High-resolution shallow structures of the Red River fault zone in the southeastern Tibetan Plateau of China revealed by multiple seismic dense linear arrays

The complex Red River fault zone (RRFZ) which is situated in the southwestern region of China and separates the Indochina plate and South China blocks has diverse seismic activities in different segments. To reveal the detailed geometric characteristics of the RRFZ at different sections and to better understand the seismogenic environment, we deployed 7 seismic dense linear arrays consisting of 574 nodal stations across the RRFZ in the northern and southern segments near the towns Midu, Gasa, Zhega, Dazhai, Xinzhai, and Taoyuan in 2022 and 2023. The linear arrays which extend from 2.4 to 12.5 km in length with station intervals ranging between 40 and 140 m recorded seismic ambient noise for approximately one month. Using the extended range phase shift method, we extract the phase velocity dispersion curves of the Rayleigh waves between 0.9 and 10 Hz, which are then used to invert for the high resolution shear-wave velocity structures across the RRFZ under the linear arrays. The key findings are: (1) the 7 imaged sections of the RRFZ exhibit quite similar structures with higher velocities on the SW side and lower velocities on the NE side and the velocity variation is consistent with the surface geological structures along the RRFZ; (2) the shear-wave velocities on the SW side of the RRFZ at the northern Midu section and southern Gasa-Dazhai sections are generally higher than their counterparts in the southern Xinzhai-Taoyuan sections, which reflects lithological variations from the marble-dominated Paleoproterozoic Along basement to the gneiss dominated Paleoproterozoic Qingshuihe basement; (3) from the northern Midu section to the southern region where the RRFZ intersects with the Xiaojiang Fault, the major faults of the RRFZ exhibit a consistent high-angle, NE-dipping structure; (4) the low shear-wave velocities immediately to the NE of the velocity boundary may indicate a faulted zone due to long term shearing, where excessive amplifications of ground motions could occur. This study provides new insights into the characteristics of the shallow structures of the RRFZ.