Variation of shear-wave residuals and splitting parameters from array observations in southern Tibet

A tight array of seismographs spanning a 500 km traverse of southern Tibet resolved anisotropy from SKS with a spatial variation of its direction and an increase northward of the splitting delay, as well as of its first arrival residual. Both waves split by velocity anisotropy are slow relative to P and their waveform analysis may be interpreted to suggest attenuation anisotropy. The array here provides examples of residuals and splitting of other S waves which do not tightly conform to the anisotropy assumed in the simplest model of olivine of transverse isotropy with horizontal symmetry axis. S waves are also split, with parameters which vary along the array, and hence are relevant to near-receiver structure like those of SKS. Their splitting delay, for non-vertical incidence and polarization, appears larger than that for SKS. Residuals of S-wave first arrivals and splitting delays increase less northwards for S than SKS. Anisotropy with a slow vertical axis may account for these observations. Its origin may be related to horizontal shear or how in low-velocity layers.