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In the present study seismicity of the Southern Marmara Region before and after the 1999 İzmit earthquake are analyzed and compared. A homogeneous seismicity catalogue that have completeness above the MC=2.9 and covers the time period between 1978 and 2020 is used. Comparison of the spatial mapping of the frequency-magnitude distribution before and after the 1999 İzmit earthquake revealed that b-values demonstrate a general increase after the earthquake indicating a general stress decrease in the region. The shortest computed TL value of about 450 years in the east of city of Bursa vanished after the 1999 earthquake. The computed time variations of b-value have shown an increase from 0.8 to 1.6 between 1978 and 1997 and an anomalous increase from 1.1 to 2.1 between 2000 and 2006. After 2006, b-values have decreased from 2.1 to 0.8, implying that decreased stress after the 1999 İzmit earthquake begun to increase after that year. Since the historical seismicity indicates no large earthquake on the fault segments of the North Anatolian Fault Zone extending along the towns of Geyve, İznik, Gemlik and Bandırma are considered to be most likely faults to host the next destructive earthquakes in the region.