A data set was constructed to analyze regional landslide-susceptibility in San Mateo County, California. The data set consists of 2,639 cells spaced at 2,100 ft intervals. Six input variables were used to define the characteristics of each cell. Information was compiled from maps that had been published by others pertaining to slope angle, rock type, rainfall, soil type, vegetation type and the distribution of landslide deposits. A digitizing tablet was used to input and store the data in digital form on a computer.
The geologic units were separated into four categories of relative susceptibility on the basis of a statistical analysis of the data: highly susceptible, susceptible, moderately susceptible, and least susceptible. The Purisima Formation, Monterey Shale, Mindego volcanics, San Lorenzo Formation and Tertiary intrusives were designated as highly susceptible.
The distribution of landslides was found to be sensitive to slope angle only up to a slope of about 15 percent (9 degrees). Other factors appear to be more significant in controlling landslide distribution at steeper slope angles.
Susceptibility to slope failures was found to be directly related to the mean annual rainfall. This can be explained theoretically by the relationship between pore-water pressure and soil shear strength.
Soil type and vegetation type were found to be secondary factors in determining the distribution of slope failures. This may reflect the fact that only landslides with long dimension in excess of 500 ft were considered in this study.
The methodologies of Brabb and others (1972), and Radbruch and Crowther (1973) used to create regional susceptibility maps were found to be consistent with the statistical analyses performed in this study.