Global Tomographic Models Research Articles

A fast velocity anomaly to the west of the Australian‐Antarctic discordance

To examine the hypothesis that the Australian‐Antarctic Discordance overlays an anomalously cold upper mantle, we have measured Rayleigh wave group velocities along paths between the Southeast Indian Ridge and the Australian station NWAO. The group velocity peaks for events in the vicinity of 117°E, rather than for those within the discordance zone between 120 and 127°E. This variation is resolved at least for periods of 35–45 s. To model the cause, assumed velocity anomalies with simple geometry were tested using the surface wave Gaussian beam method in a forward sense. The preferred model consists of an elongated structure centered 300–500 km west of the discordance and stretching northward for at least 1000 km. The position of the anomalous structure is consistent with recent global tomographic models in this region.

Comparison between tomographic structures and models of convection in the upper mantle

SUMMARY Gruneisen's and third-order finite-strain theories are used to compute the density and seismic-wave velocities of minerals. Assuming the minealogical model of Ito & Takahashi (1987), seismic velocities of the upper mantle are calculated using the Hashin–Shtrikman averaging procedure. 1-D profiles are first obtained along adiabats, and compared to the IASP91 model. Different adiabats are considered in order to take into account the thermal effect of phase transitions. The best results are found with adiabats initiated at 1473, 1573 and 1613 K for α-olivine, β- and γ-spinel, respectively. The incorporation of thermal effects resulting from phase transitions gives velocity jumps at discontinuities close to those of IASP91. Next, a model of convection constructed by Dupeyrat, Sotin & Parmentier (1995), incorporating plate tectonics, is used to compute 1-D profiles and 2-D fields of seismic anomalies in the upper mantle. Averaged profiles show seismic-velocity gradients very close to those of IASP91, but individual values are much too high, suggesting that the mean temperature profile of the convection model is too cold by 400 K. When low-pass filtered to the resolution scale of presently available tomogrphic models, both the amplitude and shape of the computed seismic anomalies are consistent with the results of tomographic studies. The amplitude of the anomalies ranges between – 2.7 and 3.8% for P-wave slownesses, and from – 3.3 to 4.5% for S-wave slownesses. These anomalies correspond to lateral temperature variations of –465 to 520 K. These calculations are used (1) as an aid to the interpretation of global tomographic models, for instance by computing spectra of lateral heterogeneities, and (2) to test the adequacy of the basic assumptions used in the computation of numerical models of mantle convection, and to build a theoretical temperature profile that would give the best fit to IASP91. In the uppermost mantle this theoretical model has a shape close to both the convection model and the 1473 K adiabat, but in the transition zone the profile is highly subadiabatic. The spectra obtained for the synthetic seismic anomalies resemble that of tomographic studies, with most of the energy contained within gravest angular orders l, and a fast decrease of energy with increasing l. The spatial filtering has clearly different effects on heterogeneities, depending on their respective wavelengths. It is suggested that the change of decreasing rate observed in tomographic models at l= 7 is closely related to the filter wavelength and may correspond at a lesser extent to a characteristic wavelength to mantle heterogeneities.

A global tomographic model of shear attenuation in the upper mantle

We present a global three‐dimensional model of shear attenuation in the upper mantle, based on the measurement of amplitudes of low‐frequency (100–300s) Rayleigh waves observed at stations of the Geoscope and Iris networks. Attenuation coefficients are measured on R1 and R2 paths using a method which minimizes the effects of focussing due to propagation in a three‐dimensional elastic Earth. Through a series of tests which, in particular, involve the computation of synthetic models of attenuation and focussing, we demonstrate that long wavelength lateral variations in attenuation in the first 400–500 km of the mantle can indeed be resolved. The model is obtained in a two‐step procedure. The first step consists in the computation of maps of Rayleigh wave attenuation at different periods, using an inversion method without a priori parametrisation, which involves the introduction of a correlation length, chosen here at 3000 km to optimize the trade‐off between resolution and variance in the model. In the second step, after corrections for shallow structure, an inversion with depth is performed, assuming lateral heterogeneity is confined to depths between 80 and 650 km. The resulting model presents lateral variations in Qβ that are correlated with tectonic features, in particular ridges and shields in the first 250 km of the upper mantle. Below that depth the pattern shifts and becomes correlated with the hotspot distribution, particularly so if the buoyancy strength of hotspots is taken into account. Two major low‐velocity zones appear to be located in the central pacific and beneath northern Africa, in the depth range 300–500 km. This pattern seems to continue at greater depth, but resolution becomes insufficient below 500 km to draw definitive conclusions. The smooth lateral variations retrieved are on the order of ±50% down to 400 km. We propose an interpretation in terms of plume/lithosphere/ridge interaction in the upper mantle, arguing for deflection of the bulk of hot upwelling material from plumes towards ridges, which may be occurring between 200 and 300 km depth.

