A nonlinear mathematical model of an unsteady-state slip on a contact betwe
en samples is proposed. The effect of friction and geometrical irregulariti
es on the evolution rate of creeping waves is taken into account. An unstea
dy-state (stick-slip) motion on an uneven surface of a rock fault is shown
to be a nonlinear process and can be described by the generalized sine-Gord
on equation. The resulting theoretical estimates agree with data of laborat
ory measurements.