  We discuss the stick-slip motion of an elastic block sliding along a rigid
substrate. We argue that for a given external shear stress this system shows a
discontinuous nonequilibrium transition from a uniform stick state to uniform
sliding at some critical stress which is nothing but the Griffith threshold for
crack propagation. An inhomogeneous mode of sliding occurs, when the driving
velocity is prescribed instead of the external stress. A transition to
homogeneous sliding occurs at a critical velocity, which is related to the
critical stress. We solve the elastic problem for a steady-state motion of a
periodic stick-slip pattern and derive equations of motion for the tip and
resticking end of the slip pulses. In the slip regions we use the linear
viscous friction law and do not assume any intrinsic instabilities even at
small sliding velocities. We find that, as in many other pattern forming
system, the steady-state analysis itself does not select uniquely all the
internal parameters of the pattern, especially the primary wavelength. Using
some plausible analogy to first order phase transitions we discuss a ``soft''
selection mechanism. This allows to estimate internal parameters such as crack
velocities, primary wavelength and relative fraction of the slip phase as
function of the driving velocity. The relevance of our results to recent
experiments is discussed.
