## 4 Results  
Figure 4 shows key parameters as functions of time and a zoomed view of the final two stages. Figure 5 shows a section image of the three-dimensional (3D) X-ray computed tomography (CT) scan performed after the test, the 3D fracture surface, and stereo plots of well-determined AE hypocenters determined by at least 12 arrival times for each event. The CT images showed that the involved fracture had a rough surface. Acoustic emission hypocenters were clustered on the fracture surface within a narrow offset depth. As shown in Figure 4, AE activity was initiated at a very low stress level. Many events were observed during the hydrostatic loading stage. Such features were different for similar intact rocks, in which AE activity was initiated at a stress level approximately 60% of the strength [20]. Thus, the pre-existing immature fault exhibited a very low stress threshold for producing AE activity. In total, more than 100,000 AE events were detected by the peak detectors. A total of 64,100 events were recorded with waveforms, and 42,900 events were located by at least eight arrival times. The evolutions of AE rate, \(b\)-value, and fractal dimension of AE activity, together with rock deformation, demonstrated three pre-failure phases or stages, referred to hereinafter as stages I, II, and III, and a dynamic-aftershock stage, i.e., stage IV. The second stage can be further divided into two substages, stages II-1 and II-2. Figure 6 shows normalized density maps of AE hypocenters (determined by at least 12 arrival times) on the fault surface for the pre-failure phases, stages I, II-1, II-2, and III. Figure 3: Examples of (**a**) frequency-magnitude distribution and (**b**) and correlation function for selected data points A–F. Figure 4: Plots show (**a**) key parameters with respect to time and (**b**) close-up plots of the final two stages. Acoustic emission rates were calculated for every 10 s. The \(b\)-values were estimated using the maximum likelihood method for a running window of \(n=2000\) events with a step of 500 events. The fractal dimension (\(D_{2}\)) was estimated for a running window and a step of \(n=1000\) and 500 hypocenters. Axial, circumferential, and volumetric strains (\(\varepsilon_{a}\), \(\varepsilon_{c}\), and \(\varepsilon_{b}\)) were obtained as the average of data recorded by seven pairs of stain gauges adhered directly to the surface of the specimen. The axial shortening of the specimen was estimated based on piston displacement and was calibrated using the elastic portion of the mean axial strain, \(\varepsilon_{a}\). The shaded period in (b) corresponds to the dynamic rupture stage, in which AE occurred at a very high rate and several events could not be separated, and thus the estimated \(b\)-values were inaccurate. Stage I was the hydrostatic compression stage, in which confining pressure and axial stress were simultaneously increased to 40 MPa. During stage I, P velocity increased from 4.5 km/s to 5.4 km/s (Figure 4a), indicating closure of major microcracks in both the fracture zone and the host rock volume. The fractal dimension decreased from approximately 1.8 to approximately 1.7, while \(b\)-value increased slightly from 1.1 to 1.2. Acoustic emission hypocenters were concentrated in many small clusters having sizes of 2 to 4 mm, which is comparable to major grain sizes (Figures 5 and 6). Acoustic emission clusters were distributed along the entire domain of the fault surface, but edge areas showed a relatively low density (Figure 6). Stage II-1, which corresponds to an axial loading to a stress level of approximately 50% strength, was characterized by (1) a gradually decreasing \(b\)-value from the maximum value of 1.2 to approximately  
Figure 5: (**a**) Vertical section of three-dimensional X-ray computed tomography (CT) scan performed after the experiment. The fracture zone is characterized by very low CT value. (**b**) Three-dimensional fracture surface. (**c**) Stereo plots of well-determined (by at least 12 P arrival times) AE hypocenters.