Microseismic monitoring is generally the most reliable method for estimating stimulated fractured volume. Receivers used in microseismic monitoring measure only seismic events. That limitation explains why only a small portion of the energy budget during hydraulic fracturing can be estimated by information obtained from microseismic monitoring. We performed a series of numerical experiments to investigate the effects of rock mechanical properties and fracture friction characteristics on seismic efficiency and rupture velocity. We conducted numerical experiments using acoustic emission for saw-cut samples under triaxial loads and applied slip-weakening constitutive modeling for natural fractures to study how the Young's modulus and slip-weakening distance affect seismic efficiency and rupture velocity. Perhaps surprisingly, our results show that rocks with higher values of the Young's modulus have lower seismic efficiency generated from sliding on pre-existing natural fractures, while lower rigidity leads to higher seismic efficiency. These results do not contradict general beliefs about the effect of rigidity on fracability. More rigid rocks are more favorable for hydraulic fracturing and generate larger fracture networks; however, compared with less rigid rocks, fewer events would be detected seismically. The results also give insight into how to connect geomechanical numerical modeling of hydraulic fractures in naturally fractured reservoirs with microseismic data from the field and actual subsurface-generated fractured networks.