Shallow shear wave velocity structure and azimuthal anisotropy in Yibin，Sichuan

Based on the 44-day continuous waveform data recorded by 30 temporary stations in Yibin and its adjacent regions, Rayleigh wave phase velocity dispersion curves in the period range 1−15 s are extracted by using the ambient noise cross-correlation method. Then the direct surface wave tomographic method with period-dependent ray tracing is applied to obtain the 3-D shear-wave velocity structure. Finally, by taking the inverted shear-wave velocity model as the initial model, we obtained the azimuthal anisotropic structure in the same inversion framework. The results show that the distribution of shallow shear-wave velocity is heterogeneous, and the degree of heterogeneity decreases with the depth. At about 2 km depth, high velocity is distributed beneath the Junlian-Gaoxian-Gongxian-Changning anticline, which may be related to the folding, uplift, weathering and denudation of the strata in this area. The northwest of Shizitan-Shuanghechang-Changning anticline shows relatively low velocity compared with the southeast with a depth deeper than 5 km. As we known, the medium with relatively low wave velocity are usually weaker compared with high velocity, so it may be one of the reasons for the northwestward rupture of the Changning earthquake sequence in 2019. Our results of azimuthal anisotropy show that near the Kongtan anticline, the fast wave direction is approximately parallel to the strike of the anticline, and the anisotropic strength becomes weaker with the increase of depth.We also found that the fast wave direction gradually converges to the north and spreads to the south, showing a brush-type structure. Under the influence of regional geological structure and stress field, the fast wave directions in some local areas show complex characteristics at different depths.