Some properties of unstable slip on rough surfaces

In this paper we report results obtained from various friction experiments under direct and oblique shear loading conditions. We used four rock types of varying brittleness (quartzite, anhydrite, limestone, pyrophyllite) with different surface roughness. The observations concentrate on the time span several milliseconds before dynamic failure occurs. During this period a premonitory, unstable phase of slip (slip 2) occurs. This differs importantly from a premonitory, stable process (slip 1) with durations of hundreds of seconds. On smooth surfaces slip 2 is usually observed with ductile rocks and less reliably with brittle rocks. Slip 2 is mostly accompanied by acoustic emissions, which increase in rate of occurrence and in magnitude until the stick-slip event. Foreshocks are observed during approximately 50% of the slip 2 events on rough surfaces. Foreshocks far exceed the acoustic noise level, which is also prevalent before stick-slip events on rough surfaces. In the direct shear experiment, where two faults are being loaded simultaneously, in about 20% of the cases precursory slip 2 was observed on the opposite side on which the final stick-slip event occurred.