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Intersecting orthogonal strike‐slip faults with opposite senses of slip pose the question of what allows rupture to propagate through the junction and through both faults versus confining rupture to a single fault. I conduct dynamic rupture simulations on simplified orthogonal strike‐slip fault systems, to determine which conditions produce rupture on both component faults. In models with uniform initial tractions on both faults, slip on the first fault must reduce normal stress on the second fault for it to rupture. If the first fault ends at the cross fault, a stopping phase causes the cross fault to rupture. In models where I resolve a uniform regional stress field on the faults, only a narrow range of stress orientations allow multifault ruptures. These results will be helpful for evaluating hazard near orthogonal strike‐slip faults. Plain Language Summary: There are many examples around the world where two strike‐slip earthquake faults cross each other at nearly 90° angles. This is not remarkable when only one of the faults in a pair causes an earthquake, but it becomes notable when two or more crossing faults move at the same time. This raises the question of what causes the second fault to get involved, or not. To address this question, I use computer simulations of the physics of the earthquake process to test dozens of different fault configurations and earthquake starting points. I find that the location where the earthquake starts on the first fault controls whether the second fault is made stronger versus weaker, and therefore whether both faults can move together in one earthquake. These results can help us understand earthquake hazard around crossing faults. Key Points: Nucleation location effectively controls whether multifault rupture occurs on orthogonal strike‐slip fault systemsA stopping phase from rupture reaching the end of one fault is often required to initiate rupture on the cross faultOnly a narrow range of regional stress orientations allows both cross‐faults to rupture [ABSTRACT FROM AUTHOR]

The 2016 Kaikoura (New Zealand) earthquake is characterized as one of the most complex multifault rupture events ever observed. We perform dynamic rupture simulations to evaluate to what extent relatively simple forward models accounting for realistic fault geometry can explain the characteristics of coseismic observations. Without fine parameter tuning, our model reproduces many observed features including the multifault rupture, overall slip distribution, and the locations of the maximum slip and rupture arrest. In particular, our model shows spontaneous arrest of dynamic rupture at the both ends of the ruptured fault system due to smaller prestress levels expected from a regional tectonic stress field. Both the simulated and the observationally inferred source time functions show similar double peaks with a larger second peak. The results illuminate the importance of the 3‐D fault geometry in understanding the dynamics of complex multifault rupture. Plain Language Summary: The 2016 Kaikoura earthquake in the northern South Island of New Zealand was one of the most complex faulting events ever observed. Previous studies identified surface ruptures along at least a dozen major faults extending over 150 km. Here we use computer simulations to investigate how the complex three‐dimensional geometry of faults plays a role in the rupture propagation and termination during the Kaikoura earthquake. We find that our model can explain a number of observations associated with the earthquake. In particular, the model suggests that the termination of propagating rupture at the ends of faults was caused by the unfavorable orientations of these faults with respect to regional tectonic stresses. The results illuminate the importance of three‐dimensional fault geometry in understanding the dynamics of multifault earthquakes. Key Points: Dynamic model considering realistic fault geometry and regional stress field reproduces multifault rupture during the Kaikoura earthquakeModel shows spontaneous rupture arrest on the western Humps and Needles faults, which are unfavorably oriented in a regional stress fieldDynamic triggering of fault slip may have played an important role in transferring the rupture from the southern to northern faults [ABSTRACT FROM AUTHOR]