Effects of loading rate and normal stress on stress drop and stick-slip recurrence interval

In this paper we report on investigations of frictional rcstrcngthcning during repetitive stick-slip on initially bare granite surfaces (nominal contact area 25 cm²). The tests were conducted in a double-direct shear apparatus at room-temperature and humidity. Normal stress was held constant, and shear was induced by controlling the velocity of a loading piston. Samples exhibited quasi-periodic stick-slip instabilities. Appreciable yielding and precursory creep occur prior to failure. Stress drop amplitudes range from 0.1 to 3.1 MPa (or 4-49% of the failure strength). We study the effects of loading rate (0.5 to 300 µm/s) and normal load (5 and 10 MPa) on stress drop and stick-slip recurrence interval. At a given loading rate, there is a positive correlation between stress drop and recurrence interval, indicating nealing rates of ~4 MPa per decade increase in recurrence time. However, the combined data from all velocities show a lower rate, suggesting an apparent healing rate of ~1 MPa per decade increase in recurrence time. We find a consistent scaling between different loading rates when stress drop is compared to inter-event loading displacement. The data for all velocities delineate a single trend and show larger stress drops for greater pre-failure loading displacement. Failure strength and post-slip stress levels converge with increasing velocity. Our data indicate that stress drop and post-seismic stress levels depend on loading rate and recurrence interval in a complex way.