Assessing Energy Budget of Laboratory Fault Slip Using Quantitative Micro-CT Image Analysis

Off-fault fracturing that occurs out of the main fault is observed at all scales, from laboratory to plate boundaries. Understanding these co-seismic fracturing phenomena in the laboratory under controlled conditions can provide insight on the u dynamic stress and fault conditions of the earthquakes. Studies regarding the relative amount of energy consumed by fracturing (E_G) is inconclusive, and the disagreement about the relative size of E_G is related to the difficulty in assessing the fracture surface area. We conducted a rotary shear experiment under X-ray micro-CT, which allowed not only the measurement of macroscopic stresses but also the imaging of the newly formed fractures inside the sample. With the careful analysis of the micro-CT images, we quantitatively assessed the fracture surface area. Then, we used Griffith theory of brittle fracture to estimate E_G, which accounted for only 0.15–0.43% of the total energy consumption during slipping. The E_G we estimated may imply the lower bound of the actual fracture energy, because it did not include the micro-fractures below the resolution of the micro-CT or the energy consumed by non-elastic deformation. Friction work, which is calculated from the macroscopic shear stress and angular slipping distance, consumed most (>80%) of the total energy. Less than 18% energy may have radiated in form of stress waves.