Locked segment of fault as earthquake barrier: Laboratory and field evidence of preseismic stress drop

Natural fault systems exhibit significant geometric heterogeneity, where locked segments act as strong, unbroken regions that impede fault slip and influence seismic nucleation. This study integrates laboratory triaxial shear experiments on granite samples with field observations from piezomagnetic stress monitoring stations to investigate the rupture dynamics, stress evolution, and precursory characteristics of locked patches. Experimental results reveal four distinct phases of fault slip: elastic deformation, microcrack propagation, stress accumulation, and stress drop/instability. Locked patches exhibit 3–5 times higher peak shear strength than continuous faults, with failure modes transitioning from tensile (low confining pressure) to shear-dominated (high confining pressure). Notably, precursory stress drops lasting tens to hundreds of seconds—significantly longer than smooth fault precursors were observed, offering extended early-warning windows. Field validation from the Qiaoqi station (Longmen Shan Fault Zone) captured 15% stress fluctuations 4 days before the 2013 Lushan earthquake, aligning with laboratory-derived stress-time signatures. These findings provide a mechanistic basis for interpreting crustal stress anomalies as precursors, bridging laboratory-scale physics with natural fault systems to advance earthquake forecasting strategies. • Distinct concave stress drop emerges during microcrack initiation in locked segments. • Irreversible stress drop marks the onset of dynamic instability in locked faults. • Precursory stress drops offer extended windows for earthquake prediction. • Field data highlight preseismic stress changes before Lushan earthquake.