We discuss the role of the long-range elastic interaction between the
contacts inside an inhomogeneous frictional interface. The interaction produces
a characteristic elastic correlation length λc=a2E/kc (where
a is the distance between the contacts, kc is the elastic constant of a
contact, and E is the Young modulus of the sliding body), below which the
slider may be considered as a rigid body. The strong inter-contact interaction
leads to a narrowing of the effective threshold distribution for contact
breaking and enhances the chances for an elastic instability to appear. Above
the correlation length, r>λc, the interaction leads to screening of
local perturbations in the interface, or to appearance of collective modes ---
frictional cracks propagating as solitary waves