Summary: rock samples. These results led to the slip- weakening
model, in which friction was assumed to decrease as slip on the
fault increased. The key model parameter was the slip-weakening
distance (the amount of slip required to reduce friction at high
speeds). This phenomenological model avoided the difficult
question of the origin of friction. Mechanical studies of rocks
in the late 1970s provided the first experimental evi- dence
that steady-state friction indeed decreased logarithmically with
slip rate. Friction also depends on several parameters
representing the state of the slipping surface (3). In these
experiments, designed to under- stand friction at low slip
rates, the slip-weak- ening distance is very small, on the order
of a fraction of 1 mm. In the past 15 years, seismologists were
able to study in detail several major earth-