We investigate spatial clustering of 2414 aftershocks along the Izmit M-w = 7.4 August 17,1999 earthquake rupture zone. 25 days prior to the Duzce earthquake M-w=7.2 (November 12, 1999), we analyze two spatial clusters, namely Sakarya (SC) and Karadere-Duzce (KDC). We determine the earthquake frequency-magnitude distribution (b-value) for both clusters. We find two high b-value zones in SC and one high b-value zone in KDC which are in agreement with large coseismic surface displacements along the Izmit rupture. The b-values are significantly lower at the eastern end of the Izmit rupture where the Duzce mainshock occurred. These low b-values at depth are correlated with low postseismic slip rate and positive Coloumb stress change along KDC. Since low b-values are hypothesized with high stress levels, we propose that at the depth of the Duzce hypocenter (12.5 km), earthquakes are triggered at higher stresses compared to shallower crustal earthquake. The decrease in b-value from the Karadere segment towards the Duzce Basin supports this low b-value high stress hypothesis at the eastern end of the Izmit rupture. Consequently, we detect three asperity regions which are correlated with high b-value zones along the Izmit rupture. According to aftershock distribution the half of the Duzce fault segment was active before the 12 November 1999 Duzce mainshock. This part is correlated with low b-values which mean high stress concentration in the Duzce Basin. This high density aftershock activity presumably helped to trigger the Duzce event (M-w = 7.2) after the Izmit M-w 7.4 mainshock. (C) 2009 Elsevier B.V. All rights reserved.