Based on the assumption that major class of rock instabilities are
created by discontinuities, a method is proposed to estimate the
fracture density by means of a digital elevation model (DEM). By
using the mean orientation, the mean spacing and the mean trace length
of discontinuity sets potentially involved in slope instabilities
and a DEM, it is possible to calculate the mean number of discontinuities
of a given set per cell of the DEM. This would allow for an estimation
of the probability of the presence of at least one discontinuity
in a given area or simply in a topographic cell of the DEM. This
analysis highlights sites potentially affected by rockslides within
a region. Depending on the available data, the mean number can be
calculated either by area, or along a line parallel to the mean apparent
spacing. The effective use of the probability of occurrence is dependent
on the size of the discontinuities because short and closely spaced
discontinuities will have a 100% probability of occurrence in each
favorable location. The a posteriori prediction of a recent rockslide
is discussed as an example.