A dense seismological profile with 45 stations at a spacing of 5–10 km across the Longmenshan fault zone was deployed from 2018 to 2020. It complemented an array of 80 stations deployed in the region from 2012 to 2013. Using data from both deployments, dispersion curves were obtained from ambient noise Green’s functions. Subsequently, a 3D shear (S) velocity model was derived using tomographic methods. Combining this S-wave velocity model with an existing 3D compressional velocity model, a 3D Poisson’s ratio model was derived. These models show that the Precambrian Pengguan complex, which cores this part of the Longmenshan, is approximately 20-km thick and displays high velocities and low Poisson’s ratios. The result of this study is that, along the dense profile, the complex is underlain between 20 and 40 km depth by low S velocities, which likely extend out beneath the Sichuan Basin in a 10-km thick basal crustal layer. These low S velocity zones have high Poisson’s ratios and are best explained as a result of the compressive forces and push exerted by the Tibetan Plateau, causing processes such as ductile shearing and fluid/melt infiltration to occur.