S-Velocity Structure of the Upper Mantle

S-Velocity Structure of the Upper Mantle Cross-section depths and color-scale limits are specified within each frame. The reference Sv-wave velocity values are, for increasing depths: 4.38, 4.39, 4.69, 5.27 km/s—all at a reference period of 50 s.
Sv-velocity structure of the upper mantle (crust–660 km) is constrained by the Automated Multimode Inversion of surface and S-wave forms [Lebedev et al., 2005], applied to a large global data set retreived from IRIS facilities [Lebedev and van der Hilst, 2008]. Structure of the mantle and crust is constrained by waveform information from both the fundamental mode Rayleigh waves (periods from 20 to 400 s) and S and multiple S waves (higher modes). The reference model used in the waveform inversions and the tomographic linear inversion incorporates 3-D crustal structure. The tomographic model includes isotropic variations in S- and P-wave velocities and also S-wave azimuthal anisotropy. The lateral resolution of the imaging is a few hundred kilometres, varying with data sampling.
</p><p>The technique has been benchmarked with a ‘spectral-element’ resolution test: inverting published global synthetic data sets computed with the spectral-element method for laterally heterogeneous mantle models, Lebedev and van der Hilst [2008] have been able to reconstruct the synthetic models accurately.
</p><p>The tomographic model displays low-Sv-velocity anomalies beneath mid-ocean ridges and back-arc basins that extend down to ~100 km depth only. In the seismic lithosphere beneath cratons, strong high velocity anomalies extend to ~200 km. Pronounced low-velocity zones beneath cratonic lithosphere are rare; where present (South America; Tanzania) they are often neighboured by volcanic areas near cratonic boundaries. The images of these low-velocity zones may indicate hot material possibly of mantle-plume origin— trapped or spreading beneath the thick cratonic lithosphere [Lebedev and van der Hilst, 2008].
Lebedev, S., G. Nolet, T. Meier, R. D. van der Hilst, Automated multimode inversion of surface and S waveforms, Geophys. J. Int. 162, 951-964, 2005.
</p><p>Lebedev, S., R. D. van der Hilst, Global upper-mantle tomography with the automated multimode inversion of surface and S-wave forms, Geophys. J. Int., 173, 505-518, 2008.</p>


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