The mechanics of brittle failure is a well-described multi-scale process that involves a rapid transition from distributed microcracks to localisation along a single macroscopic rupture plane. However, considerable uncertainty exists regarding both the length scale at which this transition occurs and the underlying causes that prompt this shift from a distributed to a localised assemblage of cracks or fractures. For the first time, we used an image analysis tool developed to investigate orientation changes at different scales in images of fracture patterns in faulted materials, based on a two-dimensional continuous wavelet analysis. We detected the abrupt change in the fracture pattern from distributed tensile microcracks to localised shear failure in a fracture network produced by triaxial deformation of a sandstone core plug. The presented method will contribute to our ability of unravelling the physical processes at the base of catastrophic rock failure, including the nucleation of earthquakes, landslides and volcanic eruptions.