Initiation mechanism and movement characteristics of the catastrophic Caotan landslide triggered by the Ms 6.2 Jishishan earthquake on 18 December 2023 in Gansu, northwestern China

Background On 18 December 2023 (CST), an Ms 6.2 earthquake struck Jishishan County, Gansu Province, northwestern China. The catastrophic Caotan loess landslide–mudflow, located approximately 14.8 km from the epicenter, was triggered simultaneously. Because the landslide caused extensive damage, affected a large area, and resulted in substantial casualties, we conducted a detailed investigation comprising field surveys, UAV surveys, borehole drilling, trench observations, and in situ tests. We analyzed the topography, geological setting, hydrological conditions, and geometry of the landslide. Results Strong seismic shaking initiated slope failure in the source area. The displaced mass rapidly transitioned into a mudflow and traveled down the Qijia Valley, producing an unusually long runout over gentle terrain. We discuss the probable initiation mechanism, the transition process from sliding to flow, and the movement characteristics using evidence from the field investigation. Conclusions We identify the principal factors controlling the initiation and long-runout movement of the Caotan landslide. These findings provide insight for mitigating similar hazards in northwestern China and other loess-covered regions worldwide.