Upper mantle Anisotropy from Surface Wave studies (original) (raw)

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 ...

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...

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...

Anisotropic shear‐wave velocity structure of the Earth's mantle: A global model

Journal of Geophysical Research: Solid Earth, 2008

We combine a new, large data set of surface wave phase anomalies, long‐period waveforms, and body wave travel times to construct a three‐dimensional model of the anisotropic shear wave velocity in the Earth's mantle. Our modeling approach is improved and more comprehensive compared to our earlier studies and involves the development and implementation of a new spherically symmetric reference model, simultaneous inversion for velocity and anisotropy, as well as discontinuity topographies, and implementation of nonlinear crustal corrections for waveforms. A comparison of our new three‐dimensional model, S362ANI, with two other models derived from comparable data sets but using different techniques reveals persistent features: (1) strong, ∼200‐km‐thick, high‐velocity anomalies beneath cratons, likely representing the continental lithosphere, underlain by weaker, fast anomalies extending below 250 km, which may represent continental roots, (2) weak velocity heterogeneity between 250...

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