<p>Surface deformation observed in subduction zone forearcs helps to determine the locking state of the megathrust beneath, and therefore seismic and tsunami hazards. The vertical component of such deformation is particularly important, but measurements of this component in various subduction zones show a level of complexity that is poorly understood. Here we demonstrate from numerical simulations and a global compilation of observations that this apparent complexity can be readily explained in terms of earthquake cycles in a viscoelastic Earth. We show that subduction zones follow a common process of earthquake cycle evolution but are currently at various stages of this cycle, and that, during interseismic deformation, there is a previously overlooked secondary zone of subsidence around the volcanic arc, in addition to the primary zone of subsidence near the trench. We propose that this secondary zone is a key signature of megathrust locking that is absent from the elastic models commonly used to infer the locking state. The importance of this signature is demonstrated by the Lesser Antilles subduction zone, where we argue that the ongoing subsidence of the island arc is strong evidence for the presence of such a secondary zone; this implies that the megathrust is locked and building energy for a future earthquake, contrary to the prevailing understanding.</p>