Recent observations suggest that the presence of frictionally weak minerals in a majority frictionally strong matrix may explain the reduced strength and instability in faults. Experimental results on synthetic fault gouges using a mixture of a frictionally strong phase and a frictionally weak phase indicate that the fault can be weakened by even a small amount of frictionally weak minerals. These frictionally weak minerals weaken the fault by either acting as weak spots/clusters or as a through-going weak layer in the bulk gouge. A two-dimensional Distinct Element Method (DEM) numerical model using the Particle Flow Code 2D (PFC 2D) is developed to investigate the effect of frictionally weak minerals on the bulk shear strength of fault gouge. Mechanical response of particles is modeled using a linear-elastic contact model and Coulomb's friction law. Numerical direct shear experiments were performed on homogeneous mixtures of weak and strong mineral particles and also on heterogeneous mixtures consisting of a frictionally weak layer sandwiched in frictionally strong minerals. The weight percentage (wt%) of the frictionally weak mineral in the homogeneous mixtures and the relative thickness of the frictionally weak mineral layer in the heterogeneous mixtures are adjusted schematically to obtain the weakening regime of the bulk shear strength. A transition from high to low residual coefficient of friction is observed. Specifically, for homogenous mixtures a sharp drop of bulk shear strength is observed with 25% of frictionally weak mineral presented in the mixture, and a dominant influence occurs at 50%; for heterogeneous mixtures, noticeable weakening is shown at a relative weak layer thickness of 0.05, and a dominant influence quickly follows at a relative thickness of 0.10. The observed weakening regime matches well with previous lab results using talc/quartz mixtures.
In nature, tectonic faults tend to slip at much lower resolved shear stress than the stresses inferred from rock mechanics experiments (Engelder et al., 1975; Dieterich, 1978; Marone et al., 1990). Explanations to this difference between lab observations and natural phenomenon include low effective stress, elevated pore pressures (Rice, 1992; Faulkner and Rutter, 2001) and dynamic weakening in which friction decreases above a threshold slip rate (Melosh, 1996; Di Toro et al., 2006; Ampuero and Ben-Zion, 2008). Recent field observations of the San Andreas fault (Moore and Rymer, 2007) and an exhumed low angle normal fault in Italy (Collettini et al., 2009a, 2009b) show that the weakness of natural faults can be explained by the presence of talc; a frictionally weak mineral. Earlier experiments using synthetic mixtures of salts and muscovite/kaolinite (Bos and Spiers, 2002; Niemeijer and Spiers, 2005, 2006) showed that weakening can occur with as low as 10% of frictional weak minerals. The shear strength of a fault greatly depends on its mineralogical composition (Ikari et al., 2011; Fang et al., 2015). Shear experiments using mixtures of talc and quartz sand (Carpenter et al., 2009) suggested that in order to weaken the composite gouge, 30%-50% frictional weak mineral is needed. However, there were only 2-3 wt% of talc presented in these weak faults which is surprisingly less than the previous suggestions. The difference can be explained by the generation of a through-going localization zone during dynamic shearing, which greatly weakens the fault. This strong weakening effect of frictionally weak mineral in a majority frictionally strong matrix derives a question of how much the frictionally weak mineral in weight percentage (wt%) is needed; and in terms of localized weakening effect, how thick the weak localization zone is enough to weaken the fault. Experiments have been conducted on synthetic gouge consisting of quartz sand as a strong phase and a through-going talc layer as a weak phase (Niemeijer et al., 2009; Moore and Lockner, 2011). The findings suggest that the frictional strength of the sample gouge decreases systematically with the increase in thickness of the talc layer, two critical values were proposed for a starting point of weakening relative to pure quartz sand and weakening domination point relative to pure talc, Additionally, findings suggest that the permeability evolution of fractures is likely linked to such mineralogical properties (Ishibashi et al., 2015).