The majority of global seismicity originates at subduction zones, either within the converging plates or along the plate interface. In particular, events with Mw ≥ 8.0 usually occur at the subduction megathrust, which is the frictional interface between subducting and overriding plates. Consequently, seismicity at subduction megathrusts is responsible for most of the seismic energy globally released during the last century [Pacheco and Sykes, 1992]. What's more, during the last decade giant megathrust earthquakes occurred at an increased rate with respect to the last century [Ammon et al., 2010], often revealing unexpected characteristics and resulting in catastrophic effects. Determining the controlling factors of these events would have fundamental implications for earthquake and tsunami hazard assessment.