We have modeled linear viscoelastic relaxation of a uniform-viscosity asthenosphere following large earthquakes in the Aegean-Anatolian region during the 20th century to evaluate whether this process contributes significantly to the regional velocity field. We address in particular whether southward motion of the Aegean Sea region relative to central Anatolia (and thus extension of western Turkey) can be attributed to a postseismic strain pulse. Our models show that postseismic relaxation of a thick (250 km) viscoelastic substrate cannot generate the observed extension in western Turkey, regardless of the choice of viscosity. We also find that the asthenosphere viscosity must exceed about 5 ¿ 1020 Pa s, at least north of the North Anatolian Fault Zone (NAFZ), to be consistent with low GPS site velocities in the Black Sea region and highly localized strain around the fault.