Surface deformation transients measured with the Global Positioning System during the 87 days between the 17 August 1999 İzmit earthquake and the 12 November 1999 Düzce earthquake indicate rapidly decaying aseismic fault slip on and well below the coseismic rupture. Elastic model inversions for time-dependent distributed fault slip, using a network inversion filter approach, show that afterslip was highest between and below the regions of maximum coseismic slip and propagated downward to, or even below, the base of the crust. Maximum afterslip rates decayed from greater than 2 m/yr, immediately after the İzmit earthquake to about 1.2 m/yr just prior to the Düzce earthquake. Maximum afterslip occurred below the eastern Karadere rupture segment and near the İzmit hypocenter. Afterslip in the upper 16 km decayed more rapidly than that below the seismogenic zone. These observations are consistent with a phase of rapid aseismic fault slip concentrated near the base of the seismogenic zone. Continued loading from the rapid deep afterslip along the eastern rupture zone is a plausible mechanism that helped trigger the nearby, Mw 7.2, 12 November Düzce earthquake.