Shear Velocity Anomalies Research Articles

On the Origin of the Hawaiian Swell: Lithosphere and Asthenosphere Seismic Structure From Rayleigh Wave Dispersion

AbstractIn this study, we revisit the shear‐wave velocity structure of the lithosphere and asthenosphere surrounding the Hawaiian hotspot and Hawaiian swell using Rayleigh wave data spanning periods of 20–125 s from the PLUME project. A primary goal of this investigation is to probe the origin of the Hawaiian swell and the mechanism that elevates the topography, providing insights into mantle dynamics beneath hotspot swells. In the shear velocity model, the 30–70 km depth range is largely featureless with weak and local anomalies, indicating that the elevation of the Hawaiian swell cannot be attributed to upper lithospheric reheating or replacement. In contrast, at 80–150 km depth, a pronounced region of anomalously low velocities is well‐resolved, with the lowest velocities found beneath the Hawaii‐Maui‐Molokai part of the island chain. Minimum shear velocities are approximately 4.0 km/s at 100–120 km depth, which is an ∼8%‐10% velocity decrease relative to the surrounding velocities away from the swell. This pattern suggests that hot, buoyant mantle from deeper plume sources laterally spread out near the top of the normal oceanic asthenosphere. We find that the low‐velocity pattern in the asthenosphere exhibits a strong correlation with the overall shape of the Hawaiian swell topography. Assuming that density anomalies are proportional to shear velocity anomalies, we demonstrate that the anomalous elevation of the swell can be explained by the uplift of a 30‐km‐thick elastic plate loaded from below by this buoyant, low‐seismic‐velocity layer in the asthenosphere.

Iron silicate perovskite and postperovskite in the deep lower mantle

Ferromagnesian silicates are the dominant constituents of the Earth's mantle, which comprise more than 80% of our planet by volume. To interpret the low shear-velocity anomalies in the lower mantle, we need to construct a reliable transformation diagram of ferromagnesian silicates over a wide pressure-temperature (P-T) range. While MgSiO3 in the perovskite structure has been extensively studied due to its dominance on Earth, phase transformations of iron silicates under the lower mantle conditions remain unresolved. In this study, we have obtained an iron silicate phase in the perovskite (Pv) structure using synthetic fayalite (Fe2SiO4) as the starting material under P-T conditions of the lower mantle. Chemical analyses revealed an unexpectedly high Fe/Si ratio of 1.72(3) for the Pv phase in coexistence with metallic iron particles, indicating incorporation of about 25 mol% Fe2O3 in the Pv phase with an approximate chemical formula (Fe2+0.75Fe3+0.25)(Fe3+0.25Si0.75)O3. We further obtained an iron silicate phase in the postperovskite (PPv) structure above 95 GPa. The calculated curves of compressional (VP) and shear velocity (VS) of iron silicate Pv and PPv as a function of pressure are nearly parallel to those of MgSiO3, respectively. To the best of our knowledge, the iron silicate Pv and PPv are the densest phases among all the reported silicates stable at P-T conditions of the lower mantle. The high ferric iron content in the silicate phase and the spin-crossover of ferric iron at the Si-site above ~55 GPa should be taken into account in order to interpret the seismic observations. Our results would provide crucial information for constraining the geophysical and geochemical models of the lower mantle.

Inferring the relationship between core-mantle heat flux and seismic tomography from mantle convection simulations

The heat flux pattern at Earth’s core-mantle boundary (CMB) imposes a heterogeneous boundary condition on core dynamics that may profoundly affect the geodynamo. Owing to the expected temperature dependence of seismic velocities, this pattern is classically approximated as proportional to the lowermost layer of seismic tomography models for the global mantle. Two biases however undermine such a simple linear relationship: 1) other contributions than thermal (compositional and mineralogical) influence seismic velocities and 2) the radial average is inherent to tomographic models whereas the local thermal state at the CMB is relevant for the heat flux. We analyze here simulations of thermochemical mantle convection where, owing to their spatial characteristics, specific mantle components are readily identified: hot thermochemical piles (TCPs), “normal” mantle (NM) and, when post-peroskite (pPv) is included, a cold region where this phase is present. Synthetic seismic velocities (i.e. from the mantle simulations) are then computed based on thermal, compositional and mineralogical sensitivities. A formalism to infer the CMB heat flux from these seismic shear velocity anomalies is derived. In this formalism, within each mantle population (i.e. TCPs, NM or pPv) the CMB heat flux vs. seismic anomalies follows a unique fitting function. The transition from one mantle population to another is marked by a jump in the seismic anomaly, i.e. a range of seismic anomalies in between two mantle populations corresponds to a similar CMB heat flux. Applying our formalism to the seismic anomalies from the mantle convection simulations provides far superior fits than the commonly used linear fits. The results highlight reduced negative heat flux anomalies beneath large low shear velocity provinces (LLSVPs), while positive heat flux anomalies are enhanced, both with respect to the classical linear interpretation.

