Strain Localization in Sandstone-Derived Fault Gouges Under Conditions Relevant to Earthquake Nucleation

Constraining strain localization and the growth of shear fabrics within brittle fault zones at sub-seismic slip rates is important for understanding fault strength and frictional stability. We conducted direct shear experiments on simulated sandstone-derived fault gouges at an effective normal stress of 40 MPa, a pore pressure of 15 MPa, and a temperature of 100°C. Using a passive strain marker and X-ray Computed Tomography, we analyzed the spatial distribution of deformation in gouges deformed in the strain-hardening, subsequent strain-softening, and then steady-state regimes at displacement rates of 1, 30, and 1,000 µm/s. We developed a machine-learning-based automatic boundary detection method to recognize the shear fabrics and quantify displacement partitioning between each fabric element. Our results show fabrics oriented along R₁ and Y (including boundary) shears are the two major fabric elements. At rates of 1 and 30 µm/s, the relative amount of displacement on R₁ shears is displacement dependent, increasing to ∼20% of the total displacement up to the strain-softening stage, then decreasing to ∼10%–18% at the steady state. This trend is absent at the high rate where ∼18% of the displacement occurs on R₁ shears throughout all investigated stages. At all rates, the relative amount of displacement on Y shears increases linearly with displacement to a total of larger than 50% at the steady state. Our study provides constraints on the development of the active slip zone, which is an important factor controlling heating and weakening associated with small-magnitude earthquakes with limited displacement (mm-dm), such as induced seismicity.