Role of fault branches in earthquake rupture dynamics

Abstract

We analyze earthquake ruptures propagating along a straight “main” fault and encountering a finite‐length branch fault. Such intersections are often observed in natural fault systems. The predicted effects of the interaction with the branch that we report can be remarkable; they can strongly perturb the propagation velocity on the main fault and, in some cases, even arrest that propagation. Earlier work (Kame et al., 2003; Bhat et al., 2004) emphasized the role of the fault pre‐stress state, branch geometry (i.e., branching angle), and the incoming rupture velocity at the branching junction in determining whether the rupture would follow the branch or continue on the main fault or both, through simulations which did not let a rupture on the branch encounter a barrier or a fault end (called ‘infinite’ branch cases henceforth). In this study we look at “finite” branch cases, and study the effect also of branch length, with rupture being blocked from propagation beyond the branch end. It is known that sudden stoppage of a dynamic rupture front leads to the propagation of large dynamic stress perturbations in the medium. These have been known to nucleate or terminate ruptures on adjacent fault segments (Harris et al., 1991; Harris and Day, 1993, 1999; Harris et al., 2002; Fliss et al., 2005, among others). We thus anticipate interaction between the rupture on the main fault and the branched one at two stages, when the rupture is propagating on the branch and when it is suddenly blocked at the branch end. We show that in general rupture termination on a compressional branch little affects propagation on the main fault compared to the infinite branch cases. For branches on the extensional side, we show in some cases, that whereas an infinite' branch would have allowed (or stopped) rupture propagation on the main fault, a finite branch stops (or allows) propagation on the main fault. Such results have a dependence on branch length that we document. We also illustrate branch‐related complexities in rupture velocity evolution which could be one of the sources of the high‐frequency content of strong ground motion record. Complexities in the slip distribution, often associated with a presumed heterogeneous strength distribution along the fault, can also be observed when rupture is terminated on a branch.