Resolving Continental Magma Reservoirs With 3D Surface Wave Tomography

AbstractSurface wave tomography is widely used to improve our understanding of continental magma reservoirs that may be capable of fueling explosive volcanic eruptions. However, traditional surface wave tomography based on inversions for phase velocity maps and locally 1D shear velocity may have difficulty resolving strong 3D low‐velocity anomalies associated with crustal magma reservoirs. Here, we perform synthetic tomography experiments based on 3D seismic waveform simulations to understand how the limitations of surface wave tomography could affect interpretations of tomography in volcanic settings. We focus our modeling on the Yellowstone volcanic system, one of the largest and most thoroughly studied continental magmatic systems, and explore scenarios in which the maximum shear velocity anomaly associated with the crustal magma reservoir ranges between −10% and −66%. We find that even with the well‐instrumented setting near Yellowstone, the recovered shear velocity anomalies in the mid‐to‐upper crust are severely diminished due to the small spatial scale of the reservoir with respect to the seismic wavelengths that sample it. In particular, recovered VS anomalies could be reduced by a factor of two or more, implying that the inferred melt fraction of large‐scale continental magma reservoirs may be considerably underestimated.

Mapping the mantle transition zone beneath the Indian Ocean geoid low from Ps receiver functions

The most pronounced high-amplitude (−40 to −106 m) geoid deficit with respect to the Earth's reference shape straddles the Indian Ocean. The existing hypotheses suggest a possible linkage of low density and low-velocity anomalies in the mid-and upper mantle and high-velocity anomalies near the base of the mantle to the source behind the lowest geoid. We obtained P to S wave radial receiver functions to investigate these mantle anomalies from an extensive array of Ocean Bottom Seismometers (OBSs) positioned in the centre of the Indian Ocean Geoid Low (IOGL). Our modelling suggests that the mean depths for d410 and d660 range from 386.0 ± 13.6 to 459.7 ± 2.9 km and 643.1 ± 7.4 to 710.2 ± 5.4 km with an average of 432.6 km and 680.2 km, respectively. The average thickness of the transition zone varies from 199.6 to 289.8 km. New results confirm an extensive ~800 km wide depression at d410 and d660 towards the centre of the geoid. The depression potentially implies a rather hot mantle material, in which majorite garnet to perovskite transition may become dominant at 660 km depth. The excess temperature calculated for d410 and d660 topography ranges from 139.5 to 557.5 K and 206 to >1000 K, respectively. The shear velocity anomalies derived from the excess temperature range from −0.89 to −3.52 (%) at d410, and at dd60 range from −1.13 to −7.87 (%). The velocity anomalies are highly underestimated at d660 and could be explained by hydrous mantle upwelling at the lower mantle transition zone. The present study outcomes offer new avenues towards exploring a mantle upwelling in the Indian Ocean and its linkage with geoid undulations.

Elasticity of Hydrated Al‐Bearing Stishovite and Post‐Stishovite: Implications for Understanding Regional Seismic VS Anomalies Along Subducting Slabs in the Lower Mantle

