Propagation of shear failure may occur in an upslope or downslope direction depending on
the point of maximum strain. Where the slope is undercut or where the interface daylights on
the slope, it is likely that shear failure will propagate upslope due to unloading. Where the
slope is in tension, usually marked by curvi-linear tension cracks defining the potential
headscarp, shear failure will propagate downslope due to loading. Bulging of the landslide toe
is a characteristic of the latter mechanism which marks the location where the shear surface
ruptures to the ground surface. In both cases, when 50% or more of the shear surface has
developed, rapid failure is likely to develop given a favourable pore water pressure regime.