Effects of seismic anisotropy on P-velocity tomography of the Baltic Shield (original) (raw)
Related papers
Seismic Anisotropy Tomography and Mantle Dynamics
Surveys in Geophysics
Seismic anisotropy tomography is the updated geophysical imaging technology that can reveal 3-D variations of both structural heterogeneity and seismic anisotropy, providing unique constraints on geodynamic processes in the Earth’s crust and mantle. Here we introduce recent advances in the theory and application of seismic anisotropy tomography, thanks to abundant and high-quality data sets recorded by dense seismic networks deployed in many regions in the past decades. Applications of the novel techniques led to new discoveries in the 3-D structure and dynamics of subduction zones and continental regions. The most significant findings are constraints on seismic anisotropy in the subducting slabs. Fast-velocity directions (FVDs) of azimuthal anisotropy in the slabs are generally trench-parallel, reflecting fossil lattice-preferred orientation of aligned anisotropic minerals and/or shape-preferred orientation due to transform faults produced at the mid-ocean ridge and intraslab hydra...
Geophysical Journal International
Seismic anisotropy provides a unique constraint on the past and present dynamics of the lithosphere and sublithospheric mantle. To contribute to studies of large-scale tectonic fabric, we have developed code AniTomo for regional anisotropic tomography. AniTomo allows us to invert simultaneously relative traveltime residuals of teleseismic P waves for 3-D distribution of isotropic-velocity perturbations and velocity anisotropy in the upper mantle. Weak hexagonal anisotropy with the symmetry axis oriented generally in 3-D is considered. The first application of novel code AniTomo to data from passive seismic experiment LAPNET results in a model of anisotropic velocities of the upper mantle beneath northern Fennoscandia. We have opted for northern Fennoscandia for the first application because it is a tectonically stable Precambrian region with a thick anisotropic mantle lithosphere without significant thermal heterogeneities. We carefully analyse the distribution of the rays to limit the fully anisotropic inversion only to the volume with the sufficient directional ray coverage. Capability of the given inversion setup to reveal large-scale anisotropic structures in the upper mantle is documented by a series of synthetic tests. The strongest anisotropy and the largest velocity perturbations concentrate at depths corresponding to the mantle lithosphere, while in deeper parts of the tomographic model, the lateral variations are insignificant. We delimit regions of laterally and vertically consistent anisotropy in the mantle-lithospheric part of the model. We attribute the retrieved domain-like anisotropic structure of the mantle lithosphere in northern Fennoscandia to preserved fossil fabrics of the Archean microplates, accreted during the Precambrian orogenic processes.
Isotropic and Anisotropic P and S Velocities of the Baltic Shield Mantle
2009
Eken, T. 2009. Isotropic and Anisotropic P and S Velocities of the Baltic Shield Mantle. Results from Analyses of Teleseismic Body Waves. Acta Universitatis Upsaliensis. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 653. 110 pp. Uppsala. ISBN 978-91-554-7548-2.
How robust is isotropic delay time tomography for anisotropic mantle?
Geophysical Research Letters, 1999
Using isotropic inversion of teleseismic P-travel times computed for laterally varying anisotropic structures, we investigate how anisotropy may show up in classical tomographic studies based on isotropic 3-D inversions. Anisotropic bodies of several tens of kilometers in the upper mantle may significantly bias the estimation of the isotropic part of a Pwave velocity image. The largest artifacts are introduced by anisotropic bodies with a dipping olivine "a" axis which could be associated with fossil subduction zones. However, anisotropy-induced artifacts are negligible for anisotropy caused by pure shear deformation of the lithosphere due to horizontal extension or compression. The negative P-wave velocity anomaly arising from isotropic inversion beneath a rift zone may be enhanced if the olivine crystal's "a" axes are preferentially oriented parallel to the asthenospheric flow along the rift axis. Examination of recently published tomographic models shows that the image of the Rhine Graben may contain significant anisotropic artifacts.
Global upper mantle tomography of seismic velocities and anisotropies
Journal of Geophysical Research, 1991
A data set of 2600 paths for Rayleigh waves and 2170 paths for Love waves enabled us to retrieve three-dimensional distributions of different seismic parameters. Shallow layer corrections have been carefully performed on phase velocity data before regionalization and inversion at depth. The different seismic parameters include the five parameters of a radially anisotropic medium and the eight azimuthal anisotropic parameters as defined by Montagner and Nataf. It is found that the lateral heterogeneities of velocities and anisotropies in the upper mantle are dominated down to 250-30 km by plate tectonics with slow velocities below ridges, high velocities below continents and a velocity increasing with the age of the seafloor. Anisotropy is present in this whole depth range and the directions of maximum velocities are in good agreement with absolute plate velocities. Below 300 km, there is a sharp decreasing of the amplitude of lateral heterogeneities of seismic velocities and anisotropies. Below 450 km, lateral heterogeneities display a degree 2 and to a less extent a degree 6 pattem. Therefore, between 250 km and 450 km, there is a transition region where vertical circulation of matter is possible as shown by subducted slabs and "plumes" of slow velocities but which probably separates two types of convection. The first one is closely related to plate tectonics and to the distribution of continents. The second one dominates below 450 km and is characterized by two downgoing and two upgoing flOWS.
