A graphical technique to predict slip along a pre-existing plane of weakness

A graphical technique is proposed to determine whether a pre-existing plane of weakness will be reactivated by slip under a stress field. This technique is based on Coulomb-Navier criteria and the method of Yin and Ranalli (Yin, Z., Ranalli, G., 1992. Critical stress difference, fault orientation and slip direction in anisotropic rocks under non-Andersonian stress systems. J. Struct. Geol. 14, 237–244). It consists of calculating which mechanism, rupture or sliding, needs the smaller stress difference to liberate the deformation. Using the results of calculations over a wide range of plane orientations, we plotted, in an equal-area net, the line which separates the orientation fields where rupture needs less stress difference from the fields where slip on pre-existing plane is favored. We named these plots slip-rupture graphs. For the three Andersonian fault regimes, the graphs are presented as dendrograms. These dendrograms show the variation of the range of orientations favorable for reactivation as a function of cohesion and friction of the plane of weakness, depth, pore fluid pressure and the stress ratio. The slip-rupture graphs are compared with the Mohr diagram and slip-tendency graphs (Morris et al., 1996). Relative to Mohr diagrams, our graphs have the advantage that it is possible to work with geographic orientations of planes and principal stresses, and it is not necessary to transform the field data to a stress space. The slip-rupture graphs are similar to slip-tendency graphs; however, the former can lead to estimate physical parameters that make reactivation possible along planes with unfavorable orientations.