Figure 2

Two mechanisms that might account for low-strength fault zones imbedded in a stronger crust. Both mechanisms are capable of satisfying the heat flow and directional constraints discussed in the text. In each case, the state of stress in the fault zone and in the adjacent crust are indicated by shaded and open Mohr circles, respectively. The pore fluid pressure, least horizontal principal stress and greatest horizontal principal stress are shown as P_p, S_hmin and S_Hmax, respectively. a) Assumes there is superlithostatic pore pressure (and correspondingly high stress) within the fault zone (after Rice, 1992). b) Assumes fault zone rocks have intrinsically low friction but that pore pressure is the same within the fault zone and the surrounding crust.