Shear-velocity structure of the crust and upper mantle beneath the Tibetan Plateau and southeastern China

S U M M A R Y This paper addresses the velocity structure of the crust and upper mantle beneath southern China with special emphasis on the Tibet region. Waveform data from 48 earthquakes as recorded on the WWSSN and GDSN are used in this detailed forward modelling study. Constraints on the upper crustal section are derived from modelling local Love waves in the time domain applying the mode-sum modelling technique. Lower crustal constraints are derived by modelling the PnI-wavetrain with the reflectivity method. -An average crustal thickness of 70 km is obtained beneath the Tibetan Plateau with a modest increase of velocity with depth. The lithospheric and upper mantle structure is deduced from modelling S and SS triplication waveform data and relative traveltimes by applying a combination of WKBJ and generalized ray methods. S-SS seismograms chosen with bounce-points directly under Tibet allow remote sensing of this inaccessible region. The resulting model is an averaged 1-D model where corrections for lateral variation have been applied. We conclude that the upper mantle structure in the entire region is basically shield-like below 200km (SNA). However, the velocity of the lithosphere is abnormally slow, roughly 5 per cent beneath Tibet. The model for Tibet derived does not have a distinct lid, and has a positive velocity gradient in the crust, suggesting crustal shortening. A preliminary velocity model for southeastern China is also suggested.