Surface Wave Group Velocity Distribution Based On Ambient Noise Analysis In Northeast Qinhai-Tibet Plateau

We obtained surface wave group velocity distribution in northeastern Tibet from ambient noise analysis with the broadband seismic data recorded from May to December 2008 at 240 stations from the Qinghai, Gansu and Sichuan Earthquake Administration digital seismic network. Firstly we removed instrument response ,remove mean,remove trend,band-pass filter,cut to length of 1-hour and used the multiple-filter analysis method to extract more than one thousand three-component surface wave group velocity dispersion caves from inter-station paths at periods from 10s to 40s. Then the study area was discretized into a 0.4°×0.4°grid, and using O’ccam inversion method to obtain the Rayleigh and Love 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 (8~20s), Sichuan basin is clearly resolved with low group velocity due to its thick sedimentary layer, and the eastern part 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 is possibly correlate with the low-velocity zone exhibiting in the middle crust beneath the eastern part of the Tibetan Plateau, it also means that the crust in our studying area has conspicuous lateral heterogeneity. Our group velocity distribution maps clearly show that Longmenshan fault is not only the boundary of topography and tectonic zone, but the transition zone of high and low group velocity.This results of the study area consistent with geological structure, show a clear lateral heterogeneity, reflecting the crust in the region which caused by the intense activity of lateral heterogeneity and construction complexity. At the same time, through different cycles of the speed with regional comparative study to verify the NE-SW direction along the Qinghai-Tibet Plateau by a thick crust and thin speculation.