In this work, the effect of in-situ pressure on natural fracture shear and hydraulic fracturing-induced microseismicity generation was evaluated using a discrete element model. Overall, the initial in-situ pressure was found to play a significant role in the ability to shear natural fractures during a hydraulic fracture stimulation, largely through a reduction in the effective stresses acting on the natural fractures. Simply, the greater the initial in-situ pressure relative to the in-situ principal stresses, the greater the shear on natural fractures for a given stimulation treatment. Shear slip events on natural fractures were divided into "wet" events and "dry" events to help understand the shear failure mechanism and the induced-microseismicity complexity during the hydraulic fracturing process. The results showed that the density and distribution area of the slip events were affected by a combination of multiple factors. Dry events were mainly affected by the total stress change or shear stress change and the initial strength of the natural fractures while the wet events were affected by the total stress change or shear stress change, the initial strength of the natural fractures and the fluid leakoff area. The stimulated reservoir volume (SRV), measured by the scope of the wet events or the shape of the leakoff area, was affected by the trace of the hydraulic fracture and the fracture network connectivity in the matrix. These results provide further understanding and ability to interpret the different microseismicity responses seen in the field, which, ultimately, will allow for improved optimization of stimulation and completion strategies.