Direct observations are presented of the micromechanical events that contribute to the localization of deformation within brittle compressive shear faults. The observations were made on ice and show that faults are composed of both wing cracks and splay cracks. The latter features initiate from one side of inclined parent cracks and create sets of slender microcolumns fixed on one end and free on the other. It is proposed that the fault-triggering mechanism is the breaking of near-surface microcolumns owing to frictional sliding across their free ends. A lower-bound estimate of the compressive strength of ice is found to be in order of magnitude agreement with experiment.