Figure 2 shows the surface volumetric strains measured at the sample surface around the central part of the sample. An X-ray CT image of a horizontal cross section through the specimen at the strain gauge locations, obtained after the experiment, is superimposed. To show the deformation features, the volumetric strains are plotted at each of the indicated loading times. Strains measured until just before the rapid increase in AE activity are plotted. Therefore, this strain diagram shows the deformation from the start of the experiment to just before the volumetric increase due to dilatancy. During the loading, the volumetric strain at location S3, where the fracture plane eventually appeared at the sample surface, decreased more than the strain at the other locations. Although the AE activity increased just before the fracture, the strain was inhomogeneous in the early stage of loading. In symmetric compression tests of a homogeneous rock sample, microfractures in the sample are distributed homogeneously in sections perpendicular to the loading axis until the axial load reaches its peak value (e.g., Zang et al. 1998). In contrast, strain in this sample that included a healed plane became localized at the very early stage of loading. This result suggests that strength recovery is not uniform along the entire length of the fault, and at least some parts of the healed fault may be weaker than the host rock, leading to strain and AE localization during the early stage of loading.
AEs occurred late in the loading period (after 12,500 s) with the increase in the applied differential stress, and sample failure occurred abruptly at 13,740 s (Fig. 3a). The main fracture surface formed nearly parallel to the plane of the existing healed fracture. AE activity was concentrated around the fault plane (Fig. 3b) until 20 s before failure, when AEs became distributed throughout the sample (Fig. 3c). As the sample approached failure, acoustic activity occurred first along the future fault surface between 12,500 and 13,000 s (Fig. 3d), then in a wider region around the future fault zone (Fig. 3e), and finally in a zone extending farther along the future fault plane (Fig. 3f). Early in the initial period of AE activity (12,600–12,900 s), AEs began near the lower left corner of the vertical cross section selected for study (Fig. 3g). The upward propagation of AE activity was apparently accompanied by on and off AE activity in the lower left. The temporal variation in the AE source locations in the direction normal to the cross section was random, and they showed no systematic movement. The apparent upward propagation speed of AE activity along the fault plane on the cross section, estimated from the movement of the averaged positions for each time period corresponding to Fig. 3g-i was about 0.2 mm/s.