An empirical search for short-wavelength heterogeneities in D″ using ISC P delays

The spectrum of the heterogeneities in the D″ layer at the base of the mantle is investigated using a comparison of P delays for rays bottoming respectively inside and above D″. Data come from the International Seismological Centre (ISC) bulletins. They reveal a decoupling between D″ and the overlying structures, and the presence in D″ of a short-wavelength signal which may contaminate global tomographic models.

Upper mantle anisotropy and coupled-mode long-period surface waves

SUMMARY We investigate surface wave propagation effects caused by P and S-wave velocity anisotropy and its orientation in the upper mantle. We compare coupled-mode synthetic seismograms, constructed from sums of free oscillations for different types of upper mantle models, in order to understand better the observable aspects of anisotropy and its fast direction in the earth. Our calculations suggest that upper mantle anisotropy, which is not transversely isotropic, of few per cent can generate significant waveform anomalies in long-period seismic records of 0–20 mHz. Those anomalies are termed ‘quasi-Love’ waves and are caused by spheroidal—toroidal free-oscillation coupling. Large quasi-Love waveforms are difficult to produce with levels of isotropic heterogeneity consistent with global tomographic models of the mantle. Coupled-mode waveform anomalies in long-period seismic records at a single station can be used to diagnose the presence of anisotropic structure in the upper mantle. Because 0Sl, —0Tl coupling pairs have weak sensitivity to aspherical wavenumbers s 4 mHz, smoother/rougher structure generates longer/shorter-period fundamental-branch waveform anomalies. Anisotropy in our models is assumed to have hexagonal symmetry, that is, a single axis of symmetry. However, the symmetry axis can be arbitrarily oriented. The waveform anomalies associated with spheroidal-toroidal coupling are sensitive to the orientation of the fast-velocity direction of upper mantle anisotropy. In particular, for the same level of velocity perturbation, coupled-mode waveform anomalies increase as the fast axis of anisotropic structure rotates from vertical to horizontal. The orientation of the fast axis can influence both phase shifts and coupled-mode waveforms of long-period surface waves. The coupled-mode waveform anomalies predicted by our theoretical calculations are observed in the data and seem to diagnose the presence of upper mantle anisotropy which is not transversely isotropic.

Three‐dimensional structure of the upper mantle beneath the Atlantic Ocean inferred from long‐period Rayleigh waves: 2. Inversion

The three‐dimensional shear wave velocity structure of the upper mantle beneath the Atlantic Ocean is investigated by inversion of fundamental mode of Rayleigh wave group and phase velocities. The results are reliable down to 300 km depth and improve upon the image of upper mantle structure given by global tomographic models. The inversion of two‐dimensional lateral distributions of group and phase velocities, regionalized without a priori constraints, shows that shear wave velocities are correlated with lithospheric age beneath the North Atlantic. Our data set fails to quantify the correlation between shear velocities and lithospheric age, because of the small lateral extent of young regions (<22 Ma) and of strong transverse gradients. In contrast with similar studies in the Indian and Pacific oceans, slow velocities associated with the location of the Mid‐Atlantic Ridge are restricted to the lithosphere. Therefore the Atlantic mid‐oceanic ridge behaves as a shallow structure, which is weakly related to deep upwelling asthenospheric flows. At depths greater than 200 km, the central part of the Atlantic is characterized by the highest velocity anomaly. This anomaly can be interpreted in different ways. Part of this anomaly might correspond to a negative (VSH–VSV)/VSV ratio implicitely included in the isotropic inversion. We also tentatively interpret this anomaly as an important contributor to a shallow degree two pattern of shear wave velocity distribution for the Atlantic area.

Seismic constraints on the morphology of deep slabs

Residual sphere images from deep earthquakes not only detect the presence of slab‐associated velocity anomalies but also lend insight into the flow and deformation of lithosphere subducted into the lower mantle. We have compared travel times from deep events in the Kuril and Mariana arcs with the seismic velocity anomalies implied by kinematical models that thicken the slab perpendicular to its plane by reducing the vertical velocity of the flow with depth. We assume that the details of the deformation (whether the slab buckles, imbricates, fragments, etc.) are averaged out along the ray paths, and hence our models constrain the scale, not the mode, of slab thickening. The deep event travel times are best fit by undeformed models, but the ability of the residual sphere method to resolve slab thickness is limited by ray bending effects. Although the Mariana times are consistent with advective thickening factors of 5 or more, factors larger than 3 are ruled out by the Kuril data. For all models examined, the data require that slab material extends to depths of 900–1000 km. Global tomographic models and regional studies which delineate high‐velocity anomalies in the lower mantle beneath zones of Cenozoic subduction are consistent with our results, as is recent work on pulse distortion by slab gradients. Comparison of observed and predicted rates of seismic moment release suggests that if substantial advective thickening does occur, it is largely aseismic.