Along the North Anatolian fault (NAF), the surface deformation associated with tectonic block motions, elastic strain accumulation, and the viscoelastic response to past earthquakes has been geodetically observed over the last two decades. These observations include campaign‐mode Global Positioning System (GPS) velocities from the decade prior to the 1999 Mw 7.4 İzmit earthquake and seven years of continuously recorded postseismic deformation following the seismic event. Here, we develop a 3D viscoelastic block model of the greater NAF region, including the last 2000 yrs of earthquake history across Anatolia, to simultaneously explain geodetic observations from both before and after the İzmit earthquake. With a phenomenologically motivated simple two‐layer structure (schizosphere and plastosphere) and a Burgers rheology (with Maxwell viscosity log10ηM≈18.6–19.0  Pa·s and Kelvin viscosity log10ηK≈18.0–19.0  Pa·s), a block model that incorporates tectonic plate motions, interseismic elastic strain accumulation, transient viscoelastic perturbations, and internal strain can explain both the pre‐ and post‐İzmit earthquake observations with a single unified model. Viscoelastic corrections to the interseismic GPS velocity field with the unified model reach magnitudes of ∼2.9  mm/yr. Geodetically constrained slip‐deficit rate estimates along the central NAF and northern strand of the NAF in the Sea of Marmara vary nonmonotonically with Maxwell viscosity and change by up to 23% (∼4  mm/yr) for viscosities ranging from 1018 to 1023  Pa·s. For the best‐fit viscosity structures, central NAF slip‐deficit rates reach 22  mm/yr, increasing to 28  mm/yr in the Sea of Marmara. Along the central NAF, these rates are similar to the fastest geologic slip‐rate estimates. The fastest slip‐deficit rate estimates along the entire fault system (∼27–28  mm/yr) occur less than 50 km from Istanbul, along the northern strand of the NAF in the Sea of Marmara.

Electronic Supplement:Figure of sensitivity of viscoelastic block model slip‐deficit rate estimates and contour plot of mean residual improvement.

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