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. Plain Language Summary: In the past few decades, several studies have focused on the mechanical behavior of simulated fault zones to understand earthquake nucleation. However, minor attention has been paid to the microstructural characterization of fault‐zone gouges due to the difficulties in quantifying deformation. An understanding of brittle fault‐zone fabrics and their development provides crucial constraints on the mechanical strength and stability of faults. We conducted laboratory experiments on simulated sandstone‐derived fault gouges with a passive strain marker under the conditions relevant to earthquake nucleation to explore the development and evolution of shear zone fabrics and their relations to fault strength. We combined X‐ray Computed Tomography (XCT) and a custom‐designed machine‐learning‐based automatic boundary detection method to analyze the spatial distribution of gouge deformation and to quantify displacement partitioning between deformation features. The results show that our samples have similar evolution of the shear fabrics, partitioning of displacement, and mechanical response with increasing shear strain at all tested nucleation velocities. The evolution of the mechanical behavior from strain‐hardening, to softening, to steady‐state stages is related to the transformation of R1 to shear‐parallel shear bands. Up to 50% of the total displacement can be accommodated within shear‐parallel shear bands, facilitating strain weakening of the materials. Key Points: We performed 3‐D analyses on the evolution of shear zone fabrics within sandstone‐derived fault gouges utilizing the X‐ray CT techniqueOur samples show similar evolution of shear fabrics, slip partitioning, and mechanical response with shear strain at all tested velocitiesUp to 50% of the total imposed displacement can be accommodated within shear‐parallel shear bands during earthquake nucleation [ABSTRACT FROM AUTHOR]