Three-Dimensional Crustal Azimuthal Anisotropy Reveals Multi-Stage Deformation in the Suqian and Zhangbaling Segments of the Tanlu Fault Zone

The Tanlu Fault Zone, extending approximately 2400 km in a NNE orientation, constitutes the most extensive fault system in eastern China and has undergone complex multi-stage tectonic evolution since the Mesozoic. Notwithstanding substantial geological and geophysical investigations, the shallow crustal deformation characteristics across distinct segments and their intrinsic relationship with seismicity distribution remain inadequately constrained. This study presents high-resolution three-dimensional S-wave velocity and azimuthal anisotropy models for the Suqian and Zhangbaling segments, utilizing Rayleigh wave dispersion data extracted from ambient noise recordings acquired by dense portable seismic arrays comprising 238 and 192 short-period seismometers, respectively.

Ambient noise cross-correlation techniques were employed to extract inter-station Rayleigh wave empirical Green's functions, with phase velocity dispersion curves measured across the period range of 0.5 to 8.8 seconds. The DAzimSurfTomo direct inversion method was applied to jointly determine three-dimensional isotropic shear-wave velocity structures and azimuthal anisotropy distributions within the shallow crust at depths of 0 to 8 kilometers. Checkerboard and recovery tests demonstrate that the obtained azimuthal anisotropy models possess reliable resolution capability in regions with adequate ray path coverage.

The inversion results reveal velocity structures corresponding to distinct tectonic units, with high-velocity anomalies associated with uplifted regions and low-velocity anomalies with sedimentary basins. The Suqian segment exhibits systematic depth-dependent variations in fast-wave directions: NNE orientations parallel to the fault strike at shallow depths of 1 to 3 kilometers, transitioning to NE orientations at greater depths of 4 to 8 kilometers, with anisotropic magnitude of 2% to 3%. The Zhangbaling segment displays pronounced anisotropic contrasts between tectonic units, characterized by NE fast-wave directions in the Zhangbaling uplift and NW directions in the Hefei Basin, reflecting fundamentally different deformation characteristics between the North China Plate and South China Plate. Notably, fast-wave directions at the fault-uplift interface exhibit complex depth-dependent variations, progressing from NE (1 to 3 kilometers) to NNE (3 to 6 kilometers) and reverting to NE (6 to 10 kilometers).

Pronounced spatial correlations between anisotropic structure and earthquake distribution indicate that pre-existing crustal fabric exerts primary control on contemporary rupture patterns, with seismicity concentrated at depths of 3 to 5 kilometers where anisotropic transitions occur. The two segments manifest contrasting deformation characteristics: distributed deformation in Suqian versus localized deformation along the sharp anisotropic boundary in Zhangbaling, reflecting distinct evolutionary stages and present-day tectonic regimes. These depth-dependent anisotropic patterns preserve signatures of multi-stage tectonic evolution encompassing early sinistral strike-slip motion, subsequent extensional deformation, and recent compressional tectonics, thereby providing novel constraints on deformation processes and the contemporary stress field within this significant intracontinental fault system.