Asthenosphere rheology and the continental-scale interseismic deformation driven by repeated megathrust earthquakes: Insights from the South American plate

Hugo Boulze , Luce Fleitout, Emilie Klein , Christophe Vigny

This study presents a framework for quantifying large-scale interseismic deformation of a tectonic plate overlying a viscoelastic asthenosphere. We explain and compare four common methods for modelling upper-plate surface deformation induced by time-dependent slip on the subduction interface. One method is a widely used, purely elastic model. The other three are viscoelastic models, in which slip on the subduction interface is either steady over time, steady over time but with the impact of the last earthquake taken into account, or represented as periodic earthquake cycles.

For the three viscoelastic models, two asthenospheric rheologies are investigated. The first is a Maxwell rheology with a single viscosity, inferred from early postseismic deformation (3 x 10^18 Pa.s). This model reasonably reproduces short-term deformation. The second is a Burgers rheology with a comparable short-term viscosity, but with a higher long-term effective viscosity (3 x 10^19 Pa.s). This higher long-term viscosity is constrained by decadal-to-centennial observations. Therefore, this model alone accounts for deformation across all timescales and spatial scales. These observations include a trenchward deformation persisting 50 years after the 1960 M_w9.5 Valdivia earthquake, and deformation from northern Chile, where deformation reflects coupling along segments that have not ruptured for decades.

All models are evaluated using a finite-element mesh spanning an 8000 km-wide region around the Chilean subduction margin. For the same coupling, near the coast, all models yield similar deformation, with differences smaller than 20%. However, discrepancies increase markedly around 200 km inland. In the continental interior, the elastic model consistently underestimates interseismic velocities. Only the Burgers rheology, combined with a realistic cyclic slip history, reproduces the observed deformation over the entire continent and across annual to centennial timescales. We constrain the acceptable parameter range of this simplified Burgers rheology. This requires a Kelvin-Voigt shear modulus 4 to 15 times smaller than the elastic shear modulus.

These results demonstrate that the effective viscosity of the asthenosphere increases markedly from annual to decadal or longer-term timescales. They also show that millennium-scale strain accumulation must be accounted for when interpreting interseismic deformation. They call into question estimates of coupling depth and sliver motion based on purely elastic models.