Upper crustal azimuthal anisotropy and seismogenic tectonics of the Hefei segment of the Tan-Lu fault zone from ambient noise tomography

The Tan-Lu fault zone is a large NNE-trending fault zone, which has a significant effect on the development and earthquake disaster prevention of eastern China. Aiming at the azimuthal anisotropy structure in the upper crust and seismogenic tectonics in the Hefei segment of this fault, we collected phase velocity dispersion data of fundamental mode Rayleigh waves from ambient noise cross-correlation functions of ~400 temporal seismographs in the area about 80 km×70 km along the fault zone. The period band of the dispersion data is 0.5~10 s. We inverted for the upper crustal 3-D shear velocity model with azimuthal anisotropy from surface to 10 km depth by using a 3-D direct azimuthal anisotropy inversion method. The inversion result shows the spatial distribution characteristics of the tectonic units in the upper crust. Meanwhile, the deformation of Tan-Lu fault zone and its conjugated fault systems can be inferred from the anisotropy model. Especially, the faults that kept active from early and middle Pleistocene, control the anisotropic characteristics of the upper crustal structure in this area. The direction of fast axes near the fault zone area in the upper crust is consistent with the strike of the faults. While for the region far away from the fault zone, the direction of fast axes is consistent with the direction of regional principal stress caused by plate movement. Combined with the azimuthal anisotropy models in the deep crust and uppermost mantle from surface wave and Pn wave, the different anisotropic patterns caused by the Tan-Lu fault zone and its conjugated fault system nearby are shown in the upper and lower crust. Furthermore, by using the double-difference method, we relocated the Lujiang earthquake series, which contain 32 earthquakes with the depth shallow than 10 km. The result shows these earthquakes are mainly located in the transition zone between high and low velocities. Together with the tectonic deformation characteristics from azimuthal anisotropy results, we constructed a seismogenic model for these upper crustal earthquakes in this area.