A transportable array with 297 broadband seismic stations was deployed in the western Sichuan (26 degrees N similar to 32 degrees N, 100 degrees E similar to 105 degrees E) in 2006 by the State Key Laboratory of Earthquake Dynamics,Institute of Geology, China Earthquake Administration. From the ambient noise data recorded at 156 stations of this array to the north of 29 degrees N from January to December of 2007, we have obtained the surface wave empirical Green's functions (EGF) using cross-correlation technique and measured the Rayleigh-wave phase velocity dispersion curves for all possible station pairs. These dispersion measurements were then used to invert for phase velocity maps of Rayleigh waves between periods 2 similar to 35 s. Our results manifest the significant discrepancies between the crustal structures of the Chuandian block, Songpan-Garze block and Sichuan basin, which can be summarized as follows. (1) The phase velocity maps at the short periods (2 similar to 8 s) are well-correlated with the surface geological feature, and the upper crust structures of the Chuandian block,Songpan-Garze block and Sichuan basin are well confined by the Longmen Shan faults and Xianshuihe faults, especially in the foreland of the Sichuan basin, where the significant low-velocity structure suggests that a thick sediment does exist; (2) The phase velocity maps at the intermediate periods (12 similar to 18 s) manifest that the middle and lower crust velocity structure of the Chuandian and Songpan-Garze block has apparent lateral heterogeneities, including low-velocity zones of different sizes, and the upper and middle crust beneath the Sichuan basin shows high velocity structure; (3) The phase velocity maps at the long periods (25 similar to 35 s) manifest that the Songpan-Garze block and Chuandian block have an evident low-velocity anomaly in the middle-lower crust,suggesting that the middle-lower crust of these two blocks is relatively weak, and the middle-lower crust of Sichuan basin has high-velocity anomaly, suggesting a mechanically strong middle-lower crust in the basin. In particular, the crustal velocity structure along the Longmen Shan faults has a high-velocity anomaly to the north and a low-velocity anomaly to the south of the hypocenter of the Wenchuan earthquake.
