Shear and normal load perturbations on a two‐dimensional continuous fault: 1. Static triggering

Abstract

The influence of normal and shear stress static perturbations on a strike‐slip fault is addressed on the basis of a two‐dimensional continuous and quasi‐dynamic model. Friction along the fault plane is described using a rate‐and‐state friction law with depth variable properties. Normal and shear stress perturbations result in similar effects in terms of earthquake triggering if Δτ − μ*Δσ is constant, Δτ and Δσ being the amplitude of the shear and normal stress fluctuations, respectively, and μ* being a constant which can be interpreted as the static friction coefficient on the fault in a Coulomb failure model. Therefore the Coulomb stress change ΔCFF = Δτ − μ*Δσ is a useful tool to account simultaneously for normal and shear stress variations in our model. We also show that when estimating the clock advance or clock delay of an earthquake, the simple Coulomb failure model is at first order in good agreement with our results during the first 90% of the earthquake cycle. However, it differs significantly during the last 10% due to the sharp velocity increase predicted by the rate‐and‐state friction law before rupture. This suggests that as long as static variations of stress are concerned, realistic fault models using rich, laboratory‐based, friction laws like rate‐and‐state friction laws may lead to predictions fairly close to the ones made using one of the simplest failure model, i.e., the Coulomb failure model. This may explain why Coulomb stress change computations, although often based on drastic approximations, have been able in many occasions to explain earthquake triggering sequences.