Brittle strength of rocks and gouges at various pressures and temperatures. (a) Frictional strength of dry rocks (upper) and gouges (lower) plotted in the dotted rectangular area of the upper diagram. The dotted blue line represents the frictional strength model given by Equations 3 and 4. (b) Mohr circles and fracture strength of intact rocks. The thick green line is the fracture strength model given by Equation 5. RT, room temperature; S, Stesky et al. (1974); H, He et al. (2007); B, Blanpied et al. (1995); Ch, Chester (1995); NS, Nakatani and Scholz (2004a); T, quartz+montmorillonite (Mont) mixtures at a shear displacement of 2.2 mm after Takahashi et al. (2007). The clay contents in weight percentage were given by Takahashi (personal communication). Cr, quartz+kaolinite (Kao) mixtures at 5% shear strain after Crawford et al. (2008); dH13, quartz+muscovite (Msc) mixture (den Hartog et al.2013); dH12, illite-rich shales (den Hartog et al.2012); WB, Wong and Biegel (1985); K, Kato et al. (2003).