Interseismic deformation at subduction zones investigated by 2D numerical modeling: case study before the 2010 Maule earthquake

Abstract

We study interseismic deformation preceding the Mw8.8 2010 Maule earthquake by means of two-dimensional finite-element modeling. Our goal is to gain insight into the fundamental factors controlling elastic strain build-up and release in subduction zones, and to evaluate different modeling approaches of surface displacement as observed by GPS. We developed a linear elasticity solver that allows us to implement a realistic subducting plate geometry constrained by geophysical data. We test the influence of subducting plate thickness, variations in the updip and downdip limit of a 100% locked interplate zone, elastic parameters, and velocity reduction at the base of the subducted slab. We compared our modeled predictions with interseismic GPS observations along an EW profile crossing the Maule earthquake rupture area, in order to determine best fitting parameters. Our results indicate little influence of the subducting plate thickness at a given downdip limit, which itself has a strong influence on surface deformation. However, the fit to observations is achieved only after reducing the velocity at the base of the subducted slab below the trench region to 10% of the far-field convergence rate. We link this novel result to complementary numerical models that gradually evolve toward considering longer time-scales and complex rheology in order to evaluate the mechanical meaning of the above mentioned inferred kinematic conditions. This allowed us to link the velocity reduction at the base of subducting slabs with a long-term state of high flexural stress resulting from the mechanical interaction of the slab with the underlying mantle. Even a small amount of theses high deviatoric stresses may transfer towards the upper portion of the slab as strain energy that could participate into the mechanical loading of the megathrust and therefore in triggering large earthquakes there.