The Tanlu fault is the largest strike-slip fault in eastern China. It has experienced multiple episodes of tectonic activity suggested mostly by near-surface geological analysis. However, little evidence comes from deformation at different depths in the crust. This study presents a three-dimensional (3-D) high-resolution upper crustal azimuthally anisotropic model of shear-wave velocity beneath the Chao Lake segment of the Tanlu fault zone (TFZ-CL) using surface wave dispersion data (0.5–12 s) extracted from ambient seismic noise. The results show that fast directions of azimuthal anisotropy are consistent with local geological units, hence interpreted as shape-preferred orientations (SPOs). A special depth-dependent anisotropic pattern is revealed beneath eastern Chao Lake, where the fast directions vary from nearly NE-SW at shallow depths, to NNE-SSW or nearly N-S at intermediate depths and back to NE-SW at great depths. The anisotropic model is combined with other previously published datasets to construct a deformation model at multiple depths. The deformation model suggests that lithospheric deformation is decoupled from the asthenosphere beneath the TFZ-CL, implying that the deep tectonic setting could have changed dramatically since the Mesozoic. By connecting the new anisotropic model and the local tectonic episodes, we further propose a four-stage tectonic evolution model of the TFZ-CL. The model can well interpret the generation of anisotropies, especially the specific anisotropic pattern beneath eastern Chao Lake, providing new evidence of deformation at great depths to support the multistage tectonic episodes along the Tanlu fault.

The approach of direct inversion of surface wave dispersion data to obtain shallow earth structure becomes popular compared with the traditional two-step surface wave tomographic method. However, some fundamental problems still exist on selecting proper grids to parameterize model. In this study, we apply model parameterization with a multiplegrid scheme to the direct inversion method. A multiple-grid scheme represents a combination of several collocated grids, such as staggered grids, multiscale grids and multiscale-staggered grids. At each iteration, direct inversion is applied to each individual collocated grid to invert for S-wave velocity perturbations that is then interpolated to a predefined base grid using a 3-D B-spline interpolation method. At the end of each iteration, the S-wave velocity perturbations of all collocated grids are averaged together. The averaged perturbations are used to update the 3-D S-wave velocity model that is then used as an initial model for the next iteration. The efficiency of the multiple-grid direct inversion method is demonstrated by applying it to a dense array in Chaohu lake, a special area with the Tanlu fault zone (TFZ) passed through. Synthetic examples confirm a better performance of the multiple-grid direct inversion method than the traditional one. A detailed shallow 3-D S-wave velocity model of the Chaohu lake is obtained after applying the multiple-grid direct inversion method to real data. The velocity model reveals a staggered pattern of velocity variations with low-velocity anomalies in the Hefei basin and eastern Chaohu lake and prominent deep rooted and high-velocity anomalies beneath the TFZ and Yinping mountain, which may associated with the multi-stage tectonic activities in the Tanlu fault. The multiple-grid scheme of this study can further be applied to other tomographic approaches in the near future.