Radial Anisotropy in the Crust beneath the Northeastern Tibetan Plateau from Ambient Noise Tomography

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.