Impact of Locked Segments in Laboratory Activated Faults: Insights From Near‐Field Strain Monitoring in Analog Rock Materials Under Uniaxial Stress

ABSTRACT Understanding fault activation and slip is vital for earthquake mechanics and underground structure stability. Fault stability, beyond in situ stresses, depends on their architecture, including asperities, segments, and gouge materials. This study investigates locked fault segments of varying lengths using mortar specimens with prefabricated faults filled with montmorillonite and quartz sand gouge. Uniaxial loading was applied, with embedded strain gauges and digital image speckle techniques monitoring internal stress, surface strain, and displacement near the fault. Results show increased model strength with longer locked segments. Local stress field deflection angles vary more in the dilatation quadrant than the compression quadrant. Gouge material disperses stress concentrations during loading. Slip occurs along the structural surface during failure, with the relative sliding rate of blocks correlating strongly with stress field deflection angle changes. As the system transitions from elastic to stable fracture stages, this correlation in the dilatation quadrant shifts. These findings offer insights into complex fault mechanisms and stability compared to simple planar saw‐cut discontinuities.