Geophysical Journal International, 2021
Despite the well-established anisotropic nature of Earth's upper mantle, the influence of elastic anisotropy on teleseismic P-wave imaging remains largely ignored. Unmodelled anisotropic heterogeneity can lead to substantial isotropic velocity artefacts that may be misinterpreted as compositional heterogeneities. Recent studies have demonstrated the possibility of inverting P-wave delay times for the strength and orientation of seismic anisotropy. However, the ability of P-wave delay times to constrain complex anisotropic patterns, such as those expected in subduction settings, remains unclear as synthetic testing has been restricted to the recovery of simplified block-like structures using ideal self-consistent data (i.e. data produced using the assumptions built into the tomography algorithm). Here, we present a modified parametrization for imaging arbitrarily oriented hexagonal anisotropy and test the method by reconstructing geodynamic simulations of subduction. Our inversion approach allows for isotropic starting models and includes approximate analytic finite-frequency sensitivity kernels for the simplified anisotropic parameters. Synthetic seismic data are created by propagating teleseismic waves through an elastically anisotropic subduction zone model created via petrologic-thermomechanical modelling. Delay times across a synthetic seismic array are measured using conventional cross-correlation techniques. We find that our imaging algorithm is capable of resolving large-scale features in subduction zone anisotropic structure (e.g. toroidal flow pattern and dipping fabrics associated with the descending slab). Allowing for arbitrarily oriented anisotropy also results in a more accurate reconstruction of isotropic slab structure. In comparison, models created assuming isotropy or only azimuthal anisotropy contain significant isotropic and anisotropic imaging artefacts that may lead to spurious interpretations. We conclude that teleseismic P-wave traveltimes are a useful observable for probing the 3-D distribution of upper mantle anisotropy and that anisotropic inversions should be explored to better understand the nature of isotropic velocity anomalies particularly in subduction settings.
Global Tomography of Seismic Anisotropy and Interpretations
Seismic anisotropy, in spite of its inherent complexity is becoming an important ingredient for explaining various kinds of seismic data. Global tomographic models have been improved over years not only by an increase in the number of data but more importantly by using more general parameterizations, now including general anisotropy (both radial and azimuthal anisotropies). Different physical processes (lattice preferred orientation of crystals, cracks or fluid inclusions, fine layering...) related to strain field and/or stress field, give rise to observable seismic anisotropy (S-wave splitting, surface wave radial and azimuthal anisotropies), which makes its interpretation sometimes difficult and non-unique. Surface waves are well suited for imaging large scale (>1000km) lateral heterogeneities of velocity and anisotropy in the mantle by using fundamental and higher modes, since they provide an almost uniform lateral and azimuthal coverages, particularly below oceanic areas. The...
Geophysical Journal International, 1996
A new formulation is given for inverting teleseismic P-wave residuals in terms of both 3-D variations of seismic velocity and anisotropy of the subcrustal lithosphere. Two restrictive assumptions are made on the anisotropic elastic tensor to reduce the number of model parameters. First, the upper-mantle anisotropy is modelled by a preferred orientation of olivine crystals with b-and c-axes randomly distributed around the a-axis, and second, the elastic properties of polycrystalline aggregates are computed according to the Voigt averaging rule. The latter assumption allows us to link linearly the observed P-wave residuals to the proportion of oriented crystals in a realistic polycrystalline assemblage. In the inversion process, the orientation of the symmetry axis is fixed, either by a trial procedure or by using a priori hypotheses from independent data. A linear inversion scheme is then applied to the data to retrieve the 3-D variations of two scalar parameters giving the isotropic velocity perturbation and the amount of anisotropy. The inversion algorithm is tested for different source-receiver configurations. Using several idealistic ray geometries, it is concluded that the teleseismic P-wave residuals could potentially carry enough information for retrieving both the 3-D isotropic velocity variations and the amount of anisotropy. However, from a more practical point of view, by looking at the distributions of rays for two recent tomographic experiments in the southern Rhine Graben area and in the Pyrenees, it is concluded that the distributions of rays in these experiments do not fulfil the necessary requirements for retrieving reliable information about the lithospheric anisotropy. Additional data are thus needed to constrain the anisotropic model. If the symmetry axis is set as horizontal, the inversion of the P-wave residual of the Pyrenees leads to an east-west fast P-wave velocity orientation. The resulting anisotropy is compatible with SKS observations made in the Pyrenees. Whatever the anisotropy is, the numerical tests performed with the actual ray configurations show the robustness of the isotropic components in the inverted model. A long-wavelength anisotropy in the lithosphere, if present, should not drastically alter the isotropic 3-D velocity distributions that are obtained from classical methods of teleseismic tomography.
Upper mantle Anisotropy from Surface Wave studies
2008
Major advances in Structural Seismology during the last twenty years, are related to the emergence and development of more and more sophisticated 3D imaging techniques, usually named seismic tomography, at different scales from local to global. Progress has been made possible by the rapid developments in seismic instrumentation and by the extensive use of massive computation facilities. The scope of this chapter is limited to the tomographic elastic structure of the upper mantle. In order to obtain a good spatial coverage of this part of the Earth, it is necessary to make use of dispersive properties of surface waves. Most global tomographic models are still suffering severe limitations in lateral resolution, due to the imperfect data coverage, and to crude theoretical approximations. It is usually assumed that the propagating elastic medium is isotropic, which is a poor approximation. It is shown in this chapter how to take account of anisotropy of Earth’s materials and a complete ...