North Anatolian Fault off the southern coast of Istanbul. This suggests that the 150-km long submarine seismic gap below the Sea of Marmara could point out to a future, large earthquake. These possibilities are quite important, with respect to the segmentation of major fault ruptures along the North Anatolian Fault Zone in north-western Turkey. With the possible activation of segments towards the metropolitan areas of Istanbul, the Princes' Islands gap should be considered to have an impact on the large seismic hazard potential for Istanbul. Despite a long-term earthquake catalogue existing for the North Anatolian Fault Zone and for the Istanbul area in particular, the basic understanding of the seismicity there is still a long way off. The observation of a seismic gap in vicinity to the Istanbul metropolitan area was made possible through deploying a dense network of seismic stations and small arrays near the fault trace south of the Princes' Islands. This improved monitoring along the Princes' Islands segment, which is west of the İzmit 1999 rupture and southeast of Istanbul's city center is highlighting the location of likely rupture points for the pending Marmara earthquake. It also limits the maximum size of future events along the whole Marmara seismic gap in case of cascade behavior. Knowing this, the need of a regional earthquake early warning system for Istanbul and surroundings could be justified. The aseismic part of the Princes' Islands segment could represent a potential and likely high-slip area in a future, large earthquake. Fault characterization is likewise very relevant to determine the directivity of earthquake waves approaching Istanbul. Recently made modelling of potential impacts to Istanbul from different scenarios have shown to improve the estimation of hazards the seismic gaps pose. In a similar way, more improved and dense seismic monitoring is expected from on-going efforts to install an underground (borehole-based) seismograph network in the eastern Sea of Marmara region. Istanbul, being the most populated city in Turkey, lies right near the segments of the North Anatolian Fault Zone, making it at very high risk to an earthquake disaster which could possibly cause thousands of casualties and severe damage. Following the large earthquake in 1999, there was a great urgency for the government to mitigate these risks. With the help of organizations like the World Bank, hundreds of buildings have been retrofitted and reconstructed, and thousands of citizens have been trained in disaster preparedness. Gallery See