Surface Wave Dispersion Measurements and Tomography from Ambient Seismic Noise Correlation in China

We perform ambient noise tomography of China using the data from the China National Seismic Network and surrounding global and PASSCAL stations. The results so far are summarized below. (1) Dispersion measurements and tomography. For most of the station pairs, we retrieve good Rayleigh waveforms from ambient noise correlations using 18-months of continuous data at all distance ranges across the entire region (over 5000 km) and for periods from 70 s down to about 8 s. We obtain Rayleigh wave group and phase velocity dispersion measurements using a frequency-time analysis method and invert for Rayleigh dispersion maps for periods from 8 to 60 s. The dispersion maps correlate nicely with surface geology. (2) Error estimates using bootstrap analysis. A major feature of the ambient noise method is that the whole process is completely repeatable with different time segments, which make it possible to evaluate the uncertainties. We adopt a bootstrap method to quantify the errors in the Rayleigh wave group velocity dispersion measurements and the tomographic maps. Most of the pairs show similar dispersion curves between different runs and small standard deviations, indicating good data quality and convergence of the Green function. Group velocity at long period end generally has a larger error, which is consistent with the notion that the long period needs longer time to converge. The best retrieved periods are from 10 to 30 s with the optimal period of around 15 to 20 s. Pairs with large errors do not depend on the orientations of the paths or the locations of the stations. Rather, they are associated with a few stations with large average standard errors. The likely causes are missing data and poor instrumentation (or site conditions). Where ray coverage is good, there is only subtle difference in tomography maps between different runs, suggesting that our solution is very stable. (3) 3D structure. We invert the Rayleigh group and phase dispersion maps for 3D shear-wave velocity structure. The 3D model shows some remarkable features, including slow sedimentary layers of all the major basins in China at the shallow depth, Moho depth variation, fast (strong) mid-lower crust and mantle lithosphere in major basins surrounding the Tibetan Plateau (TP) (Tarim, Ordos, and Sichuan). These strong blocks thus seem to play an important role in confining the deformation of the TP to be a triangular shape. The Moho change from plateau to Tarim is very sharp; the change to the Sichuan Basin is gentler but still steep. The India lithosphere seems to terminate around the Bangong Nujiang Suture as indicated by the fast-slow velocity contrast in the mantle lithosphere, but it seems to extend further north under E. Tibet. Widespread, prominent low-velocity zone is observed in mid-crust in the TP, providing strong support for the channel flow model. These slow velocities are generally connected and seem to reach to the surface near the margins of the TP, consistent with the thermalmechanical modeling of the ductile extrusion of the Greater Himalayan Sequence.