Time-dependent stress changes associated with the 1999 İzmit (MW = 7.5) and Düzce (MW = 7.2) earthquakes in NW Türkiye: Implications for seismicity changes and earthquake hazard

In this study, we calculated coseismic and time-dependent viscoelastic (VE) postseismic Coulomb stress changes of the 1999 İzmit (M_W = 7.5) and Düzce (M_W = 7.2) earthquakes along the North Anatolian Fault Zone (NAFZ). We assessed these changes in relation to the seismicity surrounding M_W ≥ 5.0 earthquakes and the selected faults and locations. Excluding the 2009–2011–2012 Simav and 2020 Kırkağaç earthquake sequences, the calculated stress changes coincided with the observed seismicity changes. Our findings suggest that stress changes from nearby M_W > 6.0 earthquakes provided approximately 1.9 bars of stress load for the 2009 Simav earthquake, yet only a minor stress increase (about 0.07 bars) for the 2020 Kırkağaç earthquake compared to the stress decreases (about −0.3 and − 0.18 bars, respectively) caused by the 1999 mainshocks. Nevertheless, geological and seismological evidence from published studies indicates that magma-driven stress changes may have influenced occurrence of the Kırkağaç earthquake. For the 2006 Gemlik earthquake, the coseismic stress shadow was neutralized by VE postseismic stress load at its hypocenter in 2006, suggesting it may be a long-standing off-fault aftershock of the 1999 İzmit earthquake. We also identified a seismicity shutdown due to stress shadowing along the Bursa and İnegöl faults, whereas significant stress loads caused high seismic activity along the Elmalık and Bakacak faults, as well as at the Akyazı and Gölyaka locations. Additionally, stress change calculations along the Middle Strand of the NAFZ reveal that VE postseismic stresses have been generally rising since 1999, effectively reducing negative or augmenting positive coseismically imposed stress changes. Given the last ruptures of the Middle Strand in 1419 and 1556, this fault strand may be capable of producing two earthquakes in the order of M_W = 7.2–7.3.