Shallow characteristics of Chenghai Fault Zone, Yunnan, China, from ambient noise tomography and horizontal-to-vertical spectral ratio with two dense linear arrays

High-resolution imaging of the shallow structure of a fault zone contributes valuable information about regional earthquake hazard and tectonic evolution. The Chenghai Fault (CHF), located in the eastern margin of the collision zone between the Indian and Eurasia plates, has accumulated significant shear strain and has hosted several strong earthquakes, but our understanding of its shallow structure remains limited. Therefore, in this study, we deploy ~4.3 km and ~ 7 km long dense linear arrays across the Qina and Pianjiao segments of the CHF from October 24 to November 29, 2020. The shallow structure is imaged using ambient noise tomography, which identifies distinct low-velocity zones (LVZs) beneath the Qina and Pianjiao segments, with widths of ~2.0 km and ~ 3.3 km and depths of ~0.7 km and ~ 0.9 km, respectively. These LVZs exhibit a shear wave velocity contrast of 25%–50% relative to the surrounding regions. We also use the horizontal-to-vertical spectral ratio (HVSR) method to constrain the subsurface structure, and the geometry and thickness of the LVZs match well with the interface from the HVSR analysis. The distribution of the high- and low-velocity features is consistent with the regional geology. The fault damage zone is usually on the order of ~100 m, which is smaller than our result (width of 2–3 km). We interpret the LVZs as sedimentary basins controlled by long-term fault movements and evolution processes rather than fault damage zones. Moreover, the LVZs exhibit notable variations along the strike of the CHF, indicating that the CHF is the factor controlling the development and evolution of the sedimentary basin in this region.