Fault heterogeneity increases the complexity of earthquake precursors: Insights from direct shear tests with AE activity

Fault heterogeneity is ubiquitous and critical to the slip behaviors of faults. Strong heterogeneity arises from macroscopic fault irregularities or local healing, but its effect on seismicity remains unclear. We report the acoustic emission (AE) activity during shear failure of discontinuous laboratory faults to reveal the impact of geometrical fault heterogeneity on temporal variations in seismicity. AE characteristics (AE event rate and b -value of size distribution) effectively identify the progressive failure phases of discontinuous laboratory faults with different fractions of rock bridges. Two types of AE anomalies dictated by fault geometry have been observed as precursors of failure: (1) the power-law acceleration of AE event rate preceding the shear localization phase; (2) the fluctuating decrease of b -value during the shear localization phase and the dynamic failure phase. The scale of fault heterogeneity plays an important role on the properties of shear rupture and precursory AE activities. These precursory AE anomalies, attributed to successive damage and off-fault macro-fractures ahead of the full failure of the fault, illuminate why earthquake forewarning is made more complex due to fault heterogeneity.