AbstractSeismic studies have found seismic scatterers with −2 to −12% shear velocity anomalies along some subducting slabs at 700–1900 km depth. The ferroelastic post‐stishovite transition in subducted mid‐ocean ridge basalt (MORB) has been linked to these seismic features, but compressional and shear wave velocities (VP and VS) and full elastic moduli (Cij) of Al,H‐bearing stishovite and post‐stishovite at high pressure remain uncertain. Here we have determined Raman shifts of optic modes and equation of state parameters of two hydrated Al‐bearing stishovite crystals, Al1.3‐SiO2 (1.34 mol% Al and 0.55 mol% H) and Al2.1‐SiO2 (2.10 mol% Al and 0.59 mol% H), up to ∼70 GPa in diamond anvil cells coupled with Raman spectroscopy and X‐ray diffraction. The experimental data are modeled using a pseudoproper Landau theory to derive full Cij and sound velocities across the post‐stishovite transition at high pressure. The Al and H dissolution in stishovite significantly reduces the transition pressure to 21.1 GPa in Al1.3‐SiO2 and to 16.1 GPa in Al2.1‐SiO2, where the transition is manifested by approximately 29% VS reduction. Considering that stishovite with approximately 1.3 mol% Al and 0.6 mol% H could account for 20 vol% in subducted MORB at the top‐lower mantle, the Al,H‐bearing post‐stishovite transition with a Clapeyron slope of 65 K/GPa would occur at about 1060 km depth with −7(4)% VS anomaly. The VS anomalies across the Al,H‐bearing post‐stishovite transition can help explain the seismically‐observed depth‐dependent VS anomalies along some subducting slabs in the top‐ to mid‐lower‐mantle depths including the Tonga subducting slab.

Imaging crustal melt beneath northeast Japan with Ps receiver functions

To investigate crustal structures related to melt storage, Ps converted waves were used to image crustal velocity discontinuities across the Tohoku region of northeast Japan. Ps receiver functions from 127 permanent seismic stations were migrated to depth, accounting for variations in crustal velocity structure. In the 5-15 km depth range, negative Ps phases indicative of velocity decreases with depth are prevalent in central and western Tohoku, where Holocene volcanoes occur and surface wave tomography indicates low crustal velocities. Large negative Ps phases are largely absent in the crust of the old mountain terranes in eastern Tohoku where crustal velocities are higher. In central and western Tohoku, velocity gradients inferred from Ps phases and shear velocity anomalies are consistent with models for crustal melt storage that involve trans-crustal zones of crystal-melt mush where the highest concentrations of melt are localized at the top of the mush column. Negative Ps phases are concentrated at depths of 5-10 km, and many correlate with the upper margins of localized low velocity zones beneath volcanoes. These correlated features are consistent with the roofs of high melt fraction layers. At depths of 10-30 km, positive Ps phases are consistent with gradual velocity positive gradients that correspond to decreasing melt fraction with depth in the mush column, although some groups of deeper negative Ps phases at depths of 10-20 km may reflect local maxima in melt fraction. The amplitudes of the negative Ps phases at 5-10 km depth vary significantly. In a higher amplitude example, waveform modeling of Ps receiver functions from a station near Hijori volcano indicates a 16% ± 5% Vs drop at 10 km depth, implying melt fractions of 10% ± 3% if Gassmann's equations are assumed, above a velocity increase from 10 to 20 km that indicates decreasing melt fractions with depth. However, modeling of lower amplitude Ps phases in the 5-10 km depth range indicates a 7% ± 3% velocity drop and a maximum melt fraction of 5% ± 2%. The geographic distribution of apparent melt fraction at 5-10 km depth suggests that in some cases the same upper crustal source can supply multiple volcanoes, and in others high melt fraction zones are significantly laterally offset from the nearest volcano.

Shear Velocity Structure Beneath Saudi Arabia From the Joint Inversion of P and S Wave Receiver Functions, and Rayleigh Wave Group Velocity Dispersion Data

AbstractWe develop a new 3‐D shear velocity model for the lithosphere and sublithospheric mantle under Saudi Arabia by jointly inverting P wave receiver functions, S wave receiver functions, and fundamental‐mode Rayleigh wave group velocities. P and S receiver functions are calculated from earthquakes recorded between 2012 and 2015 at 156 Saudi National Seismic Network stations operated by the Saudi Geological Survey. Rayleigh wave dispersion data are extracted from independent tomographic studies. Our model reveals significant lateral variations in crustal and upper‐mantle S velocity below Saudi Arabia. Particularly, a low‐velocity zone, with minimum S velocity of ~4.0 km/s in the depth range of 70‐190 km, is observed under the Arabian Shield coinciding with Cenozoic surface volcanism. The low‐velocity zone is found consistent with the presence of partial melts in the mantle and interpreted as a potential deep magma source for the volcanism in western Arabia. We propose that magmas responsible for the Arabian volcanism may be derived from multiple sources. Both lithospheric thinning and the Afar plume trigger magma production beneath southwestern Arabia, while decompression melting caused by the lithospheric thinning may be the main factor in the central and northern portions. Furthermore, we find evidence for localized crustal low shear velocity anomalies (2–4% reductions) that appear spatially correlated to the volcanism; these may be due to fractures caused by magma ascent and small amounts of partial melt in the crust. This spatial distribution of S velocity reductions may indicate the plumbing system for magma ascent underneath western Arabia.

