Research on the Spatial Structure Characteristics and Formation Mechanism of the Deda Ancient Landslide Based on Microtremor Survey Method in Sichuan Province, China

The topography and geological structure of the eastern Tibetan Plateau are extremely complex. The high development density and intricate spatial structures of large ancient landslides in alpine canyon areas, such as those in the Jinsha River Basin. The hazards resulting from the creep deformation and resurgence of these ancient landslides are particularly severe. The Deda ancient landslide, situated in Deda Township, Batang County, Sichuan Province, is a significant ancient landslide influenced by the Chalong-ranbu fault activity. The spatial structural characteristics of this ancient landslide are complex, with localized resurgence deformation observed at the landslide front. This study employs various methods, including remote sensing interpretation, on-site investigations, Microtremor Survey Method (MSM), and engineering geological drilling, to further elucidate the spatial structural characteristics of the Deda ancient landslide. The research reveals that the Deda ancient landslide can be divided into three parts in plan view: Deda I landslide (I), Deda II landslide (II), and the rear wall of the Deda ancient landslide (Ⅲ). Through MSM combined with drilling verification, the study proposes a scheme for classifying the shear wave velocities of shallow and deep sliding zones, with a relative error in sliding zone depth identification generally ranging from 2.6% to 4.8%. The research uncovers that Deda I landslide features two sliding zones, with a burial depth of 18.7-20.1 m for the shallow sliding zone (S1) and 36.2-49.9 m for the deep sliding zone (S2). And the volume of Deda I landslide is approximately 8.7-12.0×10⁶ m³. Deda II landslide has one sliding zone, with a burial depth ranging from 25.2 to 38.6 m and a landslide volume of approximately 6.3-9.6×10⁶ m³. A comprehensive analysis suggests that the formation of the Deda ancient landslide is the result of various factors, including fault structures, rainfall infiltration, and river erosion. The complex landslide structure and its genesis are identified as the primary controlling factors for the landslide being in a state of creep deformation. The research methods and insights presented in this study can serve as a reference for the spatial identification and risk prevention of large ancient landslides in the eastern Tibetan Plateau.