  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 $\lambda_c = a^2 E / k_c$ (where
$a$ is the distance between the contacts, $k_c$ 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, $r > \lambda_c$, the interaction leads to screening of
local perturbations in the interface, or to appearance of collective modes ---
frictional cracks propagating as solitary waves.
