THE ROLES OF TIME AND DISPLACEMENT IN THE EVOLUTION EFFECT IN ROCK FRICTION

Room temperature friction experiments on quartzofeldspathic rocks obey a velocity dependence of strength which consists of two opposite-sens ed effects. The second of these effects has a negative velocity depend ence and evolves over a characteristic displacement. This evolution ef fect was originally attributed by Dieterich [1978; 1979] to an underly ing time-dependent process but is often described by either of two emp irical evolution laws. One depends explicitly on displacement (slip la w) and the other retains time dependence (slowness law). The slip law is favored in representing behavior around steady-state as seen in vel ocity stepping experiments. However, in this study slide-hold-slide te sts conducted at different machine stiffnesses show that the evolution effect depends on time, not slip. For the slowness law the coefficien t of time-dependent strengthening b is measured directly in slide-hold -slide tests. Existing empirical evolution laws may not be sufficient to describe both near steady-state and non steady-state behavior. Prov ided a more correct form can be found, time-dependent evolution may im prove frictional models of the seismic cycle by reducing the amount of inter-seismic slip.