Hydrothermal Friction Experiments on Simulated Basaltic Fault Gouge and Implications for Megathrust Earthquakes

Nucleation of earthquake slip at the plate boundary fault (décollement) in subduction zones has been widely linked to the frictional properties of subducting sedimentary facies. However, recent seismological and geological observations suggest that the décollement develops in the subducting oceanic crust in the depth range of the seismogenic zone, at least in some cases. To understand the frictional properties of oceanic crustal material and their influence on seismogenesis, we performed hydrothermal friction experiments on simulated fault gouges of altered basalt, at temperatures of 100–550°C. The friction coefficient (μ) lies around 0.6 at most temperature conditions but a low μ down to 0.3 was observed at the highest temperature and lowest velocity condition. The velocity dependence of μ, (a−b), changes with increasing temperature from positive to negative at ∼100°C and from negative to positive at ∼450°C. Compared to gouges derived from sedimentary facies, the altered basalt gouge showed potentially unstable velocity weakening over a wider temperature range. Microstructural observations and microphysical interpretation infer that competition between dilatant granular flow and viscous compaction through pressure‐solution creep of albite contributed to the observed transition in (a−b). Alteration of oceanic crust during subduction produces fine grains of albite and chlorite through interactions with interstitial water, leading to reduction in its frictional strength and an increase in its seismogenic potential. Therefore, shear deformation possibly localizes within the altered oceanic crust leading to a larger potential for the nucleation of a megathrust earthquake in the depth range of the seismogenic zone. Plain Language Summary Megathrust earthquakes in subduction zones have long been studied to mitigate damage by ground shakings and tsunamis. Frictional properties of plate boundary fault are fundamental information to understand the slip activities causing megathrust earthquakes. In this study, we focused on altered basalt, which is a rock of subducting oceanic crust and has not been focused on as much as sedimentary rocks. Laboratory friction experiments were performed at a wide temperature range with water pressurized conditions. We found that the altered basalt shows frictionally unstable behavior of the type that is prerequisite for generating an earthquake in the temperature conditions of seismogenic zone. This unstable behavior is likely to be