An earthquake-induced stress drop on a megathrust instigates different responses on the upper plate and slab. We mimic homogenous and heterogeneous megathrust interfaces at the laboratory scale to monitor the strain relaxation on the two elastically non-identical plates by establishing analog velocity weakening and strengthening materials. A sequential elastic rebound follows the coseismic shear-stress drop in our elastic-frictional models: a fast rebound of the upper plate and the delayed and smaller rebound on the slab. A combination of the delayed rebound of the slab and the rapid relaxation of the upper plate after an elastic overshooting may accelerate the relocking of the megathrust. This acceleration triggers/antedates the failure of a nearby asperity and enhances the early backslip in the rupture area. Consequently, the trench-normal rearward displacement in the upper plate may reach a significant amount of the entire interseismic backslip and speeds up the stress build-up on upper plate faults.