Hydrothermal friction experiments on simulated basaltic fault gouge and
implications for megathrust earthquakes
Abstract
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 ℃. 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 100-200 ºC and from negative to positive at
450-500 ºC. 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.