Evidence of Subduction‐Related Thermal and Compositional Heterogeneity Below the United States From Transition Zone Receiver Functions

AbstractThe subduction of the Farallon Plate has altered the temperature and composition of the mantle transition zone (MTZ) beneath the United States. We investigate MTZ structure by mapping P‐to‐S conversions at mineralogical phase changes using USArray waveform data and theoretical seismic profiles based on experimental constraints of phase transition properties as a function of temperature and composition. The width of the MTZ varies by about 35 km over the study region, corresponding to a temperature variation of more than 300 K. The MTZ is coldest and thickest beneath the eastern United States where high shear velocity anomalies are tomographically resolved. We detect intermittent P‐to‐S conversions at depths of 520 km and 730 km. The conversions at 730‐km depth are coherent beneath the southeastern United States and are consistent with basalt enrichment of about 50%, possibly due to the emplacement of a fragment of an oceanic plateau (i.e., the Hess conjugate).

High seismic attenuation at a mid-ocean ridge reveals the distribution of deep melt.

At most mid-ocean ridges, a wide region of decompression melting must be reconciled with a narrow neovolcanic zone and the establishment of full oceanic crustal thickness close to the rift axis. Two competing paradigms have been proposed to explain melt focusing: narrow mantle upwelling due to dynamic effects related to in situ melt or wide mantle upwelling with lateral melt transport in inclined channels. Measurements of seismic attenuation provide a tool for identifying and characterizing the presence of melt and thermal heterogeneity in the upper mantle. We use a unique data set of teleseismic body waves recorded on the Cascadia Initiative's Amphibious Array to simultaneously measure seismic attenuation and velocity across an entire oceanic microplate. We observe maximal differential attenuation and the largest delays ([Formula: see text] s and δTS ~ 2 s) in a narrow zone <50 km from the Juan de Fuca and Gorda ridge axes, with values that are not consistent with laboratory estimates of temperature or water effects. The implied seismic quality factor (Qs ≤ 25) is among the lowest observed worldwide. Models harnessing experimentally derived anelastic scaling relationships require a 150-km-deep subridge region containing up to 2% in situ melt. The low viscosity and low density associated with this deep, narrow melt column provide the conditions for dynamic mantle upwelling, explaining a suite of geophysical observations at ridges, including electrical conductivity and shear velocity anomalies.

Elasticity of single‐crystal superhydrous phase B at simultaneous high pressure‐temperature conditions

AbstractWe investigated the combined effect of pressure and temperature on the elasticity of single‐crystal superhydrous phase B (Shy‐B) using Brillouin scattering and X‐ray diffraction up to 12 GPa and 700 K. Using the obtained elasticity, we modeled the anisotropy of Shy‐B along slab geotherms, showing that Shy‐B has a low anisotropy and cannot be the major cause of the observed anisotropy in the region. Modeled velocities of Shy‐B show that Shy‐B will be shown as positive velocity anomalies at the bottom transition zone. Once Shy‐B is transported to the topmost lower mantle, it will exhibit a seismic signature of lower velocities than topmost lower mantle. We speculate that an accumulation of hydrous phases, such as Shy‐B, at the topmost lower mantle with a weight percentage of ~17–26% in the peridotite layer as subduction progresses could help explain the observed 2–3% low shear velocity anomalies in the region.

Inferring shear-velocity structure of the upper 200 m using cultural ambient noise at the Ngatamariki geothermal field, Central North Island, New Zealand

We have determined subsurface structure using the refraction microtremor (ReMi) method at the Ngatamariki geothermal field, Central North Island, New Zealand. The local geology is such that refraction and reflection studies are hindered by energy scattering and attenuation in the near-surface layers. The ReMi method uses surface waves from ambient noise and active sources to determine S-wave velocities in the shallow subsurface. We have deployed two lines of 72-channel, 10 Hz vertical geophones with 10 m spacing, and we were able to model near-surface S-wave velocity to depths of 57–93 m for 2D profiles and as much as 165 m for 1D profiles. Shear-velocity anomalies were detected on one line that were spatially correlated with a fault. The location of the fault was previously inferred from stratigraphic offset in the geothermal wells, suggesting that the ReMi method can provide important constraints on near-surface geology in noisy geothermal settings.

