F) Anisotropy:

        Modeling seismic anisotropy is a new technique that developed rapidly in the late 1990's to provide information on strain or motion induced fabric and older tectonic signatures through crustal and mantle levels. For calculation, teleseismic analysis of SKS splitting and P to SV splitting were commonly used by that time. SKS splitting results, in principle occurs due to strain-induced lattice preferred orientations of anisotropic minerals in upper mantle such as olivine and represents orientation of strain or flow in the uppermost mantle. Strong seismic anisotropy that coincides spatially with the region of highly attenuated low Sn and Pn velocities has been reported from SKS shear-wave splitting studies in northern Tibet (McNamara et al. 1994) (figure 19).
        Fast direction of the split mainly similar to the orientations of left-lateral shear planes inferred from GPS and tectonic slip rates. Some authors tried to explain this similarity by coupling between the crust-mantle (Lave et al. 1996). In this respect, major strike-slip faulting in northern Tibet was interpreted as lithospheric scaled structures which is consistent with the tomographic results on Altyn-Tagh fault (Wittlinger et al. 1998).
        In southern Tibet, measured SKS splitting is rather weak or not detectable.  A dense seismic array deployed between Qiangtang and Lhasa terranes identified a sharp onset of splitting north of the surface trace of the Banggong-Nujiang suture (32 N) which mostly likely marks the northern limit of the underthrusting Indian lithosphere (Huang et al. 2000).
 

Figure 19. Compiled seismic data for Tibet including; Pn tomography, uppermost mantle anisotropy
(SKS Splitting) and Inefficient shear-wave (Sn) propagation [McNamara et al. 1997]
 
 

        On the other hand  analysis of Moho converted P to SV waves which sample the crust, provides well constrained orientations of near surface and middle crustal anisotropy. In northern Tibet, results indicate a dominant NE orientation of the fast polarization which also rotates progressively clockwise towards north into a direction mainly parallel to the strike of Kunlun fault and remains quite constant from Kunlun to the north to the Qaidam basin (Herquel et al. 1995). This crustal anisotropy can be explained by preferential orientation of minerals and rotated strain field in the vicinity of the Kunlun fault where abundant vertical fractures lies parallel. In southern Tibet, preliminary results of Heather Folsom indicates varying near surface crustal anisotropy and NW trending mid-crustal anisotropy south of Jinsha suture which contradicts with shear plane orientations and SKS splitting results (Heather & Zandt, 2002). Her interpretation requires a possible decoupled middle crust beneath the Qiangtang terrane.