  We model the progressive maturation of a heterogeneous mass towards a
gravity-driven instability, characterized by the competition between frictional
sliding and tension cracking, using array of slider blocks on an inclined basal
surface, which interact via elastic-brittle springs. A realistic state- and
rate-dependent friction law describes the block-surface interaction. The inner
material damage occurs via stress corrosion. Three regimes, controlling the
mass instability and its precursory behavior, are classified as a function of
the ratio $T_c/T_f$ of two characteristic time scales associated with internal
damage/creep and with frictional sliding. For $T_c/T_f \gg 1$, the whole mass
undergoes a series of internal stick and slip events, associated with an
initial slow average downward motion of the whole mass, and progressively
accelerates until a global coherent runaway is observed. For $T_c/T_f \ll 1$,
creep/damage occurs sufficiently fast compared with nucleation of sliding,
causing bonds to break, and the bottom part of the mass undergoes a
fragmentation process with the creation of a heterogeneous population of
sliding blocks. For the intermediate regime $T_c/T_f \sim 1$, a macroscopic
crack nucleates and propagates along the location of the largest curvature
associated with the change of slope from the stable frictional state in the
upper part to the unstable frictional sliding state in the lower part. The
other important parameter is the Young modulus $Y$ which controls the
correlation length of displacements in the system.
