Large earthquakes on plate boundary megathrust faults exhibit substantial variations in rupture characteristics determined from seismic wave observations. We analyze large earthquakes with Mw ≥ 7.0 identified as being on megathrust faults, examining the source scaling behavior, spatial patterns, and depth-dependence of the source parameters. Depth-dependent characteristics have previously been recognized in the occurrence of shallow tsunami earthquakes with depleted short-period source spectra and unusually long source durations, and in enhanced coherent short-period energy release from the deeper portions of megathrusts that fail in huge ruptures. There is little depth-dependence of stress drop or total radiated energy release for megathrust events that are not tsunami earthquakes, but there is systematic increase in relative short-period energy content and reduced high-frequency fall-off slope of the source spectra as depth increases. We consider the implications of the new source parameter data set for mechanisms operating in the megathrust environment.