Accurate measurements of residual topography from the oceanic realm

In the oceans, our understanding of plate subsidence as a function of age permits residual depth anomalies to be identified and mapped. These anomalies may reflect dynamic topography and could be an important means for constraining convective circulation of the sublithospheric mantle. Here we analyze a global database of seismic reflection and wide-angle profiles from heavily sedimented oceanic crust, which abuts continental lithosphere. At 449 locations, we calculated water-loaded subsidence, compared it with a reference age-depth relationship, and determined residual depth. We then combined these spot measurements of residual depth with observations from mid-oceanic ridges and from selected ship track bathymetry to construct a global map of residual depth. Our results suggest that the amplitude of residual depth varies by up to ±1 km with wavelengths of order 103 km. We compare our residual depths with free-air gravity and seismic tomographic anomalies. Our results show that residual depths correlate with long-wavelength gravity anomalies. In contrast, correlations between residual depths and vertically averaged shear velocity anomalies within the upper and/or the lower mantle are weaker. The largest discrepancies occur at short (∼1000 km) wavelengths. These combined observations suggest that residual depth anomalies could be generate by density variations within a thin (∼102 km) low-viscosity layer beneath the lithosphere. Our global compilation should play a significant role in helping to refine predictive geodynamical models.

Receiver function imaging of lithospheric structure and the onset of melting beneath the Galápagos Archipelago

The Galápagos Archipelago represents an opportunity to investigate the properties of young oceanic lithosphere, the effects of a hotspot anomaly on lithospheric thickness, and melting dynamics in a hotspot-ridge interaction. Here we use data recorded by the SIGNET array and permanent station PAYG on the Islands Santa Cruz and Isabela, respectively. We used P-to-S (Ps) and S-to-P (Sp) receiver functions to constrain crust and mantle structure. A simultaneous deconvolution method was used to constrain 1-D structure and also for the modeling of robust features. A migrated extended multitaper method was used to investigate 3-D structural variations. Ps images a velocity increase with depth at 11±7 km, probably the base of the pre-plume crust, or old Moho. Sp imaging and modeling images a second, deeper velocity increase at 37±7 km depth. A velocity decrease with depth is imaged on average at 75±12 km likely associated with the lithosphere–asthenosphere boundary. This discontinuity is imaged deeper, 82 km, in the southwest and shallower, 66 km, in the northeast near the spreading ridge. Although the trend is consistent with lithospheric thickening with age, the thickness is much larger than predicted by conductive cooling models of 0–10 My oceanic lithosphere. We infer a compositional contribution to velocity variations. Finally, a velocity increase with depth is imaged at ∼125 to 145±15 km depth that is likely associated with the onset of melting. The discontinuity is imaged deeper in 3 sectors of the Galápagos platform-ridge region, all coincident with the slowest surface wave shear velocity anomalies in the upper 100 km. One is located in the southwest in a hypothesized plume location. The other two are to the northwest and northeast, possibly illuminating multiple plume diversions related to complex plume–ridge interactions.

Synthetic images of dynamically predicted plumes and comparison with a global tomographic model

