Loading rate changes following megathrust earthquakes explored with viscoelastic models

Viscoelastic relaxation following large subduction earthquakes is known to last from years to decades, and affect the interseismic loading rate up to hundreds of kilometers in the trench perpendicular direction. Post seismic relaxation also generates a rotation pattern close to the edges of the ruptured asperity. Recently, several observations reported an accelerated loading rate coeval with megathrust ruptures, at along-trench distances from the epicenter of hundreds of kilometers.

Proposed models involved so far viscoelastic relaxation in the mantle wedge and the oceanic mantle, as well as a weak oceanic LAB layer. However those models often fail to explain simultaneously the amplitude and the spatio-temporal patterns of the observations.

Here, we perform 3D viscoelastic models of post seismic relaxation and explore a range of structural and rheological settings to investigate the mechanisms responsible for the complex loading variations observed. The tested scenarios include a Burgers rheology, viscosity contrasts between the continental and oceanic mantles, a weak LAB, and a low-viscosity layer overlying the subducting slab.

The relevance of these different models is evaluated by comparing their predictions with geodetic observations following several large earthquakes along the Chile–Peru subduction zone, allowing us to assess to assess the relative importance of the proposed mechanisms.