High-Resolution 3D Shallow Crustal Shear-Wave Velocity and Azimuthal Anisotropy Structures in the Middle Segment of Longriba Fault Zone, Eastern Tibetan Plateau

Abstract

The Longriba fault zone (LRBFZ) represents a key tectonic transition zone where the eastward extrusion of the eastern Tibetan plateau is obstructed by the rigid Yangtze block, making it crucial for understanding regional crustal deformation and seismic hazard. However, the lack of dense seismic arrays has prevented previous studies from resolving its shallow, high-resolution velocity structure. In this study, we deployed 309 short-period nodal seismometers across the middle segment of the fault zone from April to May 2024. Using ambient noise tomography, we constructed a high-resolution 3D anisotropic model of the shallow crust (0–12 km), including both isotropic shear-wave velocity (VS) and azimuthal anisotropy. Our isotropic VS model reveals (1) a pronounced low- to high-velocity contrast across the fault zone in a southeastward direction that is consistent with regional geological outcrops and (2) an enhanced low-velocity anomaly near the fault linkage zone of LRBFZ and the Aba South fault (ASF), likely indicating intense rock fracturing and fluid enrichment. Azimuthal anisotropy further indicates that fast velocity directions are generally northwest-west–southeast-east (NWW-SEE) oriented, in agreement with the regional stress field driven by the eastward extrusion, but exhibit local rotations in the fault linkage zone caused by stress redistribution along ASF. Our study provides new constraints on the structural heterogeneity and deformation processes governing seismic behavior and fault zone dynamics of the LRBFZ.