EXPERIMENTAL STUDIES OF FLUID-ROCK INTERACTIONS AND SEISMIC CYCLE IN GEOTHERMAL FIELDS (KRAFLA GEOTHERMAL FIELD, ICELAND)

Fault frictional properties in the presence of hydrothermal fluids control natural and human-induced earthquakes in high-enthalpy geothermal (fluid-rich) systems. While the mechanical and chemical impact of fluid-rock interaction on the seismic cycle has been studied, the role of the physical state of fluids remains overlooked. Here, I performed rotary shear friction experiments on simulated fault gouges made of basaltic and felsic rocks collected from the main lithologies of Krafla Geothermal Field (Iceland). I performed both slide-hold-slide and velocity-stepping experiments at aseismic slip rates (3-100 μm/s) under temperature (Tf=100-400°C), pore pressure (Pf=3-30 MPa), and effective normal stress (σneff=10-50 MPa) conditions representative of the hydrogeologic system of Krafla. Under these conditions, water can be in liquid (Lq), vapor (Vp), and supercritical (Sc) states. To understand the underlying deformation and healing mechanisms, the sheared gouge layers were analyzed with a FESEM, XRPD, and XRF. Results showed that both rock mineralogy and ambient conditions, particularly Tf and the physical state of water, control the frictional strength μss, frictional healing Δμ, and the seismic vs. aseismic behavior of the experimental faults. For instance, the presence of frictionally weak phyllosilicates lowered the bulk friction and inhibited Δμ, resulting in a lower μss and Δμ measured in altered basaltic samples than in non-altered basaltic samples. Rock mineralogy affected the dependence of μss and Δμ on Tf. For instance, μss either increased (powders of fresh basalt and epidote-actinolite altered basalt), decreased (powders of chlorite-altered basalt and of rhyolite), or remained similar (powders of basaltic dyke) at elevated Tf conditions by different fluid-rock interactions. Also, Δμ increased with the hold time due to increasing contact area during holds and, in some cases, also with Tf when temperature-sensitive pressure solution was activated, particularly when Sc water is present. For Tf>200-300°C, all tested rock types showed a transition from stable sliding to stick-slip, due to the decreasing slip frictional weakening distance (i.e., the distance over which the friction coefficient evolves from static to μss). Regarding the role of the physical state of water, the presence of Vp increased μss in all tested rock types. Instead, the relatively low μss measured in Lq and Sc water was explained by a more efficient lubrication of grain contacts due to the combination effect of relatively viscous Lq and Sc water (compared to Vp water) and the formation of compacted and less permeable slip zones (SZs). Phase transition between Lq and Vp water changed the stress drop magnitudes, particularly at high σneff (30-50 MPa), regardless of the tested rock types, as explained by the efficient mechanical pressurization of relatively viscous Lq water that was temporarily trapped within the compacted SZ during sudden stress drop. Transferred to nature, the experimental observation of stick-slip (laboratory earthquakes) at Tf>200-300°C is consistent with the typical nucleation temperature of earthquakes in the Krafla geothermal field. Also, stick-slip observed in powders made of epidote-actinolite altered basalt, of basaltic dyke, and of granophyre under the lower reservoir conditions, is consistent with the seismic data showing that most hypocenters are localized at ~1.4-1.6 km b.s.l. In conclusion, this work implies that in high-enthalpy hydrothermal systems, the liquid-dominated reservoirs may host few but large in magnitude earthquakes, while vapor-dominated reservoirs may host frequent but small in magnitude earthquakes. This work also implies that during geothermal energy production, the re-injection of condensed water into the deep reservoir may have the potential to modulate the magnitudes of seismicity.