Deep Postseismic Viscoelastic Relaxation Excited By an Intra-Slab Normal Faulting Earthquake in the Chile Subduction Zone

Abstract:

Geodetic observations of postseismic deformation have been widely modeled to constrain the rheology of lower crust and/or upper mantle in variable tectonic environments. In subduction zones, postseismic deformation following megathrust earthquakes is often explained by afterslip and/or viscoelastic relaxation (VER) processes, and explicitly modeled to infer the viscosity of the mantle wedge. Besides the megathrust events, large intermediate-depth normal faulting earthquakes also induce stresses in surrounding lithosphere. Presumably, these stresses will be gradually released by viscoelastic relaxation in any nearby weak layer(s) and hence provide opportunities to infer rheological properties.

The 2005 M_w 7.8 Tarapaca earthquake is identified as a result of normal faulting on a west-dipping plane at depths between 90 and 115 km within the subducting slab of northern Chile (Delouis and Legrand, 2007). The Envisat-recorded short-term postseismic deformation shows an elliptical pattern of subsidence across the fault’s surface projection. We construct a semi-analytical three-dimensional model, which takes into account the vertical and horizontal heterogeneities of viscosity. Our preliminary results of VER modelling show a first-order similarity in spatial pattern with the InSAR observed deformation, assuming a Maxwell viscosity of 5e18 Pa s for the continental upper mantle and 1e20 Pa s for the oceanic upper mantle. This study demonstrates the feasibility of using normal faulting earthquakes to constrain deep subduction zone rheology.