High Resolution Surface-Wave Tomography in SE Tibet from Ambient Seismic Noise and a Two-station Method

The surface-wave Green's function between two passive receivers can be estimated from the longtime cross-correlation of ambient seismic noise. We obtain short-period surface-wave dispersion measurements for interstation paths, which can be used for the high-resolution tomography studies of the crust and shallow mantle. Previous studies used group velocity dispersion, but we demonstrate that phase velocity dispersion can also be obtained from the estimated Green's functions by using the representation of the surface wave Green's tensor in the far field and a phase imaging analysis technique. We apply this method to the MIT-CIGMR broad-band network (25 stations) in southeastern Tibet. Rayleigh wave Green's functions (RGFs) in the period band 10-60s of all possible interstation paths for four months (April, May, June, July) in 2004 are estimated from vertical component ambient seismic noise. RGFs are quite stable over different months but predominant directions of noise sources may produce one-sided feature for some station pairs. Phase velocity dispersion measurements are made from the RGFs, and phase velocity maps are made at different periods (10-30s). For the same period, the inversions for four separate months show similar features. Phase velocities at different periods are then averaged over the four months to construct the final phase velocity maps. The path coverage is very good in the region under study and the lateral resolution is about 100 km. A classical two-station method is also used to extract the interstation Rayleigh-wave phase velocity dispersion in the period band 20-120s and phase velocity maps are inverted. Compared with the RGF results, the spatial pattern of phase velocity variation from classical two-station method is similar to that inferred from RGFs but (at 20s) the average phase velocity is ~ 1-3% higher. In addition to the methods, we will present preliminary results and discuss the implications for our understanding of the regional tectonics.