## 4 Results  
As in previous studies [13], the \(b\)-value, event rate, and strain data for the intact specimen (SA) had three distinct phases: the primary, secondary, and nucleation phases (indicated by P, S, and N, respectively in Figure 3). The \(b\)-values were estimated using the maximum likelihood method for a running window of 1000 events with a step of 256 events. Acoustic emission rates were calculated for every 10 s. Axial, circumferential, and volumetric strains were obtained as the average of data recorded by six pairs of stain gauges, glued directly onto the surface of the specimen. The axial shortening of the specimen was estimated from piston displacement and was calibrated using the elastic portion of the mean axial strain. Acoustic emission activity was initiated when the axial stress reached 75% (approximately 370 MPa) of the failure strength (495 MPa). The AE initiation also coincides with the start of dilatation, as indicated by the mean volumetric strain. In the primary phase, the \(b\)-values increase with increasing stress from approximately 1.4 to 1.7 and then decrease during the secondary phase from approximately 1.7 to 1.0. Finally, the \(b\)-values drop quickly to approximately 0.5 during the nucleation phase and grow quickly during the final failure stage. Overall, the results for the pre-cut specimens (S1 and S2) were similar. Experiment S1 were stopped after 7 stick-slip events due to leakage of confining oil into the pre-cut fault. In this paper, we thus mainly focus on results of experiment S2, in which 18 stick-slip events were recorded. As shown in Figure 4, AE activity was initiated at a very low differential stress level. In fact, some events were observed during the hydrostatic loading stage. In addition, the maximum axial stress was 210 MPa, which was less than 60% of the stress level (approximately 370 MPa) at which AE activity was initiated in the intact specimen, as described above. Since both the shear and normal stress acting on the fault surface could be calculated, we therefore estimated the static and dynamic frictional coefficients [12]. During the dynamic stress drop associated with the first stick-slip event, the static and dynamic frictional coefficients were 0.75 and 0.5, respectively (Figure 4b). With increasing axial stress, the seismic \(b\)-value gradually decreased from approximately 1.8 to 1.0 prior to the first stick-slip event. We estimated the slip along the fault from sample shortening and elastic strain data. Prior to the first stick-slip, the fault had statically or quasi-statically slipped approximately 0.6 mm (Figure 4a). Since the initiation of the fault slip coincides with the initiation of the AE activity, and the AE event rate increases continuously with increasing fault slip, we concluded that the AE activity and the pre-failure fault slip were coupled with each other. Following the first stick-slip, 17 stick-slip events were observed (Figure 4b). The experimental results indicate that the static frictional coefficients decreased from the maximum value prior to the first stick-slip event to a stable mean value of approximately 0.6 after several stick-slip events, while the dynamic frictional coefficient fluctuated between 0.45 and 0.52. After each stick-slip event, there was a quiescent period with a relatively low AE event rate. When the shear stress reached 50 to 90% of the shear strength, the AE event rate increased rapidly and reached a maximum value immediately before the onset of the next dynamic slip (Figure 5). In total, the number of AE events per stick-slip cycle exhibits a tendency to gradually decrease as the number of stick-slips events increases. The mean value  
Figure 3: Results of the intact specimen for reference. AE rates (red line), \(b\)-values (blue line), axial stress (\(\sigma_{a}\)), confining (\(P_{c}\)) pressure, axial/circumferential/volumetric strains (\(\varepsilon_{a}/\varepsilon_{c}/\varepsilon_{v}\)), and axial shortening are plotted against time.