Thermo-hydro-mechanical mechanisms in sandstone-derived fault gouges during simulated small-magnitude earthquakes from experiments and models

Laboratory studies have demonstrated that faults undergo dynamic weakening during large displacements (>1 m) at seismic slip velocities (>0.1 m/s). However, 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. To address this, we conducted seismic slip-pulse experiments on Slochteren sandstone gouges (SSG), derived from the gas reservoir, using a rotary-shear apparatus to provide decimeter-scale constraints on the dynamic fault slip of quartz-rich gouges. Pre-sheared gouge layers, confined between ~1.5 mm thick sandstone host blocks, were subjected to slip pulses under initial effective normal stresses of 4.9–16.6 MPa and pore fluid pressures of 0.1 and 1 MPa under undrained conditions. The experiments achieved peak velocities of 1.8 m/s, accelerations up to 42 m/s², and displacements of 7.5–15 cm, using either dry Argon or water as pore fluid at ambient temperatures. Our results reveal that water-saturated gouges weaken rapidly from a peak friction of ~0.7 to ~0.3, accompanied by early fast dilatancy followed by slower ongoing dilation, with minimal dependence on normal stress, slip acceleration, or displacement. In contrast, Argon-filled samples exhibited only minor weakening. Microstructural analysis shows no systematic relationship between the width of the principal slip zone (PSZ) and frictional work or power input densities, indicating that wear or heat production alone does not control PSZ growth. Instead, our thermo-hydro-mechanical (THM) numerical modeling suggests that thermal pore fluid pressurization, potentially involving water phase transitions at asperity scales, drives weakening in short-displacement, induced seismic events. To extend these small-scale laboratory findings to reservoir-scale processes, ongoing research focuses on discrete element modeling (DEM) at particle and gouge scales coupled with THM solutions. This includes calibrating the THM-DEM models at both grain and gouge scales using laboratory data from Groningen sandstone-derived samples. The calibrated models will be validated with recent data generated under fast slip conditions with varying pore fluids.