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Love and Rayleigh Wave Tomography of the Qinghai-Tibet Plateau and Surrounding Areas

JOS BADAL  
【摘要】:正Surface wave data were initially collected from events of magnitude Ms≥5.0 and shallow or moderate focal depth occurred between 1980 and 2002:713 of them generated Rayleigh waves and 660 Love waves,which were recorded by 13 broadband digital stations in Eurasia and India.Up to 1,525 source-station Rayleigh waveforms and 1,464 Love wave trains have been processed by frequency-time analysis to obtain group velocities.After inverting the path-averaged group times by means of a damped least-squares approach,we have retrieved location-dependent group velocities on a 2°×2°-sized grid and constructed Rayleighand Love-wave group velocity maps at periods 10.4-105.0 s.Resolution and covariance matrices and the rms group velocity misfit have been computed in order to check the quality of the results.Afterwards,depth-dependent SV-and SH-wave velocity models of the crust and upper mantle are obtained by inversion of local Rayleigh- and Love-wave group velocities using a differential damped least-squares method.The results provide:(a) Rayleigh-and Love-wave group velocities at various periods;(b) SV-and SH-wave differential velocity maps at different depths;(c) sharp images of the subducted lithosphere by velocity cross sections along prefixed profiles;(d) regionalized dispersion curves and velocity-depth models related to the main geological formations.The lithospheric root presents a depth that can be substantiated at~40 km (Qiangtang Block) and exceptionally at~80 km in some places (Lhasa Block),and which exhibits laterally varying fast velocity very close to that of some shields that even reaches~4.8 km/s under the northern Lhasa Block and the Qiangtang Block.Slow-velocity anomalies of 7-10% or more beneath southern Tibet and the eastern edge of the Plateau support the idea of a mechanically weak middle-to-lower crust and the existence of crustal flow in Tibet.

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