Upper plate dynamic response to a sequential elastic rebound and slab acceleration in laboratory-scale subduction megathrust

Abstract

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 bi-material plates by establishing analog velocity weakening and neutral 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 rebound of the slab and the rapid relaxation (i.e., elastic restoration) 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 slip reversal in the rupture area. Hence, the trench-normal rearward displacement in the upper plate may reach a significant amount of the entire interseismic slip reversal and speeds up the stress build-up on upper plate backthrust. Moreover, the backthrust switches its kinematic mode from a normal to reverse mechanism reflecting the sense of shear on the interface during coseismic and postseismic stages.