Observations of Frequency-Dependent Sn Blockage in Northern Tibet

We present new observations of the frequency-dependent propagation efficiency of the seismic phase Sn over the Tibetan plateau. Our measurements are the ratio of the Sn amplitude to the Pcoda amplitude on Tibetan regional seismograms. 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 Sn blockage identified by previous studies to be better constrained. An important result is that at low frequencies (0.2 Hz), Sn propagates efficiently over the entire plateau, while at higher frequencies, Sn is blocked for a region of the northern plateau. The area of inefficient Sn propagation extends farther southward as higher frequencies (between 0.2 and 1.0 Hz) are brought into consideration. The observation that low frequency Sn propagates efficiently across the whole plateau suggests that the lithosphere beneath Tibet is still intact and has not delaminated as previously proposed. The observation that high frequency Sn does not propagate beneath northern Tibet suggests that the Tibetan upper mantle has been heated and possibly contains partial melt.