Upper to middle crustal structure beneath the Longmen Shan fault zone: Implications for the growth of eastern Tibet

•1. Ambient noise tomography with topography correction is used to study the 3D crustal velocity structure.•2. The S-wave velocity structure controls the downward stretch of the Longmen Shan faults.•3. The expansion of the middle-lower crustal low-velocity layer is responsible for the growth of eastern Tibet. The Longmen Shan (LMS) fault zone is located on the eastern margin of the Tibetan Plateau and is a representative area for studying the eastward growth of the plateau. In this study, we present a 3-D crustal S-wave velocity structure beneath the LMS fault zone and its adjacent regions with a lateral resolution of ∼ 10 km down to a depth of 25 km from data recorded at 57 stations over one year using seismic ambient noise imaging techniques. The high and steep topography of the LMS is considered when building the high-resolution image of the upper and middle crust. The imaging provides further evidence for the high-angle listric characteristics of the LMS faults south of Beichuan, and the feature disappears toward the north, indicating differences in deformation from south to north. The structural characteristics are well correlated with the aftershock distribution and the rupture of the Wenchuan earthquake, which indicate a change in the mechanism from mostly thrust in the south to mostly strike slip in the north. The velocity structure controls the downward stretch of the faults. The results support that crustal channel flow is responsible for the growth of eastern Tibet. The eastward movement of the weak crustal material was blocked by the hard crust surrounding the Minshan uplift, resulting in great differences in the velocity structures and fault properties south and north of Beichuan.