Seismic detection of a mantle plume may resolve the debate about the origin of hotspots and the role of plumes in mantle convection. In this paper, we test the hypothesis that whole-mantle plumes exist below major hotspots, by quantitatively comparing physically plausible plume models with seismic images following three steps. We (1) simulate a set of representative thermal plumes by solving the governing equations for Earth-like parameters in an axisymmetric spherical shell, (2) convert the thermal structure into shear-velocity anomalies using self-consistent thermo-dynamic relationships, and (3) project the theoretical plumes as seismic images using the S40RTS tomographic filter to account for finite seismic resolution. Simulated plumes with excess potential temperatures of 375 K map into negative shear-wave anomalies of up to 4–8% between 300 and 660 km depth, and 2.0–3.5% in the mid-lower mantle. Given the heterogeneous resolution of S40RTS, plumes of this strength are not easily detectable if tails are narrower than 150–250 km in the upper-mantle or 400–700 km in the lower mantle. In S40RTS, more than half of the forty hotspots we studied overlie low-velocity anomalies that extend through most of the lower mantle. These anomalies exceed 0.6% in the lower mantle, compatible with thermal plume strengths. They have widths mostly with the range 800–1200 km, which is at the high end of plausible thermal plume structures, and at the low end to be resolved in S40RTS. In the upper mantle, the shear velocity is low beneath more than ninety percent of the hotspots. For about ten, including Iceland, the East African hotspots, Hawaii, and the Samoa/Tahiti and Cobb/Bowie pairs, S40RTS low-velocity anomalies extending through the transition zone imply 200–300 K excess temperatures over a ~ 1000 km wide region. This is substantially broader than expected for thermal plume tails. Such anomalies may be compatible with deep-seated plumes, but only if plume flux is strongly variable due to, for example, interaction with phase transitions and/or chemical entrainment.

Upper mantle thermochemical structure from seismic–geodynamic flow models: constraints from the Lithoprobe initiativeThis article is one of a series of papers published in this Special Issue on the theme Lithoprobe — parameters, processes, and the evolution of a continent.

High-resolution seismic models of three-dimensional mantle heterogeneity are interpreted in terms of upper mantle thermal and compositional anomalies. These anomalies produce density perturbations that drive mantle flow and corresponding convection-related geophysical observables, such as the nonhydrostatic geoid, free-air gravity anomalies, and dynamic surface topography, and provide constraints on internal mantle density structure. The convection related observables are corrected for the isostatically compensated crustal heterogeneity and compared with those predicted by tomography-based mantle flow models. Occam inversions of the surface topography and gravity data provide inferences of the velocity–density scaling coefficients, which characterize mantle density anomalies below North America. The inferred density anomalies require simultaneous contributions from temperature and composition. The density and seismic shear velocity anomalies place constraints on the thermochemical structure of the mantle beneath the North American craton. Perturbations in the molar ratio of iron, R = XFe/(XFe + XMg), are used to quantify the compositional anomalies in terms of iron depletion in the sub-continental mantle. Estimates of the extent of basalt depletion in the tectosphere beneath North America are obtained. This depletion is interpreted to produce a local balance between positive chemical buoyancy and the negative thermal buoyancy that would otherwise be produced by the colder temperatures of the sub-cratonic mantle relative to its sub-oceanic counterpart.

Characterizing the 410 km discontinuity low‐velocity layer beneath the LA RISTRA array in the North American Southwest

Receiver functions recorded by the 54‐station 920 km long Program for Array Seismic Studies of the Continental Lithosphere–Incorporated Research Institutions for Seismology Colorado Plateau/Rio Grande Rift Seismic Transect Experiment (LA RISTRA) line array display a pervasive negative polarity P to S conversion (Pds) arrival preceding the positive polarity 410 km discontinuity arrival. These arrivals are modeled as a low‐velocity layer atop the 410 km discontinuity (410‐LVL) and are inverted for a velocity profile via a grid search using a five‐parameter linear gradient velocity model. Model parameter likelihood and correlations are assessed via calculation of one‐ and two‐dimensional marginal posterior probability distributions. The maximum likelihood model parameter values found are top velocity gradient thickness of 0.0 km with a 4.6% (−0.22 km/s) shear velocity reduction, a 19.8 km constant velocity layer, and bottom gradient thickness of 25.0 km with a 3.5% (+0.17 km/s) shear velocity increase. The estimated mean thickness of the 410‐LVL is 32.3 km. The top gradient of the 410‐LVL is sharp within vertical resolution limits of P to S conversion (&lt;10 km), and the diffuse 410 km velocity gradient is consistent with hydration of the olivine‐wadsleyite phase transformation. The 410‐LVL is interpreted as a melt layer created by the Transition Zone Water Filter model. Two secondary observations are found: (1) the 410‐LVL is absent from the SE end of the array and (2) an intermittent negative polarity P525s arrival is observed. We speculate that upper mantle shear velocity anomalies above the 410 km discontinuity may manifest Rayleigh‐Taylor instabilities nucleated from the 410‐LVL melt layer that are being shed upward on time scales of tens of millions of years.

