Fig. 7. S-wave velocity at different depths, a: 10 km, b: 20 km, c: 30 km and d: 46 km. The thick black lines mark the boundaries of the major geological/tectonic units. Black dotted lines and arrow represent the suggested eastward channel flow.
According to the 1-D shear wave velocity structure of each grid node obtained in this study, we construct a 3-D shear wave velocity structure ranging from a depth of 0 km to a depth 60 km in this area (Fig. 7). At the depth of 10 km (Fig. 7a), the Songpan-Ganzi Block, the northern part of the SYRB, the Cathaysia Block and the eastern part of the Yangtze Block exhibit high-velocity anomalies (Fig. 7a, HV1, HV2 and HV3), whereas the Sichuan Basin exhibits low-velocity structure (LV1), which may be from the influence of the sedimentary strata. The Longmen Mountain fault zone is located on the boundary between the high-velocity (west) and the low-velocity (east) anomalies.
At the depth of the 20 km (Fig. 7 b), the Sichuan Basin and the Cathaysia Block shows high-velocity structures (Fig. 7 b, HV4 and HV5), whereas the Songpan-Garzi Block, the SYSB and the Yangtze Block are characterized by low-velocity structure (Fig. 7 b, LV2). Chen et al. (2014) revealed a low-velocity anomaly using the ambient noise adjoint tomography, which is similar to our LV2. At the depth of the 30 km-46 km, the Sichuan Basin and the Cathaysia Block shows high-velocity structures (Fig. 7 c and d, HV4 and HV5), whereas the Songpan-Garzi Block, the SYDB and the Yangtze Block have low-velocity structure (Fig. 7 c and d, LV2).
Fig. 8. Vertical cross sections of S-wave velocity. The profile locations are in Fig. 1. SGB: Songpan-Ganzi Block, SB: Sichuan Basin, SYDB: Sichuan-Yunnan diamond-shaped block, YB: Yangtze Block, CB: Cathaysia Block.
We also analysed 6 profiles of S-wave velocity (Fig. 8). The Sichuan Basin (Fig. 8 a and b, HV5) and Cathaysia Block (Fig. 8 d and f, HV4) show high-velocity anomalies at the middle and lower crust. The low-velocity anomaly (LV2) mainly exists in the western part of the study region (Fig. 8 a-f) and extend to the eastern part locally, or beneath the Yangtze Block (Fig. 8 c, d and f). The seismic activities are mainly distributed at the high-velocity region (Fig. 8 b and d) or the boundary region of the low- and high-velocity (Fig. 8 a, c, e and f).