  The rock heterogeneity exists widely in fault zones; however, the intrinsic
mechanism of how it affects the mechanical behavior of faults is poorly
understood. To develop a quantitative understanding of the effect of the rock
heterogeneity on the strength and stability of faults, here we investigate a
pore-pressure model based on rate- and-state friction in the manner of
two-degree-of-freedom spring-sliders and analyze the reasons of fault weakening
and the conditions of frictional instability by carrying out nonlinear
simulations and a linear stability analysis. We find that the strength of
heterogeneous faults depends largely on the compaction difference (or
differential compaction) between the two gouges (e.g. quartz and clay), and the
stability is affected by the proportion of the two gouges patches. Our model
implies that the rock heterogeneity is likely to weaken faults and reduce the
stability of faults.