The relative wavelengths of fast and slow velocity anomalies in the lower mantle: Contrary to the expectations of dynamics?

We quantify the spherical harmonic characteristics of shear velocity anomalies as a function of depth for a range of tomographic models of the Earth's mantle. In particular, the amplitude spectra of fast and slow anomalies are analyzed separately. Thus, the higher amplitude harmonic degrees and relative scale lengths of both possible thermal upwellings and downwellings can be determined. Our results are consistent with prior studies in that we find that the amplitude spectra of heterogeneity (fast and slow) in the mid-mantle are essentially flat. However, where heterogeneity is known to be stronger in the deep mantle, slow anomalies are observed to be consistently higher amplitude at harmonic degrees 1–12, while fast anomalies dominate at higher degrees. This result is at odds with many dynamic models in which upwellings tend to be narrow and downwellings tend to be broad. This inconsistency with isochemical dynamic models may be associated with variations in phase, chemistry and/or viscosity in the Earth's lowermost mantle. The technique presented here provides a means for discriminating between these effects when comparing tomographic and dynamic models of the Earth's mantle.

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We present a shear velocity model of the crust and upper mantle beneath central Eurasia by simultaneous inversion of broadband group and phase velocity maps of fundamental-mode Love and Rayleigh waves. The model is parameterized in terms of velocity depth profiles on a discrete 2°×2° grid. The model is isotropic for the crust and for the upper mantle below 220 km but, to fit simultaneously long period Love and Rayleigh waves, the model is transversely isotropic in the uppermost mantle, from the Moho discontinuity to 220 km depth. We have used newly available a priori models for the crust and sedimentary cover as starting models for the inversion. Therefore, the crustal part of the estimated model shows good correlation with known surface features such as sedimentary basins and mountain ranges. The velocity anomalies in the upper mantle are related to differences between tectonic and stable regions. Old, stable regions such as the East European, Siberian, and Indian cratons are characterized by high upper-mantle shear velocities. Other large high velocity anomalies occur beneath the Persian Gulf and the Tarim block. Slow shear velocity anomalies are related to regions of current extension (Red Sea and Andaman ridges) and are also found beneath the Tibetan and Turkish–Iranian Plateaus, structures originated by continent–continent collision. A large low velocity anomaly beneath western Mongolia corresponds to the location of a hypothesized mantle plume. A clear low velocity zone in vSH between Moho and 220 km exists across most of Eurasia, but is absent for vSV. The character and magnitude of anisotropy in the model is on average similar to PREM, with the most prominent anisotropic region occurring beneath the Tibetan Plateau.

J362D28: a new joint model of compressional and shear velocity in the Earth's mantle

The discrimination between chemical and thermal heterogeneity in the Earth's mantle remains one of the most important and challenging questions to be answered by observational and theoretical geophysics. To answer this question requires a thorough knowledge of the ratio between compressional and shear velocity anomalies. We describe results of a joint inversion for compressional and shear velocity in the mantle using a large and diverse data set consisting of traveltimes, complete waveforms and surface wave dispersion measurements. A horizontal tessellation consisting of 362 spherical splines is used to parametrize the model, which is approximately equivalent to a spherical harmonic of degree 18 in resolution. The model contains peak variations (from PREM) of up to ±7 per cent in S velocity and ±2.5 per cent in P velocity in the upper mantle. These variations decrease to ±1.5 and ±0.6 per cent, respectively, at 1000 km depth and reach ±2.5 and ±1.0 per cent, respectively, in the D″ region. The rms ratio of S to P velocity perturbations is fairly constant between 2.0 and 2.5 in the lower mantle, but a local minimum in this ratio occurs at a depth of approximately 1700 km. Resolution tests show that the recovery of P and S velocity is not geographically uniform, but also show that this amplitude ratio is well resolved between 670 and 2700 km depth. We also observe a persistent negative correlation between bulk-sound and shear velocity throughout the same region of the mantle. In addition, the model contains a minimum correlation between P and S velocity between 670 and ∼1100 km. This feature is supported by both the favourable outcome of resolution experiments and the poor fit provided by the starting model from the inversion (in which the P and S velocities are perfectly correlated) to our data set of P-wave traveltimes. The power spectra of both P and S velocity heterogeneity are similar, although we note a slightly larger dominance of degree two in the spectra for P velocity in the mid-mantle where resolution is highest.