Summary In this paper we study teleseismic shear‐wave splitting due to multiple anisotropic layers. The effects can be described in terms of a multilayered splitting operator that may be calculated using a matrix formulation. As a special case, we consider smooth variations of anisotropy with depth. The model is characterized by the (horizontal)fast‐axis directions at the top (φt) and bottom (φb) of the anisotropic region, and the total delay time (Δt). Numerical results show that the particle motion at relatively long periods (T/Δtnbb 5) is clearly elliptical and is a function of the polarization direction of the incident wave. At shorter periods (T/Δtnab1); however, the seismogram is characterized by two distinctly separated arrivals corresponding to the fast‐ and slow‐axis directions at the top of the anisotropic region. Relative amplitudes of the fast and slow arrivals depend on the orientation of the initial polarization with respect to the axis directions at the bottom. Only the high‐frequency results agree with predictions based on ray theory. At fixed initial polarization, the longer‐period results can be described in terms of an apparent fast‐polarization direction (φa) and an apparent delay time (δta). The apparent splitting parameters exhibit a π/2‐periodicity as functions of initial polarization (or backazimuth), which is similar to the two‐layer case considered by Silver & Savage (1994). Assuming that φt−φb < 45 °, we find variations of φa and δta to be smaller than 10 per cent over most ( ≈ 2/3) of the backazimuth range. In this case, φa≃ (φt+φb)/2 and δta≃Δt. The calculated apparent splitting parameters agree well with direct measurements using synthetic waveforms and may thus be applied to the inversion of observations Long‐period apparent splitting parameters for different types of depth variations cannot be resolved uniquely without further constraints from the deformation history at a given station. For example, we find that recently observed azimuthal variations of splitting parameters (Özalaybey & Savage 1994; Brechner et al. 1998) can be modelled equally well in terms of two distinct layers (a four‐parameter model) or smooth variations of fast‐axis directions (a three‐parameter model). Depth‐dependent models with randomly chosen fast‐axis directions and delay times in each layer display a characteristic decrease of the most probable apparent delay time, δta (Pmax), with increasing number of layers N. We find that δta (Pmax)∼ 1/√N. The fact that most measurements yield values of δta≃ 1 s (Silver 1996) suggests that the majority of observations can be explained in terms of one or two anisotropic layers, provided the fast‐axis directions in different layers are independent.

The AlpArray programme is a multinational, European consortium to advance our understanding of orogenesis and its relationship to mantle dynamics, plate reorganizations, surface processes and seismic hazard in the Alps-Apennines-Carpathians-Dinarides orogenic system. The AlpArray Seismic Network has been deployed with contributions from 36 institutions from 11 countries to map physical properties of the lithosphere and asthenosphere in 3D and thus to obtain new, high-resolution geophysical images of structures from the surface down to the base of the mantle transition zone. With over 600 broadband stations Electronic supplementary material The online version of this article (https ://doi.org/10.1007/s1071 2-018-9472-4) contains supplementary material, which is available to authorized users. operated for 2 years, this seismic experiment is one of the largest simultaneously operated seismological networks in the academic domain, employing hexagonal coverage with station spacing at less than 52 km. This dense and regularly spaced experiment is made possible by the coordinated coeval deployment of temporary stations from numerous national pools, including ocean-bottom seismometers, which were funded by different national agencies. They combine with permanent networks, which also required the cooperation of many different operators. Together these stations ultimately fill coverage gaps. Following a short overview of previous large-scale seismological experiments in the Alpine region, we here present the goals, construction, deployment, characteristics and data management of the AlpArray Seismic Network, which will provide data that is expected to be unprecedented in quality to image the complex Alpine mountains at depth.

S U M M A R YSeismic anisotropy is an important tool for studying the nature, origin and dynamics of the lowermost mantle (D ). We introduce differential S-ScS splitting as a tool for removing the effect of near-source and near-receiver anisotropy to estimate splitting accrued in the D region. This is applicable to events recorded at epicentral distances between 60 • and 85 • . Near-source anisotropy has often been ignored in previous studies of lowermost mantle anisotropy. We apply differential S-ScS splitting to records from Canadian National Seismic Network stations of western Pacific earthquakes; these sample the lowermost mantle beneath the north Pacific. The residual splitting in ScS, which we attribute to D , shows lag times between 1.0 and 3.9 s. Given the near horizontal ray path of ScS in D , we interpret the recovered fast directions as the orientation of the fast shear wave in the plane defined by the vertical and transverse directions and observe a clearly non-VTI (transverse isotropy with a vertical axis of symmetry) style of anisotropy. The largest population of results shows an approximately southeasterly dipping symmetry axis which we speculate might be explained by descending palaeoslab material being swept horizontally across the core-mantle boundary towards an upwelling region beneath the central Pacific. Non-VTI symmetry and the many possible contributions to D anisotropy from lower-mantle minerals, melt and subducted materials suggest that our understanding of the lowermost mantle could be greatly improved by trying to resolve a more general style of anisotropy.

Mantle plumes and consequent plate extension have been invoked as the likely cause of East African Rift volcanism. However, the nature of mantle upwelling is debated, with proposed configurations ranging from a single broad plume connected to the large low-shear-velocity province beneath Southern Africa, the so-called African Superplume, to multiple lower-mantle sources along the rift. We present a new P-wave travel-time tomography model below the northern East-African, Red Sea, and Gulf of Aden rifts and surrounding areas. Data are from stations that span an area from Madagascar to Saudi Arabia. The aperture of the integrated data set allows us to image structures of 100 km length-scale down to depths of 700-800 km beneath the study region. Our images provide evidence of two clusters of low-velocity structures consisting of features with diameter of 100-200 km that extend through the transition zone, the first beneath Afar and a second just west of the Main Ethiopian Rift, a region with off-rift volcanism. Considering seismic sensitivity to temperature, we interpret these features as upwellings with excess temperatures of 100 6 50 K. The scale of the upwellings is smaller than expected for lower mantle plume sources. This, together with the change in pattern of the low-velocity anomalies across the base of the transition zone, suggests that ponding or flow of deep-plume material below the transition zone may be spawning these upper mantle upwellings.