High-resolution 3D shallow crustal structure of Urumqi foreland thrust tectonic belt, northern Tianshan, based on ambient noise tomography

The Urumqi foreland thrust tectonic belt exhibits complex geological structures and strong seismicity. Imaging its shallow crustal structure is of great significance for understanding its tectonic mechanism and seismogenic environment. We obtained a high-resolution S-wave velocity model of the shallow crust at depths of 0–8 km using ambient noise tomography applied to data from a dense seismic array. Sediments are generally thinner in the southeast and thicker in the northwest, with a maximum thickness of more than 8 km. Variations in the velocity structure near the Xishan, Wanyaogou, and Yamalike faults indicate that their formation was related to differences in the physical properties on either side of the fault. In addition, the faults exhibit thrusting of the low-velocity sides towards the high-velocity sides. In the study area, earthquakes rarely occur at depths of less than 3 km and are mostly concentrated in the high-velocity zone in the southern part. Below 3 km depth, more earthquakes were observed, mainly distributed near faults or in relatively high-velocity areas in the southern part. This suggests that high-velocity structures are more prone to stress accumulation, resulting in earthquakes. At 6–8 km depth, the densely distributed earthquakes in the northwestern part of the Bogda mountains are well-aligned with the northwest-oriented low-velocity zone observed in this study, suggesting that this weak zone likely controls seismicity in this area.