| The frictional properties of fault rocks and, in particular, the velocity dependence of friction
and associated rate-and-state parameters, are thought to exert an important control on
earthquake nucleation and propagation. Experimental results obtained from natural fault
gouges typically show that the velocity dependence of friction is a function of both
temperature and sliding velocity, indicating that thermally activated time-dependent processes
are fundamentally responsible for causing velocity-weakening behavior in silicate-bearing
gouges at earthquake “nucleation velocities” (∼ 1 μm/s) and temperatures around 150-300 ˚
C. In addition, slow experiments at velocities of 10s of nm/s using three different fault gouge
types all exhibit major weakening with ongoing displacement at constant velocity.
Microstructural and microanalytical analyses demonstrate that the development
of a weak through-going foliation as well as the (shear-enhanced) formation of
new, weak minerals such as talc or muscovite occurred, which both presumably
contributed to the observed weakening. Importantly, the slow deformation rates allow for
time-dependent viscous deformation (e.g. pressure solution) to occur at low shear stress
within the hard, frictionally strong minerals such as quartz. The results highlight the
importance of the chemical effects of fluids and microstructural development on
long-term fault weakening under slow loading conditions. The resultant frictionally
weak fault gouges allow strain to remain localized, yield a strong permeability
anisotropy and provide a barrier for rupture propagation. Along-fault variations in the
chemical conditions thus have the potential to produce strong contrasts in frictional
properties, which can have a large effect on potential earthquake rupture size and style. |