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

Abstract

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.

Dynamics of axial torsional libration under the mantle‐inner core gravitational interaction

“…The most definite information, by virtue of equation , comes from the observed whole‐Earth Q 22 or the [2, 2] gravitational Stokes coefficient that is an integrated quantity heavily weighted toward the upper mantle, not on , which is heavily weighted toward the lower mantle. On the other hand, seismic tomography has asserted the presence of strong anomalies in the lowermost mantle several hundred kilometer thick right above the CMB known as the Large Low‐Shear‐Velocity Provinces (LLSVP) [e.g., Gu et al ., ; Trampert et al ., ; Ishii and Tromp , ; Garnero and McNamara , ; Moulik and Ekström , ], whose physical origin is presently undetermined. Characterized by anomalies of low seismic wave velocities and (arguably positive) anomalies in density, two LLSVPs, one under Africa and one under the Pacific Ocean, form an antipodal pair straddling the equator, indeed constituting a predominantly [2, 2] density distribution.…”

Morphology of seismically slow lower-mantle structures

“…(v) ME2016-P (Moulik & Ekström 2016): P-wave model is jointly inverted with the S-wave model used here and similarly parametrized.…”

Compositional layering within the large low shear‐wave velocity provinces in the lower mantle

“… (a) Seismic profiles through the African and (b) Pacific LLSVP. Colors show average S wave anomalies across five recent tomographic models: SPani [ Tesoniero et al ., ], SAVANI [ Auer et al ., ], S40RTS [ Ritsema et al ., ], “ME2016” [ Moulik and Ekström , ], and UCBSEM‐WM1 [ French and Romanowicz , ]. Before averaging, all these models were truncated at spherical harmonic degree 18, and had the global average shear‐wave speed at each depth removed.…”

“…Using cluster analysis, Cottaar and Lekic [] demonstrate that five different shear‐wave velocity models agree well with each other in terms of the geometry and depth extent of “slow clusters” (i.e., LLSVPs according to our definition here; see their Figures and ). Based on observations of abrupt lateral gradients in seismic velocity across their boundaries [ Ni et al ., ; Ni and Helmberger , ; To et al ., ; He and Wen , ; Kawai and Geller , ; Sun and Miller , ; Thorne et al ., ], anticorrelation between shear‐wave and bulk‐sound velocity ( v s and v Φ ) anomalies [ Su and Dziewonski , ; Masters et al ., ; Romanowicz , ; Trampert et al ., ; Mosca et al ., ; Koelemeijer et al ., ; Tesoniero et al ., ], as well as decorrelation of v s with density anomalies [ Ishii and Tromp , ; Simmons et al ., ; Mosca et al ., ; Moulik and Ekström , ], a purely thermal origin of LLSVPs remains unlikely. In turn, LLSVPs are commonly interpreted as being hot but compositionally dense piles that are shaped by ambient‐mantle convection [ Davaille , ; McNamara and Zhong , ; Tan and Gurnis , ; Deschamps and Tackley , ; Steinberger and Torsvik , ].…”