Initiation of Friction Instability on a Plane Fault System

Abstract

We study an unstable elastodynamic process during the initiation phase (i.e. the period between a perturbation of a unstable state and the onset of rupture propagation associated with the seismic wave radiation). We consider the elastic antiplane problem for a system of finite faults under a slip-weakening friction law. A spectral analysis is used to determine the existence, or not, of a catastrophic evolution of the slip. We find that long initiation durations are expected. We also investigate the possibility of defining an effective friction law for a finite fault with a small scale heterogeneity. The “spectral equivalence” between an heterogeneous fault system and an homogeneous fault is pointed out. Surprisingly good agreements are found between the heterogeneous fault model and the homogeneous fault with an effective friction law. Finally we analyze the initiation pattern as a possible signature of instability and we show how the weakening rate is transmitted in the elastic medium through a “domain of confidence”.