Shear-parallel displacement measurements are fundamental for understanding the deformation and stability of rock fractures in laboratory shear tests. However, these measurements are not direct records of fracture slip but represent combined contributions from slip of the fracture and deformation of the sample matrix, shear box, and testing machine, which complicates interpretation. In this study, we critically examine the role of measurement location, servo-control mode, and sampling frequency in shaping shear test results. Using a spring-slider framework, we show that stiffness corrections are necessary to recover true fracture displacement and to assess slip stability, which is commonly governed by machine stiffness in hard rock systems. We recover the underlying physical process influencing oscillations and stress drops under servo-control based on different measurement locations. Furthermore, we show that adequate sampling frequency is necessary for capturing peak slip velocity and dynamic rupture processes, while excessively high frequency amplifies noise and data redundancy. These results underscore the need for deliberate design of measurement configurations and rigorous data interpretation of shear-parallel displacement in laboratory shear testing, in order to understand natural fracture shearing and faulting processes.
Meng et al. (Wed,) studied this question.