Jeannot Trampert

Shear velocity structure of central Eurasia from inversion of surface wave velocities

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.

Sensitivities of seismic velocities to temperature, pressure and composition in the lower mantle

We calculated temperature, pressure and compositional sensitivities of seismic velocities in the lower mantle using latest mineral physics data. The compositional variable refers to the volume proportion of perovskite in a simplified perovskite– magnesiowustite mantle assemblage. The novelty of our approach is the exploration of a reasonable range of input parameters which enter the lower mantle extrapolations. This leads to realistic error bars on the sensitivities. Temperature variations can be inferred throughout the lower mantle within a good degree of precision. Contrary to the uppermost mantle, modest compositional changes in the lower mantle can be detected by seismic tomography, with a larger uncertainty though. A likely trade-off between temperature and composition will be largely determined by uncertainties in tomography itself. Given current sources of uncertainties on recent data, anelastic contributions to the temperature sensitivities (calculated using Karato’s approach) appear less significant than previously thought. Recent seismological determinations of the ratio of relative S to P velocity heterogeneity can be entirely explain by thermal effects, although isolated spots beneath Africa and the Central Pacific in the lowermost mantle may ask for a compositional origin.

Anomalies of temperature and iron in the uppermost mantle inferred from gravity data and tomographic models

We propose a method to interpret seismic tomography in terms of thermal and compositional anomalies. In addition to the tomographic model, we use gravity data, which provide information on the density expressed as a relative density-to-shear wave velocity scaling factor (ζ = ∂ ln ρ/∂ ln Vs). The inferred values of ζ are not consistent with the presence of thermal anomalies alone. However, simultaneous anomalies of temperature and composition explain the observations. Compositional anomalies can have several origins, but we find the most relevant parameter to be the global volumic fraction of iron ( xFe=Fe/(Fe +Mg)). We invert the tomographic model S16RLBM (Woodhouse and Trampert, 1995) and the density anomalies correlated to Vs-anomalies (δρ/ρ0 = ζδ Vs/V0) for anomalies of temperature (δT) and iron (δFe). The partial derivatives are provided by a numerical method that reconstructs density and seismic velocity for given temperatures and petrologic models (Vacher et al., 1998). Down to z = 300 km depth, the distribution of temperature and iron anomalies strongly depends on the surface tectonics. The continental mantle below old cratons and stable platforms is colder than average and depleted in iron, whereas the oceanic mantle is mostly homogeneous. Due to uncertainties on the reference state of the mantle, error bars on δT and δFe reach 10% of the inverted values. Finally, we apply these results to the stability of continental roots and test the hypothesis that the negative buoyancy induced by lower than average temperatures is balanced by the positive buoyancy induced by the depletion in iron. We find that continental roots are stable only if the viscosity of the mantle is strongly temperature-dependent. However, some uncertainties remain on the real effects and importance of rheology.

Eurasian fundamental mode surface wave phase velocities and their relationship with tectonic structures

We automatically analyzed 32,000 fundamental mode Love and Rayleigh wave signals with earthquake-station paths traversing Eurasia and Indonesia and obtained robust average phase velocity measurements between 20 s and 170 s periods along 4389 Love and 4020 Rayleigh paths. These were inverted to give phase velocity maps at 14 fixed periods. Resolution tests suggest that features with diameter >750 km and >500 km are resolved over most of Eurasia and central/SE Asia respectively. Low-period Love waves image areas with thick sedimentary cover as low-velocity zones, and almost all periods image mountainous regions since these have thick crust and hence low average lithospheric shear velocity. At long periods, both Love and Rayleigh waves define high phase velocity zones across shield and cratonic areas reflecting their deep lithospheric roots. We observe significant along-strike heterogeneity in the Zagros fold belt and Tien Shan-Altai system. Taking sections across Eurasian phase velocity space allows us to make approximate interpretations in terms of shear velocity structure directly. For example, the Red River and East Vietnam Boundary faults are traced on their eastern side by low velocities which extend at depth into Indonesia. We relate this to mantle upwelling associated with early Eocene rotation of Indochina and reversal of the sense of shear across the Red River fault post-20 Ma. We observe dipping subduction of the Mediterranean beneath the Aegean, of the Philippine Sea beneath Indonesia, and of the Indian shield beneath Tibet. We also image a fossil subducted plate beneath NE Borneo which we associate with subduction of the proto-South China Sea between 50 Ma and 15 Ma.

Thermal structure of continental upper mantle inferred from S-wave velocity and surface heat flow

Results from seismic tomography provide information on the thermal structure of the continental upper mantle. This is borne out by the good agreement between tectonic age, surface heat flow and a tomographic S-wave velocity model for depths less than 180 km. The velocity anomalies of tomographic layers deeper than 230 km have relatively small amplitudes and show little correlation with surface heat flow or shallow velocities. We associate the drop in correlation and amplitude of the velocity perturbations between 180 and 230 km depth with the maximum thickness of the thermal boundary layer (TBL), in which larger variations in temperature and possibly composition than in the underlying convecting mantle can be sustained. Velocity profiles for different tectonic provinces are converted to temperature using mineralogical data. Both anharmonic and anelastic effects on the wave speeds are taken into account. The resulting geotherms differ most at depths of 60^120 km with variations of up to 900‡C. Below 230 km, differences do not exceed 300‡C. These geotherms agree well with one-dimensional conductive geotherms for the observed range of continental heat flow values using the empirical relationship that 40% of the surface heat flux stems from upper crustal radiogenic heat production. The S-wave velocity in the continental upper mantle appears to be adequately explained (within the uncertainties of the tomography and the conversion to temperature) by a thermal signature. A compositional component can, however, not be ruled out as it may have only a minor effect on the velocity and the heat flow. The surface heat flow is controlled by the shallow heat production and the thickness of the TBL. Seismology helps to determine the relative importance of the two factors and our results confirm the similar importance of both factors. Variations of TBL thickness could be controlled by compositional differences and/or by the effect of temperature on the rheology. fl 2000 Elsevier Science B.V. All rights reserved.

