Mitigating rainfall-induced landslides requires an understanding of the failure mechanisms leading to their initiation. Evaluating how predicted failure mechanisms relate to the landscape’s topography, geomorphology, antecedent hydrological conditions, and other subsurface characteristics such as shear strengths and hydraulic properties is essential for identifying the factors that control landslide occurrence across the landscape. Most existing regional mechanistic models estimate outputs such as landslide area density or characteristics of individual landslides, but, to our knowledge, they do not distinguish between the underlying failure mechanisms that trigger these landslides. The objective of this study is to analyze the impact of various factors on the occurrence of different failure mechanisms across the topography. We present a novel mechanistic regional model that integrates a pseudo-three-dimensional slope stability approach with the Parallel Integrated Hydrologic Model (ParFlow). The model, named CRISIS, predicts regional rainfall-induced landslides and classifies them into two distinct failure mechanisms: (1) a bottom-up saturation-induced failure caused by a rise of the groundwater table; and (2) top-down wetting-induced failure driven by either a downward propagating wetting front or the formation of a perched water table above less permeable subsurface layers. CRISIS was applied to a watershed in Utuado, Puerto Rico, that was affected by Hurricane Maria in 2017. It was first run with near-realistic conditions to provide best-estimate predictions of the observed landslides. It was then used in a series of hypothetical simulations with varied subsurface properties to investigate the conditions controlling different failure mechanisms. Results show that failure mechanisms were strongly controlled by geomorphic position and initial groundwater conditions. Bottom-up failures occurred mainly on steep hillslopes near valley bottoms, where initial groundwater tables were shallow, while top-down failures dominated steep slopes near ridge tops, where initial groundwater tables were deeper. Failure depth was primarily controlled by subsurface cohesion; shallower landslides occurred at lower cohesion values. Subsurface material type, defined by both shear strength and hydraulic properties, significantly affected the frequency, mechanism, and timing of failures. Notably, a sandy subsurface triggered the highest number of failures, all initiated by the bottom-up saturation-induced mechanism within a short timeframe.
