Laboratory Shear Behavior of Tensile- and Shear-Induced Fractures in Sandstone: Insights from Acoustic Emission

The distinction between the shear behavior of tensile- and shear-induced fractures is critical to understanding the deformation and failure of geologic discontinuities at different scales. To investigate these differences, a series of direct shear tests were performed on sandstone specimens with a continuous fracture created by either splitting or shearing. The acoustic emission (AE) technique was used to examine variations in grain-size cracking behavior between specimens with tensile- and shear-induced fractures. An increase in normal stress for both fracture types correlates with increased microcrack density and energy release. However, there are notable differences: during the shear process, tensile-induced fractures produce AE sequences similar to the seismic patterns observed along natural tectonic faults, with foreshocks, mainshocks, and aftershocks. In contrast, the AE sequence for shear-induced fractures during the shear process lacks prominent mainshocks and deviates progressively from the power-law function with time as normal stress increases. In addition, the AE b-value for tension-induced fractures initially shows a gradual decrease as the mainshock approaches and then slowly increases during the aftershock period. In contrast, the b-value remains nearly constant for shear-induced fractures due to the low roughness and heterogeneity of the fracture surface. These differences highlight the strong correlation between AE responses and fault heterogeneity, paving the way for fault characterization and risk assessment in subsurface energy extraction. The cracking behavior of both tensile- and shear-induced fractures in direct shear tests is investigated using the AE technique.In direct shear tests, the AE sequences of tensile fractures follow a power law, while a significant deviation from the power law is observed in the AE sequence of shear fractures.The power-law evolution of the AE sequence before and after the mainshock, together with anomalous -values, can be used as indicators to distinguish young faults from mature faults.