  We report on the multi-contact frictional dynamics of model elastomer
surfaces rubbed against bare glass slides. The surfaces consist of layers
patterned with thousands spherical caps (radius of curvature 100 $\mu$m)
distributed both spatially and in height, regularly or randomly. Use of
spherical asperities yields circular micro-contacts whose radius is a direct
measure of the contact pressure distribution. In addition, optical tracking of
individual contacts provides the in-plane deformations of the tangentially
loaded interface, yielding the shear force distribution. We then investigate
the stick-slip frictional dynamics of a regular hexagonal array. For all stick
phases including the initial one, slip precursors are evidenced. They are found
to propagate quasi-statically, normally to the iso- pressure contours. A simple
quasi-static model relying on the existence of interfacial stress gradients is
derived and predicts qualitatively the position of slip precursors.
