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Abstract: The radial attenuation profile of the Earth is needed to account for dispersion effects when interpreting seismic velocities and can provide important constraints on composition. To date, most radial Q models have been produced using traditional damped inversions of free oscillation and surface wave data. Because such inversions can severely underestimate the model uncertainties that are needed to guide mineralogical and dynamic interpretation, and because the quality of data has continued to improve, we revisit this seismic inverse problem using a model space search approach already proven effective with similar data. We do, indeed, observe model uncertainties at least an order of magnitude greater than earlier estimates. At the same time, we find that Q is determined well enough to confirm that the data favor several important features previously disputed because of questions of consistency. These include shear attenuation that drops significantly in the lower third of the lower mantle and bulk attenuation that is negligible in the inner core but stronger in the outer core and lower mantle than suggested by most models. [Copyright &y& Elsevier]

We aim to constrain the lower mantle geotherm and average composition from 1D seismic models and experimental mineralogy data, explicitly accounting for possible sources of uncertainty. We employ an isentropic third-order Birch–Murnaghan equation of state, which is in excellent agreement with recent ab initio calculations of density and bulk modulus for Mg-perovskite. Furthermore, ab initio and experimental data are reasonably consistent with each other. Modelling the shear modulus is not as straightforward, but is needed because density and the bulk modulus alone do not sufficiently constrain temperature and composition. To correctly predict ab initio calculations for the shear modulus of Mg-perovskite, we needed to prescribe a cross-derivative at zero pressure, which we determined by trial and errors. Unless this ad hoc cross-derivative is confirmed by further experimental results, there seems to be an inconsistency between ab initio and experimental data. Purely experimental data most likely require a non-adiabatic temperature profile, but it is difficult to infer the number and location(s) of the non-adiabatic increase(s). If ab initio data are used, at least one thermal boundary layer seems reasonable, but its location depends on the modelling of the iron content. A strong chemical density contrast in the mid-mantle (≥2%) is not supported by ab initio data, but is possible with experimental data. Other major sources of uncertainty are the trade-off between thermal and compositional effects, the possible influence of aluminium perovskite, and poorly understood frequency effects. [Copyright &y& Elsevier]

The presence of radial anisotropy in the upper mantle, transition zone and top of the lower mantle is investigated by applying a model space search technique to Rayleigh and Love wave phase velocity models. Probability density functions are obtained independently for S-wave anisotropy, P-wave anisotropy, intermediate parameter η, Vp, Vs and density anomalies. The likelihoods for P-wave and S-wave anisotropy beneath continents cannot be explained by a dry olivine-rich upper mantle at depths larger than 220 km. Indeed, while shear-wave anisotropy tends to disappear below 220 km depth in continental areas, P-wave anisotropy is still present but its sign changes compared to the uppermost mantle. This could be due to an increase with depth of the amount of pyroxene relative to olivine in these regions, although the presence of water, partial melt or a change in the deformation mechanism cannot be ruled out as yet. A similar observation is made for old oceans, but not for young ones where VSH>VSV appears likely down to 670 km depth and VPH>VPV down to 400 km depth. The change of sign in P-wave anisotropy seems to be qualitatively correlated with the presence of the Lehmann discontinuity, generally observed beneath continents and some oceans but not beneath ridges. Parameter η shows a similar age-related depth pattern as shear-wave anisotropy in the uppermost mantle and it undergoes the same change of sign as P-wave anisotropy at 220 km depth. The ratio between dlnVs and dlnVp suggests that a chemical component is needed to explain the anomalies in most places at depths greater than 220 km. More tests are needed to infer the robustness of the results for density, but they do not affect the results for anisotropy. [Copyright &y& Elsevier]

We use a neighborhood algorithm to explore the fit to long period seismic data of a wide variety of long wavelength mantle models. This approach to the global tomographic inverse problem yields probability distributions for seismic velocities, density, and related properties as functions of depth. Such distributions can be robust even when individual models are not, and allow us to test several assumptions about the Earth that have long been enforced a priori in inversions. In particular, we are able to test the paradigm of deep mantle heterogeneity that is dominantly thermal in origin, producing velocity and density perturbations that are well correlated and have relative amplitudes given by δlnρ/δlnvs<0.5. Our distributions show that such relationships are unlikely, and even though the results are consistent with recent best fitting models from damped seismic inversions, they demonstrate that many specific properties of such models are not robust. The data clearly favor density perturbations that are poorly or negatively correlated with velocity heterogeneity and have amplitudes several times larger (yielding δlnρ/δlnvs>1.0) than damped inversions allow. These characteristics are most pronounced in the upper mantle transition zone and the base of the lower mantle, suggesting layered convection. The negative density–velocity correlations favored at these depths imply dominantly chemical heterogeneity, while the likelihood of relatively high amplitude density variations suggests that variable iron content is an important component of this heterogeneity. These results, which we show to be consistent with independent gravity constraints, represent a profound change in the interpretation of seismic constraints. In addition, the distributions show that even though best fitting density models from recent inversions or our sampling are consistent with the data, most specific properties of such models are not robust. This implies that it is more appropriate to use seismic model distributions, rather than individual models, to make geodynamic and geochemical inferences. [Copyright &y& Elsevier]

