Finite Element Modeling of Branched Ruptures Including Off-Fault Plasticity

Fault intersections are a geometric complexity that frequently occurs in nature. Here we focus on earthquake rupture behavior when a continuous planar main fault has a second fault branching off of it. We use the finite element (FE) method to examine which faults are activated and how the surrounding material responds for both elastic and elastic-plastic off-fault descriptions. Compared to an elastic model, a noncohesive elastic-plastic material, intended to account for zones of damaged rock bordering maturely slipped faults, will inhibit rupture on compressional side branches and promote rupture of extensional side branches. Activation of extensional side branches can be delayed and is triggered by continued rupture propagation on the main fault. We examine the deformation near the branching junction and find that fault opening is common for elastic materials, especially for compressional side branches. An elastic-plastic material is more realistic because elevated stresses around the propagating rupture tip and at the branching junction should bring the surrounding material to failure. With an elastic-plastic material model, fault opening is inhibited for a range of realistic material parameters. For large cohesive strengths, opening can occur, but with material softening, a real feature of plastically deforming rocks, opening can be prevented. We also discuss algorithmic artifacts that may arise due to the presence of such a triple junction. When opening does not occur, the behavior at the triple junction is simplified and standard contact routines in FE programs are able to properly represent the physical situation.