Spherical versus flat models of coseismic and postseismic deformations

We perform an exhaustive study of coseismic and postseismic surface deforma tions induced by shear dislocations using flat and spherical Earth:models. Our aim is to examine the effects of the spherical geometry, the vertical l ayering, and the self-gravitation on surface displacement field. For a 100 km long fault, spherical and flat models produce comparable coseismic defor mations up to a distance of similar to 300 km from the epicenter. This dist ance is sensibly reduced in the postseismic regime and when infinitely long strike-slip faults are considered. The differences between predictions bas ed on hat and spherical models are due both to their global geometry and th e effect of the gravity forces. Self-gravitation has a minor role with resp ect to that of sphericity for surface coseismic deformations, while in the postseismic regime its effects increase considerably. As a case study, we c onsider the coseismic and postseismic deformations due to the great 1960 Ch ilean earthquake. The results of the spherical stratified model differ sens ibly from those of a flat uniform model. Moreover, within the framework of spherical Earth models, the rheological stratification plays a major role i n determining the pattern of the displacement field. We show that the prese nt-day rates of vertical and horizontal deformations are considerably large (similar to 10(-2) m yr(-1)) for an asthenospheric viscosity ranging from 10(19) to 10(20) Pa s. These rates, which could possibly be detected by geo detic investigations, are found to be also sensitive to the rheological pro perties of the mantle beneath the asthenosphere.