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 (EG) is inconclusive, and the disagreement about the relative size of EG 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 EG, which accounted for only 0.15–0.43% of the total energy consumption during slipping. The EG 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.

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