Rock deformation texture from deeply exhumed fault zones in quartzo-feldspathic crust are considered in relation to likely rates of energy dissipation and hence the levels of shear resistance operative during seismic slip in the upper, frictional regimes of major crustal dislocation. Available evidence suggests that both low-stress (&tgr;?100 bars) and, less commonly, high-stress (&tgr;?1 kbar) faulting occur, depending on local conditions of which the most important is the ratio of fluid to overburden pressure. The higher stresses are usually associated with immature fault systems, especially reverse faults developed in crystalline host rocks. As a result of initial power dissipation at the onset of slip, feedback mechanisms involving either friction melting or the creation of high transient fluid pressures may drastically diminish kinetic shear resistance over all or part of the rupture surface, the effects becoming more pronounced the greater the inital shear stress. For shallow strike slip earthquakes, values of radiant flux (the wave power radiated per unit area of a fault) range from 0.1 to 10 MW/m2. This suggests that there are considerable varations in seismic efficiency and/or total energy release, even for events of similar magnitude occurring on faults of the same type.