CHANNEL FLOW WITHIN THE TIBETAN CRUST: NEW OBSERVATIONAL EVIDENCE FROM SEISMIC ANISOTROPY

The dispersion of Love and Rayleigh waves propagating across Tibet requires radial anisotropy within the middle and lower crust. We observe that the group speed of Rayleigh waves crossing Tibet decreases between 10 sec and 35 sec period and then increases again at longer periods, whereas the dispersion of Love waves at these periods remains near normal and increases approximately monotonically with period. Simple models of isotropic seismic wave speeds in the crust cannot fit simultaneously the Rayleigh and Love wave group-speed dispersion curves observed across Tibet and a radial anisotropy (transverse isotropy with a vertical symmetry axis) is required within the Tibetan crust with horizontally polarized SH shear wave speed greater than shear wave speed for a vertically polarized SV wave. The radial anisotropy in the Tibetan crust is reminiscent of similar anisotropy in the mantle that manifests as a much longer period "Rayleigh-Love discrepancy", but must have different causes because the mineralogy of the crust and mantle differ from one another. To study the spatial extent and strength of the radial anisotropy in the Tibetan crust, we combined surface wave dispersion measurements from more than 45,000 crossing paths to constrain the three-dimensional distribution of shear wave speeds in the Tibetan crust and uppermost mantle. We followed a two-step inversion procedure. First, we used surface-wave diffraction tomography to construct dispersion maps at periods ranging from 18 sec to 200 sec for group speeds and from 40 sec to 150 sec for phase speeds. This was followed by a Monte-Carlo inversion for the shear wave speed of the crust and the uppermost mantle. Results of the inversion show that the mid-crustal radial anisotropy stands out most clearly beneath the high plateau between ~80E and ~95E where moment tensors of earthquakes indicate active crustal thinning. The preferred orientation of mica crystals resulting from the crustal thinning can account for the observed anisotropy. The mid-to-lower crust of Tibet appears to have thinned more than the upper crust, consistent with deformation of a mechanically weak layer in the mid-to-lower Tibetan crust that flows as if confined to a channel.