How landslides happen without heavy rainfall: Early strong runoff coincides with weak geomaterials

Intense rainfall is widely recognized as the primary trigger for landslides and is commonly used in early warning systems. Yet, many unexpected failures occur during periods of little or even no rainfall, leading to ineffective warnings and failed evacuations. The mechanisms by which landslides occur without heavy rainfall remain unexplored. Integrating field investigations, machine learning, remote sensing, and numerical simulations, we found that landslide disasters without heavy rainfall are widespread: 75.7% of 1118 catastrophic cases exhibited delayed onset, controlled by antecedent rainfall, the topographic wetness index, and landslide scale. For the first time, we identified three runoff-supply patterns (slope, gully, and creek supply) and two migration stages (surface and subsurface) that together govern strong runoff supply in confluence zones. Our work contributes an innovative perspective on the coupling of early strong runoff and weak geomaterials that triggers delayed failures, with three subsurface runoff stages-interflow, sliding-face flow, and return flow. These results clarify the hydro-geomaterial coupling behind delayed landslides and support improved early warning and prediction to reduce risk.