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 various structural and rheological settings in order to test the mechanisms responsible for the complex loading variations observed. These involve a Burgers rheology, a contrast of viscosity between the continental and the oceanic mantles, a weak LAB, and a low viscosity layer atop the slab.

The pertinence of these different models is discussed against the fit to observations done after several earthquakes along the Chile-Peru subduction, in order to assess the importance of the different mechanisms.