ALBERT

Description: Previous studies have shown that radial seismic anisotropy as estimated from flow models is in good agreement with results from tomography at global scale, in particular underlying oceanic basins. However, the fit is typically poor at smaller scale lengths, particularly in tectonically complex regions. We conduct a comparative analysis of tomographically mapped and dynamically modeled radial anisotropy at the scale of Europe and the Mediterranean Basin, including three tomographic models based on different observations and/or methods. We find that adaptive-grid surface-wave tomography, with parametrization density depending locally on the spatial and azimuthal density of data coverage, leads to the seismic model closest to (albeit still far from) geodynamic predictions. The ability to map regional-scale seismic anisotropy may provide a new constraint, complementary to isotropic tomography, to the nature of upper mantle flow.

Description: ABSTRACT In this study, we aim to close the gap between regional and global traveltime tomography in the context of surface wave tomography of the upper mantle implementing the principle of adaptive parametrization. Observations of seismic surface waves are a very powerful tool to constrain the 3-D structure of the Earth’s upper mantle, including its anisotropy, because they sample this volume efficiently due to their sensitivity over a wide depth range along the ray path. On a global scale, surface wave tomography models are often parametrized uniformly, without accounting for inhomogeneities in data coverage and, as a result, in resolution, that are caused by effective under- or overparametrization in many areas. If the local resolving power of seismic data is not taken into account when parametrizing the model, features will be smeared and distorted in tomographic maps, with subsequent misinterpretation. Parametrization density has to change locally, for models to be robustly constrained without losing any accurate information available in the best sampled regions. We have implemented a new algorithm for upper mantle surface wave tomography, based on adaptive-voxel parametrization, with voxel size defined by both the ‘hit count’ (number of observations sampling the voxel) and ‘azimuthal coverage’ (how well different azimuths with respect to the voxel are covered by the source-station distribution). High image resolution is achieved in regions with dense data coverage, while lower image resolution is kept in regions where data coverage is poorer. This way, parametrization is everywhere tuned to optimal resolution, minimizing both the computational costs, and the non-uniqueness of the solution. The spacing of our global grid is locally as small as ∼50 km. We apply our method to identify a new global model of vertically and horizontally polarized shear velocity, with resolution particularly enhanced in the European lithosphere and upper mantle. We find our new model to resolve lithospheric thickness and radial anisotropy better than earlier results based on the same data. Robust features of our model include, for example, the Trans-European Suture Zone, the Panonnian Basin, thinned lithosphere in the Aegean and Western Mediterranean, possible small-scale mantle upwellings under Iberia and Massif Central, subduction under the Aegean arc and a very deep cratonic root underneath southern Finland.

Description: [1]  Ambient-noise seismology is of great relevance to high-resolution crustal imaging, thanks to the unprecedented dense data coverage it affords in regions of little seismicity. Under the assumption of uniformly distributed noise sources, it has been used to extract the Green’s function between two receivers. We determine the imprint of this assumption by means of wave propagation and adjoint methods in realistic 3D Earth models. In this context, we quantify the sensitivity of ambient-noise cross correlations from central Europe with respect to noise-source locations and shear wavespeed structure. We use ambient noise recorded over one year at 196 stations, resulting in a database of 864 cross-correlations. Our mesh is built upon a combined crustal and 3D tomographic model. We simulate synthetic ambient-noise cross-correlations in different frequency bands using a 3D spectral-element method. Traveltime cross-correlation measurements in these different frequency bands define the misfit between synthetics and observations as a basis to compute sensitivity kernels using the adjoint method. We perform a comprehensive analysis varying geographic station and noise-source distributions around the European seas. The deterministic sensitivity analysis allows for estimating where the starting crustal model shows better accordance with our dataset and gain insight into the distribution of noise sources in the European region. This highlights the potential importance to consider localized noise distributions for tomographic imaging and forms the basis of a tomographic inversion in which the distribution of noise sources may be treated as a free parameter similar to earthquake tomography.

Description: [1]  We present a new tomographic model of radially anisotropic shear-velocity variations in the Earth's mantle based on a new compilation of previously published datasets and a variable block parameterization, adapted to local ray-path density. We employ ray-theoretical sensitivity functions to relate surface-wave and body wave data with radially anisotropic velocity perturbations. Our database includes surface-wave phase delays from fundamental modes up to the 6 th overtone, measured at periodsbetween 25 and 350 s, as well as cross-correlation traveltimes of major body-wave phases. Prior to inversion we apply a crustal correction using the crustal model CRUST2.0 [ Bassin et al., 2000] and we account for azimuthal anisotropy in the upper mantle using ray-theoretical corrections based on a global model of azimuthal anisotropy. While being well correlated with earlier models at long spatial wavelength, our preferred solution, savani , additionally delineates a number of previously unidentified structures, due to its improved resolution in areas of dense coverage. This is because the density of the inverse grid ranges between 1.25 ° in well sampled to 5 ° in poorly sampled regions, allowing us to resolve regional structure better than it is typically the case in global S -wave tomography. Important features of our model include: (i) A distinct ocean-continent anisotropic signature in the uppermost mantle; (ii) an oceanic peak in above average ξ  〉 1 which is shallower than in previous models and thus in better agreement with estimates of lithosphere thickness; (iii) a long wavelength pattern of ξ  〈 1 associated with the large low-shear-velocity provinces in the lowermost mantle. Furthermore we conduct a comprehensive comparison between various published isotropic and anisotropic upper- and whole-mantle tomographic models to identify regions in which anisotropic images have reached a stage of maturity, comparable to that of their isotropic counterparts.

