On the basis of elastodynamic stress fields for singular crack and nonsingular slip-weakening models of propagating rupture, we develop preliminary answers to such questions as follows: If a rupturing fault is intersected by another, providing a possible bend in the failure path, when will stressing be consistent with rupture along the bend? What secondary fault locations and orientations, in a damaged region bordering a major fault, will be stressed to failure by the main rupture? Stresses that could initiate rupture on a bend are shown to increase dramatically with crack speed, especially near the limiting speed (Rayleigh for mode II, shear for mode III). Whether a bend path, once begun, can be continued to larger scales depends on principal stress directions and ratios in the prestress field. Conditions should often be met in mode II for which bend paths encouraged by stressing very near the rupture tip are discouraged by the larger-scale stressing, a basis for intermittent rupture propagation and spontaneous arrest. Secondary failure in the damage zone likewise increases markedly as the limiting speed is approached. Such may make the fracture energy much greater than for slip on a single surface. The extent of secondary faulting is strongly affected by prestress directions and the ratio of residual to peak strength. For mode II, prestress controls whether activation occurs primarily on the extensional side, which we show to be the typical case, or on the compressional side too. Natural examples are consistent with the concepts developed.