Crustal Flow beneath Eastern Tibet Revealed by Rayleigh-wave Tomography

We explore the Rayleigh-wave phase-velocity structure of eastern Tibet in a broad period range (20-100 s). Rayleigh-wave dispersion curves are measured by the two-station technique for a total of 531 inter-station paths using vertical-component broad-band waveforms at 32 seismic stations from the China National Seismic Network (CNSN) from 467 global earthquakes.These interstation dispersion curves are then inverted for the high-resolution isotropic and azimuthally anisotropic phase-velocity maps at various periods between 20 and 100 s.In the period range of 20-40 s, sampling the middle and lower crust, the isotropic structure exhibits a clear contrast between the strong fast anomaly below the Yangtze and Sino-Korean Cratons in the east and the very slow anomaly beneath eastern Tibet. At periods sampling the crust (20-60 s), the anisotropy we observe suggests that the Tibetan crust flows eastwards up to longitude around 100°E, and then bends southwards.At longer periods, the anisotropic structures are in agreement with absolute plate motion and the overall upper-mantle flow. The anisotropic patterns we observe support a mechanical decoupling between the lower crust and lithospheric upper mantle.