<p>Upper-plate aftershocks following megathrust earthquakes are particularly dangerous as they may occur close to densely populated regions. Aftershock numbers decay with time, imposing a time-dependent seismic hazard that is assessed with statistical forecast models. While coseismic static stress transfer cannot explain this time-dependency, transient postseismic deformation due to afterslip, viscoelastic relaxation, and pore-pressure diffusion are potential candidates. Here we demonstrate which postseismic process is the key driver of the upper-plate aftershocks pattern following the 2014 M_<i>w</i> = 8.2 Iquique earthquake in northern Chile. We first use a 4D (space and time) model approach to reproduce the postseismic deformation observed in geodetic data. We then analyze the spatiotemporal stress changes produced by individual postseismic processes and compare them to the upper-plate aftershocks distribution. Our results reveal that stress changes produced by coseismically-induced pore-pressure diffusion best correlate in space and time with increased upper-plate aftershock activity. Moreover, an increase in pore-pressure reduces the three effective principal stress magnitudes likewise. Hence, all faults, regardless of their orientations, are brought closer to failure. This explains the higher diversity of the aftershocks faulting styles. Our findings provide further insights into the link between pore-pressure diffusion and upper-plate deformation in subduction zones and provide grounds for a physics-based aftershock forecast.</p>