Abstract:

 We obtained surface wave group velocity distribution in eastern Tibet from ambient noise analysis with the broadband seismic data recorded from January to December of 2008 at 76 stations from the Qinghai, Gansu and Sichuan Earthquake Administration digital seismic network. First, we used the multiplefilter analysis method to extract more than one thousand threecomponent surface wave group velocity dispersion curves from interstation paths at periods from 5 to 50 s. Then the study area was reticulated into a 0.2°×0.2° grid, and using Occam inversion method to obtain the Rayleigh（RR）and Love（TT） wave group velocity distribution. The results of group velocity distribution maps generally demonstrate good correlations with surface geological and tectonic features; they also clearly revealed the lateral velocity variation in the crust. The results of the group velocity tomography show that at the short periods (820 s), Sichuan Basin is clearly resolved with low group velocity due to its thick sedimentary layer, and the eastern part of Tibet shows relative higher group velocity distribution. With the increase of period (>20 s), the group velocity distribution appears the contrary characteristics compared to that of the short periods, and the group velocity beneath the eastern part of Tibetan Plateau is much lower than Sichuan Basin, which possibly correlates with the lowvelocity zone exhibited in the middle crust beneath the eastern part of the Tibetan Plateau, and also means that the crust in our study area has conspicuous lateral heterogeneity. Our group velocity distribution maps clearly show that the Longmenshan fault is not only the boundary of topography and tectonic zone, but also the transition zone of high and low group velocity.  

Key words:  ambient noise, surface wave, group velocity distribution property, crust tectonic, lateral heterogeneity