Interseismic crustal deformation between successive subduction earthquakes is controlled by the locking state of the megathrust and viscoelastic Earth rheology. At the late stage of the interseismic phase, such as Cascadia today, deformation features wholesale landward motion, subsidence over the locked zone, and uplift landward of it. However, improved geodetic observations revealed the common presence of a Secondary Zone of Subsidence (SZS) farther landward around the volcanic arc. Here we use numerical models of megathrust earthquake cycles in a viscoelastic Earth to investigate this phenomenon. Besides successfully reproducing deformation patterns observed at global subduction zones that are at various stages of their earthquake cycles, our models elucidate the process of SZS development. After an earthquake, the arc area first undergoes pronounced uplift – a recently recognized geodetic signature of the cold forearc mantle wedge. Less than halfway through the interseismic phase, the uplift reverses to subsidence, giving rise to the SZS. A longer recurrence interval or lower mantle wedge viscosity results in a more pronounced SZS. The SZS is a fundamental consequence of Earth’s viscoelasticity and is conspicuously absent in elastic models. Recognition of the SZS is important to understanding megathrust earthquake potential, especially in regions lacking adequate horizontal geodetic constraints. For example, it is under heavy debate whether the Lesser Antilles megathrust is currently locked. GPS measurements indicate subsidence along the arc that cannot be explained by elastic models. Our viscoelastic modelling shows it to be the SZS due to interseismic megathrust locking, implying potential for future large earthquakes.