Estimation of Seismic Source Parameters Based on Focal Mechanisms During Stress-Relaxation in Rocks

ABSTRACT: Stress-relaxation is a key process in the time-dependent behavior of rocks, influencing the stability of structures in rocks over long periods. This study focuses on estimating source parameters based on focal mechanisms during stress-relaxation experiments in crystalline rocks. The experiments were conducted on double-flawed prismatic Barre granite specimens under uniaxial compression in controlled laboratory conditions. Acoustic emission (AE) monitoring was employed to capture microseismic events, allowing for a detailed analysis of the focal mechanisms associated with the fracture propagation. Focal mechanisms, indicative of the type of fracture mode - non-shear or shear - were determined and analyzed about the stress relaxation process. Source parameters such as seismic moment, stress drop, source radius, and energy release were estimated using spectral fitting methods based on the displacement spectra from the non-shear and shear AE events. The temporal evolution of these parameters during relaxation at different stress levels was analyzed. Results revealed that during stress relaxation, non double-couple (NDC)-non shear events dominated in the early stages, with an increasing presence of double-couple (DC)-shear events as relaxation progressed. Higher values of the seismic moment, radiated seismic energy, and stress drop were observed for the DC events compared to the NDC events. This study provides a deeper understanding of the micromechanical processes driving time-dependent failure in rocks, which are critical for predicting the long-term behavior of rock masses in engineering applications.