The asthenosphere viscosity variations during the 2010 Maule Chile earthquake cycle inferred from finite-element modeling of GPS observations

The modeling of modern geodetic measurements has revealed that viscoelastic deformation is a prevalent process during the cycle of great subduction zone earthquakes. Simple models are sufficient for fitting the interseismic contributions of viscoelastic deformation to the measured surface velocities. However, the modeling of postseismic deformation usually involves complex rheological heterogeneities, which may suggest variations of viscoelastic behavior over the earthquake cycle. Here, we use 3-D viscoelastic finite-element models and continuous GPS observations to investigate the evolution of the apparent asthenosphere viscosities before and after the 2010 Mw 8.8 Maule earthquake in South-Central Chile. Apparent viscosities are here defined as the best-fitting Maxwell viscosities for pre-determined post-seismic time windows. Our results reveal a steady-state high interseismic viscosity of a few 1020 Pa.s, followed by heterogeneous patterns of viscosity variation during the postseismic phase. The near field (at distances <500 km inland from the trench) shows a sudden decrease in apparent viscosity of up to three orders of magnitude (i.e. 1017 Pa.s) immediately after the earthquake followed by a slow recovery of one order of magnitude (i.e. 1018 Pa.s) in the 6 years after the earthquake. This recovery is consistent with a power law rheology. The viscosity variation in far field (at distances > 800 km) is not well resolvable because part of the measured transient deformation may be from relaxation closer to the rupture. Our study hence suggests that the megathrust rupture disturbs the asthenosphere strength in the near field from a steady-state flow in the late interseismic period into a spatiotemporally heterogeneous flow in the postseismic period.