Lateral Variation of the Tibetan Lithospheric Structure Inferred from Teleseismic Waveforms

I present a teleseismic waveform study of the lithospheric structure beneath the Tibetan Plateau, using data collected during the 1991-1992 SinoUS Tibet seismic recording experiment. Crustal thickness at each of the 11 broadband stations is estimated from the differential travel time between the Moho P -to-S converted phase and direct P . I demonstrated that this estimation is not sensitive to the average crustal P or S velocity, but is strongly dependent on the V p/V s ratio. The ambiguity is removed by adding a secondary converted phase to the analysis so that both thickness and V p/V s ratio can be determined. I found crustal V p/V s ratios ranging from 1.75 to 2.0 and thicknesses from 55 to 80 km. On average the northern Tibetan crust is 20 km thinner and has a higher V p/V s ratio than the southern part. Teleseismic P and S arrival delays accurately measured by a multichannel correlation technique exhibit large azimuthal and lateral variations. The delays after crustal correction indicate that the upper mantle velocities beneath the northern plateau are significantly slower too. The uniform surface elevation of Tibet coupled with the large variations of crustal thickness and upper mantle velocities suggests that the north-central plateau is supported partly by a hot upper mantle.