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-