Through analysis of Rayleigh wave and Love wave Green's functions estimated from ambient noise tomography, we obtain radial anisotropy and shear wave velocity structure beneath the northeastern Tibetan Plateau. With two hundred and twenty three broadband seismic stations deployed by China Earthquake Administration, a collaborative seismic experiment of northern Tibet(ACSENT) experiment and northeastern Tibet seismic(NETS) experiment provide the unprecedented opportunity to resolve the spatial distribution of the radial anisotropy within the crust of the northeastern Tibetan Plateau. Discrepancies between Love(sh) and Rayleigh(sv) wave velocities show complex anisotropic patterns associated with the dynamic processes of the collision between the Indian and Eurasian plates:(1) In the upper crust, V_(sv)>V_(sh) anisotropy is dominant throughout the study area which probably reflects fossil microcracks induced by the uplift, folding and erosion geodynamic processes;(2) in the middle crust, V_(sh)>V_(sv) observed beneath the Songpan-Ganzi terrane and the northwestern Qilian orogen correlates well with a mid-crustal low velocity zone(LVZ);(3) at depths deeper than 40 km, V_(sh)>V_(sv) is still found in the Songpan-Ganzi terrane. This anisotropy could be caused by the sub-horizontal alignment of anisotropic minerals that has followed the collision between India and Eurasia. However, the northwestern Qilian orogen is associated with V_(sv)>V_(sh) anisotropy which may be related to the vertically aligned seismic anisotropic minerals caused by the crustal thickening.
We apply ambient noise tomography to continuous three-component broadband seismic data between January 1,2008 and December 31,2008 from the regional networks of 76 stations de-ployed by China Earthquake Administration.Ambient noise cross-correlations were performed to produce the Green's functions of each station-pair.Within the period from 6 to 50 s,Rayleigh and Love wave dispersion curves were measured using the multiple filter analysis method.Then three-dimensional(3-D) S-wave velocity structures from the surface down to 70 km are inverted from both Rayleigh and Love wave dispersion results.The obtained S-wave velocity maps show strong lateral variations and correlate well with the distinct geological and tectonic features in the study area.The Sichuan basin displays low velocity in shallow depth due to thick sedimentary deposits but high velocity in the mid-lower crust;the eastern Tibetan plateau is clearly featured with a low-velocity zone in its mid-to-lower crust which is consistent with the crustal flow model proposed to explain the mechanism of uplift and pattern of deformation for the Tibetan plateau.Meanwhile,our results also exhibit that the crustal thickness decreased from the eastern Tibetan plateau to the Sichuan basin.
