Tectonic and sedimentary controls on widespread gas emissions in the Sea of Marmara: Results from systematic, shipborne multibeam echo sounder water column imaging

Abstract

Understanding of the evolution of fluid-fault interactions during earthquake cycles is a challenge that acoustic gas emission studies can contribute. A survey of the Sea of Marmara using a shipborne, multibeam echo sounder, with water column records, provided an accurate spatial distribution of offshore seeps. 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 and 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 microseismicity. 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, e.g., the northern part of the locked Princes Islands segment.

Pore water geochemistry at two seismogenic areas in the Sea of Marmara

“…The Sea of Marmara is also characterized by a large number of widespread gas expulsion sites at the seafloor, bubbling up to several tens to hundreds of meters into the water column [ Burnard et al ., ; Dupré et al ., ; Gasperini et al ., ; Géli et al ., ; Kuscu et al ., ; Tary et al ., ; Zitter et al ., ]. Such gases are primarily of thermogenic origin in the western Marmara, which is geologically connected to the hydrocarbon reservoirs of the Thrace basin, while mainly biogenic gases, i.e., generated by microbial activity, have been collected from the seafloor of the Cinarcik Basin and the Gulf of Izmit (Figure ) [ Bourry et al ., ; Gasperini et al ., ; Kuscu et al ., ; Ruffine et al ., ].…”

“…A recent long‐term monitoring experiment carried out close to the eastern termination of the 1999 Mw = 7.4 Izmit earthquake has confirmed the potential of combining seismological and gas‐geochemical observations of the seafloor to study the seismic cycle [ Embriaco et al ., ]. Previous studies of the water column in the Sea of Marmara have shown a systematic association between fluid expulsion sites along the North Anatolian Fault and the fault network, and the basin edges [ Dupré et al ., ; Kuscu et al ., , ]. Thus, those faults play an important role in the migration of deep‐sourced fluids to the seafloor.…”

Grain‐Displacive Gas Migration in Fine‐Grained Sediments

“…Gas migration mechanisms in soft sediments control gas release events (Dupre et al, 2015;Scandella et al, 2017), determine the formation of craters and pockmarks (Andreassen et al, 2017;de Prunele et al, 2017;Riboulot et al, 2016), and constrain the potential gas recovery strategies from methane hydrate accumulations in shallow marine sediments (Fauria & Rempel, 2011;Jang, 2014;Moridis et al, 2011). Gas migration pathways are also relevant to other degassing processes such as subsurface CO 2 leakage (Bang et al, 2013;Cevatoglu et al, 2015;Lewicki et al, 2007) and gas-driven volcanic eruptions (Suckale et al, 2016).…”

Increased Fluid Flow Activity in Shallow Sediments at the 3 km Long Hugin Fracture in the Central North Sea

“…The discovery of reduced fluids and evidence for the presence of shallow methane (high sulfide concentrations, bacterial mats) in the Hugin Fracture sediments suggests that fractures may also be important fluid migration pathways in the North Sea. Fault‐related methane seepage structures have mainly been observed in tectonically active areas such as the Marmara Sea (Dupre et al, ), the Santa Barbara Basin (Eichhubl et al, ) and the Nankai Trough (Henry et al, ). Our study shows that the Hugin Fracture is an active conduit transferring fluids from the deeper in the sediment column (<550 m below the seafloor) into surface sediments, with visual and geophysical indicators for near‐surface fluid flow, biogeochemical turnover of methane and large spatial variabilities.…”