Description: A large data set of surface-wave phase-velocity measurements is compiled to study the structures of the crust and upper mantle underneath the Alpine continental collision zone. Records from both ambient-noise and earthquake-based methods are combined to obtain a high-resolution 3-D model of seismic shear velocity. The applied techniques allow us to image the shallow crust and sedimentary basins with a lateral resolution of about 25 km. We find that complex lateral variations in Moho depth as mapped in our model are highly compatible with those obtained from receiver-function studies; this agreement with entirely independent data is a strong indication of the reliability of our results, and we infer that our model has the potential to serve as reference crustal map of shear velocity in the Alpine region. Mantle structures show nearly vertical subducting lithospheric slabs of the European and Adriatic plates. Pronounced differences between the western, central and eastern Alps provide indications of the respective geodynamic evolution: we propose that in the southwestern and northeastern Alps, the European slab has broken off. The complex anomaly pattern in the upper mantle may be explained by combination of remnant European slab and Adriatic subduction. Along-strike changes in the upper mantle structure are observed beneath the Apennines with an attached Adriatic slab in the northern Apennines and a slab window in the central Apennines. There is also evidence for subduction of Adriatic lithosphere to the east beneath the Pannonian Basin and the Dinarides down to a maximum depth of about 150 km.

Description: 〈span〉〈div〉Summary〈/div〉We derive a theoretical relationship between the cross correlation of ambient Rayleigh waves (seismic ambient noise) and the attenuation parameter α associated with Rayleigh-wave propagation. In particular, we derive a mathematical expression for the multiplicative factor relating normalized cross correlation to the Rayleigh-wave Green’s function. Based on this expression, we formulate an inverse problem to determine α from cross correlations of recorded ambient signal. We conduct a preliminary application of our algorithm to a relatively small instrument array, conveniently deployed on an island. In our setup, the mentioned multiplicative factor has values of about 2.5 to 3, which, if neglected, could result in a significant underestimate of α. We find that our inferred values of α are reasonable, in comparison with independently obtained estimates found in the literature. Allowing α to vary with respect to frequency results in a reduction of misfit between observed and predicted cross correlations.〈/span〉

Description: We image the lithospheric and upper asthenospheric structure beneath the central and eastern parts of the northern Gulf of Aden rifted passive continental margin with 59 broadband stations to evaluate the role of transform fault zones on the evolution of magma-poor continental margins. We used teleseismic tomography to compute a relative P wave velocity model in eastern Yemen and southern Oman down to 400 km depth. Our model shows low-velocity anomalies located in the vicinities of five major fracture zones and regions of recent volcanism. These low-velocity anomalies are likely caused by localized asthenospheric upwelling and partial melting, caused by small-scale convection promoted by gradients in the lithosphere-asthenosphere boundary topography near the fracture zones. In addition, low velocities underlie regions of elevated topography between major sedimentary basins. We suggest that locally buoyant mantle creates uplift and dynamic topography on the rift margin that affects the course of seasonal rivers and the sedimentation at the mouth of those rivers. Our new P wave velocity model suggests that the dynamic topography and recent volcanism in the central and eastern Gulf of Aden could be due to small-scale convection at the edge of the Arabian plate and/or in the vicinity of fracture zones.

Description: Cross-correlation of ambient seismic noise recorded by two seismic stations may result in an estimate of the Green's function between those two receivers. Several authors have recently attempted to measure attenuation based on these interferometric, receiver–receiver surface waves. By now, however, it is well established that the loss of coherence of the cross-correlation as a function of space depends strongly on the excitation of the medium. In fact, in a homogeneous dissipative medium, uniform excitation is required to correctly recover attenuation. Applied to fundamental-mode ambient seismic surface waves, this implies that the cross-correlation will decay at the local attenuation rate only if noise sources are distributed uniformly on the Earth's surface. In this study we show that this constraint can be relaxed in case the observed loss of coherence is due to multiple scattering instead of dissipation of energy. We describe the scattering medium as an effective medium whose phase velocity and rate of attenuation are a function of the scatterer density and the average strength of the scatterers. We find that the decay of the cross-correlation in the effective medium coincides with the local attenuation of the effective medium in case the scattering medium is illuminated uniformly from all angles. Consequently, uniform excitation is not a necessary condition for the correct retrieval of scattering attenuation. We exemplify the implications of this finding for studies using the spectrally whitened cross-correlation to infer subsurface attenuation.