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Earthquake moment-area scaling relations play a key role in both earthquake physics studies and earthquake hazard assessment. A three-stage moment-area relation, based on advances in earthquake source inversion, is currently in use in Japan. The second stage has a scaling exponent outside the range of commonly accepted models of small and very large earthquakes. We develop theoretical insight on the mechanical origin of this second-stage scaling. We utilize an analytical dislocation model, a numerical crack model and multi-cycle rate-and-state simulations of strike-slip faults with heterogeneous friction properties. We find that the second stage in earthquake moment-area scaling results from a combination of surface rupture effects, comprising an effective rupture elongation along-dip due to a mirror effect and systematic changes of the shape factor relating slip to stress drop. Based on this physical …