Coseismic displacement interferences and patterns along subduction megathrusts: insights from analog modelling

Finding a deformation pattern that is representative of a given stage of the seismic cycle of subduction megathrusts is crucial as this might provide clues about the upcoming earthquake. Here we focus on the short term interaction between seismic asperities and in particular on how geodetic velocities change in response to ruptures of an along-strike neighbor portion of the megathrust. Enhanced megathrust coupling, slab acceleration, in plane bending of the overriding plate, continental-scale viscoelastic mantle relaxation have been proposed as potentially responsible driving mechanisms. However, the paucity of observations from natural cases and the multiple- interrelated contributions that act at different spatial and temporal scales complicate the understanding of this process.

We use an analog model that simulates a series of laterally partial ruptures and analyze systematically the effect of slip episodes on deformation history of the neighbor “receiver” region. The analog model has the advantage of reproducing tens of seismic cycles with well controlled boundary conditions. The model shows that the deformation pattern associated to slip episodes has a characteristic twisting about a vertical axis. Such twisting interfere positively (causing velocity increase) or negatively (causing velocity decrease) with local interseismic velocity field depending on time since the last earthquake. Identifying accelerating or decelerating velocities in geodetic timeseries could be therefore informative of the seismic evolution of a subduction zone.