Frictional properties and fluid-induced reactivation of fault rocks from a granitic EGS reservoir

In 2017, an M_W 5.5 earthquake struck the Pohang region, representing the most damaging seismic event in South Korea, and has been linked in previous studies to hydraulic stimulation at the Pohang Enhanced Geothermal System (EGS) site. However, the relative roles of fluid injection, imposed stress state and fault-zone structure in nucleating this event remain a matter of debate, and the laboratory results presented here are intended to illuminate one mechanically plausible scenario rather than provide a unique causal explanation. Despite the scientific and societal importance of this earthquake, the frictional properties of rocks from the Pohang system are still poorly constrained. Here we experimentally characterize the frictional properties and slip behavior during fluid-induced reactivation of granodiorite wall rock powder and fault gouge recovered from the Pohang PX-2 borehole (~3.8 km depth). We first assessed the mineralogical assemblages of the two fault materials, which consist of mixtures of quartz, K-feldspar, plagioclase and phyllosilicates (mostly chlorite), with phyllosilicate contents varying between 15% and 23% for the wall rock and the fault gouge, respectively. We then measured friction, healing rate and the velocity dependence of friction for both materials under water-saturated conditions at normal stresses of 20–100 MPa using the BRAVA apparatus hosted at the National Institute of Geophysics and Volcanology (INGV). We performed frictional experiments in a double-direct-shear configuration, using a protocol consisting of a run-in at 10 µm/s, slide-hold-slide tests (SHS; hold times ranging between 3 and 3000 s), velocity-stepping tests (VS; velocities ranging between 0.3 and 300 μm/s), and fluid-injection tests (pore-pressure increases of 0.25 MPa every 5 min). Steady-state friction coefficients for both materials fall within the Byerlee range (μ ≈ 0.55–0.62). SHS tests reveal that both fault gouge and wall rock exhibit relatively high healing, with β in the range ≈ 0.0046–0.0092. Conversely, velocity-stepping tests reveal that, over the tested stress and velocity range, the wall rock has a slightly velocity-weakening to neutral behavior (a–b = −0.0007 to 0.0020), while fault gouge is predominantly velocity-neutral to strengthening (a–b = 0.0005 to 0.0028). Additional fluid-injection experiments indicate that, despite these slight differences in frictional properties, both the fault gouge and the wall rock can be reactivated under elevated pore pressure, with slip accelerating from creep to millimetre-per-second rates. Accompanying microstructural observations will examine whether differences in grain-size reduction, shear localization, or porosity evolution account for the similar reactivation behavior despite the slightly contrasting frictional properties. Overall, these measurements will help quantify how lithological heterogeneity, rate-and-state parameters, and pore-pressure evolution govern slip stability and the nucleation potential of injection-induced earthquakes in geothermal settings, with important implications for induced-seismicity hazard assessment in granitic EGS reservoirs.