Detailed studies of frictional sliding of granite and implications for the earthquake mechanism

Detailed laboratory measurements have been made on the frictional characteristics of Westerly granite with ground surfaces and to normal stresses of 1 kb. These measurements show that for this type of surface there are three types of sliding: continuously stable, episodic stable, and stick-slip. All three of these modes have also been observed on the San Andreas fault in central California. In our laboratory experiments, there is a transition from the first to third mode at from 5 to 15 bars normal stress, above which stick-slip predominates. Stick-slip, however, is always preceded by a small amount of stable slip. If this occurs on a large scale on faults, it may be a promising premonitory effect for earthquake prediction. Loading rate is found to influence the frictional strength inversely, which may be an explanation for stick-slip. Stress drops were found to increase with shear stress, but not linearly, suggesting that radiation efficiency may increase with stress drop.