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The seismic potential of active low-angle normal faults (LANFs, <30° dip) remains enigmatic under Andersonian faulting theory, which predicts that normal faults dipping less than 30° should be inactive. The Alto Tiberina fault (ATF) in the northern Apennines, a partly creeping 17°-dipping LANF, has not been associated with any historical earthquakes but could potentially generate earthquakes up to Mw~7. We investigate the mechanical preconditions and dynamic plausibility of large ATF earthquakes using 3D dynamic rupture and seismic wave propagation simulations constrained by multidisciplinary data from the Alto Tiberina Near Fault Observatory (TABOO-NFO). Our models incorporate the complex non-planar ATF fault geometry, including hanging wall secondary faults and a recent geodetic coupling model. We show that potential large earthquakes (up to Mw~7.4) are mechanically viable under Andersonian extensional stress conditions if the ATF is statically relatively weak (μs=0.37). Large earthquakes only nucleate on favorably oriented, steeper fault sections (dip ≥30°), and remain confined to the coupled portion, limiting earthquake magnitude. These ruptures may dynamically trigger an intersecting synthetic branch but are unlikely to affect more distant antithetic faults. Jointly integrating fault geometry and geodetic coupling is crucial for forecasting dynamic rupture nucleation and propagation. 

Abstract The 2023 Turkey earthquake sequence involved unexpected ruptures across numerous fault segments. We present 3D dynamic rupture simulations to illuminate the complex dynamics of the earthquake doublet. Our models are constrained by observations available within days of the sequence and deliver timely, mechanically consistent explanations of the unforeseen rupture paths, diverse rupture speeds, multiple slip episodes, heterogeneous fault offsets, locally strong shaking, and fault system interactions. Our simulations link both earthquakes, matching geodetic and seismic observations and reconciling regional seismotectonics, rupture dynamics, and ground motions of a fault system represented by 10 curved dipping segments and embedded in a heterogeneous stress field. The Mw 7.8 earthquake features delayed backward branching from a steeply branching splay fault, not requiring supershear speeds. The asymmetrical dynamics of the distinct, bilateral Mw 7.7 earthquake are explained by heterogeneous fault strength, prestress orientation, fracture energy, and static stress changes from the previous earthquake. Our models explain the northward deviation of its eastern rupture and the minimal slip observed on the Sürgü fault. 3D dynamic rupture scenarios can elucidate unexpected observations shortly after major earthquakes, providing timely insights for data-driven analysis and hazard assessment toward a comprehensive, physically consistent understanding of the mechanics of multifault systems.