Summary We present an azimuthally anisotropic 3‐D shear‐wave speed model of the Australian upper mantle obtained from the dispersion of fundamental and higher modes of Rayleigh waves. We compare two tomographic techniques to map path‐average earth models into a 3‐D model for heterogeneity and azimuthal anisotropy. Method I uses a rectangular surface cell parametrization and depth basis functions that represent independently constrained estimates of radial earth structure. It performs an iterative inversion with norm damping and gradient regularization. Method II uses a direct inversion of individual depth layers constrained by Bayesian assumptions about the model covariance. We recall that Bayesian inversions and discrete regularization approaches are theoretically equivalent, and with a synthetic example we show that they can give similar results. The model we present here uses the discrete regularized inversion of independent path constraints of Method I, on an equal‐area grid. With the exception of westernmost Australia, we can retrieve structure on length scales of about 250 km laterally and 50 km in the radial direction, to within 0.8 per cent for the velocity, 20 per cent for the anisotropic magnitude and 20° for its direction. On length scales of 1000 km and longer, down to about 200 km, there is a good correlation between velocity heterogeneity and geologic age. At shorter length scales and at depths below 200 km, however, this relationship breaks down. The observed magnitude and direction of maximum anisotropy do not, in general, appear to be correlated to surface geology. The pattern of anisotropy appears to be rather complex in the upper 150 km, whereas a smoother pattern of fast axes is obtained at larger depth. If some of the deeper directions of anisotropy are aligned with the approximately N–S direction of absolute plate motion, this correspondence is not everywhere obvious, despite the fast (7 cm yr−1) northward motion of the Australian plate. More research is needed to interpret our observations in terms of continental deformation. Predictions of SKS splitting times and directions, an integrated measure of anisotropy, are poorly matched by observations of shear‐wave birefringence.

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S U M M A R YA new model, EUR-S91, of shear-wave velocity variations in the upper mantle beneath central Europe and surrounding regions, down to a depth of 670km, is presented. The model is derived from the inversion of the waveforms of 217 seismograms, using the partitioned waveform-inversion method for the seismogram in the time window from the S-wave arrival to the fundamental mode of the Rayleigh wave. The seismograms were mostly assembled from digitially recording long-period and broad-band stations in Europe. The resulting 3-D model accurately predicts most of the observed waveforms, in a wide band of frequencies. Body waves were fit for frequencies up to 60mHz. The fundamental Rayleigh mode, which at high frequencies is more prone to scattering and multipathing, was generally low passed at 25 mHz. The resolving power of the data set depends strongly on the density of available wave paths and varies as a function of geographical position. Small-scale heterogeneities like the subducted lithosphere in the Hellenic collision zone were imaged in the region with the highest density of wave paths, which indicates an optimum resolution of better than 200 km.The main new results of this study pertain to the transition between east and central Europe. We present new information about the structure below the Tornquist-Teisseyre Zone (TTZ). The TTZ is generally regarded as the boundary between the Precambrian crust of the Baltic Shield/Russian Platform and the younger crust of central Europe. Our 3-D S-velocity model reveals that below this line a sharp lateral boundary extends down to at least 140km depth, with high velocities beneath the Baltic Shield and Russian Platform contrasting to the low velocities beneath the younger regions of Europe. The observed velocity contrast across the TTZ is largest between the Pannonian Basin and the Russian Platform, where it is equal to 12 per cent at 80 km depth. At depths of 300-400 km, the TTZ is underlain by a zone of low S velocity, which indicates a local thinning of the deeply rooted high-velocity structure or tectosphere that underlies most of eastern Europe.In addition to these new results on the transition region between east and central Europe, a number of features that are present in other tomographic results have been confirmed: the Precambrian provinces of Europe are characterized by high S velocities; low velocities are found beneath the Pannonian Basin, the western Mediterranean, northern Aegean Sea/Turkey and a small region south-west of the Massif Central. The lithosphere beneath the Paris Basin has a positive S-velocity anomaly. The resolution of all these anomalies has been established with sensitivity tests.

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A strong low S velocity anomaly at 300–500 km depth coincides with the western boundary of the Russian Platform. The anomaly is too large to be explained by a simple temperature anomaly or by compositional variations, nor is it an artifact induced by seismic anisotropy. We present a model that explains the anomaly through the injection of water at the time of closure of the Tomquist Ocean that separated the continents of Avalonia and Baltica in the early Paleozoic. Dense hydrous magnesium phases are the most likely agents for transporting water to the transition zone, but an important role may also be played by nominally anhydrous clinopyroxene. When these minerals are brought out of their stability field, water is released. It may accumulate in β‐spinel or K‐amphibole and may be released when the mantle warms up after subduction halts. The effect of this water is to induce weakening of the shear modulus or even creation of a heavy melt. By a conservative estimate, a subduction episode lasting 85 m.y. would inject enough water to saturate a large volume of mantle rock with 0.3% H2O. Low velocities or low Q anomalies have also been observed in the transition zones near currently active slabs.

study of the upper mantle beneath the Australian continent. We applied a waveform inversion technique to broad-band data recorded while the SKIPPY portable arrays were positioned in eastern Australia in order to construct a 3-D model of shear velocity in the upper mantle and transition zone beneath eastern Australia and the adjacent oceanic regions. The SKIPPY data were augmented by data from the permanent seismological observatories in the region. The first step of the waveform inversion used involved the matching of the waveforms of fundamental-and higher-mode Rayleigh waves with waveforms synthesized from radially stratified models; in the second stage the linear constraints on radial variations in shear velocity were combined in a tomographic inversion for aspherical variations in shear velocity. The preferred model reduces the data variance by 90 per cent. Owing to the dense data coverage, structural features with dimensions larger than 250 km laterally and 50 km vertically are resolved.

Seismological results on the structure of the upper mantle below Europe reveal a marked contrast in seismic properties between Precambrian and younger parts of Europe. The Precambrian craton in eastern Europe is characterized by high shear-wave velocities, which can be explained by low temperatures. The transition to low seismic velocities below Phanerozoic Europe coincides with the crustal boundary zone of the craton and exists to depths of at least 140 kilometers. Despite the long and complex tectonic history of the plate boundary zone, the transition is remarkably sharp, which rules out any significant lateral transport of asthenospheric material across the suture zone.

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