Fig. 6. Rayleigh wave group velocity in the different periods obtained by inversion of noise tomography. The corresponding periods: 9 s (a), 21 s (b), 25 s (c) and 38 s (d). The thick black lines represent the boundaries of the major geological/tectonic units. Black dotted lines and arrows in the c and d is the eastward channel flow.
The group velocity of a certain period is most sensitive to the shear wave velocity of 1/3 wavelength (Lin et al., 2007; Yang et al., 2007), and the group velocity distribution of the different periods represents the structural differences at different depths. According to the characteristics of the group velocity distribution of each period, we selected representative four-period group velocity for discussion (Fig. 6). The Rayleigh wave group velocity with T=9 s mainly reflects the velocity structure of the upper crust or the shallow part of the crust (Fig. 6 a). The Sichuan Basin shows obvious low-velocity anomaly (Fig. 6 a, LV1) in the region, which is significantly affected by the sedimentary strata. Xie et al. (2013) also imaged a low-velocity anomaly from Rayleigh and Love wave phase speed at 10s in the Sichuan Basin. The Songpan-Ganzi Block, the northern part of the SYDB, the Cathaysia Bock, and the Yangtze Block mostly show high-velocity structure (Fig. 6 a, HV1, HV2 and HV3).
The Rayleigh wave group velocity with T=21 s mainly reflects the velocity structure of the middle crust (Fig. 6 b). The Sichuan Basin exhibits low-velocity structure (Fig. 6 b, LV1), with a relatively high Rayleigh wave group velocity in the center of the basin, which reflects the non-uniform feature of the basin. The Songpan Ganzi Block, the northern part of the SYDB and the Cathaysia Block all exhibit high velocity structure (Fig. 6 b, HV1, HV2 and HV3). The Rayleigh wave group velocity with meddle and long periods (25-38 s) mainly reflects the velocity structure from the lower crust to the top of the upper mantle (Fig. 6 c and d). At these periods, high-velocity structure (Fig. 6 c and d, HV4 and HV5) can be clearly seen in the Sichuan basin and the Cathaysia Block, whereas the southern part of the SYDB and most of the Yangtze Block are characterized by low-velocity structure (Fig. 6 c and d, LV2).