Destabilization of a Heterogeneous Rate-and-State Interface

Abstract:

Among all the physics-based earthquake theories, rate-and-state friction has been one of the most successful in explaining several features of the earthquake cycle. This indicates that the laboratory derived friction laws, although deduced from experiments on centimeter to meter scale rock samples, could also be used to model the behavior of kilometer scale heterogeneous fault systems where earthquakes occur. However, the question of whether the behavior of complex faults could be described with an effective rate-and-state friction law remains poorly addressed. In this framework, the study presented here focuses on fault zones experiencing both aseismic slip and earthquakes, such as the San-Andreas fault. These heterogeneous systems could be approximated by a planar rate-and-state frictional fault between elastic solids, coupling unstable velocity weakening patches and stable velocity strengthening areas (figure). The seismicity generated by such systems is computed numerically, and we show that the different regimes of activity obtained (swarms, major unstable events, continuous activity, regular events) are separated by an effective a-b friction parameter obtained as the spatial average of the steady-state a-b parameter. In order to further investigate what are the relevant effective friction parameters controlling the mechanical behavior of heterogeneous a-b faults, we analyze the transition between the regime of regular activity and the more unstable regime where accelerating swarms develop towards major failures destabilizing the entire fault. We compare in detail the nucleation process of a such major instabilities, to the acceleration of slip on a homogeneous rate-and-state fault. The similarity in the way the two systems accelerate provides a strong evidence that effective rate-and-state friction parameters control the nucleation on a frictionally heterogeneous structure.