Non-recoverable strain during the megathrust seismic cycle

Upper-plate deformation during the subduction zone seismic cycle is classically modeled as elastic, assuming the only non-reversible strain occurs on the megathrust. However, recent geomorphological studies indicate a slow build-up of distributed deformation across the upper plate over hundreds of thousands of years, with a spatial distribution that bears similarities with the interseismic strain field (e.g., Meade, 2010; Saillard et al., 2017; Malatesta et al., 2021). This suggests that non-reversible strain somehow related to seismic cycle deformation accumulates over hundreds of cycles. Oryan et al. 2024 recently suggested that portions of the upper plate could be brought to brittle failure during the interseismic period, manifesting as diffuse seismicity. Extrapolating the cumulative displacements due to this seismicity over many cycles further yielded patterns of surface uplift consistent with geomorphological observations, and correlating with the megathrust locking state. It did not, however, explicitly tie the occurrence of brittle failure to the rheological properties of the upper plate.

In this work, we investigate the hypothesis that the accumulation and release of elastic deformation between and during earthquakes can produce unrecoverable deformation, leaving a distinct signature in subduction relief. We use the commercial finite element code Zset (http://zset-software.com/) to simulate multiple cycles of loading and unloading of a wedge-shaped upper plate domain imparted by interseismic megathrust locking and coseismic slip. We model the upper plate as a Bingham elasto-visco-plastic material where irreversible viscous deformation can be activated wherever a certain yield stress threshold is exceeded. This typically occurs over the area where the megathrust transitions from fully locked to fully creeping during the interseismic phase. As a result, small increments of irreversible strain accumulate at each cycle, which manifests as persistent surface uplift above the downdip end of the locked portion of the megathrust. We perform a parametric study to examine the relationships between relief development, the plastic strength of the upper plate, and the coupling state of the megathrust. This provides a blueprint for assessing how locking patterns may become encoded in subduction landscapes, and how persistent these patterns may be.