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The 17 August 1999 İzmit (M7.4) and 12 November 1999 Düzce (M7.2) earthquakes ruptured the North Anatolian Fault Zone (NAFZ) in northwestern Türkiye and caused catastrophic damage. The offshore extensions of the central and northern strands of the NAFZ beneath the Sea of Marmara remain seismically active, having produced several Mw≥ 5.0 earthquakes since 2005. In this study, we analyse the spatiotemporal evolution of Coulomb stress changes following the 1999 earthquake doublet and examine their relationship to subsequent moderate earthquakes, including the 2006 Gemlik (Mw5.0), 2019 Silivri (Mw5.7), 2023 Mudanya (Mw5.0), and 2025 Silivri (Mw6.2) events.Our models indicate that for the 2006 Gemlik and 2023 Mudanya earthquakes, coseismically imposed stress shadows generated by the 1999 ruptures were progressively erased and reversed to positive values by viscoelastic postseismic relaxation in the lower crust and upper mantle. In contrast, at the locations of the 2019 and 2025 Silivri earthquakes, positive coseismic stress changes were substantially amplified by subsequent viscoelastic processes. These results demonstrate that stress perturbations associated with the 1999 mainshocks continue to modulate seismicity along offshore Marmara fault segments over decadal timescales.In the broader context of the seismic cycle of the Main Marmara Segment, which last ruptured in 1766, the increasing occurrence of moderate-magnitude earthquakes may reflect a transition toward a late-stage, critically stressed regime. Our results suggest that long-lived viscoelastic stress transfer following the 1999 earthquakes has imposed an additional stress load on an already mature seismic cycle, potentially accelerating its progression toward failure. Accounting for such persistent, time-dependent stress interactions is therefore essential for refining time-dependent earthquake hazard assessments in this densely populated region.