We consider refined modeling of GPS measurements of coseismic displacement for two large earthquakes occurring in the Japan and Kurile Trenches. Calculations are performed for a layered sphere and the homogeneous half-space more commonly used to model such data, using fault geometry and slip distributions from previously published studies. We show that the difference between the layered sphere and homogeneous half-space calculations can be significant, as expected from previous theoretical studies. However, it does not appear large enough to seriously affect results from previously published studies of these earthquakes. Nevertheless, the fact that the effects of sphericity and especially layering are measurable in coseismic observations of great earthquakes at moderate distances indicates that these effects should be taken into account in order to fully exploit the precision of modern GPS measurements. ¿ 1998 American Geophysical Union