Fluid emissions in the Sea of Marmara: distribution, controls and impacts

Understanding of the evolution of fluid?fault interactions during earthquake cycles is a challenge that acoustic gas emission studies can contribute. The Sea of Marmara, which results from the development of the North Anatolian Shear Zone during the late Pliocene to Pleistocene and which comprises a complex set of basins of diverse, strike-slip related origin, superimposed on a medial Miocene rift, is a good candidate to address this problematic. The submerged section of the right lateral strike-slip North Anatolian Fault system is indeed characterized by an intense seismic and fluid emission activity at the seabed. Seeps and seep-related morphologies and structures are widespread and scatter the seafloor. The nature and origin of the emitted fluids are various, with the occurrence of gas escaping bubbles mainly composed of thermogenic and microbial methane, in association with methane-derived authigenic carbonate precipitation, and more locally with heavier gases (e.g. C2 to C4), oil and (brackish) water releases. The use of acoustic ship?borne multibeam echosounder to detect fluid emissions escaping through the seabed into the water column provided an unique and unprecedented 3D picture, in particular, of the gas distribution over the Sea of Marmara. An intense and widespread seepage activity characterizes the Sea of Marmara with tens of thousands of gas bubble streams identified mainly along the Main Marmara Fault, the Western and Central highs and the edges of the Tekirdag, Central, Kumburgaz, and Çinarcik sedimentary basins. Gas emissions are spatially controlled by a combination of factors, including fault and fracture networks in connection to the Main Marmara Fault system and inherited faults, the nature and thickness of sediments (e.g. occurrence of impermeable or gas-bearing sediments, landslides), and the connectivity between the seafloor and gas sources, particularly in relation to the Eocene Thrace Basin. The relationship between seepage and fault activity is not linear, as active faults do not necessarily conduct gas, and scarps corresponding to deactivated fault strands may continue to channel fluids. Within sedimentary basins, gas is not expelled at the seafloor unless faulting, deformation, or erosional processes affect the sediments. On topographic highs, gas flares occur along the main fault scarps, but are also associated with sediment deformation. The occurrence of gas emissions appears to be correlated with the distribution of micro-seismicity. The relative absence of earthquake-induced ground shaking along parts of the Istanbul-Silivri and Princes Islands segments is likely the primary factor responsible for the comparative lack of gas emissions along these fault segments. The spatiotemporal distribution of gas seeps may thus provide a complementary way to constrain earthquake geohazards by focusing the study on some key fault segments.