Dynamic weakening in sandstone-derived fault gouges during simulated small-magnitude earthquakes under variable loading and environmental conditions

Faults exhibit dynamic weakening during large displacements (>1 m) at seismic slip velocities (>0.1 m s⁻¹), but the role of this weakening in small-displacement induced earthquakes (M 3–4), such as those in the Groningen Gas Field (the Netherlands), remains unclear. We conducted seismic slip-pulse experiments on Slochteren sandstone gouges (SSG) using a rotary-shear apparatus to investigate their dynamic behaviour. Pre-sheared gouge layers, confined between ∼1.5 mm thick sandstone host blocks, were subjected to slip pulses at initial effective normal stresses of 4.9–16.6 MPa and pore fluid pressures of 0.1 and 1 MPa under undrained conditions. Slip pulses reached peak velocities of 1.8 m s⁻¹, accelerations up to 42 m s⁻², and displacements of 7.5–15 cm, using either dry Argon or water as pore fluid at ambient temperatures. Water-saturated gouges showed rapid weakening from a peak friction of ∼0.7 to ∼0.3, with early dilatancy followed by slower ongoing dilation. In contrast, Argon-filled samples exhibited only subtle weakening. Our findings confirm that water-saturated SSG weakens substantially during slip, with minimal dependence on normal stress, slip acceleration, or displacement, while dry samples do not. Microstructural analysis indicates no systematic relationship between principle slip zone (PSZ) width and frictional work or power input densities, suggesting that wear or heat production alone does not govern PSZ growth. Instead, thermal pore fluid pressurization, potentially involving water phase transitions at asperity scales, may drive weakening in short-displacement, induced seismic events.