Gas and seismicity within the Istanbul seismic gap

Understanding micro-seismicity is a critical question for earthquake hazard assessment. Since the devastating earthquakes of Izmit and Duzce in 1999, the seismicity along the submerged section of North Anatolian Fault within the Sea of Marmara (comprising the “Istanbul seismic gap”) has been extensively studied in order to infer its mechanical behaviour (creeping vs locked). So far, the seismicity has been interpreted only in terms of being tectonic-driven, although the Main Marmara Fault (MMF) is known to strike across multiple hydrocarbon gas sources. Here, we show that a large number of the aftershocks that followed the M 5.1 earthquake of July, 25th 2011 in the western Sea of Marmara, occurred within a zone of gas overpressuring in the 1.5–5 km depth range, from where pressurized gas is expected to migrate along the MMF, up to the surface sediment layers. Hence, gas-related processes should also be considered for a complete interpretation of the micro-seismicity (~M < 3) within the Istanbul offshore domain

Contribution of high-resolution 3D seismic near-seafloor imaging to reservoir-scale studies: application to the active North Anatolian Fault, Sea of Marmara

International audienceHigh Resolution (HR) marine seismic acquisition contributes to numerous research fields. The vertical resolution is of metric scale in order to study geological processes at a short time scale or to characterise small objects. 3D seismic imaging allows optimal resolution to be reached whereas 2D images are blurred mainly by side effects. Developed for the oil industry decades ago and tailored to the exploration for hydrocarbon reservoirs, 3D seismic, as applied to higher resolution targets, is more recent. Available technological advances in acquisition have allowed research institutes to develop innovative 3D high-resolution marine seismic systems tailored to these targets. The seismic survey carried out in 2009 on the Western High, Sea of Marmara, illustrates the value of HR3D imaging. Since the destructive İzmit earthquake in 1999, an intensive international research effort has demonstrated that the Western High is one of the key structures for assessing the processes of deformation related to the North Anatolian Fault (NAF). The 30-km² HR3D survey centred on the main NAF was acquired using a dual streamers-dual source-array configuration. In spite of the minimal 3D processing sequence that was applied to the data, the fine imaging of the seabed and of the sedimentary stratigraphy and structures is much better than HR2D seismic. Comparison with an autonomous underwater vehicle (AUV) multi-beam bathymetric survey carried out at the same location enables the limits of the vertical resolution of the seismic data to be assessed. The lateral resolution is between 13.5 and 25 metres at the seabed. The HR3D seismic data highlight the interplay between tectonic processes and stratigraphy. In particular, differential uplift leads to syntectonic deposition and submarine slides. The widespread occurrence of gas in the sedimentary sequence is clearly shown by anomalously high seismic amplitudes. 3D imaging of these high amplitudes enables the identification of the pathways through faults and permeable units that gas takes as it migrates to the seabed