## 5 Discussion and Conclusions  
Early studies showed that AE activity prior to catastrophic failures in several common rock types is generally characterized by (1) increasing event rate and accelerated energy release, (2) a decrease with a subsequent precursory increase in fractal dimension and SCL, and (3) a decrease in \(b\)-value from approximately 1.5 to 0.5 for hard intact rocks and from approximately 1.1 to 0.8 for soft rocks, such as S-C cataclasite [19]. Our results for the immature fault in granite demonstrated similar patterns. However, the \(b\)-value ranged between approximately 1.2 and 0.5. The \(b\)-value of the magnitude-frequency distribution is normally used in seismic hazard assessment for predicting the maximum magnitude. Note that (1) \(b\)-value changes with stress and (2) the maximum earthquakes are sometimes outliers of the power law governing the population behavior of a seismic sequence [3; 9; 19; 29]. Therefore, the maximum magnitude of a given site should be determined based on the size of reactivated faults. The present experimental results showed that AE hypocenters clustered on the entire fault surface at the very early stages under a very low stress level. This suggests that the dimension of immature faults with active induced seismicity can be determined based on hypocenter distributions of seismicity that occurred in earlier stages. Planarly distributed hypocenters can be used to estimate the lower bound of fault size. It is reasonable to suggest that such a clue depends on the maturity of the faults involved. In the present study, the fault in the granite specimen was very young and had no slip history. For faults of different maturities, the stress level for the onset of detectable AE/microseismic activity and the start points of \(b\)-value and fractal dimension could be different. This is an area that should be investigated further. At a given site, there may be immature faults in the sediment and mature faults in the basement, as is the case for some sites in Ohio [7]. In seismic hazard assessment for injection-induced seismicity, we must pay more attention to immature faults because shear fracturing of such faults contributes a high level of injection-induced seismicity and greater ground motion [8]. Unfortunately, however, immature faults with less slip are difficult to detect by geophysical methods. Detailed monitoring and real-time analysis during injection are thus important for detecting unknown faults. For a specific site, in which out-zone seismicity was observed or expected, the maximum magnitude of potential inducing earthquakes should be estimated based on fault size rather than injection volume [2; 3]. In conclusion, the author investigated the seismic \(b\)-value and fractal dimension for AE events during shear rupture of a pre-existing immature fault in a granite specimen under triaxial compression conditions (confining pressure: 40 MPa). The naturally-created fault had an angle of 32\({}^{\circ}\) with the axial stress and thus is referred to as favorably oriented. Acoustic emission activity was initiated at a very low stress level, and the start points of \(b\)-value and fractal dimension were approximately 1.2 and 1.8, respectively. The obtained \(b\)-values decreased from approximately 1.2 to 1.0 as the shear stress increased and then quickly dropped to approximately 0.5 prior to failure. Acoustic emission hypocenters were clustered on the entire fracture surface area. With increasing stress until failure, fractal dimension exhibited the following pattern: an initial decrease from approximately 1.8 to 1.6, followed by an increase to approximately 1.8, and finally a quick increase to approximately 2.0. Our results for \(b\)-value with respect to magnitude-frequency distribution and the fractal dimension of AE hypocenters preceding the dynamic fracturing of the immature fault demonstrated that it is possible to detect some earlier signs of fault reactivation.