A GRAPHICAL TECHNIQUE TO PREDICT SLIP ALONG A PREEXISTING 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. Th is 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-An dersonian stress systems. J. Struct. Geol. 14, 237-244). It consists o f calculating which mechanism, rupture or sliding. needs the smaller s tress difference to liberate the deformation. Using the results of cal culations over a wide range of plane orientations, we plotted, in an e qual-area net, the line which separates the orientation fields where r upture needs less stress difference from the fields where slip on pre- existing planes is favored. We named these plots slip-rupture graphs. For the three Andersonian fault regimes, the graphs are presented as d endrograms. These dendrograms show the variation of the range of orien tations favorable for reactivation as a function of cohesion and frict ion of the plane of weakness, depth, pore fluid pressure and the stres s ratio. The slip-rupture graphs are compared with the Mohr diagram an d slip-tendency graphs (Morris et al., 1996). Relative to Mohr diagram s, our graphs have the advantage that it is possible to work with geog raphic orientations of planes and principal stresses, and it is not ne cessary to transform the field data to a stress space. The slip-ruptur e graphs are similar to slip-tendency graphs; however, the former can lead to estimate physical parameters that make reactivation possible a long planes with unfavorable orientations. (C) 1998 Elsevier Science B .V.