Influence of temperature-controlled non-linear viscoelastic rheology on interseismic surface deformation signals in subduction zones.

The earthquake seismic cycle consists of the gradual accumulation of elastic energy at plate boundaries during the interseismic period, followed by its release mainly during the coseismic and postseismic stages. Therefore, for the evaluation of the seismic moment accumulate rate along the plate boundary, we need to quantify the processes and rheologies that control the interseismic surface deformation that is observed by GNSS stations. 

Recent studies have shown that during the late interseismic phase, the GNSS-observed surface velocities can be explained by a combination of aseismic fault slip and viscoelastic deformation in the upper mantle. These works also demonstrate that the vertical GNSS component is particularly crucial for distinguishing between different rheological processes acting at depth. However, most of these deformation studies neglect the thermal structure of the lithosphere-asthenosphere system and its impact on the viscoelastic deformation processes in the upper mantle, and especially within the lower continental crust.

To explore the impact of the temperature field, we investigate four subduction zones with contrasting incoming plate geometries, ages, dips, and convergence rates. We use 2D interseismic deformation models  based on the Finite Element Method (FEM) with temperature-controlled viscoelastic power-law rheology that represent the Nankai, Japan, Cascadia, and northern Chile subduction systems.  We systematically compare linear and nonlinear rheological formulations across distinct thermal and tectonic environments to assess their impact on the interseismic deformation process. Our preliminary results indicate that thermally-controlled nonlinear viscoelasticity can alter both the magnitude and spatial distribution of vertical interseismic deformation. In regions with higher temperatures in the continental mantle (e.g., Nankai, Japan, and northern Chile) the nonlinear rheology can produce uplift and subsidence patterns that diverge from those predicted by linear viscoelastic models. This highlights the sensitivity of vertical deformation to the chosen rheological formulation and suggests that models with linear viscoelastic rheology may not always be sufficient to represent the details of the processes controlling the interseismic deformation signal. However, when the interseismic deformation signal is small (e.g. Cascadia), the difference between linear and non-linear rheology is too little to be resolved within the GNSS data uncertainty. 

Furthermore, our models predict differences in vertical surface deformation of ~20% near the trench and exceeding 100% in the far-field back-arc region between linear and nonlinear viscoelastic models, regions where GNSS data are generally absent or where there is poor coverage. Here seafloor geodetic observations from acoustic-GNSS and pressure gauges are especially valuable, as they provide direct constraints on near-trench deformation that cannot be resolved from land-based networks alone.

In this context, our models can help in identifying regions where nonlinear rheological effects are most likely to be observable and therefore offer guidance for the strategic deployment of offshore geodetic instrumentation to better resolve interseismic deformation processes in subduction zones.