Evolution of the seismicity in the eastern Marmara Sea a decade before and after the 17 August 1999 Izmit earthquake

We review the long term evolution of seismicity in the eastern Marmara Sea over a decade, before and after the 1999 Mw 7.6 Izmit earthquake. We analyze large scale space-time variations of seismicity in the region and illustrate the impact of the recent large strike-slip earthquakes on the background activity composed of distinct pre-existing seismic clusters. Two types of aftershocks activity are observed: the first type of enhancement is on strike-slip fault segments (Izmit Fault, Princes Island section of the Main Marmara Fault, Gemlik Fault) immediately following the main shock and related to Coulomb stress transfer; the second type of enhancement is attached to extensional clusters (Yalova, Tuzla) with a few days delay in the onset of strong activation, probably related to pore pressure increase. We observe a fast decay of the activity on strike-slip segments and slower evolution of seismic clusters with extensional features. Two years after the Izmit earthquake, seismic activity returned to the pre-earthquake pattern with most of the activity occurring within extensional clusters. It appears that the influence of the last large strike-slip event on the spatial seismicity distribution in the eastern Marmara Sea is less significant than the effect of the long term regional extension. (C) 2011 Elsevier B.V. All rights reserved.