Bedrock landslides in mountainous regions may be triggered by either storms or earthquakes; the dominant mechanism in a region affects both landscape evolution and landslide hazard. We describe a simple observational test to distinguish between storm and earthquake triggers based on a probabilistic measure of hillslope morphology. In areas that are dominated by storm-triggered landslides, steep topographic slopes are concentrated on the lowermost parts of the hillslopes. Storm triggers act primarily on the hillslope toes, and landslides preferentially remove material from those locations, giving rise to inner gorges. Areas where most landslides are earthquake triggered have more uniform spatial distributions of steep topographic slopes, because coseismic shaking causes failures at both ridge crests and hillslope toes. Earthquake-triggered landslides lead to planar hillslopes and rare or absent inner gorges.