## 1 Introduction  
Injection-induced seismicity is a by-product in several scientific and industrial applications, such as tapping deep geothermal energy through an enhanced geothermal system (EGS), fracking shale gas, enhanced oil recovery (EOR), disposal of waste fluid, and geological storage of CO\({}_{2}\). Microseismicity can be used to monitor hydraulic fracturing and for reservoir management. However, felt events of magnitude (M) 2 and above near certain projects may have economic and social impacts [1], and repeated M \(\geq\) 3 events may cause heavy damage, especially at seismically stable sites [2; 3]. Under certain conditions, earthquake ruptures can be confined to pressurized regions. In contrast, under other conditions, earthquakes are able to propagate as sustained ruptures beyond the zone that experienced a pressure perturbation [4]. At sites without large-scale pre-existing faults, induced seismicity was limited in-zone (stimulated volume) and was mainly controlled by injection parameters, and thus induced seismicity could be better controlled because seismicity faded out quickly after shut-in. For such cases, as demonstrated by an EGS project, near-real-time seismic monitoring of fluid injection has allowed control of induced earthquakes via a well-designed traffic lighting system [5]. However,for worse cases encountered in damage events, major seismicity resulted from reactivation of large-scale pre-existing faults having different maturity [6; 7]. Shear rupture of newly-created tensile fractures or pre-existing fractures opened by hydrofracturing is also a potential source inducing seismicity by later nearby injections [2; 6]. Normally, earthquakes on immature faults produce stronger ground motions at all frequencies, as compared with that generated by earthquakes on mature faults [8]. A recent study showed that mature faults in basement and immature faults in sediments demonstrated seismically-different responses to injection operations [7]. Thus, the structural maturity of faults is an important parameter that should be considered, and the shear behaviors of faults having different maturities should be fully investigated at all scales. The possibility of detecting early signs of fault reactivation, which might be a function of structure maturity, is a key issue in risk assessment and hazard mitigation of injection-inducing seismicity. Acoustic emission (AE) activity on a laboratory scale can be considered as microseismicity of a magnitude in the range of roughly from \(-11\) to \(-4\)[9]. Acoustic emission activity in stressed rocks shows similarities with seismicity in the crust via a number of power laws. The commonly accepted law is the Gutenberg and Richter (GR) relation for size distribution and fractal distribution of the AE hypocenter. Beginning with [PERSON]'s study published in 1962 [10], numerous laboratory studies have been motivated by the expectation that precursory changes in \(b\)-value may have resulted from stress changes and thus may be used for earthquake prediction and failure prediction in mines. Indeed, laboratory studies of AE events have consistently shown a decrease in \(b\)-value with increasing stress during the deformation of intact samples containing pre-existing microcracks e.g. Refs. [11; 12; 13; 14; 15]. In recent years, more studies have focused on AEs during frictional sliding on natural fractures/joints or pre-cut faults on laboratory scales because such set-ups provide a better analogue of fault rupture in nature. Systematic variations in \(b\)-value of the magnitude-frequency distribution of AE events were observed during stress buildup and release on laboratory-created fault zones [16] as well as during stable and unstable frictional sliding experiments on layers of glass beads in a double-direct shear configuration [17]. In a recent study, the results for unfavorably-oriented pre-cut faults showed similar behaviors: decreasing \(b\)-value followed by a quick jump prior to the stress drop for repeated stick-slip events [18].