocity that deviate from those expected for an isochemical, well-mixed mantle. In addition, it has been proposed that hotspots and the reconstructed eruption sites of large igneous provinces correlate in location with LLSVP margins. In this paper, we review recent results which indicate that the majority of these constraints do not require thermo-chemical piles: they are equally well (or poorly) explained by thermal heterogeneity alone. Our analyses and conclusions are largely based on comparisons between imaged seismic structure and synthetic seismic structures from a set of thermal and thermo-chemical mantle convection models, which are constrained by ~300 Myr of plate motion histories. Modelled physical structure (temperature, pressure and composition) is converted into seismic velocities via a thermodynamic approach that accounts for elastic, anelastic and phase contributions and, subsequently, a tomographic resolution filter is applied to account for the damping and geographic bias inherent to seismic imaging. Our results indicate that, in terms of large-scale seismic structure and dynamics, these two provinces are predominantly thermal features and, accordingly, that chemical heterogeneity is largely a passive component of lowermost mantle dynamics.

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Section: High ∂Ln V S /∂Ln V P and Anti-correlated ∂Ln V S And ∂Ln V φmentioning

confidence: 80%

Thermally Dominated Deep Mantle LLSVPs: A Review

Davies

Goes

Lau

2015

The Earth's Heterogeneous Mantle

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“…In particular, we determine whether wavefront healing (Wielandt 1987;Malcolm & Trampert 2011) reduces the traveltime delay due to a low-velocity deep mantle plume to an immeasurably small value at the Earth's surface.…”

Section: Introductionmentioning

confidence: 99%

Wavefront healing renders deep plumes seismically invisible

Hwang¹,

Ritsema²,

Keken³

et al. 2011

Geophysical Journal International

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S U M M A R YSince W. J. Morgan proposed that intraplate volcanism at some Pacific hotspots is caused by hot plumes rising from the lower mantle, geophysicists have been actively pursuing physical evidence for mantle plumes. Several seismic studies have mapped low-velocity anomalies below a number of hotspots. However, the association of low-velocity structures with plume tails has remained controversial given the debate on whether lower-mantle plumes impart observable traveltime or amplitude perturbations on seismic waves. Using high-resolution numerical simulations of plume ascent through the mantle and their effects on waveforms, we demonstrate that the delay of shear waves by plume tails at depths larger than 1000 km are immeasurably small (<0.2 s) at seismic periods commonly used in waveform analysis. Therefore, we conclude that narrow lower mantle plumes are not detectable.

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“…The interpretation of these anomalies may require better constraints in light of the discovery of a phase transition from perovskite to postperovskite in the lowermost ∼200 km of the mantle [e.g., Murakami et al , ; Oganov and Ono , ; Tsuchiya et al , ]. A few studies have argued that the high ν in the lower mantle could be an artifact of “wavefront healing” in traveling waves and not due to compositional heterogeneity [e.g., Nolet and Dahlen , ; Malcolm and Trampert , ; Davies et al , ]. The recent splitting measurements of v P ‐sensitive modes [ Deuss et al , ] along with travel time data sets [e.g., Masters et al , ] can provide robust constraints on the relative behavior of shear and compressional velocities, helping resolve the significance of compositional effects in the lowermost mantle.…”

Section: Introductionmentioning

confidence: 99%

The relationships between large‐scale variations in shear velocity, density, and compressional velocity in the Earth's mantle

Moulik

Ekström

2016

JGR Solid Earth

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A large data set of surface wave phase anomalies, body wave travel times, normal‐mode splitting functions, and long‐period waveforms is used to investigate the scaling between shear velocity, density, and compressional velocity in the Earth's mantle. We introduce a methodology that allows construction of joint models with various levels of scaling complexity (ϱ = d lnρ/d lnvS, ν = d lnvS/d lnvP), in order to detect seismological signatures of chemical heterogeneity. We demonstrate that the data sets considered cannot be fit concurrently with a uniform ν or a positive and uniform ϱ throughout the mantle. The variance reductions to P wave travel times and vP‐sensitive modes are up to 40% higher with our preferred model of anisotropic shear and compressional velocity than the recent anisotropic shear velocity model S362ANI+M, which was constructed assuming a uniform ν throughout the mantle. Several features reported in earlier tomographic studies persist after the inclusion of new and larger data sets; anticorrelation between bulk sound and shear velocities in the lowermost mantle as well as an increase in ν with depth in the lower mantle are largely independent of the regularization scheme. When correlations between density and shear velocity variations are imposed in the lowermost mantle, variance reductions of several spheroidal and toroidal modes deteriorate by as much as 40%. Recent measurements of the splitting of 0S2, in particular, are largely incompatible with perfectly correlated shear velocity and density heterogeneity throughout the mantle. A way to significantly improve the fits to various data sets is by allowing independent density perturbations in the lowermost mantle. Our preferred joint model consists of denser‐than‐average anomalies (∼1% peak to peak) at the base of the mantle roughly coincident with the low‐velocity superplumes. The relative variation of shear velocity, density, and compressional velocity in our study disfavors a purely thermal contribution to heterogeneity in the lowermost mantle, with implications for the long‐term stability and evolution of superplumes.

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Tomographic errors from wave front healing: more than just a fast bias

Cited by 42 publications

References 39 publications

Thermally Dominated Deep Mantle LLSVPs: A Review

Thermally Dominated Deep Mantle LLSVPs: A Review

Wavefront healing renders deep plumes seismically invisible

The relationships between large‐scale variations in shear velocity, density, and compressional velocity in the Earth's mantle

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