Gas seepage and seismogenic structures along the North Anatolian Fault in the eastern Sea of Marmara

Abstract

[1] We carried out a combined geophysical and gas-geochemical survey on an active fault strand along the North Anatolian Fault (NAF) system in the Gulf of İzmit (eastern Sea of Marmara), providing for the first time in this area data on the distribution of methane (CH 4 ) and other gases dissolved in the bottom seawater, as well as the CH 4 isotopic composition. Based on high-resolution morphobathymetric data and chirp-sonar seismic reflection profiles we selected three areas with different tectonic features associated to the NAF system, where we performed visual and instrumental seafloor inspections, including in situ measurements of dissolved CH 4 , and sampling of the bottom water. Starting from background values of 2-10 nM, methane concentration in the bottom seawater increases abruptly up to 20 nM over the main NAF trace. CH 4 concentration peaks up to $120 nM were detected above mounds related probably to gas and fluids expulsion. Methane is microbial (d 13 C CH4 : À67.3 and À76‰ versus VPDB), and was found mainly associated with pre-Holocene deposits topped by a 10-20 m thick draping of marine mud. The correlation between tectonic structures and gas-seepages at the seafloor suggests that the NAF in the Gulf of İzmit could represent a key site for long-term combined monitoring of fluid exhalations and seismicity to assess their potential as earthquake precursors.Components: 9500 words, 11 figures, 2 tables.

Detecting and Measuring Gas Seepage

“…Such instruments are typically employed in the marine environment, in vertical casts, in horizontal profilers, or in benthic platforms (e.g., Marinaro et al 2006;Camilli and Duryea 2007;Newman et al 2008;Krabbenhoeft et al 2010;Gasperini et al 2012;Embriaco et al 2014). A review of the present technology is provided in Boulart et al (2010).…”

“…In offshore petroleum exploration areas, seawater can be pumped from a ship through deep towed sample inlets at depths of 100-200 m and carried to an onboard analysis system (e.g., Sackett 1977;Philp and Crisp 1982;Gasperini et al 2012). Gases in solution can then be stripped from seawater samples and analysed via gaschromatography or other sensors.…”

Seafloor Seismic Noise at Central Eastern Mediterranean Sites

“…1b). These sites are characterized by a widely distributed seismicity that is connected to the very complex tectonic framework of the region (e.g., Vannucci et al, 2004;Ambraseys, 2009;Godey et al, 2013). Southern Italy, in particular, has been the site of both vigorous volcanic activity and destructive earthquakes, for example, the 1693 earthquake in eastern Sicily (M 7.0;Piatanesi and Tinti, 1998).…”

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 ., ]. The chemical composition of pore waters is also very heterogeneous from one site to another [ Burnard et al ., ; Cagatay et al ., ; Halbach et al ., ; Tryon et al ., ].…”

“…Does microseismicity influence the “normal” emission of gas and fluid to the seafloor? Previous studies carried out in different parts of the SoM [ Çağatay et al ., ; Gasperini et al ., ; Gasperini et al ., ; McHugh et al ., ] attempted to address these questions without providing an ultimate answer.…”