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Friction experiments have been conducted on porous sandstone, quartzite, graywacke, and granite in the 20- to 850-bar normal stress range. Sliding on clean rough-ground surfaces is initially stable for this range. However, as powered rock debris accumulates on the slip surface, stick slip becomes the dominant mode of sliding. The coefficient of static friction of surfaces with gouge exhibits a highly time-dependent behavior. Static friction increases with the logarithm of the time that adjacent blocks remain in stationary contact. Over the entire range of normal stresses the static friction for 105-sec intervals between stick-slip events is greater than the static friction for 15-sec intervals by 6 to 10%. This behavior may be significant in understanding the mechanisms of earthquake foreshocks, aftershocks, and fault creep.
James H. Dieterich (Mon,) studied this question.