Field observations indicate that failure of intact rock is associated with a rupture surface or shear band, where deformation is concentrated in a narrow zone; displacements occur with decreasing stress within the shear band, while outside the band the material remains more or less intact. When localization of deformation occurs within a rock structure, the global load-displacement behavior appears as a softening response. Therefore, understanding the initiation and propagation of the shear band is of fundamental concern, as this mechanism is associated with phenomena such as rock bursting and progressive failure. The study of localized failure under controlled conditions can be accomplished within a laboratory setting with the University of Minnesota Plane-Strain Compression Apparatus. An acoustic emission system was used to monitor the softening response of a sandstone in plane-strain compression. The recorded waveforms were analyzed for event location and source characterization. The complicated deconvolution process was eliminated by using a simplified calibration procedure. The moment tensors of the events associated with softening were evaluated, and the source mechanisms were determined. The sources of the post-peak events were characterized as being caused predominantly by slip in the direction of the global shear band. The displacement of each acoustic emission was estimated from the moment tensor by assuming a constant strain drop of 10-4 over the slip area. Within a certain post-peak regime where 110 events were captured, the AE displacement was 0.011 mm, while globally 0.033 mm was measured.