The recent 2011 Mw 9.0 Tohoku, Japan, and the 2004 Mw 9.15 Sumatra–Andaman superquakes have humbled many in earthquake research. Neither region was thought capable of earthquakes exceeding Mw∼8.4. Appealing proposed relationships to predict the size of earthquakes in subduction zones, such as that between earthquake magnitude and parameters such as lower plate age and convergence rate (Ruff and Kanamori, 1980) and plate coupling based on anchored slabs (Scholz and Campos, 1995), at least have many exceptions and may not be valid. Both earthquakes occurred where the subducting plate edge was quite...

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