Abstract：In order to quantitatively assess landslide hazard of a large mountainous area，a slope-unit-based probabilistic methodology that involves a three-dimensional(3D) deterministic model coupled with a geographical information system(GIS) is proposed. An automatic method is developed to divide a study area to slope units. For each slope unit，a GIS-based 3D slope stability analysis model is applied to the calculation of the distribution of 3D factor of safety through a Monte Carlo simulation for the unknown shape of slip surface. Assuming the resultant 3D factor of safety follows a log-normal distribution，a widely-used reliability index is then used to quantitatively evaluate the probability of unsatisfactory performance of each slope unit. At the same time，the location and the magnitude of critical slip surface(s) can also be identified by minimizing the trial 3D factor of safety. An advanced GIS is developed to efficiently implement all of the computational procedures. This method integrates the advantages of both 3D deterministic model and probabilistic method，which can quickly and efficiently complete a large number of 3D modeling and calculations of factors of safety，realizing the quantitative assessment of a natural mountainous area with complexly topographical and geological conditions. The effectiveness of the proposed method has been tested in a practical application to a landslide-prone road slope area in Japan.
