Search for a command to run...
A transportable array with 297 broadband seismic stations was deployed in the western Sichuan (26°N～32°N, 100°E～105°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°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 2～35 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 (2～8 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 (12～18 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 (25～35 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.