Accelerating seismic crustal deformation before the Izmit (NW Turkey) large mainshock of 1999 August 17 and the evolution of its aftershock sequence

The large Izmit (NW Turkey) mainshock (1999 August 17, Mw= 7.6), followed by another large earthquake on 1999 November 12 (Mw= 7.2), caused extensive damage and loss of life in a zone approximately 170 km long along the coastal area of the Gulf of Izmit and further east to Adapazar? and Düzce. On the basis of the critical earthquake concept and by applying a recently developed optimization algorithm, an elliptical area surrounding the 1999 August 17 mainshock epicentre was identified, in which accelerating moderate magnitude seismic activity started in 1981 and culminated in the generation of the mainshock. On the other hand, the space and time distribution of the aftershocks of the first mainshock (1999 August 17) and the time variations of the b value of the Gutenberg–Richter recurrence law and the mean aftershock magnitude, suggest that the second mainshock (1999 November 12) might have been anticipated.