Formation of a multi-translational reactivated ancient landslide in the Three Gorges Reservoir, China

Abstract. The fluctuation of water levels and seasonal rainfall in a reservoir may induce various types of slope movements. Some of these movements are new, whereas others are old but reactivated. The primary aim of this study is to investigate the formation mechanism and process, deposit characteristics, and identification signs of a giant multi-translational reactivated ancient landslide in the Three Gorges Reservoir region based on field observations, on-site surveys, and Electron Spin Resonance experiments. The Outang landslide, located at the south bank of the Yangtze River, has a total volume of approximately 90 million m³ and can been divided into three independent subzones with an apparent age of 120–130 ka (ka represents a thousand years) for subzone O1, 65–68 ka for subzone O2, and 47–49 ka for subzone O3. The features of mobilized material structure and slip surface morphology in each subzone are similar and are in the form of a spoon. A conceptual model, including sliding, bending, suspending, and accumulating, is deduced to explain the formation mechanism and evolutionary process of this instability. Three types of evidences are proposed to recognize the ancient landslide. Currently, landslide stability is obscure based on the significant landslide movement and reactivated features; more attention and long-term monitoring is necessary in the future.