Understanding the behavior of the shallow portion of the subduction zone, which generates the largest earthquakes and devastating tsunamis, is a vital step forward in earthquake geoscience. Monitoring only a fraction of a single megathrust earthquake cycle and the offshore location of the source of these earthquakes are the foremost reasons for the insufficient understanding. The frictional-elastoplastic interaction between the interface and its overlying wedge causes variable surface strain signals such that the wedge strain patterns may reveal the mechanical state of the interface. We employ Seismotectonic Scale Modeling and simplify elastoplastic megathrust subduction, generate hundreds of analog seismic cycles at laboratory scale, and monitor the surface strain signals over the model’s forearc over high to low temporal resolutions. We establish two coseismically compressional and extensional wedge configurations to explore the mechanical and kinematic interaction between the shallow wedge and the interface. Our results demonstrate that this interaction can partition the wedge into different segments such that the anlastic extensional segment overlays the seismogenic zone at depth. Moreover, the different segments of the wedge may switch their state from compression/extension to extension/compression domains. We highlight that a more segmented upper plate represents megathrust subduction that generates more characteristic and periodic events. Additionally, the strain time series reveals that the strain state may remain quasi-stable over a few seismic cycles in the coastal zone and then switch to the opposite mode. These observations are crucial for evaluating earthquake-related morphotectonic markers (i.e., marine terraces) and short-term interseismic GPS time-series onshore (coastal region).

Sediment progradation and spreading is a key process during gravity-driven, thin-skinned deformation in salt-bearing passive margins. However, to what degree the size and shape of a progradational sedimentary wedge control gravity-driven deformation is still not clear. We use analogue modelling to compare two endmember configurations constrained by critical wedge theory, in which the sediment wedge has different initial depositional slopes: a 5° critical (stable) slope and a 27° unstable slope. In both configurations, differential loading initiates spreading characterized by a basinward migrating system of linked proximal extension and distal contraction with a translational domain in between. With a critical frontal slope, the translational domain expands as the contractional domain migrates forward with viscous flow evenly distributed. With a steep frontal slope, both extensional and contractional domains migrate at similar rate due to more localized viscous flow under the wedge toe producing diagnostic structures of late extension overprinting early contraction. In both cases, salt flow is dominated by Poiseuille flow with only a subordinate contribution from Couette flow, contrasting to classical gravity gliding systems dominated by Couette flow. Comparison with previous studies reveal similar structural styles and viscous flow patterns. Our study highlights the geometric variations of sedimentary wedges result in variable responses in gravity spreading systems. With a steep frontal slope, the sediment wedge is more likely to collapse and develop spreading associated structures. However, such steep slope systems may not be very common in salt-bearing passive margins as they are less likely to occur in nature.