Istanbul University, Department of Geophysical Engineering, 34850 Avcýlar-Istanbul/Turkiye.

Abstract-In order to investigate the effect of aftershocks on seismic risk estimation, earthquake hazard parameters (b, l _m and M_max) have been estimated by the maximum likelihood method from the main shocks catalogue and the whole earthquakes catalogue for the north Anatolian fault zone. The main shocks catalogue has been compiled from the whole earthquakes catalogue by eliminating the aftershocks using the window method. The whole earthquakes catalogue consisted of the historical data between 1000-1900 and instrumental data between 1900 and 1992. The maximum likelihood method used in this study allows to use the mixed earthquake catalogue containing incomplete (historical) and complete (instrumental) earthquake data. The maximum regional magnitude (M_max), the seismic activity rate (l _m), the mean return period (R) and the b value of the magnitude-frequency relation have been estimated for the 24°-31°E, 31°-41°E, 41°-45°E sections of the north Anatolian fault zone from the whole earthquakes catalogue and the main shocks catalogue. Our results indicated that inclusion of aftershocks may change the b value and the seismic activity rate l about % 11-19 and % 75-106 percents respectively, while it does not significantly effect the value of the maximum regional magnitude. These changes in the earthquake hazard parameters did not significantly effect the return periods for earthquakes of magnitude M>6.0.