On the robustness of global radially anisotropic surface wave tomography

A number of recent global tomographic studies have modeled three dimensional variations in the parameters of radial anisotropy. As yet there is limited agreement among such studies, suggesting significant uncertainties in the models, which could lead to divergent geodynamical interpretations. In this study we assess the robustness of lateral variations in radial anisotropy globally in the upper mantle and in the transition zone to determine the extent to which anisotropic parameters are constrained by a data set of over 10,000,000 fundamental and higher mode surface wave dispersion measurements. We carry out inversions for isotropic and radially anisotropic shear wave velocity, systematically changing regularization and using three different crustal models to remove the effects of the crust on the data. Using crustal corrections from different crustal models has an impact on the data fit comparable or larger than that obtained by including lateral variations of radial anisotropy in the modeling. Moreover, the use of crustal corrections from different a priori crustal models may lead to different images of radial anisotropy suggesting divergent geodynamical interpretations. This work suggests that the three‐dimensional determination of global radial anisotropy in the Earths mantle using surface wave dispersion data is still an ongoing experiment.

Using probabilistic seismic tomography to test mantle velocity–density relationships

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 ofdeep mantle heterogeneity that is dominantly thermal in origin, producing velocity and density perturbations that are well correlated and have relative amplitudes given by Nlnb/Nlnvs 6 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 ofsuch models are not robust. The data clearly f density perturbations that are poorly or negatively correlated with velocity heterogeneity and have amplitudes several times larger (yielding Nlnb/Nlnvs s 1.0) than damped inversions allow. These characteristics are most pronounced in the upper mantle transition zone and the base ofthe lower mantle, suggesting layered convection. The negative density^velocity correlations f at these depths imply dominantly chemical heterogeneity, while the likelihood ofrelatively high amplitude density variations suggests that variable iron content is an important component ofthis 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.

Three-Channel Correlation Analysis: A New Technique to Measure Instrumental Noise of Digitizers and Seismic Sensors

This article describes a new method to estimate (1) the self-noise as a function of frequency of three-channel, linear systems and (2) the relative transfer functions between the channels, based on correlation analysis of recordings from a common, coherent input signal. We give expressions for a three-channel model in terms of power spectral densities. The method is robust, compared with the conven- tional two-channel approach, as both the self-noise and the relative transfer functions are extracted from the measurements only and do not require a priori information about the transfer function of each channel. We use this technique to measure and model the self-noise of digitizers and to identify the frequency range in which the digitizer can be used without precaution. As a consequence the method also reveals under which conditions the interpretation of data may be biased by the recording system. We apply the technique to a Quanterra Q4120 datalogger and to a Network of Autonomously Recording Seismographs (NARS) datalogger. At a sampling rate of 20 samples/sec, the noise of the Q4120 digitizer is modeled by superposition of a flat, 23.6-bit spectrum and a 24.7-bit spectrum with 1/f 1.55 noise. For the NARS datalogger the noise level is modeled by superposition of a 20.8-bit flat spectrum and a 23.0-bit spectrum with 1/f 1.0 noise. The measured gain ratios between the digi- tizers in the Q4120 datalogger, smoothed over a tenth of a decade between 0.01 Hz and 8 Hz for data sampled with 20 samples/sec, are within 1.6% (or 0.14 dB) of the values given by the manufacturer. Finally, we show an example of seismic background noise observations at station HGN as recorded by both an STS-1 and a STS-2 sensor. Between 0.01 and 0.001 Hz the vertical STS-2 noise levels are 10-15 dB above the STS-1 observations. The Quanterra Q4120 digitizer noise model enables us to exclude the contribution of the digitizer noise to be responsible for this difference.

Global maps of Rayleigh wave attenuation for periods between 40 and 150 seconds

Studies of seismic attenuation must account for the large amplitude deviations caused by elastic focusing of energy. In a new approach, we jointly invert phase and amplitude measurements of 19,000 minor arc Rayleigh waves between periods of 40 and 150 seconds. The simultaneous inversion ensures that attenuation and phase velocity are mutually consistent because the phase and focusing term of amplitude are modelled using a common elastic model. At the shortest periods the maps show a good correlation between attenuation and phase velocity, suggesting a common cause in the uppermost mantle, most probably thermal in origin. This correlation is lost at longer periods. The main signal beyond periods of 100 seconds is a strongly attenuating circum Pacific zone and a pronounced ring of high attenuation around Africa. This feature seems reliable in our attenuation maps but not correlated to an equivalent structure in phase velocity. We thus favour scattering of wave energy on large size structures as a possible cause.

Probability density functions for radial anisotropy: implications for the upper 1200 km of the mantle

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 R, 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 s VSV appears likely down to 670 km depth and VPH s 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 R 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.