We discover a qualitatively new behavior for systems where the load transfer has limiting stress amplification as in real fiber composites. We find that the disorder is a relevant field leading to tri-criticality, separating a first-order regime where rupture occurs without significant precursors from a secondorder regime where the macroscopic elastic coefficient exhibit power law behavior. Our results are based on analytical analysis of fiber bundle models and numerical simulations of a two-dimensional tensorial spring-block system in which stick-slip motion and fracture compete. 05.50.+q, 46.30.Nz, 81.40.Np Typeset using REVT E X 1

[1] Accelerating displacements preceding some catastrophic landslides have been found to display a finite time singularity of the velocity v $1/(t c À t) [Voight, 1988a[Voight, , 1988b. Here we provide a physical basis for this phenomenological law based on a slider block model using a state-and velocity-dependent friction law established in the laboratory. This physical model accounts for and generalizes Voight's observation: Depending on the ratio B/A of two parameters of the rate and state friction law and on the initial frictional state of the sliding surfaces characterized by a reduced parameter X i , four possible regimes are found. Two regimes can account for an acceleration of the displacement. For B/A > 1 (velocity weakening) and X i < 1 the slider block exhibits an unstable acceleration leading to a finite time singularity of the displacement and of the velocity v $ 1/(t c À t), thus rationalizing Voight's empirical law. An acceleration of the displacement can also be reproduced in the velocity-strengthening regime for B/A < 1 and X i > 1. In this case, the acceleration of the displacement evolves toward a stable sliding with a constant sliding velocity. The two other cases (B/A < 1 and X i < 1 and B/A > 1 and X i > 1) give a deceleration of the displacement. We use the slider block friction model to analyze quantitatively the displacement and velocity data preceding two landslides, Vaiont and La Clapière. The Vaiont landslide was the catastrophic culmination of an accelerated slope velocity. La Clapière landslide was characterized by a peak of slope acceleration that followed decades of ongoing accelerating displacements succeeded by a restabilization. Our inversion of the slider block model in these data sets shows good fits and suggests a classification of the Vaiont landslide as belonging to the unstable velocity-weakening sliding regime and La Clapière landslide as belonging to the stable velocity-strengthening regime.