Effects of Particle Size and Normal Stress on the Frictional Stability and Healing of Simulated Basalt Gouges: Implications for Lunar Seismicity

Basalt is a major component of the lunar soil and crust. The frictional properties, and thus the potential for seismicity and associated hazards on basaltic faults are significantly influenced by particle size distribution and normal stress conditions. We conducted velocity-stepping and slide-hold-slide shear experiments on simulated basalt gouges at gouge particle sizes of 50–250 μm, and normal stresses of 5–30 MPa under wet/dry conditions with different gouge structures to investigate the frictional stability and healing of basaltic faults. We observe a transition from velocity-strengthening (a – b = 0.52) to velocity-weakening behavior (a – b = -0.0005) and an enhanced healing rate (from 0.0075 to 0.0044), as particle size decreases. Elevated normal stress is typically associated with reduced grain size, resulting in unstable slip behavior (a – b = -0.0006), though accompanied by a diminished healing rate (from 0.0062 to 0.0050). The presence of fluid contributes to stable slip behaviors and fault compaction. These observations contribute to a better understanding of the potential and nucleation mechanism of lunar seismicity within basaltic faults. Highlights