The Dynamics of Strike-Slip Step-Overs with Linking Dip-Slip Faults

Abstract

Fault step-overs with linking dip-slip faults are common features on long strike-slip fault systems worldwide. It has been noted by various researchers that under some circumstances, earthquakes can jump across fault step-overs to cascade into large events, while under other circumstances rupture is arrested at step-overs. There is also evidence that fault step-overs may be preferential locations for earthquake nucleation. The present work uses the 3D finite element method to model the dynamics of strike-slip fault systems with step-overs and linking dip-slip faults. I find that the presence of a linking normal or thrust fault greatly increases the ability of earthquake rupture to propagate across the step-over, leading to a larger event. Additionally, dilational step-overs with linking normal faults are more prone to through-going rupture than compressional step-overs with linking thrust faults. This difference is due to the sign of the normal stress increment on the dip-slip fault caused by slip on the strike-slip segments: Slip on the strike-slip segments causes a negative (unclamping) normal stress increment on the linking normal fault in a dilational step-over, whereas the opposite effect occurs on the linking thrust fault in a compressional step-over. Even in cases for which both dilational and compressional step-overs can experience through-going rupture, dilational step-overs typically experience higher slip, particularly on the linking normal fault. In the compressional case, rupture nucleation on the linking thrust fault may increase the likelihood of through-going rupture compared to nucleation on one of the strike-slip segments. Near the intersections between the fault segments, the stress interaction between the fault segments also causes a significant rotation of rake away from that which would be inferred from the regional stress field. The results help to emphasize the importance of two-way interactions between nearby fault segments during the earthquake rupture process. The results also may have implications for the probability of large earthquakes along geometrically complex strike-slip fault systems, and may help explain why step-overs sometimes act as barriers and other times as nucleation locations for large earthquakes.