STRAIN-RATE EFFECT ON FRICTIONAL STRENGTH AND THE SLIP NUCLEATION PROCESS

The strain-rate dependence of frictional strength is investigated in relation to sliding behaviour by biaxial compression tests of large-scale granite samples. Frictional sliding is generated by increasing shear strain at a constant rate under a constant normal stress on the fault. The ram displacement is held constant for a prescribed time interval between two successive sliding events so as to have the same time effect of stationary contact between sliding surfaces. We obtained the following results. As the strain rate increases: (1) sliding becomes more unstable; (2) the nucleation zone size of stick-slip decreases; and (3) frictional strength logarithmically increases. Result (3) is consistent with the previous study by Dieterich (1979a), and (1) and (3) are similar to the strain-rate effects on failure characteristics of intact rocks reported in the literature. Result (3) can be explained by a rate- and state-dependent friction law. Our results suggest that the strain-rate dependence of the fault strength plays an important role in controlling the nucleation process of faulting.