Observations of frequency-dependent Sn propagation in Northern Tibet

P>We present new observations of the frequency-dependent propagation efficiency of the seismic phase S-n over the Tibetan Plateau. Our measurements are the ratio of the S-n amplitude to the P-g coda amplitude on Tibetan regional seismograms, and we map the lateral variation in the maximum and mean values of this ratio across the Plateau. Good path density and azimuthal coverage allow the area of S-n blockage identified by previous studies to be better constrained. An important result is that at low frequencies (similar to 0.2 Hz), S-n propagates efficiently over the entire Plateau, whereas at higher frequencies (similar to 1 Hz), S-n is blocked for a region of the northern Plateau. As we take measurements at higher frequencies, we find that the southern boundary of the region of inefficient propagation migrates to the south. The observation that low frequency S-n propagates efficiently across the whole Plateau suggests that at the length-scales sampled by these waves, the upper mantle has an overall positive shear velocity gradient, which could indicate that the high-velocity seismic lid, or seismic lithosphere, beneath Tibet is still intact, and has not delaminated as some researchers have previously proposed. The observation that high frequency S-n does not propagate beneath northern Tibet suggests that at the shorter length-scales sampled by the higher frequency waves, the upper mantle has a negative shear velocity gradient. This could indicate that at shallow depths the sub-Moho upper mantle is hot and may contain some melt.