Three-dimensional Crust Shear Wave Velocity Structure Of Western Sichuan Based On The Surface Wave Ray Tracing Method

Western Sichuan is on the eastern margin of Tibetan Plateau, and in the junction part of the uplifting and expanding Tibetan plateau and the rigid Yangtz craton. It is a focus area with high earthquake activity, dense fault distribution and complex velocity structure of the crust and upper mantle. Though many 3-D structural imaging projects have been done in western Sichuan and the surrounding areas, the results are quite different since different methods and data were adopted with their own limitations. Ambient noise cross-correlation method can avoid the limitation of earthquake distribution, thus surface wave dispersion data of short and intermediate periods can be extracted with dense and uniform path distribution. Ambient noise method provides an excellent data source for studies of the complex crust and uppermost mantle structures.For surface wave phase or group velocity tomography we usually assume that surface waves propagate along great-circle paths. However, when the velocity structure has large variations, surface waves will propagate along off-great-circle paths, and therefore there may exist considerable errors in tomographic inversion results based on great-circle propagation of surface waves. We use the fast marching surface wave tomography method to analyze how off-great-circle propagation of surface waves influences the results of phase velocity tomography in western Sichuan. During the inversion, the fast marching method is used to track the ray paths of surface waves and the subspace inversion scheme is used for the inversion step. We use synthetic data from a theoretical model and short period ambient noise phase velocity dispersion data from the western Sichuan array for the analysis, and we compare the inversion results based on off-great-circle propagation and great-circle propagation of surface waves. Results of the synthetic tests show that the off-great-circle tomographic method recovers the anomaly better when the variation of the velocity structure is large. The inversion of real data from the western Sichuan array reveals that for the 6s period data, the differences between the phase velocity tomographic results from the off-great-circle propagation and great-circle propagation methods are rather obvious, which are close to 0.2km/s in the Sichuan Basin. For the 10s period data, differences between the two methods are much smaller, which are all less than 0.1km/s. The main reason is due to that the 6s period data are more sensitive to the complex shallow structure of upper crust, so the effect of off-great-circle propagation on the inversion results is more remarkable. Our results indicate that the off-great-circle propagation effect should be considered in the tomography if the observed phase velocities along different paths have large variations, for example, more than 10% of the average velocity, otherwise the inversion results of regions with large velocity anomalies might have considerable bias.We use data of 3-45s periods Rayleigh wave interstation phase velocity dispersion obtained by the ambient noise method from the western Sichuan Array, and the direct surface wave inversion method based on surface wave ray tracing is used to invert for the three-dimensional shear wave velocity distribution of the crust and upper mantle in western Sichuan. We compare our three-dimensional shear wave velocity structure with the shear wave velocity model (Liu et al.,2014) obtained from joint inversion of ambient noise tomography based on great-circle propagation and receiver function. Although the two models are very close, our model can fit the observed dispersion data better. The shear wave velocity difference between the two models can reach 0.2km/s. This difference shows the influence of the off-great-circle propagation effect of surface waves and the empirical relationship of the compressional velocity, density and the shear velocity. Our inversion result indicates a lOkm-depth low-velocity sedimentary layer in Sichuan Basin, and below that is the high velocity crustal layer, showing the characteristics of the old and rigid craton. In Songpan-Ganzi, low velocity anomalies distribute widely at the depth of middle and lower crust; and the anomaly zone extends to the northwest of Longmen Shen area. The low velocity anomaly may be related to the middle and lower crust plastic flow from the central Tibetan Plateau to northern Sichuan Basin. And the shear wave velocity distribution may have certain relationship with the faults distribution.