The time-dependent deformation of Westerly granite containing a saw cut separated by crushed Westerly granite fault gouge was studied at 4 kbars of confining pressure. Differential stress was increased in steps and held constant at each stress level while fault creep was monitored. Logrithmically decaying primary creep occurred immediately after each increase in the differential stress level. Primary creep was followed by constant rate secondary creep above 9,200 bars of differential stress and the slip rate increased rapidly with increasing stress levels. At 10,834 bars of differential stress, primary and secondary creep were followed by accelerating tertiary creep, which culminated in violent slip. Our results may be applicable to earthquake prediction along the central San Andreas fault. They suggest that sudden seismic slip in the area of constant rate fault creep should be preceded by a period of accelerating creep.
Laboratory experiments show that several kilo bars of differential stress are required to cause fracture or seismic slip of rock under pressures that exist in the mid crust regardless of rock type or the presence of fault gouge . Stress drops calculated for crustal earthquakes, however, are generally accepted to be on the order of tens of bars , and are less than one hundred bars even for very large earthquakes. This suggests that some major faults such as the San Andreas may be under conditions of nearly constant differential stress . Although considerable attention has been devoted to the experimental creep of intact rock [4, 5, 6, 7, 8] little work has been performed on the creep of simulated faults. Johnson  noted pre-stick slip creep in experiments concerning the frictional properties of Westerly granite, Twin Sisters dunite and Spruce Pine dunite. Their experiments, however, were conducted under constant strain rate rather than constant differential stress conditions.