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Description: The Mw 7.8 Kaikōura earthquake struck New Zealand’s South Island on November 14, 2016. This event, considered the most complex rupture observed to date, caused surface rupture of at least 21 segments of the Marlborough fault system, some of them previously unknown. Puzzling features inferred from high-quality observations include a large gap separating surface rupture traces, the possibility of significant slip on the subduction interface, and slow apparent rupture speed. Here we develop a dynamic rupture model to unravel the event’s riddles in a physics-based manner. Our model reproduces key characteristics of the event and provides insights on the mechanical viability of competing hypotheses proposed to explain them. We show that the observed rupture cascade, involving strike and thrust faulting, is dynamically consistent with regional stress estimates and a crustal fault network geometry inferred from seismic and geodetic data. The rupture propagation requires a linking low-dipping shallow thrust fault, but not slip on an underlying megathrust. The complex fault system operates at low apparent friction thanks to the combined effects of overpressurized fluids, low dynamic friction and stress concentrations induced by deep fault creep.