The 1999 Hector Mine Earthquake: The Dynamics of a Branched Fault System

The 1999 M 7.1 Hector Mine earthquake ruptured a complex fault system with a branched structure in the north. This fault geometry and slip pattern presents a puzzle, because the northwest branch of this system should be in the stress shadow of the north branch, upon which nucleation likely took place. Through 3D dynamic models of this event, we show that the ability of rupture to propagate to the northwest branch most likely depends on the fact that rupture did not proceed to the surface on the north branch. This slip pattern leads to part of the northwest branch being brought above its failure stress and to subsequent rupture propagation and slip occurring on that branch. A similar effect can be seen in the case in which rupture is constrained not to propagate to the base of the north branch. Models with slip over the entire north branch do not produce rupture propagation and slip on the northwest branch. The results are robust with respect to details of the prestress pattern and hypocenter location. Large heterogeneity over small length scales in the final stress pattern is a natural product of the models, even when the initial stress field is quite homogeneous. We also find that the interaction between fault geometry and stress pattern helps to explain other observations in this event, including slow rupture propagation on the northwest branch. The results help to show that 3D dynamic effects may be crucial in determining rupture propagation and slip behavior on geometrically complex fault systems, although these effects may be hard to predict without detailed knowledge of the stress pattern and fault geometry prior to earthquakes.