Seismic ambient noise tomography for shallow velocity structures beneath Weiyuan, Sichuan

Seismic shear wave velocity is an important parameter for site characterization and strong ground motion modeling. Reconstruction of the shallow 3D seismic structures based on Ambient Noise Tomography (ANT) provides key information to the variable shallow seismic structures, the exploitation of shale gas and earthquake hazard reduction and mitigation. In this study, we perform interstation cross-correlations of vertical-component continuous waveform recorded by 50 temporary stations deployed in Weiyuan, Sichuan Province from November, 2015 to February, 2016. We then measure phase-velocity dispersion curves of fundamental-mode Rayleigh waves in the period band of 2 similar to 6 s and invert 3D shallow seismic shear-velocity structures within 5 km. Our results show that shear velocities within 2 km are relatively lower at about 2. 0 similar to 2.7 km.s(-1). Shear velocities at the depth of 2.0 similar to 5.0 km show some spatial pattern that they are faster at the northwestern of the study region (over 2.7 km.s(-1)) and slower beneath the southeastern part (2. 3 similar to 2.8 km.s(-1)), which might be related to the Weiyuan anticline and the basin sediment respectively. The cross-sections show that shear velocities generally increase with depth. And the January 7, 2016 M-L 3. 9 Weiyuan earthquake occurred near the bottom of a drilling platform where there might be the boundary between the sedimentary basin and the granite basement, which indicates the possible connection between them.