Megathrust earthquakes exhibit a ubiquitous seismic radiation style: low-frequency (LF) seismic energy is efficiently emitted from the shallowest portion of the fault, whereas high-frequency (HF) seismic energy is efficiently emitted from the deepest part of the fault. Although this is observed in many case-specific studies, we show that it is ubiquitous in global megathrust earthquakes between 1995 and 2021. Previous studies have interpreted this as an effect of systematic depth variation in either the plate interface frictional properties (Lay et al., 2012) or the P wavespeeds (Sallarès & Ranero, 2019). This work suggests an alternative hypothesis: the interaction between waves and ruptures due to the Earth’s free surface is the leading mechanism that generates this behavior. Two-dimensional dynamic rupture simulations of subduction zone earthquakes support this hypothesis. Our simulations show that the interaction between the seismic waves reflected at the Earth’s free surface and the updip propagating rupture results in LF radiation at the source. In contrast, the downdip propagation of rupture is less affected by the free surface and is thus dominated by HF radiation typical of buried faults. To a second degree, the presence of a realistic Earth structure derived from P-wave velocity (VP) tomographic images and realistic VP/VS ratio estimated in boreholes further enhances the contrast in source radiation. We conclude that the Earth’s free surface is necessary to explain the observed megathrust earthquake radiation style, and the realistic structure of subduction zone is necessary to better predict earthquake ground motion and tsunami potential.