High-Resolution Velocity and Seismic Anisotropy Structures of the 2021 Ms 6.0 Luxian Earthquake Zone in Sichuan Basin

Abstract

Abstract The Ms 6.0 Luxian earthquake, which occurred in a shallow sedimentary cover on the southern margin of the Sichuan basin in China, stands as the most powerful earthquake ever recorded in this region. This study aims to integrate multiple seismological methods to comprehensively investigate the seismic nucleation environment. Using data from 91 densely distributed seismic stations within the Luxian earthquake zone, we constructed models for velocity, anisotropy, and interfaces. Our results suggest the presence of a detachment interface at depths of ∼4.0–5.0 km, which appears to function as a stress-decoupling layer. This is evidenced by a noticeable shift in the intensity and orientation of azimuthal anisotropy, transitioning from weak to strong, and altering its alignment from a northwest–southeast to a northeast–southwest orientation. The mainshock and aftershocks are predominantly clustered along the boundaries characterized by high- and low-velocity zones, as well as the boundary of VP/VS ratios beneath the detached layer. This suggests the likely existence of a pre-existing northwest–southeast-striking fault with a southwest dip, extending from the underlying basement to the overlying sedimentary cover. The radial anisotropy analysis reveals a predominance of negative values beneath the Huaying Mountain fault, whereas positive values are prominent in the Yujisi sedimentary syncline. This distinctive pattern implies tectonic movements related to fault activities within the fault zone. Based on our findings and previous research, we speculate that the Ms 6.0 Luxian earthquake may have been influenced by local stress fields and triggered by industrial activities.