Abstract: We present a high-resolution S-velocity model of the North Atlantic region, revealing structural features in unprecedented detail down to a depth of 1300km. The model is derived using full-waveform tomography. More specifically, we minimise the instantaneous phase misfit between synthetic and observed body- as well as surface-waveforms iteratively in a full three-dimensional, adjoint inversion. Highlights of the model in the upper mantle include a well-resolved Mid-Atlantic Ridge and two distinguishable strong low-velocity regions beneath Iceland and beneath the Kolbeinsey Ridge west of Jan Mayen. A sub-lithospheric low-velocity layer is imaged beneath much of the oceanic lithosphere, consistent with the long-wavelength bathymetric high of the North Atlantic. The low-velocity layer extends locally beneath the continental lithosphere of the southern Scandinavian Mountains, the Danish Basin, part of the British Isles and eastern Greenland. All these regions experienced post-rift uplift in Neogene times, for which the underlying mechanism is not well understood. The spatial correlation between the low-velocity layer and uplifted regions suggests dynamic support by low-density asthenosphere originating from the Iceland and Jan Mayen hotspots. Our model further suggests a lower-mantle source for the Iceland and Jan Mayen hotspots. Two distinguishable low-velocity conduits are imaged, connecting the upper-mantle anomalies beneath Iceland and Jan Mayen into the lower mantle. Both conduits are tilted to the South-East, reflecting the westward motion of the Mid-Atlantic Ridge. The location of the imaged Iceland conduit is in agreement with the observation of a locally thinned transition zone south of Iceland from receiver function studies. [Copyright &y& Elsevier]

Abstract: A widely held belief is that anelasticity can contribute significantly to seismic wave-speed variations in the lower mantle. In particular it has been argued that anelasticity can strongly increase the sensitivity of V S to temperature, and that anelasticity may also increase V S relative to V P (as measured by the parameter R S/P =dlnV S/dlnV P). If true, this could significantly reduce or, in some cases, eliminate the need to explain seismic signals in the mantle with chemical heterogeneity, and that most of the variation in seismic velocities in the lower mantle could be attributed to temperature alone. We re-visit this view and find that a strong anelastic effect is unlikely in the Earth''s lower mantle. Given the present knowledge of the properties of perovskite under lower mantle conditions, we find it unlikely that anelasticity can affect dlnV S/dT by more than about 30%, and it is most likely less than 20%. We also find that the probable upper bound on dlnV S/dlnV P due to thermal variations is about 1.8 in the shallow lower mantle, and about 2.1 in the deeper lower mantle. We conclude, therefore, that anelasticity cannot be invoked to significantly reduce or remove interpretations of lower mantle chemical heterogeneity based on large-scale seismic wave-speed variations. [Copyright &y& Elsevier]

Abstract: Multi-scale full waveform inversion of complete continental- and regional-scale seismograms reveals the crustal and upper-mantle signature of the North Anatolian Fault Zone which shapes the neotectonics of Turkey and the eastern Mediterranean. Within the crust, the fault zone is mostly bounded by several high-velocity blocks, suggesting that it developed along the edges of continental fragments with high rigidity. Below the crust, the surface expression of the eastern and central parts of the North Anatolian Fault Zone correlate with a pronounced low-velocity band that extends laterally over 600km. Around 100km depth, the low-velocity band merges into the shallow Anatolian asthenosphere, thereby providing a link to the Kırka–Afyon–Isparta Volcanic Field and the Central Anatolian Volcanics. We interpret the low-velocity band beneath the North Anatolian Fault Zone as the upper-mantle expression of the Tethyan sutures that formed 60–15Ma ago as a result of Africa-Eurasian convergence. The structurally weak suture facilitated the formation of the younger (less than 10Ma) crustal fault zone. In this sense, the North Anatolian Fault Zone is not only a crustal feature, but a narrow zone of weakness that extends into the upper mantle. [Copyright &y& Elsevier]

Abstract: Evolution of continental lithosphere during orogenies and the following periods of relative stability is poorly understood, largely because of the lack of relevant observational constraints. Measurements of seismic anisotropy provide such constraints, but due to limitations in the resolving power of available data sets and, more generally, of various data types, detailed mapping of lithospheric anisotropy has remained elusive. Here we apply surface-wave array analysis to data from the East-central U.S. and determine the layering of azimuthal anisotropy beneath the Grenville–Appalachian orogen in the entire lithosphere–asthenosphere depth range. Combined measurements of Rayleigh-wave phase velocities along 60 interstation paths constrain phase-velocity maps with statistically significant anisotropy. Distinct anisotropy patterns in three different period ranges point to the existence of three distinct layers beneath the orogen, with different anisotropic fabric within each. We invert phase-velocity maps and, alternatively, pairs of selected measured dispersion curves for anisotropic shear-velocity structure. The results confirm that three anisotropic layers with different fabric within each are present, two in the lithosphere (30–70 km; 70–150 km depths) and another in the asthenosphere beneath (>150 km). Directions of fast wave propagation in the upper lithosphere are parallel to the Grenville and Appalachian fronts, suggesting that the region-scale anisotropy pattern reflects the pervasive deformation of the lower crust and uppermost mantle during the continental collisions. The fast-propagation azimuth within the lower lithosphere is different, parallel to the NNW direction of North America''s motion after the orogeny (~160–125 Ma). This suggests that the lithosphere, 70-km thick by the end of the Appalachian orogeny, gradually thickened to the present 150-km while inheriting the fabric from the sheared asthenosphere below, as the plate moved NNW. Below 150 km, the fast-propagation direction is parallel to the present plate motion, indicating fabric due to recent asthenospheric flow. Anisotropy in narrower depth ranges beneath the region has been sampled previously. Published results (from observations of P n and SKS and waveform tomography) can be accounted for and reconciled by the three-layered model of anisotropy for the lithosphere–asthenosphere depth range constrained in this study. In particular, the anisotropy we detect in the asthenosphere can account for the magnitude of SKS-wave splitting, with the fast wave-propagation directions inferred from SKS and surface-wave data also consistent, both parallel to the current plate motion. [Copyright &y& Elsevier]