Shallow S‐Wave Velocity Structure and Azimuthal Anisotropy Beneath the Qujiang‐Xiaojiang Fault Junction in Tonghai, Yunnan Province, China

Abstract

The Xiaojiang Fault (XJF) forms the eastern boundary of the Sichuan‐Yunnan rhombic block. Although its central and northern segments have hosted M  ≥ 7 earthquakes along the main trace, large earthquakes near the southern segment mainly occurred on the intersecting Qujiang Fault (QJF) and Shiping‐Jianshui Fault. The structural controls on this deformation transfer remain unclear. We investigated the QJF‐XJF junction using continuous ambient‐noise records from 229 short‐period stations deployed in 2023. Rayleigh‐ and Love‐wave dispersion measurements were directly inverted for 3‐D S‐wave velocity and azimuthal anisotropy to 6 km depth. Shallow velocity variations broadly follow surface geological units. A pronounced velocity gradient across the QJF reflects lithological contrasts, whereas a low‐velocity zone along the western branch of the southern XJF likely indicates fracture damage and material weakening. Integrated with GPS velocities and geomorphological evidence, the anisotropy pattern indicates segmented shallow deformation: the western XJF branch appears relatively less active, whereas the QJF plays a more dominant role in accommodating regional deformation. The 1970 Ms 7.7 Tonghai earthquake source region contains an upper‐crustal high‐velocity anomaly adjacent to a sharp velocity‐gradient zone. We infer that the distributed geometry and weakened materials of the southern XJF disperse deformation among multiple branches, whereas the greater structural continuity of the QJF and strong cross‐fault structural contrast favor elastic‐strain accumulation and larger ruptures. Our results constrain shallow strain transfer from the southern XJF to intersecting faults and its role in large‐earthquake nucleation and rupture.