Crustal Azimuthal Anisotropy in Eastern North China Derived from Ambient Noise Tomography

Abstract Investigating the crustal anisotropy features of eastern North China is of great significance for understanding the tectonic deformation mechanisms and the shallow response to the destruction of the North China Craton. We apply ambient noise tomography to determine the Rayleigh-wave phase velocity and azimuthal anisotropy at 8–35 s periods in eastern North China. The results show significant heterogeneity in crustal velocity and anisotropy. The short-period velocity and anisotropy are primarily related to surface structures. The pronounced anisotropy observed between the basin and uplifts, with the fast axis aligning parallel to the boundary strike, is likely attributed to the velocity and topographic contrasts. The fast axis orientation within the Bohai Bay basin exhibits northwest–southeast to north-northwest–south-southeast directions in the northern part and nearly east–west direction in the southern part, showing different deformation patterns within the basin. Strong anisotropy and sharp changes in fast axis orientation are observed in focal areas of three major earthquakes, suggesting a correlation between the genesis of strong earthquakes and the anisotropy features. The fast axis in the Yanshan Mountains aligns nearly east–west in the west to east-northeast–west-southwest in the east, likely related to the extensional tectonic environment in North China. At longer periods, the anisotropy strength decreases and fast axis orientation changes, indicating that the northwestern part of the Yanshan Mountains is influenced by the residual magmatism beneath the Quaternary volcanoes. The fast axis along the Tanlu fault zone (TLFZ) is generally oriented northeast–southwest at all periods, consistent with the fault’s strike. The anisotropy features provide evidence for the TLFZ cutting through the crust and uppermost mantle. The fault activity is currently dominated by strike-slip motion. The inconsistent anisotropy patterns in the crust and upper mantle suggest that the deformations in the crust and mantle are possibly decoupled.